Plankton: A Worldwide Guide 0691255997, 9780691255996

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PLANKTON

PLANKTON A WORLDWIDE GUIDE T O M J AC K S O N A N D J E N N I F E R PA R K E R C ON S U LTA N T E DI T OR : A N DR E W H I R S T

PRINCETON UNIVERSITY PRESS PRINCETON AND OXFORD

Published in 2024 by Princeton University Press 41 William Street, Princeton, New Jersey 08540 99 Banbury Road, Oxford OX2 6JX press.princeton.edu Copyright © 2024 by Quarto Publishing plc Conceived, designed, and produced by The Bright Press an imprint of The Quarto Group 1 Triptych Place, London, SE1 9SH, United Kingdom T (0) 20 7700 6700 www.quarto.com All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage-and-retrieval system, without written permission from the copyright holder. Requests for permission to reproduce material from this work should be sent to [email protected] Library of Congress Control Number: 2023940417 ISBN: 978-0-691-25599-6 Ebook ISBN: 978-0-691-25608-5 British Library Cataloging-in-Publication Data is available Publisher: James Evans Editorial Director: Isheeta Mustafi, Anna Southgate Art Director: James Lawrence Managing Editor: Jacqui Sayers Senior Editor: Izzie Hewitt Project Editor: Ruth Patrick Copy Editor: Caroline West Design: Anna Gatt Picture Research: Katie Greenwood Illustrations: John Woodcock Cover and prelim photos: Front cover: British Antarctic Survey/ Science Photo Library Endpapers: Norman Kruing/USGS/NASA Page 2: Alexander Semenov/Getty Images Page 4–5: Scenics & Science/Alamy Cover design: Wanda España Printed in Malaysia 10 9 8 7 6 5 4 3 2 1

CONTENTS Foreword Introduction A DRIFTER’S LIFE Chapter Chapter Chapter Chapter Chapter Chapter

1 2 3 4 5 6



06 08

A WONDERFUL DIVERSITY 12 LIFESTYLES AND ADAPTATIONS 54 FEEDING AND BREEDING 90 WHERE PLANKTON WANDER 124 FEEDING THE OCEANS 152 FACING THE FUTURE 182

Glossary Further Reading Index Picture Credits

218 219 220 224

FOREWORD The oceans, lakes, and running waters that cover our planet make it a blue and wet home in large part. Life started in these aquatic environments, and today they host diverse organisms that play an important function in the health of the planet. All the major branches of life are represented here, with some that do not even exist on land. The roles these environments and their inhabitants play are critical: photosynthetic production culminating in major sources of protein for growing populations; supplying half the oxygen we breathe; removing from the atmosphere a major portion of the carbon dioxide we have produced from burning fossil fuels. Plankton, the drifters in the water environments, are explored in this book. There are case studies covering a wide range of species, from well-known animals and plants to more obscure groups. This tour of the plankton not only provides insights into their adaptations and how they make a living, but also offers details on the techniques we use to collect, count, and assess these creatures and the roles they perform. It is not an overstatement to say that the future of humankind and our planet are intimately interwoven with the success of the plankton. Shifts in climatic temperatures, environmental impacts, and associated changes in plankton and aquatic environments are therefore also explored in this volume.

plains, there are few places to hide here. The arms race between predators and their prey has led to many planktonic species being virtually transparent and therefore difficult to see; other species have dilute watery bodies that allow extremely rapid growth in size but which make calorie-poor meals, while some have escape speeds of 500 body lengths per second, ten times the 50 body lengths per second achieved by an F16 fighter jet. This liquid environment is also a challenge for finding sufficient resources and mates, and many planktonic solutions are explained here.

The study of plankton The scientific study of plankton is in many ways a young discipline, and while scientists have raced to address many important questions, they are still working hard to provide understanding from the largest scale, tracing global elemental cycles, to the molecular scale, sequencing newly discovered genomes. In reading this book I very much hope you get a sense of the many advances being made in this discipline and the excitement of the discovery that is underway.

Professor Andrew Hirst

Planktonic form and function The striking photographs in this book highlight the magnificence of planktonic life-forms: from the “etched” silica covering of the phytoplankton Coscinodiscus, to the glittering colors of “sea sapphire” copepods Sapphirina, and the delicate feeding frills of the cnidarian jellyfish Pelagia noctiluca. We prize great art in our societies; this book perfectly highlights the art that nature has created. Yet their beautiful forms are intimately linked with their functions. Evolution has produced morphologies, life histories, and behavioral strategies that allow plankton to overcome the challenges of living in open waters. This is an environment that is viscous; in comparison to air, it is much more difficult to extract oxygen, and it is full of the hungry mouths of predators. Unlike forests or grassy 6|

FOREWORD

| A sea sapphire; the males of this marine copepod show distinctive glittering colors. The male swims in a spiral when performing a courtship dance to attract females. This movement, together with how the color is created, causes it to vary between iridescent to near invisible depending on the angle of its body.

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8|

INTRODUCTION

A DRIFTER’S LIFE Plankton are all too easy to ignore, but we do so at our collective peril and to our personal detriment. Collectively, we should be thinking a lot more about plankton because they are often characterized as the “lungs of the world.” Set aside your preconceptions about rainforests for a minute. The plankton drifting in the world’s oceans are generating almost twice as much oxygen as jungle trees. Surely that fact alone means we should give plankton our full attention. But that is not all. These tiny organisms are the basis for many of the food chains in the ocean. Billions of fish, whales, sharks, seabirds, seals, you name it … rely on plankton to form the foundations of their marine ecosystems. And many of those fish and other sea creatures are foods that we harvest from the oceans daily. Eventually the life stories of plankton will be played out at the dinner table. Taking a deep dive into plankton brings you eye to eyespot with some of the world’s most awesome organisms. There are billions of tons of microbial creatures with an unimaginable diversity simply floating in the waters. At the large scale, plankton include jellyfish that are longer than blue whales and bag-like salps with fish living inside like a wild-born aquarium. At the smaller end are crystal-cut microbes with intricate silica skeletons, tiny fish fry, mini shrimp-like krill gathering in gargantuan swarms, and even snails that build a raft of bubbles to sail the wide oceans. And among them are the phytoplankton, microscopic plant-like organisms that played the leading role in creating this green-blue planet we call home. And all of this from plankton that are almost invisible to the naked eye!

What are plankton? Defined briefly, plankton are aquatic life-forms that cannot swim very well in relation to other marine creatures, if at all. They live in almost every sizable body of water, from a modest rockpool to the greatest ocean. They are united by virtue of being passive passengers, going where the currents, tides, and waves take them. The term plankton was introduced in the 1880s and derives from the Greek word for “wanderer.”

In more detail, plankton comprise a complex diversity of all kinds of living things, some plant-like, some animal-like, and others that defy the dichotomy and are best described as something in between. Plankton can be categorized in numerous other ways as well: by their size; by the way they gain their energy and nutrients; by their life cycles; and by their ecology and their roles in watery environments. A more precise definition of plankton is very difficult to muster. Swarming in their trillions, some do have a limited ability to move under their own power. This is usually as part of a DVM—a diel (diurnal or daily, that is, 24 hours) migration, with many plankton migrating to the surface at night to feed, and returning to safer, darker deep waters in the daytime. These planktonic migrants are often dominated in terms of numbers by marine copepods, which are small crustaceans that are relatives of shrimp and crabs, as well as squid and other members of the mollusk group, and various fish. They travel upward at dusk for the hours of darkness, then down again by dawn to hide from daylight in deeper, darker waters. Despite this up-and-down motion, the selfpowered movements of these DVM hordes are very limited in horizontal directions, and usually negligible at the scale of great ocean currents.

Unsung heroes For several reasons, including being mostly very small, plankton are “sea soup” and the basis of innumerable food chains and ecosystems. Like plants on land, microscopic phytoplankton capture the sun’s light energy and use it for growth and reproduction. These humble organisms are little recognized and yet fundamental. They power a huge amount of ocean life. The phytoplankton are eaten by small animals and animal-like organisms called zooplankton. In turn, both of these groups are the primary food for bigger animals, which are consumed by still larger ones, and so on. In this way, aquatic food webs build up to support a vast array of creatures,

| The scientific name of the mauve stinger jellyfish, Pelagia noctiluca, encapsulates the creature’s habit of glowing in dark water at night.

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9

including the world’s biggest animal species—the great whales—and in many communities the ocean food webs are an intrinsic part of the human diet. So plankton, despite their unglamorous image and relative obscurity, provide the energy and sustenance for myriad forms of life in both marine and freshwater bionetworks. They replenish gaseous oxygen in our atmosphere and dissolved oxygen in aquatic environments. They also cycle the greenhouse gas carbon dioxide between the atmosphere and the watery realms. They are critical to the health of our seas and oceans. They are, indeed, unsung heroes of our hugely complex and multifaceted global ecosystem.

oceanic composition; and the vast, looming, overarching processes of climate change and global warming. Research into planktonic health and fitness can help to guide us as we strive to extract ourselves, our artificially cultivated environments, and what remains of Earth’s natural wilderness, from the challenges that humanity is generating. Plankton reveal the problem and will in many ways show us how to repair the damage we are causing. Plankton need attention. This book is a good place to start.

Guides to the future Due to their huge significance, plankton are also vital indicators of the biological and ecological status of Earth’s biosphere. Measuring, tracking, and recording them—their numbers and seasonal fluctuations in abundance, their biodiversity, distribution, and welfare—are becoming increasingly essential. Plankton are revealing the impacts of modern human-created disturbances such as rampant pollution; overfishing and food web disruption; the progressive changes of atmospheric and 10 |

INTRODUCTION

| Not all the eggs of the smooth lumpsucker (Aptocyclus ventricosus) will hatch. The viable ones have an embryo with obvious eyes visible inside.

ABOVE

OPPOSITE ABOVE | A colony of common salps (Salpa fusiformis) hangs out in the coastal waters of Balayan Bay, in the Philippines. OPPOSITE BELOW | The larva of a blind lobster, a deepsea species of Stereomastis from the tropical Atlantic Ocean, floats in the Atlantic waters off Cape Verde.

11

CHAPTER 1

A WONDERFUL DIVERSITY A bucketful of seawater does not appear to be full of life. Most plankton are too small to see with the naked eye, so the water seems largely clear when given a cursory glance. But take a closer look and you’ll find the oceans are teeming with hidden life with a diversity to rival that of a rainforest or other land habitat. Upper estimates suggest that more than 250,000 species of plankton have been identified, but that number could be only 10 percent of the total count. The panoply of plankton species contains a full range of life on Earth, from the smallest bacteria to complex multicellular life such as jellyfish, free-floating seaweeds, and the microscopic larval forms of fish. Nevertheless, plankton includes some of the most primitive or ancient groups of organisms on the planet. Multicellular plants and animals, such as worms, vertebrates, and mollusks, have their genesis in the Cambrian explosion that took place around 500 million years ago. For hundreds of millions of years preceding this astounding radiation of life, unicellular plankton were a dominant form of life on Earth, floating as they did in the later servings of the primordial “soup.”

PROKARYOTES AND EUKARYOTES For most of the natural history of our planet, the phrase “life on Earth” has mostly referred to plankton. The earliest evidence of life dates back 3.8 billion years when it was mostly confined to the oceans, floating in the waters of the young Earth. To begin with, for at least a billion years, the plankton was exclusively made up of prokaryotic life. The more familiar life-forms in and out of the ocean that we see around us today are eukaryotic, so named because their body cells have a distinct nucleus. Humans are eukaryotes, as are seaweeds, jellyfish, and the green algae in the ocean. These creatures, or rather the cell type they use to build their bodies, be they a single-celled organism or straggly salp several meters long,

evolved around 2.5 billion years ago. Again, this remarkable step in the history of life may well have occurred in a bloom of plankton. The truth is nobody knows. Eukaryotes evolved from a more primitive form of life called the prokaryotes. The latter term means “before nucleus,” and organisms under this banner have simple insides with no distinct nuclear structure or other organelles. The complex cells of the eukaryotes are, in fact, a team of simpler, smaller prokaryotic cells, which came together in a near miraculous event called symbiogenesis. The details of that story are for another time, but the prokaryotes involved are still very much part of today’s plankton community.

| The rainbow waters of the Grand Prismatic Spring, in Yellowstone National Park, Wyoming, USA, are colored by extremophile bacteria in the water and growing in mats on the bottom. Only the superheated blue center is empty of microbes.

OPPOSITE

BELOW

| The steps of endosymbiosis proposed by the theory of symbiogenesis that led to the evolution of complex cells. | Generalized diagrams of a prokaryotic cell (below) and eukaryotic cell (bottom).

BELOW

Prokaryotic cell structure Cytoplasm

Pili

Cell wall

Flagella

Nucleoid

Plasma membrane

Eukaryotic cell structure Nucleus

Nucleolus

Golgi apparatus

14 | A WONDERFUL DIVERSITY

Vesicle

Endoplasmic reticulum

Plasma membrane

The process of endosymbiosis

1 An  archaeon grows in size. As a result its cell membrane becomes folded to increase the total surface area. This allows the organism to transport enough materials in and out of the cell.

Prokaryotic cell Nucleoid (containing DNA)

Cytoplasm

1

Cell membrane Cell membrane infoldings

Archaeon

2 S  ome of the folds detach from the cell membrane. Some surround the cell’s genetic material to form a nucleus. Others become endoplasmic reticula, centers of metabolic activity now seen in all eukaryotic cells. 3 T he cell consumes a free-living bacterium, probably as an item of food to be digested inside the cell. However, the bacterium is somehow spared destruction and begins to live as a symbiont.

2 Nucleus Nuclear membrane Endoplasmic reticulum

3

Protobacterium

First eukaryote

4 Mitochondria Cyanobacterium

4 In return for a place to live, the symbiont provides the host cell with a more efficient means of handling energy. The symbiont divides inside the cell and is passed on when the host also divides. This is the origins of the eukaryotic mitochondrion. 5 T he first eukaryotes are heterotrophic. The first autotrophs arise later when a cyanobacterium enters the cell, again probably consumed as prey but which manages to thrive in its new home. This symbiont has become the chloroplast, the eukaryotic organelle for photosynthesis.

Mitochondrion

Ancestor of animals, fungi, and other heterotrophs

Chloroplasts

5

Ancestor of plants and algae

15

Primitive life A more familiar name for a prokaryote is a bacterium, but the group also includes archaea. To the layperson, and most biologists for that matter, the difference between bacteria and archaea is rather opaque. In fact, the two groups were only formally divided in the later 1970s. That division is based on the distinct chemistry of their most crucial metabolism, and this can only mean that these organisms have lived separate lives since almost the dawn of life itself. While bacteria make up a large proportion of oceanic plankton, archaea are much less common in the water column. They dominate extreme anaerobic habitats where there is no oxygen to drive metabolism. While planktonic communities are almost exclusively aerobes, archaea are able to survive in the chemical-rich, low-oxygen, and unusually warm waters of hydrothermal springs. The host cell of the endosymbiosis, as in the large cell within which the other prokaryotes gathered, is thought to be an archeon. The nucleus and other structures made from folded internal membranes evolved inside this primordial organism. Eukaryotic cells also have mitochondria, the power packs of the cell, and many have chloroplasts, which host the process of photosynthesis. The mitochondria are the descendants of a bacterium that is now spread by tick bites, so its place in the story of plankton is not clear. However, the chloroplast, the photosynthetic capsule that has turned the world green— including occasional swathes of the ocean—is related to today’s cyanobacteria. Also known, rather incorrectly, as the blue-green algae, these bacteria, barely 1 millionth of a meter long, make up about three-quarters of the phytoplankton on Earth. (Having said that, cyanobacteria are found in almost every other habitat, including soils and swamps, and even float in the air as aeroplankton.)

Other phytoplankton Prokaryotes are all single-celled organisms, although it is not uncommon for cyanobacteria and other bacterioplankton to cluster into colonies and chains. In contrast, all multicellular organisms, in plankton or elsewhere, are eukaryotic. They are built from collections of large, complex cells that take on specialist roles among the whole. 16 | A WONDERFUL DIVERSITY

That said, there is an entire kingdom of life (some say several kingdoms) made up of single-celled eukaryotes. This group is called the Protista, or less formally, protists. They include familiar life-forms like amoebas and protozoa. In terms of plankton, the most abundant protists are plant-like phytoplankton. These microscopic plankton are “plant-like” because they use photosynthesis to harness directly the energy of the sun. However, many share features with animals. For example, a euglenid, a green-colored protist, which is common in freshwater plankton, gets food and energy both from eating foods and photosynthesis.

Diatoms Out in the ocean, the photosynthetic protists are dominated by diatoms, a kind of golden algae with exquisite “shells” made from silica around their cell bodies. There are an estimated 100,000 species of these beautiful microbes, although many of them are not planktonic. (Instead, they are adapted to living in the benthic community, those organisms that survive in sediments on the seafloor.) The diatom’s shell, better known as a frustule, comes in two halves with one fitting snugly over the other to provide a rigid capsule for the cell inside. There are two main forms: Centric diatoms have rounded shells when viewed from above, while from the same angle pennate diatoms have a boat shape. Some also have elaborate spines, called setae, which create intricate, crystal-like structures when the diatoms cluster in colonies. Whiptails The other two main forms of phytoplankton protists are the flagellates and dinoflagellates. All these organisms have at least one flagellum (plural flagella), which is a long, whiplike structure that emerges from the cell membrane. Flagella are mostly used as a means of locomotion, pushing against the water as they wiggle and whirl. This movement is immaterial when flagellated plankton are borne along by ocean currents, but the appendage helps with evading capture by bigger beasts and also enables many of these photosynthesizing protists to grab prey—often diatoms—and engulf them as a supplement to their diet.

Dinoflagellates have two flagella. There are only around 2,000 known species, but they receive a lot of attention because the toxic blooms of some cause dangerous “red tides” that can decimate marine wildlife, threaten fisheries, and pose a danger to humans in the water. Dinoflagellates are independent single-celled organisms, but they often form long, chain-like colonies. There are two groups, although the division is not a clear one. Thecate dinoflagellates are covered in spines and plates made from cellulose (also the main structural material in plant cells). Athecate dinoflagellates lack these features. Flagellates are much more of a grab-bag of organisms. They all have flagella, mostly two but sometimes up to 16. Often the flagella are equal in length, although occasionally not. The cell shapes tend to be ovoid but can elongate into rods. A range of spines, plates, and shells can help distinguish one flagellate from another and sometimes the cells cluster into colonial sheets and chains. In other words, they come in a very wide range of shapes and sizes. All flagellates photosynthesize and some also feed on other plankton, engulfing their smaller prey through a furrow-shaped “mouth” on one side of the cell.

| The euglenid Phacus pleuronectes has an eyespot that it uses to seek out bright water and thus boost its rate of photosynthesis.

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ZOOPLANKTON Plankton that are exclusively heterotrophs—or “other eaters”—which survive by consuming the bodies of other organisms are grouped as zooplankton. Of course, many of these are animals, such as crustaceans and jellyfish, but there are also protists that fit this description. The main protist groups are the ciliates, foraminifera, and radiolaria.

Protozoa Due to their animal-like tendencies, ciliates, foramenifera, and radiolaria are loosely termed protozoa, or “early animals.” This early name suggested that such microbes are the simpler relatives of us, the animals. However, later scholarship has 18 | A WONDERFUL DIVERSITY

revealed that the first multicellular animals probably evolved from choanaflagellates, a group of protists, some of which are planktonic, that filter bacteria from the water. No matter, the name has stuck. There are around 8,000 species of ciliate. As the name suggests, they have multiple cilia, which are small, hairlike extensions of the cell membrane. These are used to draw particles of food to a feeding pore, or oral cavity, on one side of the cell body. Foraminifera live deep within a protective shell, or test. The shapeless cell body is at the heart of this intricate construction of calcium-rich chemicals, and thin filaments of membrane, called filopodia, connect to the water outside. There are 7,500 species described, but this is likely to be just a fraction of the true total. Foraminifera are unicellular, but their tests can be

more than 0.04 in (1 mm) wide. They are the most common shelled organisms in the oceans. Their dead remains sink to the seafloor and create a thick layer of eerily still ooze. Radiolarians are characterized by a lattice of spines that surrounds the cells, creating a spiked ball or lenticular structure that can be as big as 0.02 in (500 µm) across. These protists are hard to keep in lab cultures and so almost everything we know about them—which is not much—comes from watching them in the wild.

| The marine radiolarian Actinomma delicatulum has a beautiful siliceous skeleton.

OPPOSITE

| An example of Euplotes, a freshwater genus of a kind of ciliate known as a hypotrich. This light micrograph, magnified around 400 times, shows the dense tufts of cilia at the top and bottom of the cell body.

ABOVE LEFT

ABOVE RIGHT | This scanning electron micrograph with added color shows the fossil remains of a tiny foraminiferan test (protective shell).

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Metazoa True animals have the archaic name metazoa, which means “later animals.” In reality, any organism that has the complexity to qualify as a metazoan also meets the bar of being an animal. Animals are multicellular eukaryotic organisms that feed on other organisms to access the energy and nutrients they need. While the simplest animals of all, blob-like organisms such as sponges and placozoa, live on the seafloor, the zooplankton community is made up of members of all the major animal phyla, even the ones that have made a name for themselves in larger forms on land. The phyla include the mollusks, which are

Mycoplankton Not all heterotrophic plankton are animals (or animal-like). There is a surprisingly large contingent of fungi in plankton communities as well. Known as mycoplankton, the fungal community is mostly microscopic, although thumb-sized colonial masses of fungal filaments are commonplace. Fungi are a separate kingdom of multicellular (and eukaryotic) organisms alongside the plants and animals. They cannot photosynthesize and have no mouths or digestive systems for consuming food. Instead, they are saprotrophic, which—somewhat uncharitably— means “rot eater.” On land, that is a primary role of fungi. They grow on their food, be it a fallen log or discarded fruits, and exude digestive chemicals directly onto it. The enzymes digest the food externally and then the fungus just needs to absorb the nutrients it requires. In the sea, this mode of eating remains, although the fungi’s food is mostly the dead remains of phytoplankton. As a result, mycoplankton are most abundant in surface waters, where sunlight sustains the phytoplankton. It is estimated that there are a thousand single-celled fungi in a milliliter of seawater (compared to a million bacteria).

20 | A WONDERFUL DIVERSITY

the relatives of snails and squids; the arthropods; which include crustaceans, one of the mainstay groups of zooplankton; and, of course, chordates. Chordata is the phylum that contains the vertebrates, or animals with a spine. As well as us humans, the chordates include fish, which contribute in small ways to the plankton community, as we’ll see later. There are also many other phyla, including several types of worm, which make a showing in the plankton. Despite their apparent simplicity, worms, planktonic or not, have a bilateral body plan. In other words, the left side is a mirror image of the right, and there is a head and mouth at one end and a vent

toward the other. This seems rudimentary to animal life, but a significant proportion of zooplankton do not have this bilateral body. Instead, they have radial bodies with a mouth at the center and no discernible head or central control unit to the body. These animals are best known as jellyfish (Cnidaria), but any student of plankton will know that they are an extraordinary diversity of forms. They do indeed have a simpler body plan, but the best advice is stay back and marvel. These creatures not only glitter in the dark; they are also built to sting!

| This salmon egg is infected with the fungus Saprolegnia parasitica. The fungus causes saprolegniosis (water mold disease), which affects wild and cultured freshwater fish and fish eggs worldwide.

OPPOSITE

BELOW | Colored scanning electron micrographs of silica spicules. These tiny objects found floating in the plankton are the structural units of sponge bodies.

21

JELLYFISH

The polyp and medusa stages Tentacle

Mesoglea Mouth

Gastrovascular cavity

Endoderm

The planktonic jellyfish mostly belong to the phylum Cnidaria. This group includes organisms traditionally called jellyfish (although this belies their diversity), which are almost universally planktonic, along with sedentary bottom-dwellers like corals and sea anemones. Even so, these latter two groups spend their early days as a planktonic larval form.

Ectoderm

Mouth Endoderm

Ectoderm

Cnidaria body plan The basic body plan of a cnidaria is a bell-shaped cavity with tentacles attached. In the case of the anemones and corals, the adult body has tentacles that face upward (a form known as a polyp), while the jellyfish and their allies have a medusa body, where the tentacles hang down from the main body. The name Cnidaria is based on the Greek word for nettle, and indeed the jellyfish, anemones, and even the corals have hundreds of thousand stinging cells on their tentacles. These are primed to launch barbed stingers into whatever they touch. The stingers form a physical connection, which can be interpreted as stickiness. Anyone that has fingered an anemone in a rock pool

Mesoglea Basal disk Polyp

Gastrovascular cavity

Medusa

Tentacle

will recognize this. However, the stinger cells, known as nematocysts, also deliver a venom that is concocted to paralyze prey. Once that is achieved, the captured victim is hoisted to the mouth. Although we use the word mouth for cnidarians, this opening also acts as the genital pore that pumps out sperm and eggs and as the anus, where the waste from digestion is ejected.

The life cycle of a jellyfish

| A generalized diagram of a polyp and medusa, the two body plans of cnidarians.

ABOVE

Adult jellyfish (medusa phase) releases eggs and sperm into the water

| A generalized life cycle of a jellyfish, which sees the organism develop from an egg and through the different sedentary polyp stages to planktonic adult medusa.

Planula larva develops from a fertilized egg

LEFT

| A crown jellyfish (Cephea cephea). Also sometimes called the cauliflower jellyfish.

OPPOSITE

Polyp with buds

Polyp phase Ephyra larva

22 | A WONDERFUL DIVERSITY

23

The “jellyfish” are able to swim by flexing their muscular bodies in a rhythmic way to create a jet of water pumped from the body cavity. However, their efforts are not powerful enough for them to move against ocean currents, and so they drift wherever the water takes them.

Classifying jellyfish The animals popularly known as jellyfish fall into three orders. The true jellyfish belong to the Scyphozoa, which have the typical umbrella-like body frilled with small tentacles and more substantial oral tentacles that hang below the mouth. There are around 200 species of these “true” jellies, with some growing to 61⁄2 ft (2 m) wide. Less numerous are the Cubozoa. As their name suggests, these are sometimes called box jellyfish, or more aptly sea wasps, because they sting swimmers in shallow waters. They are about the size of a shoebox at most, including the tentacles. The squarish shape makes them more efficient swimmers and they have a top speed of 1,000 ft (300 m) an hour! Some are highly dangerous, with venoms that can stop the heart within minutes and others that cause extreme pain. (Most stings are less severe. The best treatment is a mild acid, such as vinegar.) The third group of jellies is the Hydrozoa, which includes nearly 4,000 species. Their bodies are generally small and simple compared to their cousins. There are colonial forms of hydrozoans where the body is composed of several interdependent organisms that have taken on different roles—feeding, breeding, and hunting, for example. These colonial hydrozoans—also called siphonophores—can trail stinging tentacles 130 ft (40 m) long. They are one of the most numerous hunters in the ocean.

| A box jellyfish with a cubic body and several short tentacles.

ABOVE

OPPOSITE ABOVE | A bloom of sea gooseberries (Pleurobrachia pileus) in the North Atlantic off the coast of Shetland. These comb jellies snare mesoplankton with a pair of long tentacles that dangle from the main body. OPPOSITE BELOW

24 | A WONDERFUL DIVERSITY

| Beroe abyssicola comb jelly.

Comb jellies Often mistaken for their stinging cousins, the comb jellies actually belong to a separate phylum named Ctenophore. Comb jellies are thought by some to be even more primitive than jellyfish and may be the last living relatives of the first multicellular animals to evolve. Around 100 species have been described, although there are likely to be considerably more. The difficulty is retrieving undamaged samples of these most fragile of animals. Almost all comb jellies are planktonic carnivores. Some are barely visible without a microscope, while the largest species are more than 3 ft (1 m) across. The “comb” moniker comes from the tooth-like fringes of cilia that are used for controlling swimming and buoyancy.

