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Coral reefs

Part of the Great Barrier Reef, off the northeastern coast of Australia, seen from the air Coral is found in tropical shallow waters around volcanic islands or close to rocky mainland coastlines. It is made from layers of the skeletons of tiny animals called polyps. Over many years, colonies of polyps can build up great banks of coral, known as reefs. There are many different kinds of corals; their bright colours make a coral reef look like an undersea garden. Only the living surface of the coral is coloured—the layers of dead polyps underneath are white.


Coral reefs are hosts to many colourful species of fish and other marine life-forms.
Close-up view of coral

Coral polyps

Coral is formed from the hard skeletons of tiny animals called polyps. They live together in large colonies. They are marine invertebrates, related to sea anemones, sea pens and jellyfish (cnidarians). Each measures only a few millimetres across and has a ring of food-catching tentacles around its mouth. The polyp creates a stony, cup-shaped skeleton around its soft body made of calcium carbonate—the same substance as limestone and chalk.
Coral polyps, Solomon Islands{more} The polyps breed by spawning. They release their sex cells into the water over several nights around a full moon. The polyps feed on zooplankton, which they trap using the stinging cells in their tentacles—as well as on the food provided for them by algae that live inside their bodies. 
 
Colourful corals in the Red Sea

Algae in coral

The Great Barrier Reef, off the Pacific coast of Australia, is the world’s largest coral reef. It is more than 2000 km (1200 miles) long. It is so massive a structure it can be seen from space.

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