Coral Reefs: A Natural History

CORAL

REEFS

A NATURAL HISTORY

Charles Sheppard

Consultant Editor

Russell Kelley

CONTENTS

Foreword

Introduction

CHAPTER 1

THE CORAL ANIMAL

CHAPTER 2

DIFFERENT KINDS OF REEFS

CHAPTER 3

HOW A CORAL REEF WORKS

CHAPTER 4

LOCAL AND REGIONAL DISTURBANCES TO REEFS

CHAPTER 5

CLIMATE CHANGE AND REEFS

CHAPTER 6

PEOPLE AND REEFS

Glossary

Further Reading

Index

Picture Credits

FOREWORD

Imagine swimming in a warm, tropical sea. Through your face mask you peer down into an infinite azure abyss. All you can see are shafts of light plunging into the depths like lightsabers, and iridescent, sapphire-like flashes reflecting from invisible plankton somewhere below. Now you hear something between the lazy splashes of the torpid sea. A non-stop crackling hiss rises and falls like someone tuning a distant radio station. You kick your fins and turn 180 degrees and a vertical reef wall looms, draped in a wriggling tapestry of color and movement. Fish teem around hundreds of other kinds of reef creatures. You swim closer and gawp at the hallucinogenic patterns while the entire coral reef carnival carries on without a care for an alien visitor as clumsy as yourself.

For me, there is nothing as ineffably beautiful as a coral reef on a calm, sunny day—such scenes sear vivid memories that last a lifetime. Remarkably, the story of reefs—their origins, adaptations, and achievements in the great pantheon of evolution and Nature—is equally breathtaking.

From the study of modern reefs, we know corals are not just beautiful biological baubles, but an innovative force in Nature. Corals have solved the problem of how to create something from nothing in the middle of empty, nutrient-poor oceans using an ecology based on symbiosis and collaboration. And not just a modest something, but the richest marine ecosystem on Earth.

The coral reef innovation derives from an extraordinary advent in biology—the ability for a plant to live inside an animal, the best known example being the algae cells laced through the tissues of every reef-building coral polyp.

Somewhere in the mists of geological time, shallow-water corals became infected with plant cells that not only survived but thrived within the coral animal. These tiny plant cells did what plants do: photosynthesise. Using sunlight, excess CO2, and animal waste products, they flourished inside their host. In return the plant cells released sugars and the humble coral polyp suddenly had two engines for growth—a carnivore by night and a solar farm by day. Now corals had energy to burn, energy to build!

Their larvae dispersed on ocean currents to settle on the edges of sterile volcanoes where, with nothing but sunlight and microscopic planktonic meals, they combined their construction prowess with another great evolutionary achievement—cloning. In this way the arrival of a single coral larva, transforming into a polyp, could build great coral colonies. I like to think of them as colonial castles conjured from nothing in the middle of empty oceans.

On coral reefs recycling is a way of life, as is finding a home on, or inside, another species. Over many millions of years coral reefs have attracted thousands of species to come along for the ride, weaving an evolutionary fabric so organically intricate it inspires, and deserves, the term living art.

This book explores the natural history of reefs in a human context. Written plainly and richly illustrated, it is a guide to the good, the bad, and the ugly of the coral reef story. Around the world Nature is in retreat and so it is with reefs. In the tropical world around 500 million people depend on reefs for shelter, protein, and employment. Yet in many regions reef decline is accelerating due to the impacts of growing populations of people without options.

Even more pressing is the threat of climate change and the ocean warming it causes. Underwater heatwaves are triggering mass coral bleaching, a disease (described on this page) that attacks the evolutionary bond between coral and algae, the very engine of the ecosystem’s existence.

Coral Reefs; A Natural History is a portrait of the wonders and dilemmas facing the richest of marine realms. If we want our children to see Nature’s great gifts, and if millions of people are to avoid displacement and hardship, then it is time for humanity to take our collective foot off the carbon emissions accelerator.

Russell Kelley

Townsville, Queensland, Australia

INTRODUCTION

Coral reefs mean different things to different people. To a sea captain they are potentially hazardous, often poorly marked on the chart, and always lying just beneath the surface of the water. To people living in the tropics beside a reef, or on a small island perched on one, they provide both a home and food. To many divers and snorkelers who visit tropical coasts they are a kaleidoscopic wonderland of colorful fish that circle and dart above the strangest of living structures, and these tourists bring significant revenue and foreign earnings to such regions. The scientist sees something different again, a place of huge diversity in a relatively small space, of apparently chaotic pattern and movement, something to try to make sense of. A reef is all of these things and more.

