Description:
The world"s first coral reefs occured about 500 million years ago, and the first close relatives of modern corals developed in Southern Europe about 230 million years ago. By comparison, the Great Barrier Reef is relatively young at just 5,00,000 years old. The current reer's structure is much younger at less than around 8,000 years old.
Most modern reefs have formed on hard surfaces in the ocean, such as a base of an old reef that died during a period when sea level was lower, or the edge of a rocky island. Depending on how they start out, several types of reefs can form. Some coral reefs form in the deep ocean and are called atolls. The theories on how coral reefs form were first put forward by Charles Darwin (of The Origin of Species fame) who proposed that atolls form around the edges of high volcanic islands that gradually submerge beneath the sea with changes in sea level or subsidence of the land. Thus an atoll starts life as a fringing reef, then becomes more of a ring growing on the shrinking land-mass, until the land disappears and just the coral circle remains. In some cases, the coral growth is unable to keep pace with the sinking island, and sunken dead reefs have been found.
Coral reefs are found in shallow water, ranging to depths of 60m. Some species prefer either cooler temperate water while others are found along tropical reefs, such as the Great Barrier Reef, with waters ranging in temperatures from 18 - 33degrees C. They generally occur in large numbers as colonies of individual polyps linked by tissue. Resources, such as food, are then shared amongst the individuals in the colony.