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CRUSTACEA No netting sample of plankton is complete without a plethora of crustaceans. They represent at least half the biomass of the mid-sized mesoplankton. The Crustacea is one of the largest and most diverse of all animal groups, only beaten by the insects that dominate the land. There are around 70,000 species of crustacean described so far, but many of them, especially the planktonic species, are difficult to tell apart. That and the lack of sampling and research means it is estimated that at least 90 percent of these species are as yet unknown. As arthropods, crustaceans have a body supported by an exoskeleton instead of an internal one. Breathing is via gills, the legs are highly jointed, and the animals use limb-like appendages as antennae and mouthparts. Beyond these simple

common features the diversity of crustaceans is difficult to take in, ranging as they do from spider crabs and pill bugs (woodlice) to water fleas and mantis shrimp.

Some crustacean examples Copepods alone, just one of the many subclasses of crustaceans, are considered to be one of the most abundant forms of animal on Earth. Despite this, one can be forgiven for not really knowing what they are. Adults are seldom more than a few millimeters long, and they are considerably smaller than this for much of their life cycle. Copepod means “oar foot” in reference to the paddling action of the legs when swimming. Other crustaceans include the decapods, which are perhaps the most familiar in that they include crabs, lobster, and prawns. Adult prawns and shrimp are found in vast shoals as part of the plankton, while the larval forms of the larger decapods are also mostly drifters before settling down to a life on the seabed. The same is true of mantis shrimp, which are vicious raptorial hunters that spend their younger days in the plankton, but occupy a different subclass called Stomatopoda. There are other prawn-like crustaceans, most notably the euphausids, which are better known as krill. How they differ from shrimp is rather academic and to do with body segmentation and the like. There are only 86 species of krill, but they make their presence felt in the mid-range of the plankton world. A krill swarm in cold polar waters packs 30,000 animals into every cubic meter!

ABOVE LEFT |

An adult peacock mantis shrimp (Odontodactylus scyllarus) on a coral reef in the Philippines.

| Sea sapphire males display iridescent flashes of color, possibly to attract females. The colors are structural in that they are formed by the interplay of light reflecting from layers of crystals inside cells.

LEFT

| A seething shoal of Thysanoessa spinifera, a Pacific krill, near the coast of southern California.

OPPOSITE

26 | A WONDERFUL DIVERSITY

27

SWIMMING WORMS For the uninitiated, a worm is a worm. It’s a long, legless, wiggling creature. However, this simple body plan is not due to a close relationship between wormlike animals. Evolution has produced worms many times over. The big hitters are the round worms, or nematodes; the flatworms, which are often parasitic; and the annelids, which include leeches and earthworms among a trove of other forms. The first two members of wormkind are not features of plankton, but the annelids are, specifically marine creatures called polychaetes. This name means “many hairs” and polychaetes have multiple bristle-like appendages which function as primitive limbs for walking or swimming. Several polychaetes are planktonic throughout their life cycles, while bottom-dwelling species spend their early days as tiny larval forms that drift in the water. Another significant group of planktonic wormlike animals are the chaetognaths, or arrow worms. These translucent, dart-shaped worms are mostly a few millimeters long but are present in vast numbers. The cumulative biomass of arrow worms is about a third of the weight of the copepods, their preferred food source. The predator-prey relationship between these worms and those little crustaceans is one of the driving forces of the diel vertical migrations (see page 134). The arrow worms are only distant relatives of the polychaetes. Instead, they are more closely aligned with nematodes, arthropods, and that other major phylum of animals, the Mollusca.

RIGHT | This view of an arrow worm (Sagitta sp.) shows the hooked, grasping spines on each side of the head.

28 | A WONDERFUL DIVERSITY

SNAILS AND ECHINODERMS Mollusks are yet another invertebrate group with a startling diversity on land and in the sea. They are no doubt most familiar as garden snails and slugs, but also include shellfish like scallops and limpets, plus brainy cephalopods like the giant squid and cuttlefish. The majority of mollusks are shelled creatures, and this precludes them from a free-swimming lifestyle. However, around 150 mollusks, mostly snails, have evolved to be fully planktonic. Snails are known for gliding around on their singular muscular foot. In the planktonic form, this foot has spread out and thinned into a winglike structure more suited to swimming. The snails exude strings of mucus to help with buoyancy and act as sea anchors to hold the animal steady in the water. The shell is very thin and lightweight and in some cases too small for the animal to withdraw all of its soft body parts. Another group of heavy-set creatures are the echinoderms, better known as the starfish and sea urchins. Echinoderms have bone-like skeletons in the skin, which, as well as protection, add weight that hinders swimming. The echinoderms are famed for their five-fold radial symmetry, but this is only a feature of the most familiar adults. The larval forms of starfish, brittlestars, and the like are bilaterally symmetrical. They are also small and lightweight enough to be part of the plankton, and during the breeding seasons these larval urchins and starfish can make up a significant proportion of the zooplankton.

TOP RIGHT | The free-swimming echinopluteus larva of the sea potato sea urchin (Echinocardium cordatum).

Small phyla, but not minor Oceanic zooplankton are also members of lesser known but no less interesting animal phyla with only a few thousand species between them. These include the bryozoans, which form tiny colonies on the seabed or just about anything solid and submerged. Their microscopic larvae are a common feature of plankton. The bryozoans are a close relative of the brachiopods and phoronids. The brachiopods, or lamp shells, live on the seabed when adult, standing above the sediment on little stalks, but are planktonic as larvae. Phoronids are like gnarled worms and live in the seabed of shallow seas. In the breeding seasons in spring and fall, phoronid larvae become a large part of the plankton in these waters.

ABOVE RIGHT | This swimming snail is called a sea angel. It is a type of mollusk called a pteropod, literally meaning “winged feet” in reference to its muscular swimming.

29

CHORDATA The ocean-going members of the phylum Chordata are generally hailed for their size, strength, and impressive intelligence. They include the usual suspects of dolphins, sharks, whales, and turtles. Although none of these seems to have a place in the plankton, chordates are very much present in the zooplankton community. A major component are fish eggs and the tiny larvae that hatch from them. Oceanic fish of all kinds, from hake and swordfish to sea bass and anglerfish, spawn in open waters. This means their eggs, fertilized or not, float in their trillions in the water column. The lucky few eggs, which are 30 | A WONDERFUL DIVERSITY

mostly less than 0.08 in (2 mm) wide, will hatch into tiny larval fish. These minute vertebrates, themselves only a few millimeters long, have a vaguely alien appearance, mostly because they emerge with a yolk-sac life-support system dangling from the body. This provides the energy and nutrients the baby fish needs as it starts out in life. The early days, probably early months, of that life will be spent in the plankton, where the fish steadily grow until they are able to swim against the currents and make their own way in the world.

Tunicates

| The larval stage of a flatfish in the Mediterranean Sea. The eggs and larvae of fish are known as ichthyoplankton.

OPPOSITE

| Common salp (Salpa fusiformis) with copepods inhabiting its inner chamber in Balayan Bay, in the Philippines. Salps are non-vertebrate members of the Chordata.

ABOVE

Not all members of the Chordata are vertebrates. There is a subgroup called the tunicates, which share the nervous structures that develop into our brain and spinal cord. In the tunicates, however, that process is diverted to creating a flimsy, tubeshaped creature that filters seawater for whatever morsels it can get hold of. Sometimes meeting distant relatives is a shock! Most of the 3,000 species of tunicate are benthic, and have names like sea squirts and sea tulips. However, there are some planktonic members, which fall into two groups: the salps and larvaceans. The barrel-shaped salps are mostly a few centimeters long and could be easily mistaken for a blob of gunge. However, they can form elaborate colonial chains that trail for meters through the water. The larvaceans are mostly about 0.04 in (1 mm) long. They have a tadpoleshaped body surrounded by a “house” made of jelly—a mixture of cellulose and mucus. The animals float around in the bubble-like house, which filters suitable food particles from the surrounding waters.

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TAKING SAMPLES One needs to look closely at seawater to fully appreciate the enormous diversity of plankton it contains. There are a number of methods for investigating plankton abundance and taking a snapshot of their populations across the oceans. The simplest sampling method is to filter seawater by dragging a coneshaped net behind a boat. An early pioneer of this approach was Charles Darwin (1809–1882), who used a muslin net during his days aboard HMS Beagle in the 1830s. Today’s plankton nets are made from nylon in a variety of mesh sizes. Holes smaller than 20 micrometers (µm) are used to collect single-celled life. Larger, single-celled plankton and the tiniest animals, such as rotifers, will be picked up in nets with a mesh size of about 80 µm. Smaller crustaceans and worms need a net with a mesh size of about 150 µm, while the tiny larvae of fish will get caught in nets with holes of about 500 µm (which is equal to 0.5 mm). There are, of course, many types of larger plankton than this—some can be several meters long—but macroplanktonic organisms of this size are not suited to sampling by conical net. Instead, larger nets are used with automated systems for opening and closing to keep the samples trapped inside.

More accurate measurements Plankton nets can be used at different depths to build a profile of plankton in the water column. However, because they accumulate, these samples are not a very good method for assessing what plankton lives where. A more advanced form of sampler called the continuous plankton recorder (CPR) goes some way to answering these questions. The CPR is a self-contained device which is towed behind a ship, often a merchant vessel that happens to be passing a place of interest. The CPR filters the plankton on a strip of silk, which inches through the machine as water flows through it. Every centimeter of silk represents a nautical mile (approximately 6,000 ft/1,800 m) of tow. The entire silk strip—most are about 4 in (10 cm) wide in total—will therefore present a crude spatial profile of the plankton and its relative abundance through the tow zone. 32 | A WONDERFUL DIVERSITY

The most accurate method of quantifying plankton communities is to use water bottles. As their name suggests, these are devices of various designs that collect a sample of seawater from a specified location at a specified depth. The disadvantage over netting and CPRs is that the samples contain far fewer organisms, and the sampling process must be carried out individually many times over, a laborious pursuit compared to the passive sweep of a net. Nevertheless, the results from water sampling are much more indicative of what is in the water. Plus there is the advantage of collecting plankton of all sizes, bar the largest types. The samples include tiny organisms that are smaller than 2 µm and mostly comprise bacteria. Additionally, the water contains femtoplankton, which are viral particles best measured in nanometers, not micrometers.

| An illustration from1880 of HMS Beagle sailing through a phosphorescent sea, lit by bioluminescent sea creatures, as reported by Darwin during his voyage of discovery many years before.

OPPOSITE

| A rosette of water bottles can be seen sampling plankton below the vessel used by researchers during the Tara Oceans Expeditions in 2011.

LEFT

| Small shrimp and fish are visible in this zooplankton sample taken in the Canadian Arctic.

BELOW

33

Viroplankton Viruses are not generally accepted as fully fledged living beings since they do not feed, respire, or perform other basic biological functions. Nonetheless, marine viruses are astonishingly abundant, with the vast majority being harmless to humans. Instead, most are specialist parasites of planktonic bacteria, known as bacteriophages, or attack other planktonic microorganisms. It is estimated that there are ten of these viruses for every bacterium in the ocean, which equates to a quarter of a billion viruses in every milliliter, or 75 billion in a regular drinking glass. Every day a fifth of the ocean’s community of single-celled microorganisms dies due to the actions of an infectious virus.

34 | A WONDERFUL DIVERSITY

| Bacteriophages (colored white) are attacking blue Streptococcus bacteria in this false-colored scanning electron micrograph.

ABOVE

| Bacteriophages (colored green) attacking a bacterial cell (tinted orange).

LEFT

SAMPLE ANALYSIS A water sample is intrinsically no different from a bucket of water collected at the seaside. At first glance it does not reveal much. At least three-quarters of its plankton will be microorganisms, mostly single-celled protists (such as algae and protozoans) and bacteria. Larger colonies of algae and microscopic animals will show up easily under a microscope. To highlight the smaller organisms, and begin the process of identifying them, samples are treated with fixative chemicals to stop the dead material breaking up. Next, the biological material is separated from the water, so it can be measured, sorted by type, and counted. To prevent further damage to the samples, this is most commonly achieved by a passive settling process. The device used is called an Utermöhl tower, which is helpful for standardizing results, but essentially the sample of seawater is left to settle so its

plankton contents gradually sink to the bottom of a collecting vessel. As a rule of thumb, a sample is left for two hours for every centimeter of seawater in the tower. However, smaller plankton will sink more slowly than the bigger beasties. Once settled, the sample is studied under a microscope. The best view comes from an inverted setup, where the light source is shining down on the sample and the objective lens is directed to look up from underneath. This instrument can be used to view the plankton settled at the bottom of the container. There is no need to process the sample further.

| This micrograph made with a light microscope shows a range of different marine diatoms, with a variety of shapes.

BELOW

35

Smaller organisms are counted in sectors, which represent a fraction of the total, by magnifying one part of the sample at a time. Larger members have to be counted in total across the whole sample at once, with the microscope set to a lower magnification. The anatomy of larger plankton, mostly zooplankton, is also best studied with binocular microscopes that have the objective lens above the sample. Light sources of different kinds can be positioned above and below to highlight features such as appendages, bristles, and mucus layers. As well as assessing samples manually, chemical testing can be used to identify the presence of certain organisms. Several biochemical toolkits can detect certain “fingerprint” chemicals in the sample, most notably the ribosomal DNA (rRNA) of different organisms. RNA is a close companion of DNA and used to encode genes in all forms of life. One of 36 | A WONDERFUL DIVERSITY

its roles is as a structural material for making ribosomes, the units in every cell that decode genetic material and convert its information into the functional entities that aid life processes. All life uses rRNA, and its precise makeup is an excellent signal of the lineage of plankton found in the sample, especially the small microorganisms that are harder to count and quantify in other ways.

| Marine plankton seen through a light microscope.

ABOVE LEFT

ABOVE RIGHT | A bloom of diatoms, mainly species of Thalassiosira, as seen in a differential interference contrast (DIC) light micrograph.

| Ecologists at work on Lake Greifen, near Zurich, Switzerland, using an underwater camera called an Aquascope.

OPPOSITE

Optical detectors in the wild The boom in digital video and underwater optical technology in the last couple of decades has allowed researchers to observe plankton in the wild waters that they call home. These systems are of most use for studying zooplankton, which are able to evade nets—or may simply not fit—and which exhibit behaviors that are lost when static samples are taken to the lab. Cameras are fitted to nets to capture some of the action as plankton are scooped up. They are also mounted on free-floating devices designed to capture the behavior of marine animals on video. To achieve clear imagery of plankton, most of which is microscopic, requires some special camera work. This involves flooding a small volume of water in front of the camera with light, either from LEDs or laser sources. The camera can then focus on the organisms in this

volume, many of which are largely transparent. Sometimes the camera points down onto a “light sheet,” which is a horizontal area of water illuminated by a fast flashing strobe. The camera’s focal length always matches the position of the light sheet. In turbid waters, where suspended silt makes it hard to see anything at all, shadowgraph techniques are used to scan a small volume of water. In these cases, pulses of light are used to create a three-dimensional map of the objects that block this light. This data is then used to recreate the scene and reveal the animals floating among the mud particles.

37

Antarctic krill Euphausia superba Antarctic krill are crustaceans that live in the Southern Ocean. They are thought to be among the most abundant animal species in the world, with their total biomass estimated at 500 million tons. Antarctic krill are an important food source for many marine animals, not least the world’s largest giant whales, but also seals, penguins, and fish. They also represent a food source for humans. Krill oil is a valuable source of omega-3 fatty acids. The animals are most abundant in the Antarctic Peninsula region, where they form huge swarms that have a density of 30,000 krill in every cubic meter. At first glance, Antarctic krill look very like small shrimp, in common with all krill, or euphausids. They have a transparent pinkish carapace and a long, slender body that is divided into three main sections—head, thorax, and abdomen. In decapods such as shrimp these body divisions can be less clear.

Feeding and survival Antarctic krill are filter feeders. They strain microplankton from the water using their feathery forelimbs, known as thoracopods, which sweep the water to trap a “basket” of food items in front of their open mouths. In turn, Antarctic krill are an important part of the Antarctic ecosystem and a staple food for many larger predators. When a swarm is under attack, all the krill can undergo a mass molt, where the crustaceans shed their outer carapace. This doubles the targets for hunters and raises the chances that the living krill are spared the onslaught. Another tactic is to go deep, although most of the food the swarm needs is up in the sunlit euphotic zone.

Life cycle Antarctic krill live for several years, as long as they avoid being a bigger beast’s meal, of course. They breed using internal fertilization, and mating is an elaborate five-step process. The breeding season is between December and March, the austral summer. Eggs are shed in deeper, dark waters. After hatching, the krill go through a number of larval stages with appendages added at each step. The krill transform from one stage to the next with each molt, and at all phases of its life, the krill remains planktonic.

F A M I LY: Euphausiidae DISTRIBUTION: H A B I TAT:

Southern Ocean

Open water

FEEDING HABITS: NOTES:

| An Antarctic krill (Euphausia superba) in the Southern Ocean. Its food “basket” appendages can be seen clearly.

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SIZE:

Filter feeder

Has the largest genome of any animal on Earth, with the DNA containing 42 billion bases, compared to the 3.4 billion in human DNA.

21/2 in (6 cm)

Polychaete worm Pelagobia longicirrata Pelagobia worms belong to a genus of holoplanktonic polychaetes. Polychaetes are annelid worms characterized by many bristle-like extensions called chaetae poking out of the body. Pelagobia longicirrata is the more common species and is found in all the world’s oceans. All are small, slender worms that spend their whole lives in the water column. Pelagobia are filter feeders and survive by sifting food particles of all kinds—mostly microbes and waste—from the water using feathery antennae. The mouthpart is then everted (turned inside out) to slurp up items from these feelers. The mouthpart can also be used to actively target food in the water. Pelagobia worms are an important part of the marine food web, being eaten by a variety of animals, including fish, crabs, and seabirds.

Stages to adulthood It is likely that the holoplanktonic species evolved a life cycle where they never needed to return to the seabed. The life cycle has multiple stages. Eggs are fertilized externally in the water. The larval worm starts out as a single head segment, but the body elongates as more segments are added in six developmental stages before reaching the adult form.

Body plan As annelids, the worms are divided into multiple repeating body sections, with a head at one end with a mouth and sensory organs. The body is covered with small, overlapping scales. Although they are worms, polychaetes do look as if they have legs. However, these are unjointed, soft extensions of each body segment, called parapodia (which roughly translates as “beside feet” or “almost feet”). The adult worms have between 15 and 18 segments, each with a pair of parapodia. The appendages are fringed with chaetae which enlarge their surface area so they can function as oar-like swimming limbs. Similar paddle-like appendages are developed by benthic worms so they can swim up to the surface to spawn during the breeding season.

F A M I LY: Lopadorrhynchidae DISTRIBUTION: H A B I TAT:

Worldwide

Open water

FEEDING HABITS: NOTES:

| A dorsal (top-down) view of a Pelagobia longicirrata, showing the parapodia extending from the body’s many segments.

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SIZE:



Filter feeder

The worms have symbiotic bacteria living in their digestive tract. These helpful germs break down the cellulose in the cell walls of phytoplankton, which is otherwise unavailable using animal enzymes. /16 in (4 mm)

3

Rotifer Conochilus unicornis Rotifers are minute aquatic animals that belong to a separate phylum, Rotifera. With some being as small as 50 µm across, they are among the smallest multicellular animals of all. A typical body is made of just 1,000 cells. Most rotifers can only be seen with a powerful microscope, although a few giant species reach 1/8 in (3 mm) across and are visible to the naked eye. Even so, that is easier said than done. Rotifers are transparent, and only the digestive tract, which is tinted with food, is immediately obvious. The name rotifer is derived from the Latin word for “wheel” because many of these creatures have round bodies. A colloquial name for them is wheel animals, although many are more bell-shaped. The rounded body is fringed by a “crown” of cilia at the front end, which forms a loosely defined head section. The most visible feature of the head are the tiny red eyespots, which are sensitive to light but cannot form images. The cilia are at once used for locomotion through the water and for drawing a current through the crown from which food is filtered out. The digestive system is inside the trunk section of the body, and there is a rear section, known as a foot, which is much reduced in planktonic species to a tail-like spike.

Wide-ranging habitat Being so small it is difficult to grasp just how widespread these tiny creatures are. They are primarily water animals, but do persist in damp soils and foliage on land as well. They are even found frozen in permafrost. Rotifers live in all freshwater and marine habitats and are often bottom feeders that are anchored to the seabed and form microscopic colonies. However, a large number are planktonic, and rotifers are a significant feature of freshwater plankton communities.

Parthenogenetic reproduction Rotifers are able to reproduce both sexually and asexually. The asexual process is parthenogenetic, whereby females lay eggs without needing to mate with a male. The egg hatches into a miniature version of the adult and is always female. Male rotifers are produced occasionally. They have a limited life cycle and only exist to fertilize the eggs produced by other rotifers with their sperm, thus boosting the genetic diversity of the population.

F A M I LY: Conochilidae DISTRIBUTION: H A B I TAT:

Worldwide

Marine and fresh water, as well as damp, terrestrial habitats.

FEEDING HABITS:

| A colony of rotifers, Conochilus unicornis, use their tail-like feet to cling to a central collective mass of mucus.

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NOTES: SIZE:

Filter feeder

Rotifers can even survive when trapped in ice.

0.01 in (250 µm)

Diatom Coscinodiscus sp. Diatoms are phytoplankton and make up an important part of the marine food web. They are eaten by a variety of animals, including zooplankton such as copepods, krill, and jellyfish, as well as planktivorous fish like the basking shark. Coscinodiscus is a common genus of diatoms found in seawater. It contains more than 100 species that make up part of the plankton throughout the world’s oceans. Coscinodiscus diatoms are centric diatoms, which means they have a circular shape. This form comes from the outer covering called a frustule, which is made up of two overlapping sections, or valves. The valves are highly patterned with a variety of pits, ridges, and spines. There are, however, around 200,000 species of diatom as a whole. Some have boat-shaped, pennate forms instead of a centric anatomy. The frustules are made of silica, which creates beautiful optical effects. Diatoms are sometimes called golden larvae as well as more lyrical names such as “jewels of the sea” or “living opals.”

Reproduction Diatoms are able to reproduce both sexually and asexually. The latter system is the most common, with individual diatoms dividing into two. Each daughter cell inherits one valve from the parent. In a fully grown diatom, the top valve always fits neatly over the lower one. When new cells form, the inherited valve is promoted so it is always the top valve and a smaller lower valve grows to fit perfectly inside it. As a result, one of the daughter cells—the one using the smaller lower valve— will always be smaller than its parent, a process that continues down the generations.

Value in oceanography Coscinodiscus is one of the must abundant genera of diatoms in the sea and provides an important source of data for oceanographers. The size, shape, and markings of Coscinodiscus valves identifies individual species, and the abundance of different species can be used to track changes in the environment. A bloom of these diatoms can produce damaging quantities of mucilage that clog the gills of fish and other marine life. F A M I LY: Coscinodiscaceae DISTRIBUTION: H A B I TAT:

Worldwide

Sunlit oceans

FEEDING HABITS:

| A dorsal (top-down) view of the diatom Coscinodiscus jonesianus. The epitheca, or upper valve, of the frustule can be seen here. The lower valve, or hypotheca, fits underneath.

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NOTES:

SIZE:

Photosynthetic

Some species release harmful toxins that can reach dangerous levels during a bloom.

0.008 in (200 µm)

Dinoflagellate Tripos muelleri This species of marine dinoflagellate is characterized by its distinctive shape. In common with the other 100-plus species of the Tripos genus, it has a long, slender cell body that is tapered at both ends. The cells have a distinct top—the apex, or apical horn—and two antapical horns at the lower end. The U-shaped antapical horns in this species are curved upward compared to the central axis set by the apex. Other members of the genus have straighter horns or another distinctive shape. The cell body is surrounded by a two-part shell, or frustule. The frustule is made up of upper and lower valves that are joined together at the midline by a girdle section, seen here as a faint double line. Dinoflagellates are by definition organisms with two flagella. The first is connected via a lateral groove between the valves, while the second is located near the antapex (in this case, the lowest point where the horns curve out and up). In keeping with all dinoflagellates, Tripos muelleri is a photosynthetic organism. As one of the larger examples of a dinoflagellate, this species is mostly found in coastal waters. Smaller dinoflagellates are more common out in deeper open waters, where nutrients are more sparse. Researchers are finding that many members of the Tripos genus are mixotrophic, meaning that as well as photosynthesizing, they also consume food particles, mostly bacteria, by engulfing them with their cell body.

Red tides Tripos species have a complex life cycle. Most of the reproduction is asexual, where a single cell divides into four or eight cells. To achieve that, the cell must first escape from its shell. During the sexual phase, two cells merge to create a temporary form which then divides, releasing offspring called swarmers. Dinoflagellates also form inactive cyst forms, which can sink to the seabed and lie dormant for a period. Tripos dinoflagellates are one of the types of plankton that are responsible for red tides, where the population explodes in such vast numbers that the color of the water changes—as well as red, it can also be brown and green. These plankton produce toxins in quantities that threaten fish.

F A M I LY: Ceratiaceae DISTRIBUTION: H A B I TAT:

Worldwide

Mostly coastal seas

FEEDING HABITS: NOTES:

| The orientation of the three horns indicates that this is a species of Tripos.

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SIZE:

Photosynthetic

Named for the German naturalist Johann Adam Müller (1769–1832).

0.008 in (200 µm)

Coccolithophore Emiliania huxleyi Emiliania huxleyi is a species of coccolithophore, a type of marine protist. It is one of the most abundant and widespread species of coccolithophore, and it can be found in all the world’s oceans. This species is especially widespread because it is able to withstand water temperatures ranging from 34–86°F (1–30°C). It is especially well represented in non-polar seas. E. huxleyi is a flagellated phytoplankton, although the term flagellate simply refers to the presence of the whip-like appendage and holds no particular taxonomic value. Indeed, in this species the flagellum is only present for a short sexual phase of the life cycle. Most of the time, there is no flagellum at all.

Porous skeleton As a coccolithophore, E. huxleyi is more defined by the calcium carbonate plates, known as coccoliths, which encase its body. Together the coccoliths form a porous skeleton called a coccosphere. The stark white of these structural materials makes these organisms look like tiny snowflakes. Chalk, the soft form of limestone, is a rock constructed of an immeasurable quantity of coccospheres of long-dead plankton. The heyday of the coccolithophores was about 100 million

years ago, in the Late Cretaceous Period. This period is best known for witnessing the rise and fall of the dinosaurs, but is really named after all the chalk beds created at the time by coccolithophores.

Huge blooms E. huxleyi is most evident in summer when it forms blooms that cover hundreds of thousands of square kilometers. Blooms are stimulated in part by warmer waters, and a climate feedback mechanism has been identified. As climate change heats the oceans, blooms of coccolithophores become larger and more frequent. The blooms form by vegetative growth— another term for asexual reproduction—which occurs when the coccolithophore has its white skeleton in place. The pale bloom increases the reflectivity of the surface water, pushing heat and light back into the atmosphere (and space). The end result is that deeper waters become cool. The long-term effects of this apparent climate mechanism are still being investigated.

F A M I LY: Noelaerhabdaceae DISTRIBUTION: H A B I TAT:

Worldwide

Sunlit oceans

FEEDING HABITS:

| A colorized scanning electron micrograph of Emiliana huxleyi, showing the skeleton, or coccosphere, of calcium carbonate plates, the coccoliths.

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NOTES:

SIZE:

Photosynthetic

Named for Thomas Henry Huxley (1825–1895), an English biologist.

