Coral Reefs

When you snorkel over a coral reef, you see a colourful underwater city: branching shapes in red, yellow, and violet, with schools of fish weaving between them. Much of it looks like coloured plants or stones, yet the reef is built by tiny animals. Coral reefs cover less than one percent of the sea floor, and still about a quarter of all marine species find food and shelter there. Because so many species live on so little space, coral reefs are called the rainforests of the sea.

Tiny builders

The builders of the reef are called coral polyps. A single polyp is a tiny animal, smaller than your fingertip, and it is related to jellyfish and sea anemones. From the sea water it takes lime and builds itself a hard cup to sit in. When a polyp dies its cup stays behind, and the next polyp builds on top of it. That way millions of lime cups pile up on one another and the whole reef grows upward as limestone. This happens very slowly, often only about one centimetre a year, which is why a large reef is many thousands of years old.

A partnership with algae

A polyp alone would not get far, but it has a partner. Tiny algae live inside its tissue and make food from sunlight, just like plants on land. They pass a large share of this food on to the coral, and they give the reef its glowing colours. In return the coral offers the algae a safe place to stay. At night the polyp also catches plankton from the water with its tiny tentacles, but the algae still supply the largest share of its food. Because the algae need light, reefs grow only in warm, shallow, and clear water, mostly in the tropical belt around the equator.

Yet it is exactly this warmth that also becomes dangerous for the reef. If the water stays too warm for weeks, even just one or two degrees above the usual summer level, the coral becomes stressed and pushes out its algae. Without the coloured algae the white lime shows through the clear tissue, and the reef turns ghostly pale. This is called coral bleaching. If the algae are gone too long, the coral starves, because its main supplier of food is missing. If the water cools down again quickly enough, the coral can take the algae back and recover. Because the seas are growing warmer through climate change, bleaching sadly happens more and more often today.