Anthozoa

The name "Anthozoa" comes from the Greek words ('; "flower") and ('; "animals"), hence ανθόζωα (anthozoa) = "flower animals", a reference to the floral appearance of their perennial polyp stage. Anthozoans live exclusively in marine and brackish water. 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. ==Phylogeny==

In 1995, genetic studies by J. E. N. Veron and a group of scientists that analyzed the ribosomal DNA of many species, found that the order Ceriantipatharia, now deprecated and replaced by its constituent orders Ceriantharia and Antipatharia, was "the most representative of the ancestral Anthozoa". After Ceriantipatharia became deprecated , newer phylogenetic studies determined that Ceriantharia is not the basal clade of Anthozoa, but of Hexacorallia, and that Antipatharia and the remaining orders of Hexacorallia are its descendants. }} }} ==Taxonomy==

The taxonomy of the subphylum Anthozoa is being continuously updated and modified, based on molecular biology, molecular phylogenetics, and other methods. Its most recent version can be found in the World Register of Marine Species. It subdivides Anthozoa into two monophyletic, extant classes: Hexacorallia and Octocorallia, generally showing different symmetry of polyp structure, and two extinct, fossil classes: Rugosa † and Tabulata †. Hexacorallia includes coral reef builders (Scleractinia, also called hermatypic stony corals), sea anemones (Actiniaria), and zoanthids (Zoantharia). Octocorallia comprises the sea pens (Pennatulacea), soft corals (Octocorallia), and blue coral (Helioporacea). Sea whips and sea fans, known as gorgonians, are part of Alcyonacea and historically were divided into separate orders. Classification according to the World Register of Marine Species (WoRMS) †= extinct • phylum Cnidaria • subphylum Anthozoa • class Hexacorallia • order Actiniaria • suborder Anenthemonae • suborder Enthemonae • suborder Helenmonae • order Antipatharia • order Ceriantharia • order Corallimorpharia • order Hexanthiniaria † • order Kilbuchophyllida † • order Scleractinia • order Zoantharia • class Octocorallia • order Malacalcyonacea • order Scleralcyonacea • class Rugosa † • order Heterocorallia † • order Pterocorallia † • order Stauriida † • order Stereolasmatina † • class Tabulata † • order Auloporida † • order Favositida † • order Halysitida † • order Heliolitida † • order Lichenariida † • order Sarcinulida † • order Syringoporida † • order Tetradiida Examples of some major anthozoan taxa ==Anatomy==

The basic body form of an anthozoan is the polyp. This consists of a tubular column topped by a flattened area, the oral disc, with a central mouth; a whorl of tentacles surrounds the mouth. In solitary individuals, the base of the polyp is the foot or pedal disc, which adheres to the substrate, while in colonial polyps, the base links to other polyps in the colony. Soft corals in the subclass Octocorallia are also colonial and have a skeleton formed of mesogloeal tissue, often reinforced with calcareous spicules or horny material, and some have rod-like supports internally. Other anthozoans, such as sea anemones, are naked; these rely on a hydrostatic skeleton for support. Some of these species have a sticky epidermis to which sand grains and shell fragments adhere, and zoanthids incorporate these substances into their mesogloea. ==Biology==

, a zooxanthellate species; the tentacles are armed with nematocysts to catch prey Most anthozoans are opportunistic predators, catching prey which drifts within reach of their tentacles. The prey is secured with the help of sticky mucus, spirocysts (non-venomous harpoon cells) and nematocysts (venomous harpoon cells). The tentacles then bend to push larger prey into the mouth, while smaller, plankton-size prey, is moved by cilia to the tip of the tentacles which are then inserted into the mouth. The mouth can stretch to accommodate large items, and in some species, the lips may extend to help receive the prey. The pharynx then grasps the prey, which is mixed with mucus and slowly swallowed by peristalsis and ciliary action. When the food reaches the coelenteron, extracellular digestion is initiated by the discharge of the septa-based nematocysts and the release of enzymes. The partially digested food fragments are circulated in the coelenteron by cilia, and from here they are taken up by phagocytosis by the gastrodermal cells that line the cavity. The presence of zooxanthellae is not a permanent relationship. Under some circumstances, the symbionts can be expelled, and other species may later move in to take their place. The behaviour of the anthozoan can also be affected, with it choosing to settle in a well lit spot, and competing with its neighbours for light to allow photosynthesis to take place. Where an anthozoan lives in a cave or other dark location, the symbiont may be absent in a species that, in a sunlit location, normally benefits from one. Anthozoans living at depths greater than are azooxanthellate because there is insufficient light for photosynthesis. '' with white sea anemones below, both azooxanthellate, deep water species With longitudinal, transverse and radial muscles, polyps are able to elongate and shorten, bend and twist, inflate and deflate, and extend and contract their tentacles. Most polyps extend to feed and contract when disturbed, often invaginating their oral discs and tentacles into the column. Contraction is achieved by pumping fluid out of the coelenteron, and reflation by drawing it in, a task performed by the siphonoglyphs in the pharynx which are lined with beating cilia. Most anthozoans adhere to the substrate with their pedal discs but some are able to detach themselves and move about, while others burrow into the sediment. Movement may be a passive drifting with the currents or in the case of sea anemones, may involve creeping along a surface on their base. Transverse fission is less common, but occurs in Anthopleura stellula and Gonactinia prolifera, with a rudimentary band of tentacles appearing on the column before the sea anemone tears itself apart. Zoanthids are capable of budding off new individuals. '' produces yolky eggs and the planula larvae disperse widely. Most anthozoans are unisexual but some stony corals are hermaphrodite. The germ cells originate in the endoderm and move to the gastrodermis where they differentiate. When mature, they are liberated into the coelenteron and thence to the open sea, with fertilisation being external. The zygote develops into a planula larva which swims by means of cilia and forms part of the plankton for a while before settling on the seabed and metamorphosing into a juvenile polyp. Some planulae contain yolky material and others incorporate zooxanthellae, and these adaptations enable these larvae to sustain themselves and disperse more widely. ==Ecology==

