Coral reefs grow in shallow tropical water and are formed by the plants and animals which grow within them, mainly the stony corals. Because they contain algae which need light, they only grow well near the surface of clear water. In addition molluscs, echinoderms, protozoa, sponges and sea cucumbers contribute to the growing reef. Sea grasses and mangroves grow on top of some reefs, and mats of blue-green bacteria help to trap and stabilize the structure.
There are four types of reef:
fringing reefs around islands
barrier reefs several kilometers offshore near a landmass (such as the Great Barrier reef of Australia)
atolls are hollow rings and form on submerged volcanoes (such as the islands of Bermuda)
patch reefs are small circular or irregular reefs rising from the floor of a lagoon or within an atoll.
Where corals break the surface of the water they form coral islands.
Corals form an important ecological resource. They grow in clear water which contains very little food, so no other form of life can survive there without the corals. The algae which live inside the corals form the base of a food chain which supports a wide diversity of plants and animals. The nutrients are then recycled within the reef, so it forms an ecosystem.
Coral reefs provide an economic benefit as a major tourist attraction.
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