| Corals look like plants. They are actually animals that use their
tentacles to catch tiny sea creatures to eat.
They live together in colonies of groups. Over thousands of years the
skeletons of dead corals build up on top of one another to form a coral reef.
The individual coral animal, called a coral polyp, has a soft body topped by a
ring of stinging tentacles for catching food. Some kinds of coral polyps secrete
a hard underlying skeleton made of limestone. Coral polyps often retreat into
cuplike depressions in the skeleton for protection. It builds the skeleton by
combining carbon dioxide (CO2) and calcium (Ca) in the water to make calcium
carbonate (CaCO3), also known as limestone.
When corals die, other corals build on top until a great reef is formed.
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