Coral reefs are some of the most biodiverse habitats on earth, supporting large numbers of species of corals, fish, molluscs, worms, arthropods, starfish, sea urchins, other invertebrates and algae. Because of the photosynthetic requirements of the corals, they are found in shallow waters, and many of these fringe land masses. With a three-dimensional structure, coral reefs are very productive ecosystems; they provide food for their inhabitants, hiding places of various sizes to suit many organisms, perching places, barriers to large predators and solid structures on which to grow. They are used as breeding grounds and as nurseries by many species of pelagic fish, and they influence the productivity of the ocean for miles around. Anthozoans prey on animals smaller than they are and are themselves eaten by such animals as fish, crabs, barnacles, snails and starfish. Their habitats are easily disturbed by outside factors which unbalance the ecosystem. In 1989, the invasive crown-of-thorns starfish (Acanthaster planci) caused havoc in American Samoa, killing 90% of the corals in the reefs. Corals that grow on reefs are called hermatypic, with those growing elsewhere are known as ahermatypic. Most of the latter are azooxanthellate and live in both shallow and deep sea habitats. In the deep sea they share the ecosystem with soft corals, polychaete worms, other worms, crustaceans, molluscs and sponges. In the Atlantic Ocean, the cold-water coral Lophelia pertusa forms extensive deep-water reefs which support many other species. Other fauna, such as hydrozoa, bryozoa and brittle stars, often dwell among the branches of gorgonian and coral colonies. The pygmy seahorse not only makes certain species of gorgonians its home, but closely resembles its host and is thus well camouflaged. Some organisms have an obligate relationship with their host species. The mollusc Simnialena marferula is only found on the sea whip Leptogorgia virgulata, is coloured like it and has sequestered its defensive chemicals, and the nudibranch Tritonia wellsi is another obligate symbiont, its feathery gills resembling the tentacles of the polyps. with Calliactis parasitica'' A number of sea anemone species are commensal with other organisms. Certain crabs and hermit crabs seek out sea anemones and place them on their shells for protection, and fish, shrimps and crabs live among the anemone's tentacles, gaining protection by being in close proximity to the stinging cells. Some amphipods live inside the coelenteron of the sea anemone. Despite their venomous cells, sea anemones are eaten by fish, starfish, worms, sea spiders and molluscs. The sea slug Aeolidia papillosa feeds on the aggregating anemone (Anthopleura elegantissima), accumulating the nematocysts for its own protection. == Paleontology ==

Several extinct orders of corals from the Paleozoic era (~540–252 million years ago) are thought to be close to the ancestors of modern Scleractinia: • Numidiaphyllida † • Kilbuchophyllida † • Rugosa † • Tabulata † • Cothoniida † • Tabuloconida † These are all corals and correspond to the fossil record time line. With readily-preserved hard calcareous skeletons, they comprise the majority of Anthozoan fossils. ==Interactions with humans==

Coral reefs and shallow marine environments are threatened, not only by natural events and increased sea temperatures, but also by such man-made problems as pollution, sedimentation and destructive fishing practices. Pollution may be the result of run-off from the land of sewage, agricultural products, fuel or chemicals. These may directly kill or injure marine life, or may encourage the growth of algae that smother native species, or form algal blooms with wide-ranging effects. Oil spills at sea can contaminate reefs, and also affect the eggs and larva of marine life drifting near the surface. Corals are collected for the aquarium trade, and this may be done with little care for the long-term survival of the reef. Fishing among reefs is difficult and trawling does much mechanical damage. In some parts of the world explosives are used to dislodge fish from reefs, and cyanide may be used for the same purpose; both practices not only kill reef inhabitants indiscriminately but also kill or damage the corals, sometimes stressing them so much that they expel their zooxanthellae and become bleached. ==References==