Each polyp lives inside a shell of aragonite, a type of calcium carbonate which is the hard shell we recognise as coral. The polyps join together to create forests of coloured coral in interesting fan, antler, brain and plate shapes. There are many different types of coral, some are slow growing and live to be hundreds of years old, others are faster growing. The colours of coral are created by algae. Only live coral is coloured.
Dead coral is white.
The ideal environment for coral is shallow warm water where there is a lot of water movement, plenty of light, where the water is salty and low in nutrients. Tourism may also have a negative impact, with fragile corals broken by reef walking, dropped anchors or by boats dropping fuel and other sorts of pollution. Even the number of people in the water with the associated run-off of sweat and suntan lotions may well have a negative impact on the fragile reef environment. Tourists are carried to the reef system by more than 500 commercial vessels, and tourism is permitted through nearly all the Park. One tourist who cares little for the beauty and diversity of the Great Barrier Reef system is the Crown of Thorns starfish.