Coral reefs occupy only about one percent of the Earth’s surface but they contain a high abundance of marine species, and they concentrate huge densities of it too, like an oasis. Not only this, but they also construct the reef itself. A reef is made by corals and other organisms as they extract limestone from the seawater to deposit as solid rock to make their own skeletons. They have been doing this for millennia so that now some reefs are a couple of kilometers thick in parts of the Earth that have been slowly subsiding over the ages, such as in the atolls scattered across tropical parts of the Indian and Pacific oceans. The parts of the reef that are now living form only a veneer on the top. Yet that small quantity has built the structure that supports all the rest, from the thousands of species of simple animals to the swarms of fish above, and to several entire oceanic nations.

A grouper on a Western Australian reef. Grouper are ambush predators and commonly hide in caves, though here the cave is concealed by the shoal of small, silvery fish.

Few of us realize that around half a billion to a billion people are now wholly or largely dependent on coral reefs, because they live on them or are dependent on the protein that they produce, or perhaps because they provide sheltered waters for a mainland settlement. That number does not include many millions more who use them for recreation.

Yet there are fewer than 1,000 species of reef-building coral—a thousandfold fewer than the number of species that live among the structures they have built. Corals are the key animals that deposit the limestone rock, which they do because of help from captive, symbiotic algae, in a close association between animal and plant that has lasted for a couple of hundred million years. The far greater numbers of invertebrates on reefs include many that are hidden, and this is unsurprising given the huge whirls of predatory fish above them. Many animals are gaudily colorful. All are hunting for food and mates, and many use astonishing methods of hunting while avoiding being eaten themselves.

Deeper on a reef where water is calmer, large sea fans extend into the water to trap plankton for food.

A rich mix of stony coral, soft corals, and fish on a reef slope.

A small coral island on the reef flat of one of the Federated States of Micronesia, in the Pacific Ocean.

Coral reefs have attracted people for generations, and because of that attraction to sheltered sea conditions, food, and space on which to live, they have been facing increasingly damaging pressures. So much so that by about 50 years ago probably about a fifth to a quarter had already been damaged and some destroyed. As human populations have risen steeply over the last couple of generations, this damage has increased, coming from pollution such as sewage, agricultural practices that smother reefs with too much fertilizer and sediment, and from dredging the seabed or industrial pollution because the sea has been a cheap place to dump toxic wastes. Damage has also resulted from overfishing to feed increasing populations, and overfishing is one of the most damaging impacts we inflict, because it unbalances the ecosystem.

Then, around the 1970s, the temperature of the oceans started to rise. It wasn’t noticed much at first because the increase was small and within the general variation. It occurred because for the preceding century people had been burning ever more oil, which, together with the burning of coal and land clearing, added increasing amounts of carbon dioxide (CO2) to the air. This gas traps heat from the sun. Scientists looked at every other possible cause of warming and discounted most: the temperature rise has come mainly from burning fossil fuels. Many species in tropical waters, including corals, are already very close to the maximum temperature they can survive. This warming arrives in pulses, called marine heatwaves, and repeated heatwaves are killing corals and reefs in vast numbers. Today this is the primary threat to their survival. Corals, seemingly robust because of the huge reefs they make, have a rather fragile relationship with their symbiotic algae and will die when this is disrupted. They are now facing an existential threat.

This book aims to show and describe the beauty of this most incredible ecosystem, and to explain some of the problems that it now faces and how we must respond to the threats. It is a natural history of an extraordinary but endangered ecosystem.

A rich mix of soft corals on a reef slope.

CHAPTER 1

THE

CORAL

ANIMAL

WHAT ARE CORALS AND REEFS?

Corals belong in a group of primitive, simple animals called cnidarians, a group named for the stinging cells that they all have in one form or another, in tentacles that are used for capturing tiny prey. They have an ancient lineage: over 740 million years ago, the Earth saw the emergence of the common ancestor of this group of animals, which now includes other reef-building forms as well as corals. No fossils exist from that time to confirm this age, so it has been deduced by use of the molecular clock, which is the rate at which mutations occur in their DNA. The earliest fossil cnidarians found so far are about 580 million years old, with the forms that gave rise to today’s corals appearing 100 million years later (see here).

