Water must be clear and free of sediment. They are, in effect, the rain forests of the sea. Reefs are built by tiny organisms called polyps.
Coral polyps have hard, cuplike external skeletons that form the body of the coral reef.
It takes millions of individual coral skeletons, built up over thousands of years, to form a coral reef. The coral polyps on the outside of the reef live in synbiosis with a type of algae called zooxanthellae. These algae live in polyp tissues and use sunlight to produce the oxygen and sugars that are consumed by the coral. The polyps in turn produce carbon dioxide, which the algae use to produce more oxygen and sugars. These apparently delicate organisms are surprisingly tough.