0.0004 in (10 µm)

Hydrozoan jellyfish Aequorea victoria Aequorea victoria, also known as the crystal jelly, is a small hydrozoan jellyfish from the Pacific Ocean. It is most abundant along the cold-water coastlines of North America, from the edge of the Arctic Ocean to Central America. Seen here is the adult planktonic medusa, a form with the tentacles hanging from a bell. Sexually mature individuals will have a bell above 11⁄4 in (3 cm) in diameter. The largest specimens are 2 in (5 cm) across. Larval forms of this species, in common with most hydrozoans, start out as seafloor creatures. At this stage they take a polyp form, where the tentacles reach up from the body in an inverse orientation to the medusa.

Bioluminescence The bell is transparent and decorated with blue or green spots. A. victoria is also bioluminescent, meaning that it can produce its own light. The light is created by a protein called aequorin, which is activated inside cells by calcium ions. When the calcium ions bind to aequorin, a flash of blue light is released.

That blue light can then be converted to green by another protein called GFP (somewhat uninspiredly short for green fluorescent protein). A. victoria uses its light for communication, mostly to attract mates in dark waters. Flashing lights also have a defensive function by startling predators and suggesting that the animal is much larger than is the case. In twilight environments where light is twinkling down from the surface, the light show can create a camouflage, helping the jellyfish to blend in with the shimmering surroundings. The protein aequorin has been used in a variety of scientific studies, not least into the nature of bioluminescence itself. This research has led to a burgeoning of artificial fluorescent proteins, which are used in a variety of applications, including cell biology and medical imaging. In 2008, Osamu Shimomura, Roger Tsien, and Martin Chalfie were awarded the Nobel Prize in Chemistry for their work on aequorin and GFP.

F A M I LY: Aequoreidae DISTRIBUTION: H A B I TAT:

Eastern Pacific

Coastal waters

FEEDING HABITS:

| The crystal jelly (Aequorea victoria), like many pelagic organisms, has an ability to produce bioluminescence.

OPPOSITE

50 | A WONDERFUL DIVERSITY

NOTES: SIZE:

Predators

A source of fluorescent proteins.

11/4 in (3 cm)

European eel Anguilla anguilla When adult, this snake-shaped fish lives in the freshwater rivers of western and northern Europe. However, that is just a single phase of one of nature’s most incredible life cycles, and one that even now we do not fully understand. In a turnabout from the more familiar breeding pattern, such as that used by salmon where young fish swim down river to the sea and then return as adults to breed, European eels do it the other way around. They spawn in the middle of the ocean, and then spend their larval stage, seen here and known as a leptocephalus, as marine plankton slowing drifting their way to freshwater habitats.

To the Sargasso Sea The story begins in the inland waters of Europe where sturdy, meter-long eels spend up to 20 years, using their sense of smell to find worms and aquatic insect larvae to eat. They then head down river and out to sea using that same sense of smell to swim fast and deep to the Sargasso Sea. Deep in these waters, no one is quite sure where, the eels spawn and die.

The next generation emerge from the eggs as tiny, transparent larvae called leptocephali, which are about 0.04 in (1 mm) long. They have flattened, leaflike bodies and swim weakly, meaning they can only drift in the water. The larval eels feed on smaller planktonic organisms, such as copepods, rotifers, and microscopic jellyfish, as they move in the gyre of the Sargasso. A gyre is a swirl of warm currents that corrals a vast patch of floating Sargassum seaweeds. The current spreads out the tiny eels until they are caught in the Gulf Stream, which takes them northeast across the ocean to the shores of Europe. Once in the stream, the leptocephali take about 300 days to make the crossing. After reaching neritic, or coastal, waters, the changing smells of the water stimulate the flimsy planktonic larvae to metamorphose into sturdier glass eels, or elvers, which enter the freshwater system for the first time. Sadly, the amazing European eel is now critically endangered due to overfishing of juvenile elvers as they swim into rivers. These juvenile eels are then raised in farms. Eel farmers are currently unable to breed their own stock due to the species’ unique life cycle and so rely on fishing. As a result, very few wild eels remain.

F A M I LY: Anguillidae DISTRIBUTION: H A B I TAT:

Sargasso Sea and the Gulf Stream

Open ocean

FEEDING HABITS:

| The leptocephali of a European eel (Anguilla anguilla) head for fresh water.

OPPOSITE

52 | A WONDERFUL DIVERSITY

NOTES: SIZE:

Prey on zooplankton

Critically endangered due to overfishing.

0.04 in (1 mm)

CHAPTER 2

LIFESTYLES AND ADAPTATIONS Earth has a lot of room for plankton, with the oceans covering more than 70 percent of the surface of the globe. Almost 90 percent of that ocean is more than 3,200 ft (1,000 m) deep and generally a lot deeper than this. All that water makes up more than 99 percent of the world’s habitable space where life can persist. Nevertheless, the oceans are barren compared to land, harboring only an estimated 15 percent of the planet’s species. Even so, the oceans are very diverse, with a wider array of phyla represented despite fewer total species than on land. Despite the apparent simplicity of their lifestyle, plankton are by no means a ubiquitous feature of the oceans. They are not spread evenly, either geographically or by depth. Researchers still have much to learn about why plankton communities thrive in some places but not others. Such knowledge will be crucial in a world subject to climate change and ecological damage.

FORCES AT WORK Plankton all share one ability: they stay afloat. This may be an intrinsic fact based purely on the size of the organism. Other plankton are adapted to this floating lifestyle, with simple features like gas vacuoles inside the cell body that act as tiny life preservers to reduce the density of the whole cell to below that of the surrounding seawater. Larger plankton have entire gas bladders or air bags that do the same job, while less clean-cut adaptations rely on strings of mucus splaying though the water to spread the weight and reduce the effects of gravity. Others are suspended by active swimming. They will sink when they don’t swim, so swim they must just to stay where they are. Archimedes did not have a working knowledge of plankton, nor as far as we can tell did he care much for marine biology, but he was the first to articulate the way forces work to create floating and sinking. All plankton have a weight and this all adds up. It is estimated that the cumulative mass of the world’s cyanobacteria alone is about a billion tons. But a single bacteria cell has a negligible weight of one picogram—or a trillionth of a gram. In life plankton do not sink, but once dead they cannot counter gravity and down they must go. Sinking and floating is all down to the size of the buoyant force, which is the push of all the surrounding water molecules. Archimedes might have put it in these terms: A plankton displaces its volume in water, and if it weighs less than that volume of water, it will float. This is because the force of gravity on the plankton is weaker than the buoyant force. If it is considerably weaker, then the plankton will rise to the surface. (It could even float off into the air if released from the water’s surface tension by wind-blown sprays. Like balloons, aeroplankton float in the air due to the buoyant force of the surrounding gases.) A more general case is that plankton are close to the density of water, being largely composed of water, so they achieve a neutral buoyancy, more or less staying at the same depth until the water is churned by waves and currents.

RIGHT

| A bloom of moon jellyfish (Aurelia aurita).

56 | LIFESTYLES AND ADAPTATIONS

57

Sizing up Buoyancy is obviously not a function of size, but the vast majority of plankton are small organisms. It is perhaps instructive that megaplankton, the biggest subset, refers to anything over 8 in (20 cm) wide. This group is dominated by the jellyfish but also includes less common creatures like the salps and other free-floating tunicates. Less than 8 in (20 cm) wide but larger than 3/4 in (2 cm) are the macroplankton, which again are visible to the naked eye— if, that is, you can get a good enough look in the water. This group includes the larger crustaceans, like shrimp and krill, plus some comb jellies. Below 3/4 in (2 cm), we enter the domain of the mesoplankton. This is where the community really starts 58 | LIFESTYLES AND ADAPTATIONS

to expand in diversity and sheer quantity. Mesoplankton is still dominated by animals, like the previous groupings, and this time includes the planktonic larval forms of nektonic animals. (Nekton refers to sea animals that can swim and move independently of the currents.) On top of that, the list is filled with many of the biggest hitters of the zooplankton community, such as the copepods, arrow worms, and water fleas. Smaller still are the microplankton, which occupy a size between 20 and 200 micrometers (µm, or a millionth of a meter). At this size, the single-celled protists begin to appear, including organisms like ciliates and foraminifera. There are also tiny animals, such as rotifers and the numerous larvae of copepods. Next come the nanoplankton, which are between

| Floating colonies of ciliate protozoa, a species of Stantor, showing the gas bubbles used to lift colonies.

OPPOSITE

| A chain of Anabaena cyanobacteria. The slightly larger and darker cells are heterocysts, which convert nitrogen from the water into nitrate.

LEFT

| A diagram showing the relative sizes of plankton.

BELOW

Plankton size categories Megaplankton (8–78¾ in/20–200 cm)

Mesoplankton (0.007–¾ in/0.2–20 mm)

2 and 20 µm. This size group is dominated by the protists. There are no animals this small and bacteria are smaller still. Bacteria occupy the picoplankton group, which are smaller than 2 µm. Despite their tiny stature, the members of this group, dominated as they are by cyanobacteria, play a major role in the ecology of the oceans and the planet as a whole. Additionally, bacteria are the only kind of planktonic organisms found consistently at all depths (although in diminishing concentrations). Plankton does not end with bacteria, however. The femtoplankton contains marine viruses, which are only a fraction of a micrometer in length. As discussed in Chapter 1, viruses are biologically active agents that have a major ecological impact on marine life, but they are not really living things in the accepted sense (see page 34).

Net plankton

Macroplankton (¾–8 in/2–20 cm)

Microplankton (0.0008–0.008 in/ 20–200 µm)

Nanoplankton (0.00004– 0.0008 in /2–20 µm)

Picoplankton (0.2–2 µm)

59

VERTICAL ZONES The living conditions in the ocean are by no means uniform, varying according to depth, latitude, and numerous other factors. In most places, life in the ocean is hard, but wherever one looks, there will be some plankton. The surface layers of the ocean are bathed in sunlight by day, and in certain places visibly teem with life. But with depth comes darkness, cold, and a rapid increase in water pressure that would kill an unprotected human diver still within view of the surface. Due to the variables, oceanographers divide the oceans into zones based on depth. The defining factors for these ocean zones are firstly light and, secondly, the location and topography of the seafloor beneath.

The sunlit zone The upper layer of sea is the euphotic zone, better understood as the sunlit zone. As one might expect, this upper zone is bathed in sunlight by day. However, light and water do not mix well. At most, the brightness of the sun beams will be reduced to 1 percent of their original intensity by a depth of 650 ft (200 m), and that is in the clearest waters. In turbid waters churned up with sediment, the light barely makes it 33 ft (10 m) down.

The euphotic zone is where all the action is. An approximate 90 percent of all marine life lives here. It is impossible for phytoplankton, those organisms that rely on photosynthesis as their primary source of nutrition, to live in any other part of the water column for long periods. Zooplankton and sea animals in general flock to this zone to join the food chain that starts with those phytoplankton.

Into the gloom: the twilight zone Below the sunlit waters, deeper than 650 ft (200 m), we enter the dysphotic zone, again perhaps better termed as the twilight zone. Here light arrives from the surface but not strongly enough to be used for photosynthesis and it never gets above a deep gloom. Nevertheless, the twilight zone is something of a haven for animal life, including many of the biggest shoals and swarms of zooplankton. The dark zone provides a refuge for the zooplankton, mostly in the mesoplankton groups, and also many nektonic species, from where they launch assaults on the communities of phytoplankton and zooplankton in the euphotic zone. This results in the largest mass movement of life on the planet, Ocean light zones Euphotic zone

Dysphotic zone

| Tropical fish, including the Bengal snapper (Lutjanus bengalensis) and bannerfish (Heniochus diphreutes) dive toward a coral reef in order to avoid predators, North Ari Atoll, the Maldives, in the Indian Ocean.

OPPOSITE

RIGHT | The oceanic zones are delineated by the light environment.

60 | LIFESTYLES AND ADAPTATIONS

Aphotic zone

61

known as the diel (or diurnal) vertical migration (DVM). Chapter 4 gives a more in-depth look at the DVM (see page 134), but in a nutshell, zooplankton lurk in the twilight zone until the sun sets at the surface, thus plunging the euphotic zone into darkness. Under the cover of this darkness, the zooplankton, such as copepods and krill, rise up to feed on the phytoplankton. They cannot risk foraging by day lest they are targeted by visual predators in the euphotic zone.

62 | LIFESTYLES AND ADAPTATIONS

Fish and a host of other marine animals follow the nighttime marauders up to the surface. (Lanternfish famously have their own lights to guide them as they feast in vast night-time shoals). By contrast, a few other zooplankton head to deeper waters at night to get away from this upsurge of predatory activity. They will swim back to the light come morning. In those species that show reverse DVM, this may allow them to avoid the predators that are following the DVM of their prey.

Into the midnight zone No light ever reaches below about 3,200 ft (1,000 m). From here the water is in the aphotic zone, more commonly described as the midnight zone. It is completely dark 24 hours a day, no matter what the sunlight conditions at the surface. This zone continues all the way to the seabed. Life has thinned out somewhat in these dark waters, but it is still there. In

| This dorsal (top-down) view of a copepod shows the antennae on the head are covered in setae. These hair-like structures are used to detect the tiniest motions of prey in the water, and help them forage.

OPPOSITE

terms of plankton, the ecology of the midnight zone limits them. Many have arrived here by mistake, churned in surface water by storm winds and ocean currents, and doomed never to get back to the surface. As such, these plankton become part of the marine snow (see page 100), which is a perpetual shower of organic—and mostly dead—material that sinks from the more productive zones nearer the surface. It is the ultimate source of nutrients for most of the food chains that exist in the deep ocean and on the seafloor. | Bolinopsis infundibulum comb jelly in the twilight zone.

BELOW

64 | LIFESTYLES AND ADAPTATIONS

EVADING DETECTION Marine life uses a variety of tricks to evade capture by predators, whether this is the countershading displayed by dolphins and penguins, the reflective scales of fish, or being transparent, as is the case for some plankton. These clever tricks make use of the optical properties of water, which are quite distinct from those of air. This distinction has a profound effect on the vision of marine creatures, especially the oceanic ones, which spend their days in the endless vastness of deep, open oceans. That in turn impacts the strategies used by marine life, including plankton, to stay out of sight.

The difference is clear One way to experience the different visual ecology of water for oneself is to dive a few meters under clear water on a sunny day and take a look at Snell’s Window. This is a bright patch on the surface above you. You can see through it and make out features above the water, although the full 180-degree field of view is compressed and distorted into just over 90 degrees. It is not something human eyes are good at interpreting, but this is all that marine life can ever see from below the surface. Snell’s Window is a product of refraction, and is named for the Dutch Renaissance astronomer who described the way refraction works. Refraction alters the direction of light beams as they move from one transparent medium to another, such as from air to water. The refractive properties of water focus all light that gets through from the air into this narrow window. The end result is that wherever you look up at the surface, you see an opaquely illuminated boundary, not a detailed image of what lies beyond. Look down, and there is no source of light, only a descending darkness. One obvious product of this marine light environment is the countershading that is a feature of many big sea creatures, from dolphins and whales to sharks and penguins. The idea is that the top of the body is dark and this transitions to a paler underside. When viewed from above, the dark surface blends in with the darkness emanating from the deep. Seen from below, the pale underbelly is hard to make out against a

background of bright light entering Snell’s Window. In smaller creatures like plankton, this shading may be reversed, as is the case with the violet sea snail (Janthina janthina; see page 80), which hangs upside down from the surface of the water and so has a shell that is paler on top and a bluish purple underneath.

Big blue The major factor for smaller animals is that water is blue. This is the color of the substance, just as table salt is white and charcoal is black, although the blueness of water is only apparent when observed in large quantities. Meanwhile, air is colorless (a blue sky is an optical effect, not the color of the gases). So in some senses, blue is more akin to the green in a forest. If an organism wants to blend in, that is the color it needs to be—little forest animals are more likely to be green for this reason. However, it does not quite work like this in water. A green tree frog appears that color because its skin reflects the green fraction of sunlight and absorbs the red and blue parts. In the ocean, the water has already absorbed the red and green light and is transmitting a little blue light, which illuminates the objects swimming or floating in it. To appear the same color as the water around it, an object must reflect all the light that hits it. This is why fish have mirror-like scales. The scales reflect the light between the fish and the observer, which is identical to the light coming from behind the fish. Thus, the fish disappears behind a cloak of invisibility. At least that is the plan, and for eyes that are attuned to pick out three colors like ours, it can work well. But fish themselves are less easily

OPPOSITE ABOVE | An oceanic whitetip shark (Carcharhinus longimanus) in the Red Sea, seen swimming below Snell’s Window.

| A flock of king penguins (Aptenodytes patagonicus) reveal their countershaded bodies while swimming off Macquarie Island, near Antarctica.

OPPOSITE BELOW LEFT

OPPOSITE BELOW RIGHT | The mirror scales of these fusiliers, Caesio lunaris, make them hard to see in the blue waters of Palau, in Micronesia.

65

fooled. They have dichromatic vision, which is tuned to visualizing shapes against the background of water. Fish that hunt by sight, such as sardines or anchovies, scan the water for the shapes of zooplankton. To minimize detection, therefore, many zooplankton species, including copepods, use a simple trick: the best way to stay unseen in water is to have a largely transparent body. The elaborate colorings used for camouflage on land are of no use here. So the light passes straight through the plankton, leaving no tell-tale silhouette that can be spotted by predatory fish. The internal structures of the plankton, its digestive tract especially, will show up but offer less of a target for predators. Of course, as with all biological adaptation, adopting transparency involves a trade-off. Pigments are inherently useful in that they absorb the nasty, high-energy rays of ultraviolet light, which could otherwise disrupt the cell chemistry of plankton. In lakes where fish predators are less of a threat, plankton are more pigmented for this very reason.

Staying alive Inevitably plankton cannot evade detection forever. Therefore they have a last line of defense. This may be an escape response where zooplankton flick an appendage to dart off in a random direction at high speed. Copepods make impressive “jumps” by delivering determined strokes with their cephalic appendages, the limb-like body parts on the head. This provides acceleration through the water of 500 body lengths per second, exceeding the equivalent acceleration of an F16 fighter jet. Another strategy to avoid being eaten is to grow a lot of protective jelly. Both phytoplankton and zooplankton of various kinds have developed gelatinous bodies. It allows them to grow to a large size very quickly and without expending too many resources. Predators tend to view the ball of jelly as a less-than-perfect meal.

RIGHT | The bioluminescence from a dinoflagellate bloom lights up the water in spectacular fashion along the beach at Jervis Bay, in Australia (right) and the Maldives (far right).

66 | LIFESTYLES AND ADAPTATIONS

Bioluminescence In the dark oceans, organisms make their own light in a process called bioluminescence. Many types of plankton do this, from bacteria and dinoflagellates to jellyfish. Why plankton bioluminesce is not always clear. It may be that the light is a signal to other members of the species to attract mates or even communicate in more advanced ways. In turn, the light may be a trick, designed to fool prey into thinking they are seeing friendly relatives but instead drawing them closer to their foe. Bioluminescence creates a cold light, which means no other energy, such as heat, is emitted with it. The light is produced by a chemical process. The color is not chemical but structural, which means the different hues are produced by the way the same chemicals (known as luciferins) are arranged in the light-emitting organs.

67

SEAFLOOR PROFILE As well as depth, plankton communities are also affected by the undersea topography, the ups and mostly downs of the seabed. Areas of oceans near the coast are called neritic, while farther out from land the waters are described as oceanic. Neritic waters are mostly shallow because they overlay a continental shelf.

Continental shelves A continental shelf is the lower lip of a landmass, the part of the continental crust that is below sea level. The water here is only a few tens of meters deep and so is, in effect, completely contained within the euphotic zone.

Continental slopes Eventually, the shelf gives way to a steep drop called a continental slope, which plunges thousands of meters to the deep seafloor. This marks the transition from the thick continental crust to the thinner and lower-lying oceanic crust. The continental slopes form the walls around the vast oceanic basins, which hold Earth’s surface water. Beyond the slope the waters are oceanic.

Continental rises The continental slope meets a continental rise, a less steep slope that descends more gently to the abyssal plain. The continental rise is built from materials that flow over the rim of the continental shelf, including gargantuan undersea mudslides that can create tsunamis. All the debris piles up at the base, forming a transitional zone between the physical materials and the chemical signatures derived from the continental crust and those made from the oceanic crust beyond.

Abyssal plains An abyssal plain is well named. Compared to land it is mainly flat, with areas of low hills. It is also largely empty of life, with barely 1/8 oz (4 g) of living material for every square meter. (Although, interestingly, that doubles on the abyssal hills.) Compare that with the biomass density of 3/4 oz (20 g) per square meter in a desert or 88 lb (40 kg) in a rainforest.

Ocean depth zones Pelagic Neritic

Oceanic

Low water

Epipelagic

Photic

High water

656 ft (200 m) Mesopelagic

Sublittoral o

o

50 F (10 C)

Dysphotic

Littoral

2,300–3,280 ft (700–1,000 m) Bat l

hya

Bathypelagic o

o

39 F (4 C) 6,560–13,000 ft (2,000–4,000 m)

Abyssopelagic

sa

l

ic

nth

Be

ys

Aphotic

Ab

19,670 ft (6,000 m)

Hadal

Hadalpelagic

32,800 ft (10,000 m)

Ocean zones The oceanic waters above the abyssal plain are divided into zones that correspond to the seafloor topography. The epipelagic region corresponds with the euphotic zone. The dysphotic waters are the mesopelagic region. Below this we enter the aphotic zone. The open waters down to the depth of the bottom of the continental slope or thereabouts are described as bathypelagic. Below this depth the water is uniformly cold. It is at its densest around 39°F (4°C), and water of this temperature accumulates below around

13,000 ft (4,000 m) down, creating an eerily still layer known as the abyssopelagic realm. However, the ocean does not end there. There is deeper water still.

| A deep submarine canyon plunges through the continental shelf just off the shore of Monterey, California. This connection to deep water means this area of sea is full of life.

OPPOSITE

| A diagram outlining the profile of the seafloor as it extends out and down from the coast, as well as the zones of water above.

ABOVE

69

70 | LIFESTYLES AND ADAPTATIONS

Ocean trenches and seamount

Hydrothermal vents

Volcanic regions on the seafloor appear either where the crust is breaking apart and spreading as new rock is formed, or one section of crust is being slowly destroyed as it is pushed under by another into the seething magma from whence it came. In the latter situation, the seafloor is dragged down into deep ocean trenches, the deepest being the Mariana Trench in the western Pacific—there the seafloor is more than 7 miles (11 km) down. However, there are still plankton in these trenches, in what is called the hadal zone. These plankton are mostly chemotrophic bacteria, which sustain themselves with nutrient chemicals that would not register as food for most life-forms.

In volcanic regions of the seafloor, the crust is very thin and the heat of the Earth’s interior is close to the surface. Of course, this can lead to eruptions of lava that build new seafloor or undersea volcanoes, known as seamounts, but the heat energy also reaches the surface via hydrothermal vents. These are essentially hot springs on the seabed. Cold seawater percolates down into the deep rocks, where it is superheated by the magma below. The superheating, which sees the water remain liquid but far exceed its normal boiling temperature, allows numerous minerals to dissolve, which would ordinarily stay locked in the rocks. As the hot water, which can reach temperatures of 750°F (400°C) or more, emerges through a vent at the surface, it is chilled instantly. (The water at the seabed is generally 39°F/4°C.) The sudden cold makes the dissolved minerals come out of solution and form a smoke-like precipitate that plumes into the surrounding water. These minerals include sulfates, iron, and nitrogen chemicals, and these are used as a source of energy and nutrition by bacteria in the water around the vents. These bacteria are known as chemotrophs, and in essence they “eat” inorganic chemicals. In the absence of light and phytoplankton around the deep-sea vents, the chemotrophic plankton is the basis for the food chain. It is sifted from the water by a host of strange creatures which tolerate the soupwarm waters close to the vents. They include the yeti crab (genus Kiwa), which is covered in bristles. The crab does not hunt. Instead, it waves its bristled pincers in the water, encouraging planktonic bacteria to grow there in mats. The crab then slurps up this homegrown food from its own pincers.

OPPOSITE ABOVE | The Deep Discoverer ROV (remotely operated vehicle) investigates a deep-sea coral habitat on the Retriever Seamount. OPPOSITE BELOW | The heat and minerals expelled by the hydrothermal vent on the floor of the mid-Atlantic provide energy and nutrients for a community of animals to survive around it.

| A computer model of the topography of the region surrounding the Mariana Trench (seen as a purple arc). The Pacific Ocean trench is the deepest point on Earth.

LEFT

71

FOLLOW THE NUTRIENTS Mapping the presence of plankton—and its absence—is an excellent way of tracking the motion of nutrients through the water. The neritic zones are more full of life than the sunlit oceanic waters. It is no coincidence that more plankton blooms occur in these areas, and that most of the world’s fisheries are based here. The reason is broadly that the shallower seas have a ready supply of nutrients washing into the water off the land. These nutrients include nitrates, phosphates, and minerals like iron, which are essential for the growth of phytoplankton, and in turn the zooplankton communities that rely on them for survival. The sediments floating in the neritic waters and on the seabed of the continental shelf have a particular chemical signature, with minerals present in specific proportions. This material is slowly but consistently shed down the continental slope, where the minerals end up on the cold deep seafloor, not up in the warmer euphotic zone where life needs it to thrive. Epipelagic waters have a different nutrient balance to coastal waters. The minerals coming from the land do not reach here easily. The nutrient supply there is used up quite rapidly, in a matter of a few weeks, as the phytoplankton bloom in spring, or another growing season. Then the growth rate drops off as the nitrates and other essential ingredients of life run out. For activity to continue, there needs to be a degree of mixing between the impoverished but sunlit surface waters and the dark, cold, and nutrient-rich waters deeper down.

RIGHT | The surface of the ocean, and the plankton in it, is seldom still for long. OPPOSITE | The deep blue waters of the tropical oceans—here around the Bahamas archipelago, in the Caribbean—are surprisingly empty of life.

72 | LIFESTYLES AND ADAPTATIONS

Mixing layers Despite the swell and choppiness seen on the ocean surface, the waters beneath are perhaps surprisingly still. Deeper waters hold more chemical nutrients so any influx of deep water will replenish the surface waters. There are various mechanisms for mixing. An obvious example is seasonal storms and winds that churn up the water. This effect is most marked at high latitudes. After the spring growth spurt, the phytoplankton population begins to dwindle, and it is the turn of the zooplankton to multiply. Fall storms begin to mix up the deeper layers of water, and there may be a second surge in phytoplankton before the daylength eventually shortens. This mixing process continues through winter, and come spring the epipelagic waters are renewed. In warmer latitudes plankton growth is not limited by changes in daylength but struggles due to a lack of minerals. There are few winds and storms to mix things up, and the water settles into strongly defined layers of warm water on top and colder water below. As a result, tropical seas lack the nutrients of colder seas to the north and south. Despite having the perfect, sunny climate for growth, tropical seas have a lower abundance of plankton compared to the storm-tossed waters north and south.

HNLCs A quick method for measuring the quantity of phytoplankton in water, and to gain an understanding of the abundance of plankton in general, is to look at the amount of chlorophyll in the water. It makes sense that areas without essential nutrients will also have low chlorophyll figures. However, researchers find that there are high-nutrient, low-chlorophyll areas, or HNLCs. The plankton present there are nanoplanktonic bacteria rather than larger algae. HNLCs cover about a fifth of the world’s surface waters, mostly in the equatorial Pacific and Southern Ocean. There are two possible reasons for HNLCs. The first proposes that despite there being plentiful nitrogen and other macronutrients, the waters lack iron in a form that plankton can exploit. Iron is readily available in neritic waters, but oceanic seas rely on it being delivered by dust storms that blow off the land far out to sea. HNLCs, so the theory goes, seldom receive their dose of iron and this dearth stunts phytoplankton growth. Experiments that add iron to these waters have demonstrated that phytoplankton abundance goes up, but is then limited by the supply of other macronutrients, as is seen elsewhere. The second hypothesis is that phytoplankton in HNLCs, especially diatoms and other photosynthetic protists, are limited by grazing pressure. Zooplankton lurking in the dysphotic zone are like goats on a barren hillside. Each night they come to the surface and crop back all available food to its minimum levels.