Since then, divergence in the group has resulted today in several main classes, some mostly free-swimming, including the jellyfish, and at least three that are mostly attached to the substrate when adult, which includes the corals and other reef builders.

The earliest corals were, for a long time, only a minor form of life on Earth, but they were persistent. They survived through millions of years, including the greatest biological crashes that have occurred in Earth’s history, not only once but on several occasions. When modern corals developed further and made their important impact on our planet, it was after the great global wave of extinction between the Permian and Triassic eras. Corals also nearly came to grief again during a later wave of extinction at the end of the Cretaceous, which saw the demise of the large dinosaurs. Modern corals then thrived at various times between 80 and 20 million years ago, and today’s commonest coral genus (Acropora) occurs as fossils in limestones around London and Paris, among other cities. They expanded considerably and came to dominate shallow tropical seas, as we see today.

The animals in the phylum Cnidaria are defined by the presence of stinging cells in their tentacles. The phylum contains four to six classes, the number and characters that define each being in a state of flux and dependant on the results of new research. The diagram shows likely relationships between groups of reef-dwelling Cnidarians. Blue lines indicate the eight-segmented Octocorals and green lines the six-segmented Hexacorals, all within the class Anthozoa. The red dots next to several groups indicate the important reef constructors, the main ones being the stony corals. Other classes include mainly free-swimming and tiny parasitic forms, which are not important for reef-building.

THE CORAL ANIMAL

The basic unit of a coral is a polyp, which is like a small sea anemone. Each polyp has six radial segments—they are hexacorals—and one or more rings of tentacles surrounding a mouth on top of its cylindrical, sac-like body. Its tissues secrete limestone in the form of a cup, in which the animal lives. Each cup is called a corallite, and these polyps with their corallites grow upward, and divide as they do so.

The polyps and tentacles of most species are usually contracted into their cups in the daytime, emerging at night to capture zooplankton—tiny animals floating or swimming in the water. Each tentacle is loaded with stinging cells, called nematocysts, which have barbs on threads that inject venom and pull the trapped zooplankton to the central mouth. The wall of the body (and tentacles) is composed of a simple double layer, the ectodermis on the outside and gastrodermis on the inside, sandwiching a jelly-like mass called the mesoglea. The ectodermis in the tentacles houses the stinging cells, and that on the underside of the polyp is responsible for deposition of the limestone skeleton (see here).

The gastrodermis layer of the body wall contains the digestive tissues. Importantly, this layer also contains thousands of single-celled algae living in symbiosis with the animal host. These algae take up animal waste and in return provide the products of their photosynthesis (sugars and nutrients), and give a greenish-brown color to the coral. The number of algal cells is huge, and their photosynthesis commonly provides the coral animal with more food than does the captured zooplankton.

Tentacles of most species are small and expanded at night only, when they catch zooplankton and when browsing fish do not feed.

In the polyp body, vertical structures called mesenteries partly separate the space into compartments, adding greatly to the total surface area available for digestion. These develop the eggs and sperm too, commonly in an annual cycle. Some corals have separate sexes, but most are hermaphrodites (both male and female).

Almost all species divide by budding—splitting into two individuals. The daughter polyps may remain joined or separate completely. All tissues are served by a nerve net, which can transmit signals across a colony, as can be seen when one part of a colony is touched, causing a ripple of polyp retraction across the surface. Most corals are colonial, but many are essentially solitary—single cups with polyps.

Polyps of Goniopora are especially long and are extended during the day. If touched they will almost completely retract.

One fairly large coral polyp in one limestone cup, with the polyp retracted in daytime. Tissue conceals the cup, although the radial plates (septa) show through. The green/brown color comes from the symbiotic algae in the coral. The polyp in the middle is connected to the one on the left by tissue, but not to the one on the right.

A NEMATOCYST DISCHARGING

Touch on a hair-like trigger at the opening of the nematocyst causes the sting to shoot out.

ANATOMY OF A CORAL POLYP

Anatomy of several hard coral polyps with tissue (colored) overlying skeleton (grays).

CORAL SYMBIOSIS

SYMBIOTIC ALGAE

While plants form the basis of all sunlit ecosystems on Earth, on most healthy reefs there are very few large plants or algae to be seen. Where, then, are all the plants that support the