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. Small, knoblike growths called buds appear on the body of an adult polyp, or on the connecting sheet, from time to time. These buds grow larger, separate from the parent, and begin to deposit their own limestone in the colony. Budding helps the colony increase its size. New colonies of coral polyps form when the adult polyps of an old colony produce eggs.