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The polyps sit in cup-shaped depressions in the skeleton known as corallites.Colonies of stony coral are very variable in appearance; a single species may adopt an encrusting, plate-like, bushy, columnar or massive solid structure, the various forms often being linked to different types of habitat, with variations in light level and water movement being significant.Individual heads grow by asexual reproduction of polyps.Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.In stony corals, also known as hard corals, the polyps produce a skeleton composed of calcium carbonate to strengthen and protect the organism.This is deposited by the polyps and by the coenosarc, the living tissue that connects them.The Octocorallia include blue coral, soft corals, sea pens, and gorgonians (sea fans and sea whips).These groups have polyps with 8-fold symmetry, each polyp having eight tentacles and eight mesenteries. Corals are sessile animals in the class Anthozoa and differ from most other cnidarians in not having a medusa stage in their life cycle. Most corals are colonial, the initial polyp budding to produce another and the colony gradually developing from this small start.
These are commonly known as zooxanthellae and the corals that contain them are zooxanthellate corals.Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length.A set of tentacles surround a central mouth opening. Over many generations, the colony thus creates a large skeleton that is characteristic of the species.Within the Hexacorallia, the sea anemones, coral anemones and stony corals may constitute a monophyletic grouping united by their six-fold symmetry and cnidocyte trait.The Octocorallia appears to be monophyletic, and primitive members of this group may have been stolonate.