Dubbed the rainforests of the ocean, coral reefs have survived for hundreds of millions of years. Nick Middleton looks at how they are created and why many are now threatened
If you could travel back in time about 200 million years you'd find a very different world from the one we know today. For one thing, the distribution of the planet's continents and oceans would look rather unusual because the break-up of the `super continent' known as Pangaea had yet to happen. But one familiar sight would be coral reefs. Unlike other long-term residents of Planet Earth, such as the dinosaurs who were wiped out by the impact of a huge meteorite, coral reefs have survived. Although their global distribution has changed, they have lived through numerous major environmental events like meteorite strikes, ice ages, and huge changes in the amount of heat reaching the earth from the sun.
These remarkably resilient reefs have been dubbed the rainforests of the oceans. It's an apt description, since coral reefs are among the most biologically diverse and productive of nature's ecosystems. Although coral reefs cover less than 0.2 per cent of today's ocean floor, it is estimated that they support about 25 per cent of all marine species. Nearly 5,000 species of reef fish have been identified, and there are more than 2,500 species of coral, of which almost 1,000 are reef-building hard coral.
Corals are tiny animals called polyps that live in warm, shallow seawater and are related to and look like sea anemones. Coral reefs are found mainly along tropical coastlines, although they also extend north and south of the Tropics of Cancer and Capricorn into areas where there are warm ocean currents. Reefs consist of coral colonies built on top of one another. Each coral secretes a stony skeleton around itself made of limestone. As they grow, the polyps divide to form the coral colonies. They cover between 300,000 and 600,000 sq km and are found in the waters of more than 100 countries.
Coral reefs have fascinated scientists for nearly two centuries; Charles Darwin made some of the earliest observations during his voyage on the Beagle in the 1830s. Darwin recognised three main types of reef -- fringing reefs, barrier reefs, and atolls -- and his classifications are still widely used today.
Fringing reefs are found around islands and continents. They are usually separated from the shore by narrow shallow lagoons which have a sandy bottom, sometimes with seagrasses and scattered patches of coral. The presence of a lagoon between the reef and the coastline is because erosion from the land results in sediment being deposited just offshore, which clouds the water and prevents the growth of coral. Good examples of fringing reefs are found on the southwest coast of Madagascar, on Haiti, and in the world's southernmost reefs off Lord Howe Island in the Tasman Sea between Australia and New Caledonia.
Barrier reefs grow on the edge of continental shelves, separated from the mainland by lagoons that are often wide and deep. …