Coral in Virgin Islands National Park. A coral is a tiny animal (cnidarian) with a tiny plant (an algae called zooxanthellae) living inside it. The coral takes calcium from seawater and uses it to build a limestone skeleton. The coral provides a home for the zooxanthellae, and the zooxanthellae provide energy for the coral. This is called a symbiotic relationship; two organisms live together for their mutual benefit. Zooxanthellae also give coral its color. Many corals grow together to form colonies, which are the various and fantastic shapes you see. Lots of colonies growing close together make up the growing structure that we call a coral reef. Reefs also include sponges, algae, and rock, all of which create habitat and homes for fish, mollusks, crustaceans, worms and other marine life.
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