By George Stoyle

Coral reefs, as we know them, have been around for approx. 25 million years. They are made up of hundreds of thousands of tiny organisms known as polyps belonging to the phylum Cnidaria, which also includes anemones, jellyfish and hydroids. There are 2 major types of corals: soft and hard. The hard corals are primarily responsible for the formation of reefs found throughout shallow tropical seas, made possible by the secretion of calcium carbonate which the coral organisms extract from surrounding seawater. These large and continually changing structures form the most biologically diverse ecosystem on the planet, rivalled only by tropical rainforests. The primary reason for such high levels of biodiversity is due to the physical structure of the reef providing settlement surface for a large number of organisms as well as refuge and protection for others. Coral reefs can be thought of as oases in a desert sea - tropical seas are mostly devoid of nutrients and so minimal life occurs in the open ocean.

The primary reason coral organisms have the ability to produce massive reef structures is due to the presence of microscopic algae which reside within their tissues known as zooxanthellae. These algae are also what give corals their colour. Being plants they are light dependant, relying, as all plants do, on the process of photosynthesis. As a by-product of photosynthesis carbon is converted into sugars and passed on to the host coral organism. This provides the coral with 50-95% of their metabolic functions allowing the production of calcium carbonate approximately 14 times faster than deep water corals, which do not contain zooxanthellae. In return, the waste products produced by the corals are passed back to the zooxanthellae forming what is known as mutualistic symbiosis.

Although zooxanthellae provide corals with the vast majority of their nourishment, there are other methods used for feeding. Coral polyps have tiny stinging cells in their tentacles called nematocysts which not only defend them from some predators, but are also used to catch prey which swim or float past. Prey ranges from zooplankton to small fish, depending on the size of the polyp. Another method corals use for feeding is through a mucous layer on the surface of the colony. Particles in the water get trapped in the mucous which is then ingested by the polyp. This mucous layer also serves to remove sediment from the colony preventing blocking of essential sunlight and smothering of feeding surfaces. However, these direct methods of feeding alone are not enough to support the corals and so without zooxanthellae chances of long-term survival are minimal.

Coral organisms can only survive in a limited temperature range, usually between 21-29°C (although in some unusual cases corals have become tolerant to much high and lower temperatures). This limits their distribution to around 0.2% of the world's oceans, but despite this they contain around a quarter of all marine species. As well as providing crucial biological support, coral reefs also provide numerous essential goods and services to society, supporting over 500 million people. These include direct uses such as fishing, pharmaceuticals, tourism, as well as indirect uses such as coastal protection, ecosystem interactions and a contribution to global life support. The estimated value of coral reefs in 1997 was $375 billion.