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The key life form of a coral reef is the coral itself. Coral is a simple marine animal in the phylum Cnidaria, and most corals are in the class Anthozoa. Mature coral animals are colonial, and each builds a cup-like external skeleton made of calcium carbonate. The accumulation of millions of these skeletons is what creates coral reefs.
Reef building corals usually form a symbiotic relationship with zooanthellae, single-celled algae that live inside the body of the coral polyp. The algae get carbon dioxide and a safe location, and the coral gets oxygen and nutrients from the algal cell's photosynthesis. Corals also filter feed, waving their tentacles in the water and capturing particles and single celled organisms to eat.
A large variety of fish and crustaceans are dependent on coral reefs as a habitat, but the coral depend only on the zooanthellae. In some cases the corals in a reef will suddenly eject their zooanthallae, resulting in a condition called coral bleaching. Scientists are studying coral bleaching, but at this point no one knows why it happens. Corals that have bleached often starve and die.
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