Corals are a group of cnidarian animals which are phylogenetically close to sea anemones and jelly fish. Living animals secrete a hard skeleton of calcium carbonate and form coral reefs. The majority of corals consist of genetically identical, small individuals called polyps. Each polyp feeds on planktons and also uses organic nutrients produced by photosynthetic symbiotic microalgae (zooxanthellae). Because the algae need intensive sunlight, the distribution of corals is restricted to shallow-water tropical to subtropical areas.