73

Thermohaline circulation

Arctic Ocean Heat released to air

Cold water Warm water

Heat released to air Atlantic Ocean

Indi an O

cea

n

Shallow, warm current

Southern Ocean Heat released to air Deep, cold, and saline current

OCEAN CURRENTS The motion of seawater is driven by long-term ocean currents that work together to slowly mix all the oceans’ waters. Some of the most fertile oceans, such as along the Pacific coast of South America or around the southern capes of Africa, owe their great abundance to cold, nutrient-rich currents. The motion of the ocean’s currents is driven by the prevailing winds and by the rotation of the Earth, but the flow of water is also from a system of thermohaline circulation. This is to do with the temperature and salinity, or saltiness, of the water. As this is a global phenomenon, it is best to start in polar waters, where the seawater freezes into ice. The remaining water inherits the salt of the frozen water and so its salinity rises. As water becomes more saline, its density increases and so this polar water sinks toward the sea floor. There, the cold 74 | LIFESTYLES AND ADAPTATIONS

water starts to flow (very slowly) through the deep ocean toward the equator, bringing nutrients with it. In tropical regions, waters are warm and low in salt, creating a surface layer that floats on top of the colder, denser, salty layer below. The low-saline waters spread toward the poles, drawn by a current created by the sinking waters around the ice sheets. In turn, the movement at the surface draws up water from the deep. There are major upwellings like this in the Indian and Pacific Oceans and around Antarctica. The rising water is the ultimate driver of ocean mixing, bringing nutrients locked away in the deep up to the surface. Where there are upwellings, there will also be plankton.

Upwellings On a smaller, more local scale, upwellings occur around many ocean islands and undersea mountains, or seamounts. The seafloor currents are forced up these geographical obstacles, bringing a cold soup of nutrients nearer to the surface. Upwellings also form where wind blows away from land or parallel to the shore. This displaces the surface waters and draws up deeper waters. In the case of seamounts, the submerged peak acts as an oasis in the otherwise deserted ocean. The upwelling feeds plankton and other wildlife on one side and at the summit. The waters around leeward slopes are less productive.

South North

ne

tli as

Co

ind W

Upwelling Seafloor

| The thermohaline circulation, or global conveyor belt, is a slow but steady driver of the motion of water through the global ocean.

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| A diagram showing a localized upwelling driven by winds.

ABOVE

ABOVE RIGHT | A fin whale (Balaenoptera physalus) lungefeeding, with its mouth open and throat pouch distended, in the southern Sea of Cortez, off Baja California. RIGHT | A bronze whaler shark (Carcharhinus brachyurus) swims through a sardine bait ball during the Sardine Run off the east coast of South Africa.

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Dinoflagellate Ornithocercus magnificus The Ornithocercus genus of planktonic dinoflagellates is known for the striking appearance of its members. This is especially apparent when viewed with a scanning electron microscope, which imparts a 3D view of the protists. The genus belongs to the thecate dinoflagellates, meaning they have an outer casing made from 17 or 18 overlapping plates of cellulose. This is the same structural material as that used in the cell walls of terrestrial plants, but unlike plant cells, the cellulose plates are inside the dinoflagellate’s cell membrane, not surrounding it. The species shown here is Ornithocercus magnificus, the first of the genus to be described back in the 1880s. It is found in warm tropical waters. The primary cellular feature distinguishing Ornithocercus from other members of the phylum is a ringed structure that appears as a collar around the top of the cell body. This is created by the upper valve, or section, of the casing, which is much reduced compared to the lower valve.

Survivors of the deep

However, the dinoflagellates are also very much part of the phytoplankton community in the euphotic zone of tropical seas, thanks to their symbiotic relationship with cyanobacteria. These photosynthetic bacteria are packed into the girdle region of the cell body. It is thought the symbionts benefit from the chemical conditions inside the host rather than floating freely in the water. When the host drops into darker waters, they can consume the passenger bacteria and replenish supplies on brighter days.

Global spread Being very much a warm-water species, the presence of O. magnificus and other members of the genus is used by oceanographers and climate scientists as an indication of growing conditions. One hypothesis is that these plankton will spread to the north and south as the oceans are warmed by the climate. As O. magnificus appears in new areas of the oceans, that hypothesis appears to be true.

Despite being characterized as phytoplankton, Ornithocercus cells do not contain chloroplasts. Instead, they are obligate heterotrophs that absorb organic material through the cell membrane. As such, they can survive in deeper, darker, and more turbid water, where photosynthesis is not possible.

F A M I LY: Dinophysaceae DISTRIBUTION: H A B I TAT:

Worldwide

Warm ocean water

FEEDING HABITS:

| A scanning electron micrograph of an Ornithocercus magnificus dinoflagellate from the Mediterranean Sea, magnified 560 times.

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NOTES: SIZE:

Heterotroph

Hosts symbiotic cyanobacteria.

0.004 in (100 µm)

Cyanobacteria Gloeotrichia echinulata Gloeotrichia is a genus of single-celled, photosynthetic organisms belonging to the cyanophycean class of bacteria. They are mostly lacustrine, or lake-bound, plankton and thrive in clear and cold water. They are seen as filamentous colonies that float in the water, mostly on or close to the surface. These planktonic colonies can swell to 0.08 in (2 mm) across and contain millions of cells. The cells attach to each other by forming a blob-like mucus envelope with numerous hair-like filaments or palisades. The colonies are thought to form slimy mats on rocks and stones on lakebeds, where there are plenty of nutrients. Then gas vesicles inside the colony are used to bring it up to the surface where the light is more intense.

Freshwater inhabitants

The freshwater cyanobacteria can also be toxic and can become a danger in summer when they bloom, turning lakes a distinct blue-green color. The bacteria need to be monitored on public health grounds, lest they impact lake users or get into treated drinking water supplies. Blooms of lacustrine plankton such as this are associated with a process called eutrophication (see page 189). This is where nutrients meant for terrestrial crops, or from wastewaters, are washed into watersheds and accumulate in lakes. There these chemical fertilizers stimulate the growth of lake plankton, disrupting the natural balance of the ecosystem. The bloom of phytoplankton at the surface stops light, heat, and oxygen reaching lower levels of the water column.

Gloeotrichia have been studied in many environmental and research projects as they are often the dominant bacterial species in a given freshwater habitat. They are of interest for understanding nutrient cycling in freshwater systems, being especially involved in the way nitrogen, iron, and phosphorous compounds move through these habitats. It is suspected that these bacteria are in fact meroplanktonic and only spend a portion of the year floating in the water. The rest of the time, in winter mostly, they lie dormant in the sediment.

F A M I LY: Gloeotrichiaceae DISTRIBUTION: H A B I TAT:

Worldwide

Freshwater lakes

FEEDING HABITS:

| A light micrograph of Gloeotrichia echinulata as part of the lacustrine plankton.

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NOTES: SIZE:

Photosynthetic

Can be harmful to humans and other animals.

Colonies are 0.08 in (2 mm) wide

Violet sea snail Janthina janthina The snails belong to a group of mollusks called gastropods. The violet sea snail, also known as the purple bubble raft snail, has a shell and muscular foot in keeping with its kin, but it is not like other snails. The shell is light and flimsy and provides little of the protection afforded by that of its relatives living on land or on the seabed. This is because instead of sliding slowly but surely over a solid substrate, the violet sea snail clings to the underside of the ocean’s surface. Its foot is especially slimy, with bubbles of thick mucus that form a collection of buoyancy aids to prevent the mollusk from sinking. Animals that float at the surface of the water form a special plankton community called the neuston (and the term pleuston is also used). The snail’s mucus contains the same chemicals deployed by marine snails and other mollusks to make floating egg rafts. The larvae that hatch from these floating eggs are called veligers and are generally small enough to be planktonic for this early stage of development. A thin, skin-like shell surrounds the body of the larvae, which thickens as the mollusk grows and inevitably sinks to the seafloor. In Janthina janthina, in contrast, the eggs are not left to float freely but are instead retained by the mother (herself already floating) until the veligers hatch. This species is an active feeder and uses a tongue-like radula to graze on microscopic hydrozoans floating at the surface. The snail’s favored foods are the medusae of Velella velella, or sea rafts.

Hermaphroditic advantage Violet sea snails, which are confined to warmer waters but nevertheless found floating the world over, are sequential hermaphrodites. In this case, that means younger adults are male and then become female as they age. This ordering allows the older, bigger snails to maximize the production of eggs, which can be fertilized by the essentially unlimited supply of sperm from the smaller males.

Countershading The sea snail’s paper-thin, globular shell is there primarily for camouflage by providing a reverse countershading to help it hide from predators. The top of the shell is pale and the lower parts are darker and tinged with bluish purple (hence the name). Normally these colors are seen the other way around, pale underneath and darker on top, but violet sea snails spend their lives hanging upside down.

F A M I LY: Epitoniidae DISTRIBUTION: H A B I TAT:

Worldwide

Tropical and subtropical oceans

FEEDING HABITS:

| A violet sea snail (Janthina janthina) hangs upside down from the sea surface suspended by a raft of mucuscoated bubbles secreted by the foot.

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NOTES: SIZE:

Preys on jellyfish

Uses air from above the water to fill its bubbles.

11/2 in (4 cm)

Salp Pegea confoederata Salps are barrel-shaped, planktonic tunicates, and that means they are more closely related to us than to any jellyfish or many other forms of plankton. If that were not enough, they are a fascinating group of animals in other ways. They are one of the most efficient swimmers in the animal kingdom and have a complex life cycle. Salps are found in all the world’s oceans, and they are most abundant in temperate and tropical waters. They range in size from a few millimeters to several meters long. The salp shown here, Pegea confoederata, is one of the larger species. Salps are filter feeders and consume the smaller plankton and other food particles in the water around them. The salp feeding mechanism is also tied up with the animal’s unusual method of locomotion. The salp contracts its body to pump water through a gel-filled body cavity. This creates a jet of water that propels the salp forward. For plankton, salps are high-speed swimmers, with some reaching speeds of 330 ft (100 m) per hour. Salps are an important part of the marine food web, providing food for fish, whales, and seabirds. Salp biomass makes up a significant proportion of the marine snow that sinks to deeper ocean zones. As such, salp abundance is a good way of monitoring the quantity of material that is entering long-term carbon sinks as part of the biological pump (see page 101).

Growing chains Salps have a complex life cycle, reproducing both sexually and asexually. In the sexual phase, two salps release eggs and sperm into the water. The eggs are fertilized and the resulting larvae develop into juvenile salps. The juvenile salps then undergo a process called blastogenesis, in which they divide repeatedly to form a chain of daughters. The chain of salps can contain hundreds or even thousands of these cloned units. As the chain of salp grows, the older individuals at the front of the chain eventually die. The remaining salps then break off from the chain and form new chains. This process of chain formation and breakage continues throughout the life of the salp. In ideal conditions, the salps will form swarms containing thousands of chain organisms, always lengthening, then dividing, and so on. It is said they can multiply as fast as any microorganism.

F A M I LY: Salpidae DISTRIBUTION: H A B I TAT:

Worldwide

All

FEEDING HABITS:

| A group of salps, here Pegea confoederata, hangs in the surface waters off San Diego, California.

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NOTES: SIZE:

Filter feeder

Small fish swim inside the salp to escape predators.

12 in (30 cm)

Sea wasp Chironex fleckeri As the name suggests, Cubozoa jellyfish have a boxy body, and are often called box jellyfish. That name is mostly associated with the species shown here, Chironex fleckeri from the Indo-Pacific, which can strike fear into tourists enjoying the warm, clear tropical waters in paradises across the South Seas. The jellyfish’s more targeted common name is sea wasp, which raises the alarm a little more but perhaps not enough. C. fleckeri is highly venomous; some say “the most lethal jellyfish in the world”—and by extension the deadliest plankton, too. At least 64 people have died from sea wasp stings in Australia alone since 1884. Being almost completely see-through when immersed in water, the creature is nearly impossible to spot and poses a significant threat to swimmers. They are most common near shore in the summer months.

A nasty sting The venom of C. fleckeri is a complex mixture of proteins that can cause a variety of symptoms, the most apparent being extreme pain. As well as nausea, vomiting, muscle cramps, and paralysis, the venom creates a surge of potassium in the blood and this is the real danger in that it can cause a sudden and

fatal cardiac arrest. Death can come in seconds, but in a great many cases of sea wasp stings it does not come at all. The best course of treatment is to call for help and then irrigate the stings with vinegar. The acid shuts down the action of the stinger cells (but does not neutralize the venom’s toxins), then gently brush away tentacles on the skin (a credit card is a good tool, or use tweezers).

Trailing tentacles C. fleckeri is the largest of the box jellyfish. Its bell usually reaches about 61/2 in (16 cm) in diameter but can grow up to 14 in (35 cm). A set of 15 tentacles trails from each of the four corners of the pale blue bell. (There are faint white markings on the largely transparent body as well. When viewed from certain angles these give a hint of a human skull, at least to the pattern-hungry human eye.) When the jellyfish is on the move, the tentacles contract to about 6 in (15 cm) in length and thicken to 3/16 in (5 mm) across. When the animal is ready to feed, it relaxes the tentacles, unfurling a web of stingers that trails 10 ft (3 m) behind it.

F A M I LY: Chirodropidae DISTRIBUTION: H A B I TAT:

Indo-Pacific region

Warm surface waters

FEEDING HABITS: NOTES:

| A deadly sea wasp (Chironex fleckeri) pictured off the coast of Northern Australia.

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SIZE:

Active hunter of small fish

Urinating on stings is an urban myth—it makes things worse.

61/2 in (16 cm) in diameter

Portuguese man-of-war Physalia physalis The Portuguese man-of-war is a creature of warm seas that sails the oceans, catching the wind with a sail-shaped float. The animal is named because the shape of the curved gas bladder that bobs along at the surface reminded sailors of yore of a fierce fighting ship approaching from the horizon. They were right to keep an eye on this bizarre animal, a colonial hydrozoan. It is one of the longest animals in the world. Trailing largely unseen in the waters below, its tentacles are 100 ft (30 m) long, which makes this megaplankton about the length of the largest blue whales. Those long tentacles are venomous and can deliver a substantial and very painful sting. Fatal attacks on humans have been recorded, but they are very rare.

The other structures in the colony develop from meduzoid forms, not polypoid like the rest, and these are involved in reproduction. One type is the gonophore which produces sex cells, either sperm or eggs depending on the sex of the colony—this is either male or female, not both at once. Another zooid called the nectophore has a locomotory function. It breaks off from the main colony along with the gonophores and mingles with those of other colonies. Breeding happens in the fall. A single larva buds off new clones that take up roles in the growing colony. Most of this development happens deep underwater. One of the last body parts to form is the pneumatophore, which is pumped up with carbon monoxide, and then the colony rises to the surface.

Living as a colony The Portuguese man-of-war is not actually a single animal but a colony of polyps and medusae called a siphonophore. Each member of the colony takes on a specific bodily function. At the top is the elongated, gas-filled bladder, or pneumatophore, which acts as a sail and a float. Below there are tentacles. These are mostly polyps called palpons, which are devoted to stinging prey using microscopic harpoon-like stingers delivered from cells called nematocysts. Prey for the man-of-war is mostly small fish, but it will eat whatever is paralyzed and killed by the stingers and is small enough to be hauled to the gastrozooids. These are polyps lacking in tentacles that are able to digest foods externally by secreting enzymes.

F A M I LY: Physaliidae DISTRIBUTION: H A B I TAT:

Worldwide

Warm seas

FEEDING HABITS:

| A Portuguese man-of-war (Physalia physalis) pictured above and below the water. The purple and pink coloration distinguishes this species.

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NOTES:

SIZE:

Fish

Are left- or right-handed depending on how their sail curves.

100 ft (30 m)

Water flea Scapholeberis sp. The water fleas, the main component of the crustacean class Branchiopoda, are ubiquitous members of freshwater or brackish habitats. Nevertheless, they are able to withstand the salinity of marine habitats by pumping the excess salt out of their bodies. In coastal water fed by large rivers, water fleas can form a substantial fraction of the marine mesoplankon at certain times of year, most often late summer. They even swarm with densities reaching 100,000 individuals for every cubic meter. At barely 0.04 in (1 mm) long, these rotund little creatures are busy swimming up and down in the water column, using a pair of feathery antennae as oars. Their other appendages, which include six legs, are used to catch prey. Most water fleas are active hunters, using their limbs to grab prey that is a few dozen micrometers across. The species shown here specializes in grazing on microplankton held in the surface film of water. Other water fleas are focused on even smaller food items, which they filter from the water. Another striking feature of the water flea’s rounded body is the single large eye at the top of the body. This is a compound eye, which means it can pick up motion as well as differentiate light from dark. Water fleas use the eye to time their vertical migrations, heading down to avoid sunlight and up again when night falls. They use the darkness to feed in relative safety. If they spot an approaching predator, flicks of the spike-like tail are used to effect an escape.

Aphids of the ocean The common name for these animals, water fleas, is apt perhaps, but when it comes to their system of reproduction they could be better termed water aphids. These aquatic crustaceans have adopted a similar breeding system to the one used by the sap-sucking bugs that appear in large crowds almost overnight in gardens. To swell the population and exploit food sources in summer, water fleas use parthenogenesis. Here the female churns out large numbers of daughters without the need to mate with a male. The brood of eggs can be seen inside the specimen pictured here. Male offspring are only produced after several generations. The eggs produced by sexual means often enter a dormant phase and settle to the bottom of the ocean to sit out the winter months.

F A M I LY: Daphniidae DISTRIBUTION: H A B I TAT:

Worldwide

Freshwater

FEEDING HABITS:

| A light micrograph of a water flea (Scapholeberis sp.), showing eggs inside.

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NOTES: SIZE:

Graze on microplankton

Has only one eye.

0.04 in (1 mm)

CHAPTER 3

FEEDING AND BREEDING This is a chapter of two parts which examine the two great divides that separate the world’s plankton communities. The first part looks at food and feeding and the other sex and breeding. The former is familiar already: The division between phytoplankton and zooplankton is founded on how these organisms procure the nutrients they need to build and maintain their bodies and the energy used to sustain life processes. The two main strategies are autotrophism and heterotrophism. The trophism suffix is rooted in the Greek word for nourishment. The prefixes, auto and hetero, are perhaps more familiar, meaning “self” and “other,” respectively. Phytoplankton are autotrophs, or literally “self-nourishers,” and zooplankton are heterotrophic, meaning they are “nourished by others.” In other words, zooplankton do not create food. Instead, they consume the bodies of other organisms—mostly those of phytoplankton. The second division involves plankton reproduction and life cycles: holoplankton are permanent members of the plankton, while meroplankton are temporary members as the larvae of larger organisms such as fish, mollusks, sea urchins, starfish, and so on.

PHYTOPLANKTON AND AUTOTROPHISM As we have established, all phytoplankton are autotrophs and survive using a form of photosynthesis, a process that harnesses the power of the sun to build complex organic molecules from the simple materials found in water. This quite remarkable feat forms the starting point of all food chains (almost) in the oceans and on land, and is thus the source of food energy for all living things on the planet. On top of that, the photosynthetic labors of phytoplankton supply 50 percent of the atmosphere’s oxygen supply. (The other half comes from forest trees and other land plants.) Where would life on Earth be without it? An answer to that is provided by plankton, more of which later (see page 95). 92 | FEEDING AND BREEDING

Fixing carbon Phytoplankton are just one type of autotroph, specifically a photoautotroph. Obviously this refers to a reliance of light as the source of energy that drives the process. In basic terms that process is “fixing carbon,” which is a rather archaic description of how autotrophs build their carbon-rich organic chemicals, such as sugars and proteins, from inorganic sources of carbon, namely carbon dioxide. However, chemoautotrophs are also able to do this as well. Carbon dioxide is still the source of carbon but instead of sunlight, these organisms get the energy that drives the process from a chemical source.

The bacteria in the waters around a hydrothermal vent are chemoautotrophs. They are able to thrive here because there is no light and little oxygen around, and therefore no phytoplankton competing for other resources. There is also a good supply of inorganic chemicals in the water, such as iron, sulfides, and ammonia. The bacteria use the reactive power of these chemicals to kickstart carbon fixation.

| Micrasterias are freshwater phytoplankton with an interesting bilateral symmetry. Two “semi-cells,” mirror images of each other, are joined by a narrow connection that contains the nucleus.

OPPOSITE

| These deep-sea giant tube worms (Riftia pachyptila) live around hydrothermal vents. They get their nutrients from symbiotic bacteria that extract energy from minerals in the water.

ABOVE LEFT

| A light micrograph reveals a rotifer feeding on an alga, shown in green below the tiny animal.

ABOVE RIGHT

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94 | FEEDING AND BREEDING

The Great Oxygenation Event It was once the case that all autotrophic plankton were chemoautotrophs. Back then the biosphere was anaerobic. That meant there was no free oxygen in the water or in the air. As far as we know, all life was confined to the oceans in those early eons. The sun bathing the waters did not promote biological activity. Instead, it was largely a hazard delivering destructive beams of UV. However, around 3.5 billion years ago, a new form of life evolved that made use of the vast quantities of energy hitting the oceans day after day. These were the ancestors of today’s cyanobacteria. They were the first photoautotrophs, and they gradually transformed the Earth. The waste products of a chemosynthesis, the analog of photosynthesis, vary from hydrogen gas to particles of iron or sulfur. But photosynthesis takes light and uses it to combine carbon dioxide and water molecules into sugars like glucose. The waste product is always pure oxygen. While remarkable, this does not sound outlandish today. But as the photosynthetic process began, it certainly was. Free oxygen is a highly reactive substance; all fires use it to burn, after all. So to begin with, the oxygen reacted with whatever chemicals were around. Most notable was the oxidation of dissolved iron, forming an insoluble red iron oxide. This material settled on the seabed, and with a reduction in iron in the water, the activity of the plankton was reduced. The sediment changes to silicon-rich layers made up of the remains of the plankton. Then the iron levels return and

photosynthesis restarts, creating a fresh band of iron deposit. The banded ironstones created by this process are still visible, and were produced at a time when the free oxygen released by plankton was immediately soaked up by minerals. Eventually those minerals ran out—it took at least 1 billion years!—and the atmospheric chemistry was then forever changed as oxygen became a permanent component. It was not a static volume, of course. Oxygen was and is being used by other life-forms—not least you and I as we breathe—and so it must always be replaced by more photosynthesis. As of now, the level of atmospheric oxygen has reached an equilibrium of around 20 percent. It is being added as fast as it is being taken away. From our vantage point of evolutionary history, this Great Oxygenation Event is nothing short of miraculous. Primitive phytoplankton breathed new life into the biosphere, creating a world that would nurture all kinds of organisms, something that we marvel at to this day. However, it was also potentially the biggest mass extinction in the history of the planet. The chemoautotrophs were adapted to anoxic habitats (where the waters are depleted of oxygen because demand exceeds supply), and the reactive oxygen was nothing short of poison to them. Only a handful of these primitive microbes survive in habitats too extreme for the newly evolving aerobes, or oxygen breathers. Today we find the survivors of the oxygenation in waterlogged rocks deep underground, in hot springs, and buried in muds that are too thick for oxygen to permeate.

| This banded iron formation was deposited around 2 billion years ago as Earth transitioned to an oxygen-rich atmosphere.

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95

Primary production The cyanobacteria, named after their cyan-like, blue-green coloring, are still the dominant form of life in the oceans. In fact, Prochlorococcus bacteria are the most numerous organism on the planet. It is estimated that there are an octillion of them in the oceans. (An octillion is a billion billion billion.) Together with other phytoplankton, these organisms are generating half of the world’s oxygen supply as a byproduct of carbon fixation. This fixation or harvesting of carbon is also called primary production, and it is the foundations upon which all life now rests. Primary production is the point at which useful energy enters the biosphere. It arrives as light and is packaged up into a molecule of glucose, which makes for a versatile and easy-tostore fuel. The chemistry involved is complex but is broadly divided into two phases: the light reaction (because it relies on light) and the dark reaction (which does not, although it doesn’t only happen in the dark either).

96 | FEEDING AND BREEDING

Using light The light reaction is based around the pigment chemical chlorophyll, which is bound to membranes inside the cell. (In eukaryotic phytoplankton, such as diatoms and dinoflagellates, this all happens inside the chloroplast organelles.) Chlorophyll is the main reason photosynthetic organisms are broadly green. In structure, it is a cousin chemical of the hemoglobin that soaks up oxygen in red blood cells. That compound gets its redness from an ion of iron at its core. Chlorophyll’s green comes from a magnesium ion. Yet there is another way to see this life-giving color: Pairs of chlorophyll molecules form two distinct reactive centers. One is activated by blue light and the other uses red light. The unused color is green, which simply bounces off and is reflected back as an unneeded component. Meanwhile, back at the chlorophylls, the trapped light energy is used to split water into hydrogen and oxygen ions. The oxygen is expelled before it can do too much chemical damage to the cell’s innards, while the hydrogens, also highly reactive, are handed over to the dark reaction for the final phase.

No light needed The so-called dark reaction has an unsung chemical hero, a molecule that is doing all the work while chlorophyll basks in its reflected glory (and reflected green light). Perhaps the reason this chemical is less famous is because of its name: ribulose-1,5-bisphosphate carboxylase/oxygenase. Nevertheless, biochemists shorten it to the pithy rubisco. It is the job of rubisco to actually build the glucose molecules that feed not only the phytoplankton itself but also all other life in the oceans. It does this by reacting the free hydrogens with carbon dioxide in a controlled stepwise fashion (a pathway called the Calvin cycle). However, rubisco has a conflicted character, and it is just as likely to use oxygen to break up glucose into carbon dioxide and water as run the reaction the correct way. To solve

this problem, cyanobacteria evolved carboxysomes, which are bag-like structures packed full of carbon dioxide. The high concentration of this raw material ensures that oxygen cannot interfere, and so rubisco behaves itself and builds the glucoses.

| A colored transmission electron micrograph showing some Prochlorococcus cyanobacteria in cross section.

OPPOSITE

| A diagram of the metabolic pathways carried out in the chloroplast during photosynthesis.

BELOW

Metabolic pathways in chloroplast during photosynthesis

H2O

CO2

NADP+ ADP

Light

+P RuBP

3-Phosphoglycerate Calvin cycle

Light reactions: Photosystem ll Electron transport chain Photosystem l Electron transport chain H+

ATP

G3P

NADPH

Starch (storage)

Chloroplast O2

Sucrose (export)

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ZOOPLANKTON AND HETEROTROPHISM The trophic pyramid

4 T op predators (shark, tuna)

3 Intermediate predators (large fish)

2 First order consumers (zooplankton, small fish, mollusks)

1 Primary producers (phytoplankton, seagrass, algae)

The concepts of food chains and food webs are well known, and they hold true in the oceans, just as they do on land. In the same way that an antelope eats the grass and the lions eat the antelopes in the archetypal terrestrial example, so the zooplankton in the sea eat the phyotoplankton and then the fish eat the zooplankton. In the parlance, the phytoplankton are the producers because, as has been discussed, they create the source of bioavailable energy. The zooplankton are the next link in the chain and are the primary consumers. Then there are successive links of consumers. The planktivorous fish are secondary consumers, and they in turn are eaten by larger 98 | FEEDING AND BREEDING

predators, which are the tertiary consumers. Eventually, the food chain reaches its end with top predators, such as sharks, sperm whales, and orcas.

