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 provides 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.
Coral colonies grow very slowly, sometimes only a few millimeters every year. They require specific conditions to remain healthy. Water that is too warm can cause a phenomenon known as coral bleaching; this is when corals expel their symbiotic zooxanthellae and turn white. Corals cannot survive long without their partners, and in their weakened condition, become more susceptible to disease. A growing percentage of the worlds corals are dying from coral bleaching and disease.
Corals also suffer from more direct human impacts. Stepping on or even touching corals can kill them. Some of the corals in the Virgin Islands have lived for hundreds of years. They have survived hurricanes, anchor damage, coral bleaching, smothering sediment, pollution and disease. Please remember to stand only on sand; give the corals a chance to survive.
Documents and Links
Coral diseases following massive bleaching in 2005 (Link to report summary and PDF of full report)
Discovering the Secret Gardens in the Mangroves of St. John, U.S. Virgin Islands (Link to investigations about corals living among mangrove prop-roots. Includes a slideshow and links to more articles about St. John.)
Coral Reef ID Cards in English or Spanish. Pictures and descriptions of corals as pdf files.