Most of the reef is composed of loose to well-cemented organic debris of carbonate shells and skeletons. The living part of the reef is just a thin veneer on the surface. Corals belong to the Cnidara. The animal is the coral polyp. The corallite is the exoskeleton formed by the polyp.
Corals can be either solitary or colonial.
Corals can not survive in fresh, brackish water or highly turbid water.