Coral reefs are massive calcareous rock structures that are slowly secreted by simple colonial animals that live as a thin layer on the rock surface. The living organisms continually build new structures on top of old, extending the reefs seaward toward deeper water and upward toward the surface.
Reef-building corals have algae living in their tissues in a symbiotic relationship. The algae supplies food to the coral, and the coral supplies shelter and metabolic wastes as nutrients to the algae. These reef-building corals are limited by water temperature to the tropical latitudes.
Coral reefs rival tropical rainforests as being among the most complex communities on earth. Rock-producing reef communities are some of the most ancient life forms found in the fossil record. At least 100,000 species have been named and described, but the total number inhabiting the world's reefs may exceed one million.