Most coral polyps live together in colonies. The stony corals attach themselves to each other with a flat sheet of tissue that connects to the middle of each body. Half of the coral polyp extends above the sheet and half below. Coral polyps build their limestone skeletons by taking calcium out of the seawater. As new polyps grow, the limestone formation becomes larger and larger.
Coral polyps feed mainly on tiny swimming animals, such as the larvae of many kinds of shellfish.
Reef corals cannot live without special, single-celled algae that live in the polyp's own tissue. The polyps use food manufactured and released by the algae. The algae also help the coral animals secrete their limestone skeletons. Coral reefs grow only in water with enough light for photosynthesis to occur in the algae. Coral polyps reproduce either from eggs or by budding.