Corals are gastrovascular marine cnidarians (phylum Cnidaria;
class Anthozoa) existing as small sea 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 latter are also known as stony corals inasmuch as
the living tissue thinly covers a skeleton composed of
calcium carbonate. 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 many corals of the same species in a
region releasing gametes simultaneously over a period
of one to several nights around a full moon.
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