The pyramids Another term for the largest hunters is apex predator, which points to another way of understanding the ecology of nutrition, and with a surprising result when it comes to ocean communities. This other approach is to visualize the food web as a trophic pyramid, where the top predators fill the apex, or small, pointed top section. The pyramid works well as a means of expressing the chemical energy contained within each layer.

Energy flow in a trophic pyramid

6

5 T hird-level carnivorous consumers (squid)

5

4 S  econd-level carnivorous consumers (larger fish)

4

Heat

3 F irst-level carnivorous consumers (small fish, jellyfish, crustaceans, sea stars)

energ y loss

6 T op predators (shark, orca, albatross)

3

2 H  erbivorous consumers (zooplankton, mussel, limpet)

2

1 P  rimary producers (phytoplankton, seaweed)

1

Decomposers

Sun’s energy

The primary producers create the largest layer at the base, and with every successive layer, the consumers take up less room. As a general rule of thumb, only about 10 percent of the energy captured by each layer is transmitted to the next one up. The other 90 percent is lost as heat or wasted materials. In the case of the ocean this waste material also includes “marine snow,” a steady shower of organic materials that sinks through the water. This reduction in energy continues up the pyramid, with only a tiny fraction reaching the apex. The small amount of energy available to them limits the number of apex predators in the ocean (or on land).

| Food webs are arranged in hierarchies of consumers, known as trophic levels.

OPPOSITE

| A diagram of how energy flows through a food web. It is initially harnessed by the primary producers and flows up through trophic levels before dissipating away.

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Marine snow Below the sunlit zones, there is no primary production. Still, there is no disconnect from the food chain. Instead, plankton in these deeper waters rely on marine snow. This is a constant supply of organic materials, from feces to dead bodies of all sizes, which falls from the productive sunlit waters to lower levels. It is the only source of food for zooplankton and bacteria living in deeper waters, apart from migrating animals. As such, this group of consumers are outside the trophic pyramid. Instead, they are detritivores, which means “waste eaters” for obvious reasons. Ostracods, or seed shrimp, are important detritivores. The greatest prize for an ocean detritivore is a whale fall, where the carcass of a mighty rorqual (baleen whale) descends to the seabed. This single item of detritus provides the deep-sea community with the equivalent of 2,000 years of fragmented marine snow. | It is easy to see why ostracod crustaceans are nicknamed “seed shrimp.”

ABOVE

RIGHT

| A whale fall at the Davidson Seamount.

| Biological carbon makes its way to the seabed where it is sequestered in a long-term carbon sink.

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The carbon pump

create a marine sediment called ooze. It takes about 160,000 years for the carbon pump to add one more meter of fresh ooze. The carbon pump (see also page 158) is of enormous interest to oceanographers and climate scientists. The pump forms part of the wider carbon cycle and is a means by which carbon is removed from the biosphere to fill long-term sinks. How the pump behaves as the oceans warm up and disrupts marine ecosystems is an important factor in understanding the ramifications of anthropogenic climate change (see page 192).

Not all marine snow is eaten by detritivores. Some will slip through the net and settle on the seafloor. Here it will be sequestered into sediments as part of a phenomenon called the carbon pump. This crucial process takes biological carbon and other materials out of the food chain and locks it away in the seabed, eventually as solid rocks or petroleum reservoirs. The process is slow but steady. Every year, around 900 million tons of this material arrives on the seabed, which is 7 percent of the total biomass of marine life. It arrives as fine particulates which

The carbon pump process

CO2

Carbon uptake

Large phytoplankton

Surface Aggregate formation Detritus

Detritus Sinking particles

Deep ocean 12,000 ft (3,700 m)

Decomposition

Zooplankton migration

CO2 Respiration

Consumption Archaea

Bacteria

Organic carbon Deep sea consumers

Sinking flux to sediments CO2 Burial Reactive sediments Seafloor (benthic zone)

Pathogen, pollutant, and nutrient runoff

N, P, Fe, Si

Benthic flux

Bacteria and viruses

Ventilation (upwelling)

Microzooplankton

Zooplankton

Small phytoplankton

Twilight zone 328– 3,280 ft (100–1,000 m) mesopelagic zone

CO2

Respiration

sical mixing Phy

Surface ocean 0–328 ft (0–100 m) euphotic zone

CO2

Predation by birds, fisheries

Fe, N (atmospheric deposition)

Deep water formation (downwelling)

Respiration

sical mixing Phy

CO2

Dimenthyl sulfide, nitrous oxide, methane

Biomass pyramids When the ocean food chain is organized by biomass, not by energy, something strange happens. On land, back with the grass, antelopes, and lions, the biomass of each trophic layer roughly tracks the corresponding layer of the energy pyramid in size. Suffice it to say, there are a lot more plants on land than there are lions. Somewhat counterintuitively, the situation in the oceans is reversed. The biomass of marine heterotrophs is an estimated 5 gigatonnes (Gt), while the biomass of the marine autotrophs, which are mostly phytoplankton, is just 1 Gt.

Aquatic ecosystem

Tertiary consumers: Sea lions

These data show that the biomass pyramid of a marine food chain is inverted, so the phytoplankton are outweighed by the zooplankton. Nevertheless, most of the marine biomass is unicellular, with multicellular consumers making up a third of the total biomass (2 Gt). That leaves 3 Gt for single-celled zooplankton, living short, frenzied lives. Such a setup does highlight the fragility of the marine food web. The phytoplankton are multiplying rapidly when conditions are right, and their populations are limited by the grazing power of the zooplankton as well as the supply of nutrients.

Terrestrial ecosystem

Tertiary consumers: Snakes

Secondary consumers: Fish

Secondary consumers: Rodents

Primary consumers: Zooplankton

Primary consumers: Insects

Producers: Phytoplankton

Producers: Plants

| The biomass pyramids for marine and terrestrial ecosystems.

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OPPOSITE LEFT | The ciliate Paramecium bursaria has an endosymbiotic green alga.

| A light micrograph of a Euplotes freshwater ciliate.

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Mixotrophs There is some crossover between the two dominant nutrition strategies and this muddies the waters (sometimes literally) with the abundance of different groups of plankton. Mixotrophs are organisms that make use of both autotrophism and heterotrophism. The great majority of mixotrophs are protists, and photosynthesis is usually the dominant partner in the arrangement. However, the protists are still able to take in food particles, either by engulfing them with the cell membrane (a process called phagocytosis), or by drawing foods to a mouth-like pore in the cell’s exterior. One particularly striking example of this are the ciliates, which are generally regarded as zooplankton. When some ciliates consume smaller autotrophic protists they retain their prey’s chloroplasts. Chloroplasts are the site of photosynthesis in eukaryotic cells, and by commandeering them the ciliate has an internal energy supply. This phenomenon is a form of endosymbiosis and throws light on the origins of complex cells and complex life, as discussed in Chapter 1 (see page 14).

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GRAZERS AND PREDATORS Life is hard for most zooplankton and with little reward in the lower reaches of the trophic pyramid. They are always searching for that next meal and this can take some doing because in the oceans, food is diluted. When food is found it will be low in nutrients anyway—and so the search continues.

Ocean grazers Despite the ecology being markedly different in the oceans, one term used to describe how organisms feed comes from the terrestrial realm. The majority of zooplankton are described as grazers, despite there being no grass available to eat—the term has obviously taken on a broader meaning. A grazer is any organism that is surrounded by its food. It is not able to take control of a discrete food supply or make many decisions about

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what to eat and when to move on to pastures new. This refers to animals that eat grasses and small herbs just as much as the copepods, krill, crab larvae, and protozoans that are munching their way through phytoplankton. Many of them are filter feeders that sift out food from the water in various ways. In so doing, these unfussy eaters may consume waste materials and other zooplanktons.

Plankton hunters Among the zooplankton there are some species, such as jellyfish and hydrozoans, that are active predators, where prey is snared using stinger tentacles. Nevertheless, it is most likely that a zooplankton, especially the mesoplankton and above, will be consumed by a free-swimming animal. Surviving on

plankton requires an economy of scale. Plankton are mostly water, and so the nutrient density is not always high. To survive, animals need to take in a lot of plankton with minimum energy expended, and that means going big. It is perhaps no surprise that the largest animals on Earth are planktivorous. The three largest species of fish are sharks which have dispensed with the slash of teeth and all that blood and guts. Instead, the whale shark, basking shark, and megamouth shark cruise through the waters filtering plankton from the water as they go. Similarly, the largest animals on the planet, ever, are planktoneating baleen whales that feed by gulping in gargantuan mouthfuls of water. The water is then flushed out through flexible bristle filters—actually made from the same stuff as fingernails and hair—which sift out the solid contents. These megamammals, such as the blue whale, sei whale, and right whale, mostly target swarms of meso- and macroplankton, such as krill, but will eat whatever they can get along the way.

| A larvacean “house” made from mucus, belonging to Oikopleura labradorensis.

OPPOSITE

BELOW LEFT | Green turtles (Chelonia mydas) play an important role in marine ecosystems by helping to keep jellyfish populations in check.

| A blue whale (Balaenoptera musculus) with a mouthful of water and food.

BOTTOM LEFT

| A basking shark (Cetorhinus maximus) swims with its mouth wide open to filter plankton from the surface waters close to the Scottish island of Coll.

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PLANKTON LIFE CYCLES Planktonkind are split according to how much of their life cycle is spent drifting, tossed this way and that in the water column. Holoplankton are fully committed, spending all their lives as drifters. As such, they form the larger group by species number and biomass. All phytoplankton belong to this group, while the most abundant zooplankton, such as krill and copepods, also lead purely planktonic lives and go through every developmental stage floating in the water. However, a significant proportion of zooplankton spend only a portion of their lives in the water column. These are the meroplankton. The most common trajectory is to be planktonic when young and small, an example being larval fish which become free-swimmers that join the nekton. Another route is the one followed by many benthic animals (meaning they spend the bulk of their lives on the seafloor), including crustaceans such as lobsters, echinoderms like sea urchins, and cnidarians such as sea anemones, which are all part of the plankton in their larval forms. Only once big enough will they settle on the seafloor to begin their benthic phase. By contrast, jellyfish and similar hydrozoans spend their early days anchored to the seabed and only take to the wanderer’s life in the water to complete their complex life cycles.

Breeding systems There is only one activity that diverts plankton from feeding, and that is breeding. Ecologists use a concept called r/K selection to illuminate why organisms employ different reproductive strategies. The fundamental idea is that organisms opt for quantity or quality. The former is an r-selected strategy, where the r stands for “rate.” Here organisms devote their energies to producing offspring at a high rate: the higher, the better. The trade-off is that the vast majority of these young will not survive long enough to reproduce themselves. The alternative is K-selection (capitalized because the K stands for “capacity” in German). In this approach, the rate of reproduction is kept low and aimed at replacing older members of the species as they die. So the population remains stable at the maximum capacity 106 | FEEDING AND BREEDING

that the habitat can sustain in terms of nutrients and living space. The r-selected organisms are hopelessly overshooting this capacity all the time. The trade-off with K-selection is that a large degree of parental care is needed to ensure the offspring survive. It is clear why most plankton deploy r-selected breeding strategies. Their short generation times and the vastness of their ocean habitat means they cannot hope to control their surroundings enough to raise young. There is an exception, of course. The opossum shrimp, or mysids, and some amphipods and copepods hold their larvae in a pouch during the very earliest stages of development (see page 122). Still, even then, the pressure to breed quickly means the crustaceans release their young into the water when they are still very small.

| A female pram bug, a type of Phronima amphipod, has created a nest from the remains of a salp body.

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BELOW

| A pair of marine copepods mating.

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Sex or no sex? The other consideration is whether breeding plankton should reproduce sexually or asexually. Sexual reproduction requires two parents to provide sex cells which merge into a single cell that represents a new individual. It will have a unique genetic makeup, half drawn from each parent, and this genetic diversity helps it withstand the everchanging slate of threats it will face. A disadvantage is that the parents need to meet up for long enough to complete the procedure, which is difficult for organisms lacking control over their position and trajectory. Copepods exist in such crowded conditions that males and females can find partners to copulate with easily. Krill do something similar, with specialized, limb-like appendages for delivering sperm to the eggs. Some planktonic mollusks, such as sea angels, are simultaneous hermaphrodites, meaning they have both male and female genitals at the same time. It is unlikely that these floating snails fertilize their own eggs, instead having a reciprocal exchange of sex cells with a mate. Planktonic worms of varying stripes are also hermaphrodites. Meanwhile, a few snails with planktonic larvae rely on spawning, which is when unfertilized eggs are released into the water. Sperm is then squirted over them, often synchronized by the phase of the Moon, in the hope some of it will hit a target. Spawning is a common feature of fish reproduction, and results in fertilized eggs with embryos developing inside that become part of the plankton. Jellyfish and similar creatures make the best of both worlds. They may gather in vast swarms to broadcast sperm and eggs into the water. Others take it a bit more personally, with males using tentacles to transfer packets of sperm from one animal into the body cavity of another, where young are fertilized. | A close-up view of a clutch of clownfish (Amphiprion ocellaris) eggs in the Batangas marine protected area, in the Philippines.

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| A pair of mating sea angels (Hydromyles globulosa). Sea angels are oceanic sea slugs.

TOP

| Embryos are visible inside the eggs laid by a smooth lumpsucker (Aptocyclus ventricosus) near northern Japan.

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of phytoplankton known as blooms.

Asexual growth By far the fastest way to reproduce is to produce clones, or genetically identical offspring. This allows organisms to reproduce whenever conditions are right. As a result, a single asexual plankton can populate an entire sea. However, this also runs the risk of a genetic fault wiping out that population in one go. With minimal variation between individuals there is no natural defense against an emerging threat like disease or an incoming predator. Despite this, most of the phytoplankton and single-celled zooplankton rely on asexual reproduction to boost numbers fast and make the most of good growing conditions. But they also factor in a sexual phase to their life cycle, which means they can diversify the genetic makeup of future generations. Asexual reproduction takes three forms. The first is the most complex and is called parthenogenesis. This is where the bodily machinery that evolved for sexual reproduction is repurposed for mass reproduction. It is comparatively rare in plankton, but is a feature of the life cycles of rotifers and bryozoans. It is more common for asexual plankton to use budding and binary fission. The former sees a miniature version of the adult body develop as an appendage until it breaks off to begin life as an independent organism. This is how some meroplanktonic hydrozoans reproduce. Potentially there are hydroids on the seabed that have lived for millions of years, budding off young from time to time and by chance avoiding predation, disease, or death by other means. The single-celled plankton of all kinds are more likely to use binary fission, where they simply divide in two, a fast method that leads to the population explosions 110 | FEEDING AND BREEDING

Blooms While they might look impressive, a bloom of plankton (sometimes misleadingly called an algal bloom) is sometimes unwelcome and can last from days to months. Blooms appear when phytoplankton, such as dinoflagellates and diatoms (usually in saltwater bodies) and cyanobacteria (typically in freshwater systems), multiply and accumulate rapidly in such quantities that they become visible. They can make the water brown, green, and even red. The bloom is consequential because the thick layer at the surface blocks light from reaching farther down. Also, all that life uses the oxygen in the water, which spells trouble for the organisms under the bloom. Vast blooms appear whenever environmental conditions are right for rapid growth—a combination of stratification of the water column and long daylengths—but they can also be caused by an influx of nutrients like phosphorus and nitrogen. These nutrients might arrive from a seasonal upwelling that brings materials up from deep water. Often, however, the excessive nutrients are the result of human activity—for example, sewage runoff and agrochemical pollution where fertilizers are washed into the sea and boost plankton growth instead. For more on the devastating effects blooms can have on the ocean’s food web, see page 164.

| The cylindrical cells of Aphanothece marine cyanobacteria are embedded in mucilage.

ABOVE LEFT

| A swirling, green phytoplankton bloom can be seen here in the Gulf of Finland, in the easternmost part of the Baltic Sea.

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Ocean sunfish Mola mola Weighing in at more than two tons and with long, vertical fins spanning around 11 ft (3.3 m), the oceanic sunfish is one of the biggest fish in the world. In fact, only sharks, which all have a cartilaginous skeleton, grow bigger than this. The oceanic sunfish is almost as long as it is tall, and is the largest and heaviest bony fish in the world. It is named for the way the rounded giant likes to sunbathe at the surface of tropical waters. This big fish is a slow but steady predator that hoovers up slow but steady prey. As well as the odd fish and other small nektonic creatures, plus reported meals of seaweeds and shellfish on the ocean bottom, sunfish prefer to consume salps and jellyfish, which cannot escape its slow pursuits. In the more mature stages it is nektonic, but during its development like many other fish species it is planktonic.

The fry will spend several months as plankton, at the mercy of the water and unable to swim away from danger. They already have the rounded body shape of the adult, where the tail fin of more familiar fish is much reduced. In place of the tall dorsal and ventral fins seen in the mature form, the fry bristles with protective spines, giving it the look of a tiny sheriff’s badge when viewed side on. The tiny fish feast on smaller plankton and grow fast. Within a few months they are 6 in (15 cm) long and look a lot more like the adult. At this size the fish are able to take on a more mature, nonplanktonic lifestyle.

Broadcast spawning The breeding strategy of the sunfish is one of the most r-selected in the animal kingdom. They breed through a process called broadcast spawning—and the fish don’t hold back. The female releases 300 million eggs into the water, a record for any fish. The male releases milt, a sperm-laden liquid, near the eggs in the hope that they will be fertilized. For the majority this is a vain hope, and very few of the successfully fertilized eggs reach adulthood. The eggs are very small and buoyant. Spawning is staged to occur when the water is warm enough for the young. If conditions are right, the eggs hatch after about 24 hours, and tiny larval fish, or fry (about 0.1 in/2.5 mm long) emerge. F A M I LY: Molidae DISTRIBUTION: H A B I TAT:

Worldwide

Tropical and temperate waters

FEEDING HABITS: NOTES:

| An ocean sunfish (Mola mola) assesses its chances of survival.

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SIZE:

Fish, squid, jellyfish, and salps.

Shows the greatest size difference between the larva and adult of any vertebrate animal.

Larva 0.1 in (2.5 mm)

Diatom Pseudo-nitzschia australis Pseudo-nitzschia australis is a type of pennate, or boat-shaped, diatom that is responsible for some of the most damaging algal blooms. As one of the causes of harmful algal blooms (HABs) it is closely monitored by public health officials. P. australis blooms are caused by a variety of factors, including warm water temperatures, nutrient pollution from agrochemicals or sewage discharges, and natural upwellings. Each cell secretes a long, needle-shaped, silica shell that is only a few micrometers wide. The shell overlaps with that of its neighbors, and these connections allow the diatoms to form long, chain-like colonies.  

Fisheries are routinely closed during and after HABs, often for years on end. Nevertheless, isolated incidences of ASP do occur and present for the most part an inconvenience and not a health emergency. Domoic acid is heat-sensitive, so cooking shellfish to an internal temperature greater than 150°F (65°C) will destroy the toxin.

Amnesic shellfish poisoning The diatom produces domoic acid, a neurotoxin that can cause amnesic shellfish poisoning (ASP). ASP is a serious illness caused by eating filter-feeding shellfish, such as oysters and scallops, to name only a few, which have themselves consumed the diatoms suspended in the water. The symptoms of ASP include diarrhea, vomiting, headache, and confusion. In severe—and rare—cases, sufferers have seizures and fall into a coma. Very occasionally, the disease is fatal. There is no known antidote for domoic acid poisoning, so treatment is supportive. In other words, if all else fails, life-support machines keep people alive until their body excretes the toxins. The dangers of P. australis are not limited to humans. The toxin can also be harmful to other animals, including seabirds, marine mammals, and fish. Domoic acid poisoning has led to mass die-offs of marine populations.

F A M I LY: Bacillariaceae DISTRIBUTION: H A B I TAT:

Pacific and Atlantic Oceans

Coastal waters

FEEDING HABITS:

| Pseudo-nitzschia australis diatoms observed through a light microscope.

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Photosynthetic Toxins impact shellfish harvesting.

0.004 in (100 μm)

Mauve stinger jellyfish Pelagia noctiluca Pelagia noctiluca, also known as the mauve stinger or oceanic jellyfish, is a species of jellyfish found in all the world’s oceans. It is unusual in that it does not have a bottom-dwelling polyp stage like most scyphozoan jellyfish. The species is most common far out to sea in deep, open waters. It can gather in vast swarms or blooms, and these occasionally appear on beaches in spectacular fashion when wind and current conspire to wash the jellyfish ashore. P. noctiluca is one of the most common jellyfish species, and is especially abundant in the Mediterranean. It is a venomous jellyfish with painful stings that should be avoided. It can swim freely and uses its four oral arms and eight trailing tentacles to catch prey. This includes small fish, mesoplankton, and other jellyfish.

Polyp-free reproduction The life cycle of P. noctiluca begins when females release eggs into the water. The eggs are fertilized externally by the males, and the resulting larvae are called planulae. A planula is a free-swimming larva powered by cilia, which also filter foods. After a week, the planula transforms into an ephyra, which has skipped the bottom-dwelling polyp stage used by related jellyfish. The ephyra needs warm water conditions to grow enough to then switch to the medusa adult stage. The young medusae appear in early winter and are only 1/2 in (1 cm) across. They may not grow much at all in cold conditions but are fully mature come the next breeding season the following summer. 

Ocean night-light This species is highly bioluminescent, so much so that the animal is also called the night-light jellyfish. Swarms can illuminate clear waters on dark, moonless nights. They produce the brightest light in turbulent waters, such as those churned up by a passing ship. This could suggest that the light’s primary function is to attract prey which cross paths in the fast-moving patch of water. Alternatively the current could emulate the presence of a predator and thus stimulate a defense response. Most spawning happens during the day and the night-light signals may give time for the jellyfish to congregate. 

F A M I LY: Pelagiidae DISTRIBUTION: H A B I TAT:

North Atlantic, Mediterranean

Warmer open waters

FEEDING HABITS:

| The mauve stinger jellyfish (Pelagia noctiluca) lives up to its name.

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Smaller zooplankton

Large blooms sometimes wash up on beaches.

12 in (30 cm)

Radiolarian Theocapsa sp. A radiolarian is like a microscopic sculpture drifting in the oceans. Its soft cell body is contained within a spiky sphere of silica called a test. Silica is a glass-like mineral form of silicon dioxide, which the microbe has extracted from the water. Radiolarians are classified as zooplankton. They are most common in the sunlit waters near the surface, and several classes have adopted a symbiotic relationship with tiny dinoflagellates that have evolved to live inside the larger host cell. These passengers offer sugar and oxygen in return for refuge. Nevertheless, planktonic Radiolaria are well represented in dark waters too, reaching depths of around 6,500 ft (2,000 m). Some radiolarians form colonies, but the species shown here, a member of the Nassellaria order, is a solitary roamer. The spikes and spines on the shell aid with buoyancy. The cell also has multiple thread-like pseudopods for filter feeding. These extensions trap tiny phytoplankton and particles of other organic materials.  Radiolarians are especially common in warmer, oligotrophic waters. These are surface regions that lack the nutrients needed by phytoplankton, and so there is a relative scarcity of grazing for mesozooplankton. Nonetheless, there is plenty of oxygen in the water, so smaller grazers can survive there, making the most of whatever phytoplankton foods are able to gain a foothold.

At the dawn of microbiology The Nassellaria order was first identified as a distinct group of radiolarians by the pioneer of microbiology, Christian Gottfried Ehrenberg. After a career as a field zoologist, which saw him explore North and East Africa, study corals in the Red Sea, and tour the Far East with his friend Alexander von Humboldt, Ehrenberg returned to work at Berlin University. Over the decades there, he specialized in the anatomy of microscopic organisms and described thousands of species, among them many radiolarians but also diatoms and foraminifera.

The radiolarian fossil record Radiolarian shells are varied and distinctive, and over millennia they settle on the floor of the oceans, creating a siliceous ooze. This sediment enters the rock cycle, yet the shell structure is retained as it all turns to stone. The morphology of shells and the depths of radiolarian sediments are useful indicators of what was happening during different eras of the long history of the oceans. Climate changes and mass extinctions are all written in the radiolarian fossil record.  

F A M I LY: Diacanthocapsinae DISTRIBUTION: H A B I TAT:

Worldwide

Down to 6,500 ft (2,000 m)

FEEDING HABITS:

| The skeleton of a Theocapsa radiolarian, as seen through a scanning electron microscope.

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Phytoplankton Changes color of test to match surroundings.

0.004 in (100 µm)

Barnacle nauplius Semibalanus balanoides A barnacle is a sessile creature, a crustacean glued to a rock destined to watch the world go by. Most are located in shallow, rocky seas while a few, perhaps the lucky ones, find themselves attached to the hulls of ships or the flanks of a whale, and are swept along for the rest of their lives. However, in common with many marine crustaceans, everything from crabs to copepods, every barnacle begins its life as plankton, specifically a minute creature called a nauplius. The story of a barnacle nauplius is a one of adventure and transformation. It looks almost nothing like the adult form, being equipped with a simple eye, three pairs of leg-like limbs, and feathery antennae. It uses these appendages to stay afloat and filter feed phytoplankton from the water. At less than a millimeter across, the nauplius is at risk itself of being consumed by a multitude of larger zooplankton, so it is ever ready to put on a spurt of speed to escape such an attack. 

Once the cyprid is attached, it undergoes a final molt and transforms into a barnacle. The barnacle will stay attached to this hard surface for the rest of its life. It has a tough shell and a pair of feathery appendages, called cirri, which are adapted from the original legs. The cirri are used to filter plankton from the water.

From nauplius to barnacle As the nauplius grows, it undergoes a series of molts, or shedding of its exoskeleton. After each molt, the nauplius becomes more complex, with more appendages and a more sophisticated body plan. After six molts, the nauplius reaches the cyprid stage, a tiny creature with a hard shell and a pair of antennae. The cyprid uses its antennae to feel out the surface of a rock or other object, and then it attaches itself to the surface.

F A M I LY: Balanidae DISTRIBUTION: H A B I TAT:

Worldwide

Shallow waters

FEEDING HABITS:

| The larva of a common rock barnacle (Semibalanus balanoides), a type of acorn barnacle.

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NOTES: SIZE:



Filter feeder The scientific name means “curl foot.”

0.02 in (500 µm)

Opossum shrimp Boreomysis sp. At first glance, mysids are yet another type of oceanic crustacean that looks a lot like a little decapod prawn. However, the big giveaway is that the carapace is only attached to the forward sections of the thorax. This gives mysids a longer, more flexible, tail-like abdomen and a hunched, round-shouldered profile. (In decapods the carapace covers the whole thorax.)

Brood chamber The common name for mysids is the opossum shrimp, in reference to the American marsupial mammals famed for carrying large litters of tiny young in their pouches. Female mysids have an analogous brood chamber attached to the thorax, creating a rounded belly shape to go with the hunched shoulders. The initial function of the egg sac is to improve the efficiency of fertilization. Eggs are laid inside the pouch, and their number is proportional to the length of the animal. Then the male will deliver sperm by grasping the female with a limb-like genital appendage. The female hangs on to the fertilized eggs as they develop. The eggs hatch in synchrony with the mother’s next molt, and tiny larvae, barely 0.04 in (1 mm) long, enter the water. Their short stay avoids wasting resources on eggs that are never fertilized or are eaten before hatching, although the newborn larvae are still in grave danger of being predated. Even so, the streamlined efficiency of the breeding system means that the females can produce a brood every couple of weeks and as frequently as every four days in optimal conditions.

