By night, a polyp captures plankton with its tentacles. By day, the zooxanthellae photosynthesize. Without the zooxanthellae, the polyps cannot grow fast enough to build reefs. The Great Barrier Reef has 350 named coral species. There is intense competition for space.
Corals have a rigid pecking order.
Types of coral reefs. Reefs are constructions of calcium carbonate, made literally from sunlight and sea water, by hard corals. Some algae, called coralline algae, also secrete calcium carbonate. Some encrusting invertebrates, such as sponges, complete the reef structure. Reef-building corals require water temperatures of at least 20 degrees C, and ample light.