Coral polyp, tropical waters, 5 mm (0.2 in) long Coral is a hard substance formed from the skeletons of tiny animals called polyps. Polyps live together in huge colonies in shallow warm waters fringing volcanic islands or rocky coastlines in the tropics. Each polyp measures just 5 millimetres (0.2 inch) across and is little more than a stomach with a ring of food-catching tentacles around its mouth. Polyps feed on microscopic plankton in the water. The polyps build up a stony, cup-shaped skeleton around their soft bodies. When they die, the stony casings are left behind. These may build up into huge coral reefs.
Shapes and colours
Different types of coral on a reefDifferent kinds of polyps produce different shapes of coral. Some are shaped like branching plants, others are flat like a fan, long and thin like pipes or rounded like human brains. Only the living surface of the coral is coloured by the presence of algae. The layers of polyp skeletons underneath are white. The living layer of tiny polyps feed on zooplankton that drifts by in the current which they trap using the stinging cells in their tentacles.
Corals belong to the group of animals called cnidarians. They are related to sea pens, jellyfish and sea anemones.
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