Cannibalism Opossum shrimp are filter feeders and eat whatever they can sift from the water. The shrimp shown here is a deep-sea species. It lives well below the euphotic zone, being found 6,200 ft (1,900 m) below the surface. As well as a pouch full of eggs, there is a pair of large eyes that scan the dark for signs of life and food in the gloom. Down here, food is scarce and the adults may resort to cannibalism to survive. This mostly takes the form of older mysids preying on the larvae as they emerge from the brood. 

F A M I LY: Mysidae DISTRIBUTION: H A B I TAT:

Worldwide

Marine and fresh water

FEEDING HABITS:

| A lateral (side-on) view of a deep-sea mysid or opossum shrimp, so named for the pouch where fertilized eggs are stored.

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Filter feeder Uses a pouch to hold developing eggs.

1 in (2.1 cm)

CHAPTER 4

WHERE PLANKTON WANDER Wherever there is light, phytoplankton are found in aquatic systems, from oceans and seas to lakes and ponds. Phyto and zooplankton are the most widely distributed group of organisms on Earth and can be found at all depths of the oceans. Their abundance in a given habitat depends largely on light and nutrient availability. In all of Earth’s water bodies, plankton (and especially phytoplankton) are most abundant in the euphotic zone—the uppermost layer, where there is enough light to support photosynthetic growth. Many plankton species are restricted to certain habitats, such as freshwater, estuarine, or coastal habitats, the open ocean surface, the deeper ocean, or the bottom of the sea. Other plankton species are more widespread and can exist in many different environments. Plankton move with currents and gyres, and are part of the world’s largest migration—a daily phenomenon—as well as seasonal movements through water columns.

PLANKTON DISTRIBUTION

Plankton are not distributed randomly across water bodies. While some larger members of plankton have the behavioral capability to change position in the water, their ability to move independently is limited, and active movement is simply not an option for most planktonic organisms. Their passive, floating state, or slow swimming speed in comparison to tides and currents, makes plankton extremely sensitive to physical and chemical changes in the oceans. The distribution of planktonic organisms along the vertical and horizontal planes of a water body is, therefore, influenced by the physical and chemical characteristics and dynamics of the environment. Environmental properties that affect plankton communities include water temperature and pH, nutrient availability (including phosphorus and nitrogen), and light intensity. They are further affected by factors such as water depth and surface currents, wider systems of ocean currents (including gyres), and water salinity and turbidity. Plankton are also influenced by the distribution and movements of predators and, at a more local scale, freshwater inputs, like water flowing from a river. 126 | WHERE PLANKTON WANDER

Plankton and environmental change Environmental changes resulting from human activity have caused—and continue to cause—disruption to plankton populations in marine and freshwater ecosystems. Even the slightest change in aquatic conditions can significantly alter the composition of a plankton community, and so the entire food web. As such, plankton are a vital consideration when it comes to global warming and climate change. Plankton’s sensitivity to change makes them useful indicators of environmental shifts, and given their critical part in the carbon cycle, marine plankton also offer opportunities for mitigating climate change. The future for plankton is discussed further in Chapter 6: Facing the Future.

| Single-celled diatoms are found in all marine and freshwater environments—and even in moist terrestrial habitats.

ABOVE

FRESHWATER PLANKTON Plankton are abundant in freshwater bodies, including lakes, glacial lakes, reservoirs, rivers, streams, ponds, and wetlands. Thanks to inputs from terrestrial ecosystems, freshwater systems tend to have higher concentrations of nutrients, including nitrogen and phosphorus, than saltwater bodies. The nutrient-rich waters facilitate primary production by phytoplankton, and so, since phytoplankton feed most other plankton, freshwater habitats support abundant and diverse plankton communities. While the ecology of marine and freshwater plankton is generally similar, several factors make the oceans and seas very different to freshwater environments, including higher salinity. Many species across the plankton genera are adapted to live in either fresh or salt water, but several groups of plankton exist in both environments. Some have even adapted to live in the harsh conditions of fresh water and marine anoxic environments (where waters are depleted of oxygen), like

purple and green sulfur bacteria (Chromtiaceae and Chlorobiaceae, respectively). Zooplankton common in freshwater systems include Cladocera and copepods, as well as rotifers and protozoans. Though there is more biodiversity among zooplankton in the oceans, plankton diversity is high among freshwater protists. Cyanobacteria, sometimes called blue-green algae, is most common in lakes, though some species can also be found in saltwater environments. Freshwater systems contain species from most planktonic algal groups and commonly include diatoms, silicoflagellates, dinoflagellates, and Chlorophyta, many of which are restricted to freshwater environments.

| Diatoms belonging to the genus Pinnularia are mostly found in ponds and moist soil, although they also inhabit springs, deltas, and lakes.

BELOW

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MARINE PLANKTON Marine plankton comprise a highly biodiverse group of organisms, from microscopic phytoplankton and cyanobacteria to large jellyfish. The majority of phytoplankton in saltwater systems are species of Pyrrhophyta (including a range of dinoflagellates) and Heterokontophyta (including many diatom species). Marine plankton also include a great diversity of zooplankton. Holoplankton (like copepods and ctenophores) and meroplankton, like the larvae of bivalves and crustaceans, are abundant among marine plankton. Such diversity in marine systems is hardly surprising, given that the oceans comprise around 70 percent of the Earth’s surface and the larval forms of many of the ocean’s inhabitants are planktonic. Plankton found in pelagic waters are arguably the most important group of organisms on the planet—they hold a critical ecological role in the food chain and are responsible for producing around half the world’s oxygen.

Ocean biomes It is very difficult to accurately group and divide plankton communities geographically in the world’s oceans, since their distribution is dependent on so many dynamic environmental factors. Plankton are present throughout the oceans, but their spatial distribution is also very difficult to study, so is not well understood. Sampling with plankton nets has provided some useful data on global zooplankton distribution, but many species cannot be accurately sampled with nets, so there aren’t many databases derived from consistent global sampling. It is understood that the latitudinal biodiversity gradient—a global pattern identified in terrestrial and aquatic environments, which describes heightened biodiversity at the equator (and reduced biodiversity toward the poles)—seems to hold for most plankton species. In terms of biomass, however, zooplankton is usually lower nearer the equator and higher in temperate zones, where plankton communities are supported by changes between seasons and greater upwelling of nutrients. (Upwelling occurs when winds push surface waters away from the coast and nutrient-rich, deeper water replaces them.)

Ocean mixing Phytoplankton, and so plankton species that graze on phytoplankton, are more numerous in surface waters of oceans and seas, where there is enough sunlight for photosynthesis. The surface waters also contain plenty of nutrients, particularly in areas with high levels of mixing, like coastal waters. Mixing and circulation are key to ensuring the movement of nutrients, so the physical characteristics of an ocean habitat significantly impact the local abundance and distribution of plankton. Because nutrients near the surface are utilized rapidly by phytoplankton, vertical mixing is necessary to provide enough nutrients from the rich waters beneath the mixed layer to sustain plankton in this top layer of the oceans. Mixing can occur with the main currents and seasonal changes, such as when conditions become windier in the fall, with weather events like storms and eddies, and with inputs from freshwater sources like rivers and estuaries. Mixing is, therefore, weaker and less frequent in the subtropics, where the seasons are less defined, and so mixed layers remain shallow all year. This can result in lower levels of nutrients at the surface and hence lower levels of plankton and productivity too. Coastal areas tend to have high nutrient levels, thanks to supplementation by freshwater sources and mixing through coastal upwelling and tidal mixing. Since they are nutrientrich, plankton are abundant in these habitats. Coastal and estuarine ecosystems are some of the most important marine habitats in terms of their primary production and function as a nursery habitat for key fisheries species, but they are also among the most threatened by anthropogenic activity (see page 187).

OPPOSITE ABOVE LEFT | Daphnia species inhabit a remarkable range of freshwater habitats, from temporary rock pools to lakes.

| The distribution of Daphnia is linked to the occurrence of predators; lakes with planktivorous fish tend to have smaller Daphnia species.

OPPOSITE ABOVE RIGHT

OPPOSITE BELOW | The circulation of surface currents is integral to ocean mixing, and so nutrient provision for marine plankton.

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Latitudinal diversity gradient

LOW DIVERSITY

EQUATOR

EQUATOR HIGH DIVERSITY

LOW DIVERSITY

Polar regions Plankton are important components of polar ecosystems. The extreme environmental conditions of the Arctic and Antarctic are too harsh to support species without physiological adaptations—the polar waters are not only subject to cold temperatures, but also significant seasonal changes in light and ice cover. Despite such extreme conditions, some phytoplankton species thrive in these habitats, supporting the communities of zooplankton and other marine species inhabiting the freezing waters. They live in open water during the (brief) summer and beneath layers of ice for the rest of the year. The most abundant phytoplankton in polar regions are diatoms, which are well adapted to low light conditions and utilize the nutrients from meltwater for growth. Diatoms often bloom under ice sheets, providing a food source for zooplankton, like Antarctic krill, and other marine animals. These phytoplankton form the base of the polar food webs that ultimately feed large predators like whales, seals,

penguins, and polar bears. During the parts of the year with extremely low levels of sunlight, plankton in polar waters often enter a state of reduced activity or diapause; sometimes they live out many months of the year in this hibernating state at thousands of meters depth in the ocean, safely out of reach of many predators (see page 137).

| The latitudinal diversity gradient refers to an increase in species richness toward the equator and a decrease toward the poles.

ABOVE

| Antarctic krill (Euphausia superba) live in large schools in Antarctic waters, sometimes reaching densities of up to 60,000 per 35 cubic feet (1 cubic meter).

OPPOSITE

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THROUGH THE WATER COLUMN There are four main groups of organisms in aquatic environments: neuston, plankton, nekton, and benthos. Neuston are organisms that live right at the surface of water bodies, either on top of or attached to the underside of the water’s surface film. The group includes a diverse range of taxa such as insects, spiders, worms, and protozoans. Plankton comprise the next layers; these organisms might drift at the water’s surface, but they do not live at the interface and interact with the surface film like neuston. Nekton comprises all aquatic life that actively swims and can move independently of the currents. This group includes many familiar water inhabitants, like fish, sharks, whales, turtles, and octopuses. They are found throughout the water column but become less common with increasing depth. Lastly, benthos describes all organisms that live on or within bottom sediments or on rocky substrates. The larger of these organisms include worms, sponges, and crustaceans, but many are less than a millimeter long, like nematodes and benthic copepods. Plankton play an important ecological role throughout the water column, supporting underwater life, directly or indirectly.

BELOW LEFT | Plankton even thrive in extreme environments; in Antarctic waters, phytoplankton often bloom when sea ice retreats in spring. BELOW | Phytoplankton are found primarily in the euphotic zone, where there is ample sunlight. Most zooplankton live in this zone too, but they are more widely distributed throughout the water column.

Aquatic organisms and the water column High water Low water

Coastal zone

Intertidal zone Continental shelf

Phytoplankton

Zooplankton

Sponge (Benthos)

Benthic zone (seafloor) Sea spider (Benthos)

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Plankton zones Phytoplankton, the primary producers, harness energy from sunlight, so are restricted to areas with consistent light. This group of plankton is, therefore, found primarily in the uppermost water level—the euphotic zone. The depth of the euphotic zone varies from around 160–650 ft (50–200 m) below the surface, ending at the compensation depth, where the column transitions to the dysphotic zone. At the compensation depth, light intensity reaches around 1 percent and the energy produced through photosynthesis

balances with destruction by respiration. The compensation depth changes throughout the day and with the season and becomes redundant at night. Zooplankton species that graze on phytoplankton are primarily found in the euphotic zone, but other members of the plankton group live throughout the water column, from the surface waters right down to the sediment. Bacteria, viruses, and some plankton higher up the food chain can be found in the dysphotic and aphotic zone, alongside nekton, like fish and squid.

Pelagic zone

Euphotic zone Portuguese man-of-war (Neuston)

Turtle (Nekton)

Shark (Nekton)

Zooplankton

Fish (Nekton)

3,200 ft (1,000 m)

Aphotic zone

Fish (Nekton)

Sea cucumber (Benthos)

650 ft (200 m)

Dysphotic zone

Whale (Nekton)

Octopus (Nekton)

Phytoplankton

Anglerfish (Nekton)

32,800 ft (10,000 m)

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DAILY MIGRATION The rise of aquatic life to the water’s surface to feed, followed by a deep sinking to avoid predation, is the world’s largest mass migration, and it happens every single day. Plankton are key components of the migration, which is known as the diel (or diurnal) vertical migration (DVM). The phenomenon occurs in all aquatic habitats—freshwater and marine—all over the planet, and the immense sweeping movement in the oceans can be seen from space. The DVM helps to explain the remarkable diversity in the oceans; by migrating through the water column in communities divided by trophic level, the ocean has many layered niches along the vertical plane, as well as the horizontal one. Through the DVM, the behaviors of predators from the top trophic levels are driven by those of herbivorous zooplankton.

The triggers From phytoplankton to sharks, almost all aquatic organisms undertake the migration on a 24-hour cycle. Most travel up toward surface waters at dusk, descending back into deeper water before dawn breaks, but not all organisms travel at the same time and some even follow the opposite pattern. Life of all sizes, including zooplankton, travels a remarkable distance—tens or hundreds of meters—in a matter of hours. A 0.08 in (2 mm)-long copepod, for example, can travel up to 328 feet (100 m)—equivalent to a 6½ foot (2 m)-long wildebeest traveling 62 miles (100 km) daily. That’s more than ten times the distance wildebeest actually travel each day during their famously grueling annual migrations. Most small plankton travel solely on ocean currents, without actively moving through the water levels. It is thought that the DVM patterns are driven by predator– prey relationships, a theory that is supported by evidence that some prey of non-visual feeders undertake the DVM in reverse

Depth

Classic and reverse diel vertical migration

00:00

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06:00

12:00 Time of day

18:00

00:00

to other organisms. The DVM is, therefore, usually triggered by sunlight. Even when light is scarce during dark polar winters, the migration continues, but in these conditions, the trigger changes from sunlight to moonlight. During the dark months, moonlight increases the risk of predation on zooplankton by visual hunters at the poles, so the triggers for the migration in high-latitude marine habitats shift to moonlight—as evidenced by a move from 24-hour solar cycles to 24.8-hour lunar cycles. Scientists have also recorded mass sinking of zooplankton with full moons every 29.5 days during polar winter months.

Why bother? Moving such a distance through water every day is energetically costly, especially for small organisms. There is a trade-off between saving energy and avoiding predation. Take copepods, for example. These mesozooplankton graze on phytoplankton, which photosynthesize and so live at the water’s surface. The most efficient scenario would seem to be for a copepod to feed 24 hours a day in shallow depths where phytoplankton are most abundant, but being at the illuminated surface during the daytime would also put them at the highest risk of being consumed by visual predators. Instead, copepods move away from the surface with the DVM during daylight hours—they might not be able to feed all the time, but that cost is outweighed by the benefit of reducing their risk of predation.

Impact on the carbon pump Beyond the benefits to individual organisms in the aquatic food web, the DVM plays a significant role in carbon cycling. Carbon is harnessed by phytoplankton in the surface waters and if it were not for the carbon pump (see page 101), which is facilitated by the daily migrations, much of the carbon would be consumed and then respired and hence lost back to the atmosphere. Instead, it is moved down through the water levels and a considerable amount of organic carbon, as well as inorganic carbon in structures such as the limestone shells of coccolithophores, is eventually sequestered in deep-sea sediment. This movement of carbon has a significant impact on the global carbon cycle (see page 158) and as climate change alters components of the DVM, there will be knock-on effects to the wider system.

| In the classic DVM (white arrows), visual fish predators drive larger zooplankton from the surface. Some smaller zooplankton that are less preferable to visual predators follow the reverse DVM (teal arrows).

OPPOSITE

| Zooplankton migrate to deeper water before dawn every day to avoid predation by visual feeders.

ABOVE

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LONGER-TERM MIGRATIONS Seasonal migrations

Less frequent vertical migrations are also undertaken by zooplankton inhabiting seasonal environments, such as habitats at high altitude and upwelling systems. Where the environmental conditions vary throughout the year, plankton move between vertical habitats depending on the season. While the daily migrations of aquatic life through the vertical water column have been extensively studied, these longer-term migrations of plankton are less well understood.

Seasonal movements of zooplankton were first recognized in the late 1800s, when scientists noticed that some crustaceans disappeared from the upper levels of the water column in summer and reappeared again in spring. Further investigations revealed that some jellyfish and crustaceans moved to a deeper level of the ocean for part of the year. The strategy has since been recorded in many zooplankton species living in habitats where the quality degrades for part of the year.

Water level

Seasonal depth cycle

Resting eggs

Summer

Fall Descent

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Winter Diapause

Spring

Summer Ascent

The predictable seasonal vertical migrations of plankton (and other animal species) are triggered by a change in environmental conditions. This could be a decrease in primary producers, thanks to limited light or nutrients, such as in high latitude or coastal upwelling systems, or temperature changes caused by droughts or freezing conditions, as in some freshwater systems. Predation pressure can change throughout the year, too. When conditions deteriorate, plankton migrate to deeper water and in some cases enter a dormant state until conditions improve and they can comfortably return to the upper levels of the water column.

months before returning to surface waters. In some species, the females enter diapause when they have been fertilized; they keep the sperm in their spermathacae (organs in the female reproductive system that store viable sperm) and release it to fertilize their eggs around six months later, when the conditions have improved and there is less competition for food. Other zooplankton encyst themselves in an exoskeletal layer (a cuticulin) that protects them from desiccation and predation. Some species develop the cuticulin in surface waters and this helps them sink to the sediment, while others actively swim into the sediment before developing the protective layer.

Resting stages

Wider impact

During seasons with unfavorable conditions, many zooplankton in both freshwater and marine ecosystems produce eggs with thick shells and extra lipid droplets, allowing them to survive for long periods—months, even—in a diapause state in the sediment. When conditions improve, the eggs hatch and the plankton rise to the upper pelagial for the remainder of their life cycle. Plankton that produce resting eggs include rotifers, copepods, and cladocerans. Some zooplankton can also enter resting stages in adulthood. During seasons where environmental conditions decline, these plankton sink to deeper water layers, away from hungry mouths of many predators, where they enter diapause

As with the DVM, seasonal vertical migrations of plankton have knock-on effects that influence the whole ecosystem. As herbivorous zooplankton move through the water levels, so too do their predators. Before entering a resting stage, zooplankton accumulate lipids necessary for their survival through long periods of dormancy. These plankton are therefore energy-rich prey for predators that hunt in deep waters. This movement of energy from the surface to the depths is also important for the biological pump of carbon into deep ocean, and so plays a major role in the wider carbon cycle (see page 158).

| Some zooplankton enter diapause during periods of unfavorable conditions. In this conceptual model, cladocera in a shallow pond descend and sexually reproduce, releasing resting eggs as the pond dries out over summer. When the pond refills, the resting eggs hatch and females reproduce asexually by parthenogenesis until the water level declines again the following summer.

OPPOSITE

RIGHT | Zooplankton ascend to the upper levels of the water column when local conditions improve.

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AEROPLANKTON Plankton are not only found in bodies of water—they are also floating in the atmosphere. Like aquatic plankton, aeroplankton (or air plankton) contains mostly microscopic organisms, including viruses, bacteria, fungi, and other protists—many of which are the same, or similar, species to those found in soil or aquatic environments. Aeroplankton also contains pollen, spores, seeds, and insects. The organisms hang in the atmosphere, moving with the wind—sometimes around the whole globe—before being deposited. Plankton are expelled into the atmosphere through a wide range of mechanisms—for example, animals sneezing, coughing, defecating, or scratching; ocean spray; desert dust swept up by the wind; or active-release mechanisms by fungi and plants. Two billion tons of soil are estimated to move in the atmosphere every year. Once in the air, the organisms might become attached to inorganic particles or incorporated into water droplets; they are transported higher into the atmosphere with turbulence and air currents. The amount of time they spend in the air depends on factors such as particle size and shape, the number of particles released from the source at once, and meteorological conditions. Heavier particles will drop to the ground faster than light ones, and light particles might reach the Earth’s surface by dry deposition or in droplets, like rain or snow. The abundance and diversity of aeroplankton varies with the weather, seasons, and habitat below. There are far more microbes per square meter of air above cities, for example, than over deep oceans. The circulation of air around the Earth determines the dispersal of floating organisms and, as well as dispersal of the aeroplankton species, the movements of aeroplankton affect cloud formation, precipitation, and air conditions for plants (the phyllosphere). The implications of aeroplankton movement are important for climate change and environmental health, but direct evidence of the impacts on ecosystems is only just starting to come to light. With the observation that increases in desert dust are correlated with coral reef decline, links have been

established between microbial pathogens in dust storms and diseases in marine organisms, like sea urchins, turtles, and sea fans. Despite being present throughout the atmosphere, aeroplankton are difficult to study—even more so than aquatic plankton—because of the challenges of sampling at high altitudes. They can be sampled using traps and sweep nets from aircrafts, kites, and balloons. More recently, satellite imagery has been utilized to analyze the long-distance movement of aeroplankton. Still, the atmosphere is considered to be the least understood biome on the planet.

OPPOSITE | Unlike most plants, which require insects for pollination, pine trees release pollen actively and rely on wind for dispersal. ABOVE | Many millions of microorganisms swept into the air from dust storms and sea spray are deposited around the planet every day (here over the Sahara).

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Soaring spiders Though probably not the first organism to come to mind when thinking of plankton, spiders have adapted remarkable mechanisms to exploit the atmosphere’s currents. In mass migration events, some species of arthropod climb to the highest points in their habitats before taking to the skies. Spiders are traveling hundreds of kilometers through the atmosphere—sometimes up to 21⁄2 miles (4 km)—floating on the wind in an act called ballooning. The spiders let out one or multiple strands of silk as they leap into light wind. For decades, scientists were unable to identify the precise conditions that triggered these mass migrations, with ballooning events seemingly happening whatever the weather. Humidity and wind speed have been deemed to be

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important: there should be a light breeze. This puzzled scientists further, since the light wind conditions should not allow spiders as big as those observed in the skies to balloon. Eventually, the mystery was solved. Spiders use electric fields; the atmospheric potential gradient, caused by the global atmospheric electric circuit, stimulates ballooning. Spiders have sensory hairs that are extremely sensitive to various stimuli: they respond to air and water flows, air motion close to thermal noise, sound, and, so it turns out, electric fields. When there is a vertical electric field and optimal wind conditions, spiders balloon. The atmospheric electric circuit both triggers spiders to balloon and physically enables ballooning.

Disease spread As methods for studying aeroplankton have improved, scientists have demonstrated that disease-causing microorganisms travel great distances from infectious sources on air currents, before infecting individuals who have never been in contact with the disease source. Understanding the movements of aeroplankton is, therefore, a top priority for infectious disease prevention and control. The field of study is riddled with challenges—aeroplankton are notoriously difficult to study and diseases can be spread with very small numbers of infectious organisms. For some infectious diseases, such as Influenza A and Mycobacterium tuberculosis, for example, humans only need to come into contact with a few cells for infection to take hold. The size of the particles is important when it comes to disease spread. While some infectious particles are too large to be transmitted over more than a few meters, large particles (those up to 0.004 in/100 µm) can remain suspended if the velocity of air moving through a space is high. These large particles can also desiccate rapidly, and those that have fallen from the air can become airborne again when desiccated.

The spread of infectious particles in indoor settings like offices and healthcare facilities has been extensively studied, but analyzing the spread over great distances in the Earth’s atmosphere presents bigger challenges. Because of the complexities, there is no global monitoring network for infectious microbes. But there are calls for this to change. In future, high-altitude air quality observatories (which already exist, but do not yet monitor the movement of infectious microbes) could be used to help mitigate and control infectious disease threats from aeroplankton.

| In Australia, ballooning events can see millions of spiders jumping into the air from high vantage points.

OPPOSITE

| Ballooning wouldn’t be possible without the fine but extremely strong silk threads produced by a spider’s spinneret. Here, a species of money spider (Tenuiphantes) is tiptoeing on a daisy with the silk dragline clearly visible.

ABOVE

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Copepod Calanus finmarchicus About the same size as a grain of rice, Calanus finmarchicus is a large species of copepod that is hugely abundant in the northern Atlantic Ocean. The species is a crucial component of the food web in temperate and subarctic waters, both as predator and prey. It has proven to be resilient to environmental change: The species shifted its distribution during the last ice age, and populations are on the move once again in response to global warming today. The organism’s ability to enter a state of diapause when food availability is low is key to its ability to thrive in challenging conditions.

Food source Not only is C. finmarchicus a vital primary consumer, its lipid reserves also make it a key food source for many other marine species, including commercial fishery species like larval cod, haddock, and herring. Consumption of C. finmarchicus by commercial fish has been estimated at around 20–100 percent of annual C. finmarchicus production. The species has also been shown to have a significant impact on the calving success of North Atlantic right whales—a Critically Endangered species.

Survival skills C. finmarchicus spends a remarkable six months of the year (sometimes more) hibernating deep in the North Atlantic Ocean, where individuals sink to depths of more than 328 ft (100 m)—and often more than 1,630 ft (500 m)—below the surface, and rest, relatively safe from predation, until conditions improve. During these periods, when food is scarce, they survive on stored lipid reserves, which comprise much of their body weight. Their diapause period ends in late winter or spring, and they rise to the surface in time to gorge on spring phytoplankton blooms. They mate when they return to the surface and lay eggs when food is plentiful.

F A M I LY: Calanidae DISTRIBUTION:

H A B I TAT:

Abundant in the North Sea and Norwegian Sea, as well as subartctic waters in the North Atlantic.

Found in oceans and shallow seas, and on ocean shelves, near subpolar gyres, which are centered over the deep ocean basins where C. finmarchicus sinks during overwintering.

FEEDING HABITS:

NOTES:

| Calanus finmarchicus contributes to over half the total zooplankton biomass in the northern North Atlantic, where it is most abundant.

OPPOSITE

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SIZE:

Mainly graze phytoplankton, including diatoms, dinoflagellates, and also ciliates.

During long periods of starvation, C. finmarchicus can maintain egg production, as well as its metabolism (at a low rate).

1  /16–3/16 in (2–4 mm)

Dinoflagellate Protoperidinium sp. The genus Protoperidinium comprises more than 200 species, which have been identified from waters all over the globe— though little is known about the ecology of each individual species. Members of Protoperidinium are heterotrophic dinoflagellates of fairly similar appearances. They are generally round or diamond-shaped, with thorns or spines and grooves along the center of the cell that show a division between the upper and lower halves. Protoperidinium species have plates around the cells that form a strong casing made of cellulose (called the theca) and flagella to help the organisms move through the water.

Diverse distribution Protoperidinium species are found almost universally in aquatic environments, including in many different habitats—from salt ponds to tropical seas and the Arctic Ocean. Some species are present year-round while others vary with the time of year and are only found in regions for one or two seasons. Their distribution tends to be driven by food availability, and the seasonality recorded in some species typically coincides with diatom blooms, with populations reaching their highest numbers during or straight after autotrophic biomass peaks.

Raptorial feeders All species belonging to Protoperidinium are heterotrophic. They lack chloroplasts and are raptorial feeders—using a method called pallium feeding to consume prey, whereby they exude a pseudopod (known as a pallium), which surrounds and digests the phytoplankton outside the heterotroph cell. Their diets vary between species and region; some species are generalists while others are highly specific grazers that will only grow if they consume a specific phytoplankton species.

F A M I LY: Protoperidiniaceae DISTRIBUTION:

H A B I TAT:

 Protoperidinium have been recorded in aquatic regions all over the world; they often occur seasonally and are most abundant in coastal waters.

Species are found in a wide range of habitats, including marine, brackish, and fresh water.

FEEDING HABITS:

NOTES:

| Protoperidinium use chemoreception to detect prey, circling around diatoms and other dinoflagellates to locate them before attaching to them.

