The individuals within a coral colony are called polyps, and each polyp lives in a small circular depression, called a corallite, in the surface of a hard coral structure. Polyps are typically very small, usually more-or-less cylindrical in shape, and fleshy. They are topped with a circular array of tentacles that catch their prey—usually plankton, but certain species feed on small fish as well. The tentacles direct prey toward the coral’s centralized mouth. At the base of each polyp are specialized structures that secrete calcium carbonate (in the form of aragonite or calcite, depending on species). The calcium carbonate hardens to build the trademark coral structures of coral reefs.
Between each polyp is a living tissue called the coenosarc (pronounced seen-oh-sark); it coats the outer surfaces of the stony coral colonies and connects each individual polyp. It also secretes its own calcium carbonate, contributing to the hard reef structure. This elaborate network of living tissue allows the entire colony to share nutrients and ensures the survival of the colony as a whole. It also provides a means of chemical communication, aiding in reproduction. Most corals reproduce sexually, with colonies releasing both eggs and sperm into the water en masse during simultaneous spawning events, which are usually triggered around full moons.
Polyps on the surface of a coral colony
Coral polyp in corallite (cross section)
Modern coral polyps
