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Coral platform. Aerial photograph of a coral platform in the Coral Sea. This forms part of the northern Great Barrier Reef off the coast of Australia. Coral reefs are huge colonies of marine animals (class Anthozoa) that secrete a protective skeleton around their soft bodies. This skeleton is usually composed of calcium carbonate. Its secretion is stimulated by microscopic algae that inhabit the coral body tissues. Coral reefs are found in every ocean, but they prefer warm, shallow waters. The clear waters seen here are maintained by mangrove forests on the north- eastern Australian coast. These prevent river mud from being washed into this region. (KEYSTONE/SCIENCE PHOTO LIBRARY/Ernhard Edmaier) === ===