OPPOSITE

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SIZE:

Diets vary between species; some are very specific while others are generalists. Typically, they consume large diatoms and other dinoflagellates.

Unlike most plankton species, members of Protoperidinium consume prey the same size, or even bigger than, themselves. They therefore compete with larger zooplankton, like copepods, for food.

Up to 0.01 in (300 µm)

Mantis shrimp Alima sp. Crustaceans are typically the most abundant members of the mesozooplankton—by a long way (in terms of both number of species and number of individuals). They also display some of the most impressive metamorphoses from meroplanktonic larvae to motile adults. Among these notable transformations is the development of mantis shrimp from small, transparent, free-floating organisms to stunningly colorful and famously aggressive benthic predators.

Predatory plankton Even as larvae, mantis shrimp are voracious predators. As such, they are likely to have a significant impact on food webs, though their specific ecological roles are understudied and not well understood. Around 500 species of mantis shrimp (those belonging to the order Stomatopoda) have been described, but—despite their relatively high abundances in samples—the larval stages have only been identified for a small percentage of these. Larval mantis shrimp are large zooplankton that can reach lengths of several centimeters; they prey on other zooplankton and are largely restricted to shallow, tropical waters.

Life stages Mantis shrimp have a complex life cycle. Beginning as eggs laid by females (and either carried or attached to substrate until they hatch), mantis shrimp emerge as nauplii. At this initial stage, the larvae have simple bodies and a single eye; they use their appendages for movement and feeding on small zooplankton. It is when they progress to the next stage—zoea—that they develop more complex bodies, following several molts. When they develop from zoea to megalopa larvae, they begin to resemble adult mantis shrimp but are still part of the planktonic community, since they cannot swim against currents until they settle and metamorphose into juvenile mantis shrimp.

F A M I LY: Squillidae DISTRIBUTION:

H A B I TAT:

The family is widespread in marine coastal waters, including the Pacific, Indian, and Atlantic Oceans, and the Caribbean and Mediterranean seas.

Mantis shrimp are mostly found in tropical regions with warm waters, though they are also present in subtropical and even some temperate regions.

FEEDING HABITS:

| Larval mantis shrimp use drag-powered swimming; they push water back and then move their limbs forward to propel themselves through the water.

OPPOSITE

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NOTES:

SIZE:

Predators of other smaller zooplankton, including planktonic crustaceans.

Some species come together only to mate, while others pair for life.

Up to 11/2 in (4 cm)

Brachiopod Terebratalia transversa Brachiopods are meroplanktonic, spending the first part of their life cycle as planktonic larvae and the latter part as sessile juveniles and adults that live on the seafloor. As adults, they are recognizable for having bilaterally symmetrical shells— because their shells are in two parts, the adults look similar to bivalve mollusks. Brachiopoda is an entire phylum, containing around 450 species that have traditionally been divided into two classes: Inarticulata and Articulata (which includes Terebrataliidae). As adults, brachiopods range in size from 1⁄16 in (1 mm) to 3⁄8 in (9 mm). They are solitary animals that all exhibit a free-living larval stage. The larvae of those traditionally classed as Inarticulata resemble the adults and use their protruding lophophore (a tentacle-like organ that captures food particles in the water) for feeding and movement during their several months of planktonic life. Larvae of Articulata, on the other hand, are planktonic for just a few days before settling on substrate and undergoing metamorphosis.

Predators The larvae of T. transversa and other brachiopods form an important part of the diets of numerous marine organisms, including zooplankton (like copepods), filterfeeding invertebrates (like bivalve mollusks, clams, and barnacles), and fish with filtering mechanisms (like herring and anchovies).

Filter feeding Like all brachiopods, the larvae of Terebratalia transversa use a lophophore for feeding. This is lined with cilia, which facilitate a water current that transports food particles into the organ. Their diet varies depending on the region and season, but they mostly consume phytoplankton. After the larvae metamorphose and become sedentary adults, the feeding habits of T. transversa change and it filter feeds (still using the lophophore) to capture organic particles and detritus from the nearby water; this might include bacteria, phytoplankton, and small organic particles.

F A M I LY: Terebrataliidae DISTRIBUTION: H A B I TAT:

Northeast Pacific waters, from Alaska to Mexico

Live in temperate marine waters, usually in tidal and subtidal zones.

FEEDING HABITS: NOTES:

| During the larval stage, Terebratalia transversa are free-swimming and enjoy a mobile life before settling on the seafloor.

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Primarily phytoplankton during the larval stage. T. transversa larvae are small and transparent, with a distinct hinge region between two uncalcified shells.

Larvae 0.01–0.06 in (0.3–1.5 mm); adults up to 11/2 in (4 cm)

Horseshoe worms Phoronis sp. The horseshoe worm family—Phoronidae—and, in fact, the whole phylum, Phoronida, contains just two genera (Phoronis and Phoronopsis). Their larvae are filter-feeding zooplankton known as Actinotroch. The early larval stages of horseshoe worms are very difficult to identify and there is a paucity of information around species belonging to the genera. Nevertheless, the organisms are abundant in the oceans and present at high densities (tens of thousands per square meter) when conditions are favorable in some regions.

Later life As adults, horseshoe worms are bottom dwellers; they settle in the benthic layer of oceans and attach to substrate (like rocks or shells) using a pedicle, where they metamorphose into their sessile adult form. Here, they live out their life cycle as filter feeders, consuming food particles through their lophophore— a specialist feeding structure that extends from the coelom.

The tentacle crown The larvae live near the surface—usually at a depth of up to 230 ft (70 m), but sometimes to around 1,300 ft (400 m). There, they feed on phytoplankton and small zooplankton, which they consume by generating water currents through their characteristic ciliated bands, called actinophores. These bands help them to move through and maintain their position in the water column, as well as draw in small particles to feed. (Interestingly, in adulthood, some species also incubate their eggs among their tentacles.)

F A M I LY: Phoronidae DISTRIBUTION:

H A B I TAT:

Phoronids are found at high densities in seas and oceans all around the world; they have wide geographic ranges and are likely cosmopolitan.

Surface waters of marine environments around the world.

FEEDING HABITS:

NOTES:

| During a 30-minute metamorphosis, the tentacles of the Actinotroch are replaced by a lophophore, the anus moves, and the gut changes from straight to a U-shaped bend.

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 Phoronis larvae consume phytoplankton and small zooplankton.

Phoronids were first described under their larval genus name—Actinotrocha; adult phoronids were not described until years later, and eventually the connection between the two was made (though more larval types have been described than adult).

Larvae up to 1/8 in (3 mm)

CHAPTER 5

FEEDING THE OCEANS Aquatic food webs commonly begin with plankton—the “organic soup” that provides energy for swathes of ocean, sea, and freshwater life. The energy fueling the fierce thrashing of an orca’s tail can, ultimately, be traced back to a single-celled plankton harnessing energy from the sun’s rays through photosynthesis. This remarkable process provides food for virtually all of Earth’s aquatic inhabitants (not to mention a huge proportion of the oxygen on which the world’s biosphere relies). Without plankton, the foundations of the food web would collapse, along with the entire marine ecosystem and the global carbon cycle. The aquatic food web involves a multitude of complex interactions between organisms. It is divided into two main parts: the microbial loop and the transfer of carbon between larger organisms. Plankton play an important role at both levels, but the ecological role of microscopic plankton is of particular interest. Most plankton are microscopic, and it is the interactions between these invisible organisms that kickstart—and maintain—the aquatic food web.

A BRIEF HISTORY The very beginnings of marine food webs have long been of interest to biologists, but delving into the microbial ecology of the oceans is no easy task. For decades, the extent of the aquatic microbiome was unrealized. Even with the new technologies and computational techniques widely implemented today, microbiologists are still searching for answers around the complexities of the interactions between the billions of microorganisms inhabiting Earth’s water bodies.

Counting microplankton Until the development of direct count assays using epifluorescence microscopy in the late 1970s, only organisms large enough to be seen under a light microscope could be studied. Suddenly, it was possible to count bacteria more accurately, and it became immediately clear that these microorganisms were much more abundant in oceans than biologists realized. The number of cells appearing under epifluorescence microscopes dwarfed the counts from agar plates, and estimates rocketed from hundreds of cells per milliliter of water to hundreds of thousands. Just a few decades ago, the assay count breakthrough highlighted the extent of the knowledge gap when it came to aquatic microbial ecology: the majority of bacterial strains were unknown. Though the insights it offered were invaluable, the usefulness of microscopy (even epifluorescence microscopy) was still limited when it came to analyzing the millions of microorganisms in the ocean. It was not until the dawn of molecular biology that scientists could even begin to glean a true understanding of the microscopic diversity of Earth’s water columns.

Molecular innovation With advances in molecular biology in the 1990s, microbiologists could isolate bulk DNA from ocean samples and use polymerase chain reaction (PCR) to analyze them. A technique using 16s ribosomal RNA (16s rRNA) proved particularly useful in the study of microscopic aquatic life. All microorganisms produce 16s rRNA to build proteins and, with the help of PCR, it was now possible for the gene that produces 16s rRNA to be identified in a sample of seawater. Microbiologists did just that, before analyzing the genes and recording variants of the 16s rRNA gene—each of which represented a different microorganism species. With this technique, hundreds of species were identified in each water sample. Molecular techniques improved rapidly with the Human Genome Project, and by the early 2000s scientists could analyze DNA at unprecedented rates. When Craig Venter and his team applied these techniques to samples from the Sargasso Sea, they estimated that more than 47,000 species of microorganisms were living in that well-studied (but apparently relatively biologically inactive) area near Bermuda. Molecular biology revolutionized the study of microbes in the ocean, and these rapid, inexpensive, and precise approaches are still key to the study of microscopic plankton today.

| For decades, plankton smaller than 0.2 µm were invisible to scientists, who only had access to light microscopes.

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155

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Carbon capture When scientists began analyzing ocean water from below 1,600 ft (500 m) in the 1930s, they found that the ratio of carbon, nitrogen, and phosphorus (all building blocks of cellular molecules) was the same as in phytoplankton. Some 20 years later—in 1958—Alfred Redfield, a biologist at Harvard University, proposed that phytoplankton were not just mirroring the ocean’s chemical composition; rather, they were causing it. Scientists learned that phytoplankton utilize carbon dioxide to generate the organic matter essential for life. They are vital to the carbon cycle—without phytoplankton, the oceans would be more acidic and less carbon dioxide would diffuse into the water from the air. Levels of gas in the atmosphere would be higher and the entire carbon cycle would be altered. It took a view from space to truly appreciate the impact of microscopic plankton on the carbon cycle. Phytoplankton have a pigment—chlorophyll a—that enables them to absorb light for photosynthesis. The more phytoplankton there are, the darker the water appears from space. Using data from the Coastal Zone Color Scanner, and with the help of a NASA satellite, scientists could finally estimate global phytoplankton productivity. Phytoplankton, it revealed, incorporated 50–55 billion tons of inorganic carbon—almost half of the world’s carbon primary production—despite comprising less than 1 percent of the planet’s photosynthetic biomass.

Studying zooplankton Studying larger zooplankton might sound like a simpler task, but the oceans remain a challenging environment for biologists and while many zooplankton are large enough to capture in fine nets, some are still too small, some are too fragile, and others are too savvy (and capable of avoiding capture). As such, innovative techniques are employed alongside filtration methods to measure and monitor zooplankton. These include direct observation through scuba techniques, remotely operated vehicles, acoustic sensors, and submersible technologies, like digital holographic cameras.

| Congregations of phytoplankton can now be monitored from space thanks to their bright chlorophyll pigmentation. Here, massive congregations of greenish phytoplankton swirl in the dark water around Gotland, a Swedish island in the Baltic Sea.

OPPOSITE

| Although they are bigger than phytoplankton, most zooplankton are still microscopic, so studying them often requires analyzing water samples in a laboratory.

ABOVE

157

THE CARBON CYCLE

The biological pump The biological pump is the process by which carbon is transferred into the oceans from the atmosphere and land runoff and, eventually, sequestered in the seafloor. Atmospheric carbon is fixed through photosynthesis by primary producers, while carbon that enters the system as dissolved and particulate organic matter is utilized by bacteria. The pump describes the movement of this fixed organic carbon through the water column via various biological processes. Once carbon has been fixed, and so can be used by non-photosynthetic organisms as energy, organisms higher up the trophic levels—consumers—transfer it through the food web by eating live organisms. Carbon is used as energy and integrated into the shells of calcareous cells walls of phytoplankton like coccolithophores. Decomposition, where carbon is remineralized through the microbial loop (see page 160), occurs alongside these processes. 158 | FEEDING THE OCEANS

The biological pump Carbon dioxide

Phytoplankton

Zooplankton

Vertical migration

Sinking POM Suspended POM

Euphotic zone

Carbon dioxide

Dysphotic zone

To gain insights into the intricacies of the aquatic food web, it is essential to first understand the carbon cycle. Carbon is the energy currency of aquatic organisms—the majority of an organism’s energy is stored in carbon bonds. Food webs are used to map out the cycle of carbon through the living components of ecosystems. The processes in the aquatic web provide energy for, and cycle energy between, life under water, and play an indispensable role in carbon capture (removal of carbon from the atmosphere). In aquatic webs, carbon enters the system through several routes: atmospheric carbon dioxide (which dissolves in surface waters), terrestrial labile dissolved and particulate organic matter (organic carbon from plants and soil, for example, which is broken down by bacteria), and refractory terrestrial carbon. Refractory terrestrial carbon is also organic carbon from sources like plants and soil, but it is the portion that resists bacterial degradation. It only accounts for a small portion of dissolved and particulate organic matter, but it can remain in the oceans for thousands of years, so is significant in its role as a carbon reservoir.

Bacteria Respiration

Organic compound burial

A portion of fixed carbon sinks with feces or after the death of cells. When organisms die, for example, particles often form aggregates, which makes them heavier. These particles sink faster than smaller particles and, consequently, are more likely to escape predation and decomposition and make it right down to the sea floor. These particles are known as “marine snow” and can evade decomposition by bacteria, falling to the depths of the oceans without interruption. Carbon is also pumped from the surface layer through physical mixing and transferred with the diel vertical migration (see page 134). When carbon is eventually deposited in seafloor sediments, it remains there, removed from the atmosphere, for thousands of years or more. This process, whereby carbon is sequestered from the atmosphere and terrestrial runoff and transported into the depths of the oceans, comprises the biological pump. More than 10 gigatonnes (Gt) of carbon are transferred into the deep ocean through the marine biological pump every year.

Food webs and the links between them Pelagic food web

Microbial loop Inorganic nutrients

Littoral/benthic food web

Terrestrial and aquatic detritus

Periphyton macrophytes

Bacteria and fungi Bacteria DOM Shredders and collectors (invertebrates)

Phytoplankton

Grazers and scrapers (invertebrates)

Littoral zooplankton

Zooplankton Microzooplankton and mixotrophic algae

Large predators (fish)

| Through the biological pump, organic carbon is transported deeper into the oceans and a small portion becomes buried in the seafloor, where it remains for thousands of years.

Small predators (invertebrates, fish)

Large predators (fish)

OPPOSITE

| The microbial loop, fueled by dissolved organic matter, occurs alongside the classical aquatic food web.

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THE INVISIBLE WEB Beginning with photosynthesizing single-cell organisms, the aquatic food web is a complex cycle with multi-way interactions throughout. In its simplest form, the web begins with primary producers: microscopic bacteria and plant-like phytoplankton that harness energy from the sun. The producers are eaten by primary consumers, like larger microplankton and zooplankton. Zooplankton form the diet of aquatic animals (like fish and crustaceans), and these animals might be consumed by larger animals (like carnivorous fish), then the larger animals by apex predators, like sharks. There are complex interactions between all “levels” of the food web, including top-down pressures, where organisms higher up the trophic levels regulate populations of their prey, and bottom-up pressures, where phytoplankton production regulates populations of animals higher up. The direction of these pressures varies between aquatic habitats, with ecological conditions, and over time. The invisible elements of the food web are critical to aquatic ecosystem function. They might be small but, together, the ocean’s microscopic plankton are far more abundant and have a far greater combined biomass than all multicellular ocean life. It is estimated that there are many more bacteria in the ocean than there are stars in the universe. Their role in the food web shapes entire ecosystems; without them, all of nature’s cycles would soon collapse. Earth’s oceans bring to mind images of fish, seaweed, and whales but, in reality, it would be much more accurate to envision diverse, dense pools of bacteria.

Starting small Small microscopic plankton are the basis of much ocean life. These tiny organisms—invisible to the naked eye—are divided into three groups based on their size: microplankton (20–200 µm), nanoplankton (2–20 µm), and picoplankton (0.2–2.0 µm). Aquatic viruses, which are less than 0.2µm, are classified in their own group: femtoplankton. Taxonomic classification of microscopic plankton is challenging, particularly for picoplankton and femtoplankton, due to the limitations of microscopy at such a small scale. But with the help of molecular 160 | FEEDING THE OCEANS

techniques, scientists are beginning to realize the monumental extent of diversity among the smallest plankton groups. Picoplankton, also known as bacterioplankton, are the smallest members of the plankton collective—they include cyanobacteria and bacteria. These organisms are phototrophic and heterotrophic, respectively. Cyanobacteria can perform photosynthesis—they get their energy from light radiation and can synthesize organic compounds from inorganic substrates. Bacteria, on the other hand, consume organic carbon to generate energy. Picoplankton is a hugely diverse group, containing more species than freshwater plants and animals combined. Nanoplankton—the medium-sized group of microscopic plankton—include protists, diatoms, and small algae. This group also includes both phototrophic and heterotrophic organisms. Microplankton contains the largest of the microscopic plankton, and comprises most phytoplankton, rotifers, ciliate protozoans, and juvenile copepods. Even at the microscopic level, trophic relationships between plankton groups are complex. It is useful to separate the wider food web of large (that is, non-microscopic) zooplankton and other animal species from the microbial components—the interactions between which are best understood using the microbial loop.

The microbial loop There are two classes of organic matter in the oceans: dissolved organic matter (DOM) and particulate organic matter (POM). The categories are based simply on size—pores smaller than 0.45 µm are considered DOM, while those bigger are POM. DOM is much more abundant and is one of the greatest reservoirs of organic carbon on Earth. DOM and POM play an important role in organic carbon cycling, and the flow of DOM and POM through the microbial food chain is described with the microbial loop—a process that occurs alongside (and is interlinked with) the main food web. In surface waters, phytoplankton fix organic carbon into POM through photosynthesis, and the POM is passed through

| Complex aquatic food webs are fueled by primary producing phytoplankton and their primary consumers, zooplankton.

BELOW

A complex aquatic food web

Top predators

Large sharks

Marlin

Smaller sharks

Predators

Tuna

Amphipods

Shrimp

Pteropods

Zooplankton

Lanternfish

Filterers

Mackerel

Squid

Diatoms Dinoflagellates

Phytoplankton

Copepods

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the trophic levels to grazers and fish. POM can be transported into deeper water by sinking particles—a key process for removing carbon from the surface of the oceans and sequestering it in sediment on the ocean floor. Phytoplankton also release DOM into the oceans. DOM diffuses into the water throughout a phytoplankton’s life, and also after its death. When zooplankton graze on phytoplankton, they lose much of the material through sloppy eating and the remnant DOM is released into the water. A similar release of DOM happens when a microbe is attacked by a virus and the host cell dies. Larger organisms also contribute to DOM in the water throughout their life cycles, including during the decomposition process after their death. Historically, this DOM was considered a waste product, of little use to marine organisms. The recognition that bacteria break DOM down with enzymes and incorporate it into biomass was a major breakthrough and inspired the concept of the microbial loop. Bacteria scavenge DOM floating in the water column and use what was previously considered to be waste matter to generate material for use as an energy source and to grow their own bodies—a process called secondary production. Bacteria consume vast amounts of DOM and regenerate inorganic nutrients, so the release of DOM by phytoplankton drives secondary production and bacterial growth. DOM is 162 | FEEDING THE OCEANS

The microbial loop CO2 CO2

CO2

RIGHT | In the microbial loop, bacteria release carbon via respiration and recycle organic matter so it can be reincorporated into the food web. The microbes contribute to the production of recalcitrant DOM, which resists degradation and can be sequestered in the seafloor.

Phytoplankton

OPPOSITE ABOVE | Diatoms process light into energy for zooplankton primary consumers with the help of chlorophyll and carotenoid pigments.

Zooplankton

POM DOM

| Cyanobacteria such as Chroococcus turgidus are also known as blue-green algae because of their bold coloring, caused by chlorophyll a pigments.

OPPOSITE CENTER

Microbial loop

OPPOSITE BELOW | Cryptomonads are dinoflagellates; some are mixotrophs, which means they can both photosynthesize and utilize energy from consuming bacteria.

Heterotrophic bacteria

Inorganic nutrients

Long-term storage

incorporated back into the food web as bacteria are consumed by heterotrophic nanoplankton and microplankton, and even some larger metazoans such as appendicularians, which are, in turn, consumed by larger zooplankton. In due course, the carbon released from DOM by bacteria returns to the food chain as DOM and POM, closing the loop. Despite the small biomass of individual bacteria in comparison to bigger phytoplankton and zooplankton,

bacteria have a much greater surface area. As such, they encounter and absorb more chemical substances in water than other microorganisms. Scientists now recognize that their considerable surface-to-volume ratio means bacteria have a rapid metabolic rate and, as a result, their contribution to the carbon cycle and aquatic food web is momentous.

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Effect of blooms on food webs Algal blooms are a natural component of ecosystems and can have benefits for the food web. The spring bloom in temperate seas, for example, is a key annual event that provides huge concentrations of algae for higher organisms to feed on. Blooms of plankton can, however, have serious and harmful impacts on aquatic food webs. If a bloom becomes so dense that sunlight cannot penetrate the blanket of plankton, it can prevent other organisms from getting the light they need to survive. Moreover, the plankton can clog animal’s gills and suffocate them. And when a bloom ends, the degradation of the huge population of algae can extract significant amounts of oxygen from the water, leaving insufficient levels for other organisms. Blooms can be

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responsible for mass fish die-offs—sometimes impacting millions of animals—and the deaths of shellfish and seabirds. Blooms can also release harmful substances into the ecosystem. Cyanobacteria produce hepatotoxins, neurotoxins, cytotoxins, and endotoxins, which can be difficult or impossible for water purification plants to remove from drinking water. Diatoms produce neurotoxins, which become particularly harmful as they accumulate in the food chain; they cause seizures in vertebrates and build up in shellfish and small fish which, when consumed by larger vertebrates (including humans) can cause serious health issues, and even death.

The nano and micro grazers The microbial loop is intertwined with the wider food web of plants and animals. Microplankton and nanoplankton affect populations of picoplankton directly by consuming them and, indirectly, by grazing on other picoplankton consumers, like heterotrophic nanoplankton. They control the bacteria populations in aquatic environments. Larger flagellates and cilliated protozoa consume smaller flagellates (as well as bacteria). In turn, microscopic zooplankton consume nanoplankton and other microplankton, and so the food web expands and energy is transferred between all factions of the microscopic community. This transfer of organic matter between microscopic organisms is extremely important for the efficiency of the food web. The microscopic loop reintroduces DOM, which would

otherwise be unavailable to most organisms higher up the food chain, into the food web. This valuable energy, passed on to larger microplankton that consume the bacteria, has far-reaching effects in the more traditional food web.

| Some algal blooms aren’t easy to see, but in severe cases can expand visibly over the surface of vast expanses of water, here in Lake Erie, in North America.

OPPOSITE

ABOVE |

Most toxic red tides are caused by dinoflagellates; they are typically triggered when pollution causes a nutrient imbalance in coastal waters, here in the Marmara Sea, near Turkey.

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THROUGH THE TROPHIC LEVELS The next trophic levels—primary consumers—comprise a great diversity of metazoan zooplankton, as well as microscopic nanoplankton and microplankton. Metazoan zooplankton include both primary and secondary consumers. The group includes rotifers, meroplankton (for example, echinoderm and polychaete larvae), and holoplanktonic organisms, like copepod nauplii and copepodites. Meroplankton includes the larval forms of many benthic organisms, including crustaceans, sea urchins, and worms. Their roles in the food web vary—they are diverse and

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seasonal. Many zooplankton are suspension feeders that graze on phytoplankton, but the larger species also consume smaller zooplankton. Copepod nauplii—the larval form of copepods—are the most abundant metazoans in the oceans and are found in freshwater habitats, too. They consume prey in a similar fashion to their adult counterparts and, though they are much smaller, they have a similar ecological role to adult copepods. Copepods play an important part in the food web, both as grazers of microzooplankton and as prey for forage fish.

Large zooplankton like krill and ctenophores are an important source of protein in marine habitats. These animals belong primarily to the next level in the food web—secondary consumers—since a large portion of their diet comprises smaller zooplankton. Forage fish are also important secondary consumers. These species consume small zooplankton and are prey for larger predator fish, as well as mammals and birds. Large animals like seabirds, polar bears, and seals rely on plankton’s role in the food web, as does the human fisheries industry.

BELOW LEFT | Many species of fish rely on krill as a significant component of their food intake.

| Copepods have important roles as both grazers of phytoplankton and prey for larger zooplankton, filter feeders, and fish.

BELOW

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FEEDING HUMANS Overfishing is a major issue that continues to exert huge pressure on ocean ecosystems. Not only are fish populations being depleted, but non-target species are often harmed as bycatch, including crustaceans (like krill) and decapods (like prawns and shrimp). There is global pressure to reduce the damage that overfishing causes to the marine environment, but with demand for protein rising in line with a growing human population, progress has been slow. To protect the world’s aquatic habitats, developing less damaging methods of animal production is more important than ever. Fish farming offers a more sustainable option for seafood production and, already, aquaculture accounts for a significant proportion of meat consumed by people around the world. The aquaculture industry is vital for food security and the contribution of fish to the human diet is set to increase. Today, these facilities often use unsustainable practices, but with improved processes, the industry has the potential to not only provide sustainable sources of protein, but also protein with bioavailable nutrients and fatty acids that are not naturally available in land-based foods.

Plankton in fisheries The role of plankton in aquatic ecosystems is integral to fisheries for two reasons. Firstly, the majority of farmed fish produce larvae that live among and feed on plankton. Phytoplankton and zooplankton are also crucial in the primary and secondary production that supports fisheries. As in wild ecosystems, plankton are essential to the life cycle of fish in aquaculture systems, even in ponds where fish are given manufactured feed. They are important for supplementing food in the early life stages—for fingerling fish and postlarval crustaceans. Maintaining an optimal balance of microbial plankton is a challenge in aquaculture systems. While there may be lots of species of phytoplankton present in an aquaculture pond, a few will usually dominate, comprising the majority of the phytoplankton community. Species composition in such communities can change rapidly—over several weeks— and needs to be monitored carefully. 168 | FEEDING THE OCEANS

Potential problems The concentration of nutrients in the water is monitored in aquaculture systems to control phytoplankton, but there is always a risk of an imbalance—especially in systems that use fertilizer or manufactured feed. The more inorganic nitrogen and phosphate in a water body, the more phytoplankton there will usually be, and if this balance is tipped in favor of one phytoplankton species, it can trigger a harmful algal bloom. An overabundance of some phytoplankton species, including cyanobacteria (blue-green algae), can produce odorous and toxic substances that may give fish and shrimp an “off-flavor” odor and taste. The issue is serious for aquaculture producers; off-flavor fish and shrimp will not be accepted by processors. To prevent waste due to off-flavor, producers usually undertake regular flavor tests. Some producers stock fish species that graze on cyanobacteria alongside the species they are farming to reduce the risk of blooms. If a bloom occurs, the water can be treated with copper sulfate, which kills the algae. Algal blooms can also cause toxicity—an issue most prevalent in the shellfish industry, where organisms are farmed in cages in eutrophic bays and estuaries. Toxins produced by phytoplankton do not usually impact the health of mollusks, so they can accumulate until they are present in the mollusks’ tissues at quantities that are dangerous, or even deadly, to humans. It is very difficult to prevent the growth of these toxin-producing algae in aquaculture ponds, so the emphasis is on not excessively feeding and supplementing the ecosystem with more nutrients than is necessary.

