What is Coral?
{{for|the colour|coral (colour)}} {{Taxobox | name = Coral | image = PillarCoral.jpg | image_width = 200px | image_caption = [[Pillar coral]], ''Dendrogyra cylindricus'' | regnum = [[Animal]]ia | phylum = [[Cnidaria]] | classis = [[Anthozoa]] | classis_authority = [[Christian Gottfried Ehrenberg|Ehrenberg]], 1831 | subdivision_ranks = Extant Subclasses and Orders | subdivision = }} [[File:Coral polyps in symbiosis with unicellular dinoflagellates.jpg|thumb|right|250px|Coral [[polyp]]s with unicellular algae]] '''Corals''' are formed by small [[animal]]s, the [[polyp]]s of the [[phylum]] [[Cnidaria]]. ==Characteristics== The corals are [[Ocean|marine]], either on [[continental shelf|continental shelves]] or round [[ocean]]ic islands. They live in [[Colony (biology)|colonies]]. All the polyps in a colony are [[zooid]]s: they are all [[clone]]s, genetically identical. Inside the colony they breed by [[asexual reproduction]]. They also reproduce [[Sexual reproduction|sexually]]. Colonies of the same [[species]] release [[gamete]]s together, over one, two or three nights around a full moon. Each coral animal is like a small bag. The opening on top is the [[mouth]]. [[Tentacle]]s (little arms) around the mouth carry stinging [[nematocyst]]s, which [[Paralysis|paralyse]] the small animals eaten by the coral [[polyp]]s. Coral usually grows in tropical oceans. A few corals grow in cold water, like the oceans around the [[British Isles]] and [[Norway]]. == Symbiosis == Most corals get energy and nutrients from [[symbiosis]] with [[photosynthetic]] [[unicellular]] [[algae]] called [[zooxanthella]]e. Such corals need sunlight and grow in clear, shallow water, typically at depths less than 60 metres (200&nbsp;ft).<ref name=MilneBay/> Many corals (and some other [[cnidaria]]) live with zooxanthellae of the [[genus]] ''[[Symbiodinium]]'', which are [[dinoflagellate]]s. Usually, each polyp harbours one [[species]] of algae. By photosynthesis, these provide energy for the coral, and help [[Calcium carbonate|calcification