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Brain coral (Platygyra sp.). This hard coral is named for its convoluted skeleton that resembles the surface of a brain. It is also known as a maze coral. Corals consist of colonies of small animals called polyps, which feed by filtering plankton from the passing water current. Brain coral colonies can measure up to 1 metre in diameter. Hard coral polyps secrete calcium carbonate, which forms a protective skeleton around them. When a polyp dies the skeleton remains and new polyps grow on top of it, forming a coral reef. (KEYSTONE/SCIENCE PHOTO LIBRARY/NIGEL DOWNER/SCIENCE PHOTO LIBRARY)