OPPOSITE ABOVE LEFT AND RIGHT | Global fish production reached 200 million tons in 2022; production increased by 33 million tons since 2021 and continues to rise.

| Most sea bass farming is undertaken in sea cages and involves a natural exchange of water through nets.

OPPOSITE BELOW LEFT

OPPOSITE BELOW RIGHT | Shellfish farmed in crates and cages in environments with harmful algal blooms can cause neurotoxic shellfish poisoning in people.

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Sea walnut Mnemiopsis leidyi A gelatinous zooplankton, Mnemiopsis leidyi is a species of ctenophore (a comb jelly) indigenous to temperate, subtropical estuaries along the Atlantic coast. The walnut-shaped species plays an important role in the food web as a consumer of a wide variety of zooplankton, as well as being a prey item for larger animals, including fish, birds, and aquatic mammals. The species has made headlines since it was accidentally introduced to the Black Sea, where it thrived and became a harmful invasive species.

Ocean invaders

M. leidyi depleted commercial fish stocks in the Black Sea, consuming both the commercial fish larvae and the plankton that comprised much of the commercial fish diet. The impact of the species on fisheries has since been controlled with the introduction of another ctenophore species, which feeds on Mnemiopsis. Monitoring the movement of ctenophores is challenging because of their physiology; the organisms are small and near transparent and have a delicate, gelatinous consistency, so they are not caught in nets easily without damaging them. It isn’t clear how widely or how fast Mnemiopsis is spreading.

The species was transported in the ballast waters of ships from its native home along the eastern coasts of North and South America to the Black Sea in the 1980s. The ctenophore had no natural predators to slow its population growth, so M. leidyi populations rapidly spread through the new habitat, causing widespread disruption to the ecosystem. Their rapid growth was fueled by a diet of plankton, fish eggs, and fish larvae, and the species soon had a catastrophic impact on the food web.

F A M I LY: Bolinopsidae DISTRIBUTION:

H A B I TAT:

Native to the western Atlantic Ocean but invasive in European seas, especially the Black Sea.

Marine and brackish waters; known primarily as a coastal ctenophore, but has been recorded in deeper ocean waters—the maximum reported depth is 328 ft (100 m).

FEEDING HABITS:

NOTES:

| The rows of cilia along the body of a sea walnut (Mnemiopsis leidyi) refract light and can cause rainbows to pulse along them.

OPPOSITE

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SIZE:

Actively predates on fish eggs and larvae, and zooplankton; an individual can eat up to ten times its body weight each day and survive food shortages for three weeks by reducing its body size.

The name ctenophore describes the organism’s ctenes, or combs, which run down its body and help it move in water.

Adults up to 5 in (12 cm) in length

Marine shrimp Themisto gaudichaudii All organisms living in Antarctic pelagic ecosystems have adapted to cope with extreme conditions, including the presence of sea ice and seasonal variability as a result of the annual light cycle (there can be no sunlight for days to months in winter, while in the height of summer, the sun can be visible at midnight). Due to these environmental changes, phytoplankton tend to bloom intensely for a short period each year. This means that food availability for zooplankton changes drastically with the seasons, and so organisms have developed various adaptations in their metabolism, life cycle, and feeding habits to cope. One common adaptation among organisms living in these extreme conditions—particularly primary consumers like copepods—is an increased capacity to store energy as lipids during the more productive months, which can then be released for reproduction or survival as necessary during less productive seasons. This is a vital process for Antarctic organisms throughout the food web, as it ensures energy availability year-round.

Lipid link Themisto gaudichaudii is an amphipod with a particularly important role in the Antarctic food web. As a carnivorous species, the organisms have greater access to energy throughout the year, and so do not store lipids to the same degree as herbivorous species. They are, however, extremely important for the transfer of lipids between zooplankton that mainly eat phytoplankton (primary consumers) and predators higher up the food chain. T. gaudichaudii is a great example of the vital role carnivorous zooplankton play in the food web. Populations are abundant in Antarctic waters and these secondary consumers feed mainly on copepods. They are also a key prey source for higher predators like fish, penguins, and seabirds, thus ensuring that top predators have access to enough energy year-round in the challenging polar conditions.

F A M I LY: Hyperiidae DISTRIBUTION: H A B I TAT:



Marine waters 328–1,300 ft (100–400 m) deep

FEEDING HABITS: NOTES:

| As a key part of Antarctic food webs, Themisto gaudichaudii can be used to indicate the presence of Antarctic waters in subantarctic regions.

OPPOSITE

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SIZE:

Southern Ocean, the subantarctic, and Antarctic Important predators of copepods

 T. gaudichaudii is the main prey for many planktivorous seabirds in the Southern Ocean and has a trophic role similar to that of Antarctic krill further south.

Juveniles 1/16–3/16 in (2–4 mm); adults around 1 in (2.5 cm)

Ostracod Conchoecissa ametra Ostracods—also known as seed shrimp—are an important and abundant class of crustacean found throughout the world’s oceans. Most planktonic ostracods, including Conchoecissa ametra, belong to the order Halocyprida, which describes those that lack obvious eyes. Like all ostracods, the body of C. ametra is completely enclosed within a carapace. Adults have seven pairs of limbs, while juveniles have fewer or only partially developed sets—as crustaceans, they undergo numerous molts during their development before metamorphosing into adults.

Ocean cleaners By consuming organic detritus, Halocyprida and other ostracods contribute to nutrient cycling in marine environments. The ocean’s detritivores break down dead plant and animal material and waste, then recycle the nutrients. Halocyprids are also important food sources for larger organisms. Fish, crustaceans, and other filter-feeding marine animals consume halocyprids as part of their diet.

Deep dwellers C. ametra is a detritivore and members of the Conchoecissa genus are more common in ocean samples than those taken from coastal waters. Species within Halocyprida typically live in marine environments like the deep sea, open ocean, and intertidal zones. They are recorded in the water column and at the sea floor; some have even developed adaptations to survive in extreme environments like hydrothermal vents. They have an important role in the food web as detritivores, although they also consume phytoplankton and small zooplankton.

F A M I LY: Halocyprididae DISTRIBUTION: H A B I TAT:

Pacific and Indian Oceans, and regions of the Atlantic

Marine waters, to depths of 5,000 ft (1,500 m)

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| Deep-sea-dwelling Conchoecissa ametra does not have eyes and its body is encased in a carapace.

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Filter feeder that consumes detritus as well as coccolithophores, radiolarians, crustaceans, and tintinnids.

Living organisms are bright scarlet, but appear black at the depths at which they are found, where there is no red light. /8 – 3/16 in (3.3–4.6 mm)

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Diatom Thalassiosira rotula Phytoplankton are the foundation of all aquatic food webs, performing the vital role of fixing atmospheric carbon dioxide into biological matter. Diatoms like those belonging to the Thalassiosira genus are responsible for a considerable proportion of marine primary production. When conditions are favorable, these diatoms rapidly divide and can form huge blooms, which, in turn, have an important role in the carbon cycle. As nutrients are depleted and the blooms die off, cells sink through the water column, eventually sequestering carbon deep in the ocean. Some have suggested that the role of diatoms in the carbon cycle is comparable to that of all rainforests combined.

Individual T. rotula cells are discoid (other species in the large genus are cylindrical, spherical, or box-shaped) and can join together to form defensive chains with thick bundles of threads between them. This helps the organisms to evade predation by zooplankton, which can exert control over blooms of solitary phytoplankton species. Members of the genus can be found in diverse habitats around the world, and are particularly important to food webs in temperate and polar regions.

Hardy bloom An important component of food chains in cold, turbid waters, T. rotula is a photosynthetic diatom that forms chain colonies. This species and others in the genus, which contains more than 100 species, are particularly important primary producers because they are well adapted to low temperature and light conditions. Since they can thrive in these less optimal conditions, members of Thalassiosira often bloom during spring in temperate regions, like northern European seas.

F A M I LY: Thalassiosiraceae DISTRIBUTION:

H A B I TAT:

Primarily found in temperate water regions, but more widespread if considered alongside T. gravida, which might be the same species.

Found primarily in shallow, near-shore marine habitats.

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NOTES:

| In Thalassiosira rotula thick bundles of chitin threads extend from each single-celled diatom to bind individuals together and form long chain colonies.

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Primary producers that fix carbon dioxide through photosynthesis.

Since they have small genomes, members of the Thalassiosira genus have been used in molecular studies, which have revealed important differences between diatoms and other eukaryotes.

0.0003–0.002 in (8–55 μm)

Diatom Asterionellopsis sp. Asterionellopsis is a genus of delicate, intricately shaped diatoms with needle-like projections. They are often found in chains or colonies; individual cells join together at their valve faces or form ribbon-like chains. Asterionellopsis species are important primary producers in coastal waters and the open ocean. They play a key role in the marine food web, transferring energy from the sun to zooplankton and other organisms farther up the trophic levels, as well as in the carbon cycle, since they remove carbon dioxide from the atmosphere and store it in sediments in the seafloor when they die and sink through the ocean water column.

The blooms can threaten marine ecosystems and the productivity of fisheries, and also pose human health risks. Asterionellopsis blooms have been shown to impact plankton dynamics. In some cases, meroplankton become dominant and some species of mostly carnivorous copepods notably outnumber species that are mostly herbivorous at the peak of the blooms. These changes in the balance of plankton communities can have repercussions for other parts of the food web during bloom periods.

Blooming diatoms It has been reported that some species in the genus can become large-scale blooms. Phytoplankton blooms like those caused by Asterionellopsis have become increasingly common in coastal waters. Eutrophication (see page 189) caused by anthropogenic activity, such as pollution from agricultural practices, can affect these habitats. An increase in nutrients like phosphorus and nitrogen can trigger blooms by making conditions more favorable for diatoms like Asterionellopsis, thus helping them dominate other plankton species. Coastal blooms of diatoms are being reported more frequently and across all latitudes.

F A M I LY: Fragilariaceae DISTRIBUTION: H A B I TAT:

Cosmopolitan in cold to temperate coastal waters

Shallow marine waters

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NOTES:

| Each spine projecting from this Asterionellopsis glacialis colony chain belongs to an individual diatom.

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Primary producer diatoms that transform light from the sun into energy through photosynthesis.

 Asterionellopsis species are also important palaeoenvironmental indicators; their fossilized remains are preserved in seafloor sediment and can help in the reconstruction of past conditions.

0.001–0.006 in (30–150 μm)

Golden algae Dinobryon sp. A genus of freshwater and marine plankton, Dinobryon species have a cylindrical, conical, vase- or funnel-shaped lorica (shell-like layer) on the body. Members of Dinobyron belong to the Chrysophyceae class, which are known as golden algae because of the organisms’ yellowish chloroplasts. They come together to form recognizable branching, tree-shaped colonies, which probably make them difficult for zooplankton to consume.

Mixed diet Many species of Dinobryon are mixotrophs, meaning they are able to obtain energy from both photosynthesis and phagotrophy—feeding by engulfing a food cell or particle, in this case bacteria. They inhabit lakes, estuaries, and coastal seas; Dinobryon species are more common in freshwater environments and some play a particularly important role as grazers of bacteria in oligotrophic lakes. Despite being capable of photosynthesis too, mixotrophic members of Dinobryon can consume bacterial species at a rate equal to some heterotrophic flagellates.

Eutrophic blooms Dinobryon species form stomatocysts—the organisms are able to enclose themselves in rigid cell walls, which protect them during periods of challenging environmental conditions. The cysts provide safety against predation and changes in physical conditions, so the organisms can remain in a dormant state and emerge again when conditions improve. When the environment becomes more favorable—for example, when the water temperature rises or the nutrient concentration improves—Dinobryon populations can emerge, rise to the surface, and reproduce at a rapid rate. At this time—usually during the spring and summer months in temperate regions, when the water is warm— Dinobryon species can bloom. They are most likely to bloom in eutrophic waters with plenty of sunlight. The blooms grow rapidly because Dinobryon species have defenses against predation by zooplankton; some species have spines or bristles, and colonies can form elongated shapes with slimy, mucilaginous sheaths, making them much harder to consume.

F A M I LY: Dinobryaceae DISTRIBUTION: H A B I TAT:

Cosmopolitan, but more common in temperate regions

Particularly abundant in the euphotic zone of relatively low-nutrient lakes.

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| A tree-like colony of Dinobryon contains many vase-like lorica, each with a gold-colored chloroplast inside.

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As mixotrophs they can photosynthesize and feed on other organic matter.

Each Dinobryon species forms a unique siliceous cyst, called a stomatocyst, which helps in their identification.

About 0.0008 in (20 μm)

CHAPTER 6

FACING THE FUTURE Marine ecosystems—which are fueled by plankton—regulate the planet’s climate. Ensuring the conservation of plankton is, then, essential to mitigating climate change and ensuring the survival of animal life on Earth. The diverse collection of free-floating organisms contributes hugely to global biodiversity and primary production and, in turn, the global economy. Marine biomes are vast, complex, and diverse—they are difficult to study and their components are poorly understood compared to those of terrestrial ecosystems. Only around 230,000 of an estimated 10 million marine species have been described. Because so little is known about Earth’s aquatic environments, it is difficult to predict how human activity might impact these systems in future—and what wider implications such changes might have. Considering the various ways aquatic environments are threatened by human activity can help scientists understand the pressures on plankton communities and predict how they might respond when these threats are amplified in future. There are a great many humandriven pressures on aquatic ecosystems, including habitat loss through land conversion, overexploitation of fish and marine invertebrates, invasive species, climate change (including global warming), ocean acidification, pollution, and mineral extraction.

North America

Pacific Ocean South America

May 9–May 15, 2022

May 8–May 14, 2023 Sea surface temperature anomaly (Ref. period 1985–1993) 230F (-50C)

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410F (+50C)

CLIMATE CHANGE Links have already been established between plankton and climate change. Long-term surveys have shown systematic changes in plankton communities in various locations around the globe. The distribution of Antarctic krill, for example, has contracted considerably over the last 90 years—a change that is likely linked to changing weather conditions and reduced sea ice cover. Changes in communities of zooplankton have been reported in the face of climate change too, and the effects are not limited to marine environments. While many of the recently reported changes in plankton populations can be linked to climate change, the precise causes are not always clear. Ocean mixing and global warming are thought to be important factors. The growing intensity and frequency of weather events impacts mixing, which affects light levels, surface temperatures, and nutrient recycling—all 184 | FACING THE FUTURE

important factors for phytoplankton productivity (and so all plankton and marine organisms higher up the trophic levels). Simultaneously, increasing water temperatures favor some species over others and result in changes to community compositions, as well as shifts in plankton distribution.

Ocean warming The oceans have been gradually cooling over the last 8 million years, but the last time the waters were this warm—before the cooling began—plankton were living 2,000 miles (3,200 km) from where they are found today. Global warming is bypassing 8 million years of cooling, suddenly shifting the oceans back through millennia, and organisms do not have time to adapt. As ocean temperatures continue to rise, a pattern of movement away from the tropics and toward the poles is

predicted for plankton. Other major changes associated with warming include shifts in phenology (the timing of recurring biological events) and a reduction in body size. The changes are likely to result in the loss of plankton populations and, in turn, many other marine species across the food webs. The ranges of many plankton species have already extended with increasing water temperatures. Lots of species belonging to primary producer genus Ceratium, for example, have shifted into warmer waters. Scientists are witnessing seasonal changes too, and at high latitudes, a decrease in the ratio of dinoflagellates to diatoms has been attributed to increasing

temperatures in surface waters coupled with heightened winds in summer. The distribution of phytoplankton may be changing more drastically with ocean warming than zooplankton grazers. Merozooplankton might also be affected more than holozooplankton. The differing rates of adaptation to climate change are concerning because of the potential disruption to aquatic food webs. If the abundance of those organisms at lower trophic levels shifts to peak at a different time to abundance of their predators, less energy will be transferred through the food web.

Rise in global sea surface temperature 1.0

TEMPERATURE ANOMALY (°F)

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| The temperature of the surface waters of Earth’s oceans recently hit an all-time high. This image shows sea-surface temperatures in May 2022 and May 2023 compared to a reference period of 1985–1993. Data source: NOAA.

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| The average global sea-surface temperature has risen by more than 1.5°F since the 1880s.

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185

OCEAN ACIDIFICATION The pH of oceans is decreasing with increasing atmospheric carbon dioxide. If nothing is done to prevent it, water pH is expected to change more than it has done for 300 million years. How exactly this change might impact plankton is a hot topic in marine science—there are a number of complex variables that make the overall impact of such a significant change difficult to predict. Photosynthesis by phytoplankton is stimulated by increased dissolved carbon dioxide—this could make marine plankton an increasingly useful sink as atmospheric carbon continues to rise in coming decades. Unfortunately, the overall effects of the predicted changes are not so straightforward. Different species of plankton have different sensitivities to carbon dioxide concentration. Increases in carbon dioxide levels will give some species an advantage over others, thus altering the community structure and dynamics of the whole food web.

Adding to the complexity, phytoplankton like coccolithophores use bicarbonate to produce calcium carbonate, which increases carbon dioxide levels and so could be a further source of atmospheric carbon dioxide. Acidification will hamper the ability of some plankton to build exoskeletons (including coccolithophores) and will impact zooplankton and picoplankton communities, too.

Ocean acidification | Reduced water pH will affect the structural integrity of mineralized coccospheres and might also make coccolithophores easier to digest.

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CO2

Atmospheric carbon dioxide

More acidic

Less acidic Dissolved carbon dioxide

Water CO2

H2O

Carbonic acid

Hydrogen ions

H2CO3

H+ Bicorbonate ions HCO3

Carbonte ions CO32-

Deformed shells

186 | FACING THE FUTURE

| An increase in dissolved carbon dioxide causes higher concentrations of bicarbonate and hydrogen ions, and a decrease in carbonate ions. Together, these changes result in acidification.

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OVERFISHING The overexploitation of fish and marine invertebrates has long been a major cause of biodiversity loss. Populations of large fish have already been depleted, triggering shifts in commercial fishing to deeper areas of the oceans and species farther down the trophic levels (called “fishing down the food web” as we rely more on smaller species and more on herbivorous species). It is estimated that a third of fisheries are being pushed beyond their biological limits. And the effects reach far beyond the target fish—other species are killed as bycatch, habitats are disrupted by fishing equipment, fishing debris adds to pollution, and there are knock-on effects of depleting fish populations on the food web. When top predators are targeted by the commercial fishing industry,

| Aerial view of fishermen catching fish using a huge net. The number of overfished stocks around the globe has tripled in the past 50 years.

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trophic cascades are triggered, with effects reaching down to plankton. The impacts of overfishing on plankton are heightened by environmental shifts caused by climate change. In the Central Baltic, for example, the popular commercial fish, cod, struggled to recover from overfishing when faced with decreased salinity, which causes recruitment failure. Populations of Acartia copepods and sprat increased concurrently, thanks to rising water temperatures and the reduced predation pressure from cod. Together, these changes marked a major restructuring of the trophic levels in the ecosystem, which cascaded down the trophic levels and impacted zooplankton populations. 187

MINERAL EXTRACTION

Deep-sea mining

Mining aquatic bodies for resources like sand, silt, and mud, and mineral-rich sand like ilmenite and diamonds has detrimental effects for entire ecosystems, including plankton communities. The processes of excavation and dredging in aquatic bodies cause direct harm to organisms, as well as altering the environment. Riverbeds and ocean floors are often destroyed; dredging tears up these habitats, which are particularly important for eggs and larvae (including meroplankton), so can impact species success. The physical mining process also increases turbidity and sedimentation, reducing light penetration and hampering primary production by phytoplankton; this can cause shifts in phytoplankton communities, since some species are more sensitive to low light conditions than others. Terrestrial mining operations also have negative effects on nearby aquatic systems. Heavy metals, inorganic particles, and toxic substances are often released into water bodies. Resulting changes in water quality can be detrimental to plankton growth and reproduction. Zooplankton communities are impacted through the ingestion of inorganic particles associated with phytoplankton, which reduces the nutritional value of their food intake and decreases growth. Runoff from mining activities can cause nutrient imbalances too, triggering a shift in phytoplankton community structure and, in some cases, harmful algal blooms.

The process would cause severe environmental damage vertically and horizontally throughout the water column, including a great loss of biodiversity and ecosystem functions. Seafloor habitats would be destroyed and sediment plumes would cause disruption. There would be considerable noise and light pollution, and contaminants would alter water properties. All of these activities could impact plankton communities.

RIGHT | Aquatic systems surrounding copper mines can become polluted by copper acid, which turns the water red and can cause environmental damage.

188 | FACING THE FUTURE

Pressure to extract as-yet unexploited mineral deposits in the deep sea is on the rise—particularly with an increasing global demand for metals used in advanced technologies. The development of deep-sea technology is making the mining projects more feasible, and considerable sums of money have been invested into the field. The prospect is very real, despite scientists being in agreement that the environmental consequences would be enormous, and much deep-sea life would be lost even before it becomes known to science.

EUTROPHICATION Nutrient enrichment is one of the main sources of pollution in freshwater and marine coastal systems. The frequency and extent of eutrophication have increased with human activity—in particular, as a result of increased fertilizer use in agriculture. The change in nutrient levels triggers shifts in the dynamics of aquatic ecosystems, often causing an overabundance of plants and phytoplankton. Notably, eutrophication leads to algal blooms. It causes changes in the ecosystem structure, nutrient cycles, and the food web, and alters the commercial and recreational usage of a water body. Current strategies for managing eutrophication, including top-down control of consumers and bottom-up approaches

like diverting nutrients and applying algicides, tend to be impractical, unaffordable, or ineffective. Demand for clean water is only going to increase with human population growth, so protecting these valuable resources by minimizing nutrient inputs and developing more effective management approaches to eutrophication is a priority.

| The heightened uptake of carbon by phytoplankton during a bloom increases water pH and can adversely affect plankton growth.

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189

ORGANOCHLORINE POLLUTION For decades, organochlorine pesticides were extensively used for insect pest control in agriculture and public health. Though their use has since been curbed by environmental regulations, the pesticides are still of global concern because they persist in aquatic systems, where they intoxicate aquatic life and accumulate in the tissues of aquatic and terrestrial invertebrates and vertebrates. Plankton have been identified as important in the dispersal of organochlorine pesticide particles in the oceans. Although organochlorines are not directly harmful to zooplankton, the organisms disperse particles horizontally and vertically, and bring them into the food web. The toxins accumulate at each trophic level and cause harm, or even death, to animals—especially top predators. Crustaceans, which are more closely related to insects than many animals, are particularly susceptible to the effects of pesticides. 190 | FACING THE FUTURE

Organochlorine particles also reduce photosynthesis by marine phytoplankton. As some phytoplankton species are more sensitive to the impacts of pesticides than others, this can change the composition of primary producer communities, and so higher trophic levels.

| Common organochlorine pesticides were banned in the USA and Europe in the 1970s, but they are still used in developing countries.

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PLASTIC POLLUTION Plastic pollution has been shown to impact all levels of the marine food chain, including primary production and carbon sequestration. Large plastic debris causes harm to marine life through ingestion and physical entanglement. Though harder to study, it is clear that microplastics and nanoplastics are a huge and increasing problem. There are an estimated 1014 microplastic particles drifting in marine habitats today and millions of tons more contaminate aquatic bodies every year. Floating plastics accumulate on the ocean surface, reducing light transmission to phytoplankton beneath, and therefore reducing primary production. At high concentrations, microplastics negatively affect phytoplankton growth, and this effect increases with decreasing particle size—an important consideration, given that quantities of microplastics will increase as large plastic debris degrades and fragments in water

systems over time. Toxins from microplastics can also harm zooplankton, and microplastic particles aggregate with other contaminants and transport them into the tissues of organisms when the microplastics are consumed. What’s more, when zooplankton ingest microplastics, they become satiated and their nutritional intake reduces. This has knock-on effects at a population level: their eggs become smaller and less successful. | Microplastics and the toxins associated with them accumulate in the bodies of marine animals higher up the food chain.

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| Nets and other waste from the fishing industry account for the majority of large plastic pollution (measuring over 8 in/20 cm) floating in the oceans.

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| Physical entanglement can pose a serious threat to large zooplankton, like this jellyfish.

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191

HOW PLANKTON ARE RESPONDING Aquatic environments are already undergoing noticeable change. Anthropogenic activities are causing ongoing environmental stressors for plankton, including changes to water quality, temperature, nutrient supply, light transmission, and pH, as well as direct physical threats to plankton and indirect impacts from threats to other trophic levels. Resulting adjustments in seasonal distribution, migrations, and community structure have been reported in plankton. While some taxa have been able to adapt to (or even thrive in) changing environments to date, the extent of future impacts of human activities on aquatic systems is likely to cause significant disruption to plankton populations, including widespread extinctions. Biodiversity is key to ecosystem stability. It is widely accepted that biodiversity among plankton is going to decline in a future of continued anthropogenic-induced change. Plankton communities will undergo reorganization in response to changing physical, chemical, and environmental factors, and the historic balance of diversity will be lost. This instability will have far-reaching consequences for the food web and for the global climate system by the end of the century. While there is indisputable evidence for some shifts, there are still a great many unknowns around how plankton are responding to environmental changes. Long-term research is lacking around plankton phenology (the cyclic events in biological life cycles) in the tropics and Southern Hemisphere, for example, and it is not clear how the effects of anthropogenic activities will impact important variables in the carbon cycle, like zooplankton grazing and excretion rates. The extent of change to the ocean’s biological pump is also largely unknown. Without a solid foundation of knowledge around historical and recent changes in plankton communities, predicting their future is a huge challenge. Given their vital roles, filling the knowledge gaps and making progress in this field is a priority.

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The future of phytoplankton Regional variation is expected in primary productivity; climate change will reduce suitable environments for phytoplankton at lower latitudes and extinctions will outweigh colonizations. At the same time, diversity will increase at high latitudes, where colonizations will exceed extinctions. A general trend suggests an increase in phytoplankton biomass toward the poles and a decrease toward the equator, in the tropics, and in temperate regions. This shift is anticipated to be driven by changing supplies of nutrients (leading to reduced biomass at low latitudes) and water temperature increases, which cause higher biomass in eutrophic higher latitude regions. Biodiversity and community structure will also be impacted. Increased stratification, for example, will reduce ocean mixing, which can contribute to both positive and negative changes in phytoplankton richness. Although phytoplankton colonization is expected to outweigh extinction at high latitudes, the diversity will likely be among fewer types of phytoplankton compared to the present balance. Regional variations coupled with gaps in knowledge make it very difficult to accurately model the future of phytoplankton populations. Smaller phytoplankton are expected to dominate—another important shift, since these are less effective producers for larger plankton, and nekton such as fish. Regions dominated by small phytoplankton species tend to sequester less carbon to the deep ocean and support less productive ecosystems. The effects of this change will be experienced throughout the food web. Under laboratory conditions, phytoplankton communities have adapted to new conditions (like increased water temperature or acidification) within several hundred generations—just a few years, given their short lifespans. There is hope, then, that some species of phytoplankton will adapt to changing conditions; however, this scenario is less likely given the speed of change and the great number of important environmental variables (and shifts in the whole food web) that will be changing at once.

Projected changes in diversity of the organisms shown by the end relative to the beginning of the 21st century Bacteria (P)

Bacteria (H)

Protists (P)

Anomalies (%) >50 25 Endophotosymbiont

Parasitic protists

Copepods

0 -25