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CORAL polyps create a hard skeleton around themselves and then they hide inside it if they are under threat.
Corals can grow in a wide range of colours - from pinkish red to deep red and even gold.
Coral reefs, which are made up of millions of coral skeletons that have been built up over thousands of years, are sometimes called the rainforests of the sea because there are so many different creatures that live in them.
Fantastic Ocean Facts, The Book Company