How Coral Reefs Build Themselves

A Scattered through the warm, shallow seas of the tropics lie some of the richest and most colourful habitats on the whole of the Earth. Coral reefs cover only a tiny fraction of the ocean floor, yet they shelter a quarter of all the kinds of creature that live in the sea, and for this reason they are often called the rainforests of the ocean. A large reef can stretch for hundreds of kilometres and can be seen from space, and yet, astonishingly, the whole vast structure is the work of animals no bigger than a fingernail. How such tiny builders raise so great a thing is one of the quiet wonders of the natural world.

B The builders are small, soft-bodied animals called polyps, each a little like a miniature sea anemone. A single polyp draws minerals from the seawater around it and uses them to build a hard, protective cup of limestone about its own body. When the polyp dies, its stony cup remains, and new polyps grow upon the old ones. Over hundreds and thousands of years, generation building upon generation, these countless small skeletons pile up into the mass of rock that we call a reef. The living coral is only a thin skin of animals spread across the surface of a great heap made by their ancestors.

C The polyps do not, however, work alone, for they share their bodies with a partner. Living inside the tissues of each polyp are enormous numbers of microscopic algae, far too small to see. These algae capture sunlight and use it to make food, in the same way that a plant does, and they pass much of that food to the coral that houses them; in return the coral gives the algae a safe place to live and a supply of the raw materials they need. It is this close partnership, in which each side gives something the other cannot make for itself, that allows reefs to grow so quickly and so large. The algae also lend the coral much of its brilliant colour.

D Because the algae depend on sunlight, the conditions a reef needs are strict. The water must be warm, clear and shallow enough for light to reach the coral, which is why reefs are found only in the sunlit shallows of tropical seas and never in cold or deep water. Muddy or cloudy water, which blocks the light, will not do, and even a small change in temperature can upset the delicate balance on which the whole community rests. A reef, for all its size and apparent solidity, is in truth a fragile thing that can flourish only within a narrow range of conditions.

E This fragility shows itself most clearly in an event known as bleaching. When the surrounding water grows too warm and stays warm for too long, the coral becomes stressed and drives out the algae that live within it. Robbed of its colourful partners, the coral turns a ghostly white, and, worse, it loses the chief source of its food. If cooler conditions return quickly the algae may come back and the coral may recover, but if the heat continues the starving coral will eventually die, leaving behind only bare white skeletons. As the world's seas grow warmer, such bleaching has become more frequent and more severe.

F The loss of reefs would matter far beyond the reefs themselves. They are nurseries and feeding grounds for a great host of fish and other animals, many of which people depend upon for food. They act as natural breakwaters, absorbing the force of the waves and so protecting low-lying coasts from storms and erosion. And they draw visitors from around the world, supporting the livelihoods of many coastal communities. A structure built grain by grain by the humblest of animals turns out to be one of the pillars on which both the life of the ocean and the lives of many people quietly rest.