Anthozoa

Anthozoa is a class of marine invertebrates which includes the sea anemones, stony corals, soft corals and gorgonians. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of the adult is the polyp; this consists of a cylindrical column topped by a disc with a central mouth surrounded by tentacles. Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual. Colonies are strengthened by calcium carbonate and other materials and take various massive, plate-like, bushy or leafy forms. Anthozoa is included within the phylum Cnidaria, which also includes the jellyfish, box jellies and parasitic Myxozoa and Polypodiozoa. The two main subclasses of Anthozoa are the Hexacorallia, members of which have six-fold symmetry and includes the stony corals, sea anemones, tube anemones and zoanthids; and the Octocorallia, which have eight-fold symmetry and includes the soft corals and gorgonians (sea pens, sea fans and sea whips), and sea pansies. The smaller subclass, Ceriantharia, consists of the tube-dwelling anemones. Some additional species are also included as incertae sedis until their exact taxonomic position can be ascertained. Anthozoans are carnivores, catching prey with their tentacles. Many species supplement their energy needs by making use of photosynthetic single-celled algae that live within their tissues. These species live in shallow water and many are reef-builders. Other species lack the zooxanthellae and, having no need for well-lit areas, typically live in deep-water locations. Unlike other members of this phylum, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs into the water. After fertilisation, the planula larvae form part of the plankton. When fully developed, the larvae settle on the seabed and attach to the substrate, undergoing metamorphosis into polyps. Some anthozoans can also reproduce asexually through budding or by breaking in pieces. More than 16,000 species have been described. The name 'Anthozoa' comes from the Greek words άνθος (ánthos; 'flower') and ζώα (zóa; 'animals'), hence ανθόζωα (anthozoa) = 'flower animals', a reference to the floral appearance of their perennial polyp stage. Anthozoans are exclusively marine, and include sea anemones, stony corals, soft corals, sea pens, sea fans and sea pansies. Anthozoa is the largest taxon of cnidarians; over six thousand solitary and colonial species have been described. They range in size from small individuals less than half a centimetre across to large colonies a metre or more in diameter. They include species with a wide range of colours and forms that build and enhance reef systems. Although reefs and shallow water environments exhibit a great array of species, there are in fact more species of coral living in deep water than in shallow, and many taxa have shifted during their evolutionary history from shallow to deep water and vice versa. Anthozoa is subdivided into three subclasses: Octocorallia, Hexacorallia and Ceriantharia, which form monophyletic groups and generally show differentiating reflections on symmetry of polyp structure for each subclass. The relationships within the subclasses are unresolved.