Corals are gastrovascular marine cnidarians (phylum Cnidaria, class Anthozoa) existing as small anemone-like polyps, typically forming colonies of many individuals. The group includes the important reef builders known as hermatypic corals, found in tropical oceans, and belonging to the subclass Zoantharia of order Scleractinia (formerly Madreporaria). The hermatypic corals obtain much of their nutrient requirement from symbiotic unicellular algae called zooxanthellae, and so are dependent upon growing in sunlight. As a result, these corals are usually found not far beneath the surface, although in clear waters corals can grow at depths of 60 m (200 ft). Corals breed by spawning, with all corals of the same species in a region releasing gametes simultaneously over a period of one to several nights around a full moon. (from Wikipedia)
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