Coral Reef II

A coral reef (also called "rainforest of the seas" because of its very high biodiversity) is a marine reef structure formed by hermatypic (reef-building) cnidarians that grows large enough to exert a significant physical and ecological influence on its environment. They are the largest living creature-created structures on Earth. The total area of today's coral reefs is 600,000 km², and in the case of the Maldives, the reefs rise up to 2200 metres above the seabed.

Coral reefs are mainly built up by corals from the group of stony corals (Scleractinia), but fire corals (Millepora) and, in the tropical Indo-Pacific, the blue coral (Heliopora coerulea) also contribute to reef formation. Stony corals colonise both the deep sea and the shallow water up to the water surface. They build up the reef structure from their calcareous skeletons over the course of many centuries. A coral island is formed by long-term changes in the water level. Since the coral reef can grow to the water surface, an island or a series of islands, often in the form of an atoll, forms after the sea level is later lowered or the bottom is raised.

Coral reefs are complex marine ecosystems. They are the biotope (habitat) for a biocoenosis (community) of plants and animals, for example worms, molluscs, sponges, echinoderms and crustaceans. Coral reefs are very important as "nurseries" for pelagic fish.