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Divers on a coral reef. Coral reefs are extensive and diverse marine ecosystems. Corals themselves are tiny animals that live in colonies. Corals may be hard (strengthened by secretions of calcium carbonate) or soft and branching, as seen here. The reef provides a home to a multitude of different marine organisms, including fish, molluscs and crustaceans. Photographed in the Red Sea. (KEYSTONE/SCIENCE PHOTO LIBRARY/LIONEL, TIM & ALISTAIR)