Anthozoais a class within the phylumCnidariathat contains thesea anemonesandcorals. Unlike other cnidarians, anthozoans do not have amedusastage in their development. Instead, they releasespermand eggs that form aplanula, which attaches to somesubstrateon which the cnidarian grows. Some anthozoans can also reproduce asexually throughbudding. More than 6,100 species have been described.[1]
The name comes from the Greek wordsάνθος(ánthos; "flower") andζώα(zóa; "animals"), henceanthozoa= "flower animals", a reference to the floral appearance of their perennial polyp stage.
Biology and anatomy
Like those of other cnidarians, the individual polyps have a cylindrical body crowned by a ring of tentacles surrounding the mouth. The mouth leads into a tubular pharynx which descends for some distance into the body before opening into the gastrovascular cavity that fills the interior of the body and tentacles. Unlike other cnidarians, however, the cavity is subdivided by a number of radiating partitions, or mesenteries. Thegonads are also located within the cavity walls.
All cnidarian species can feed by catching prey with nematocysts; sea anemones are capable of catching fish and corals of catching plankton. Some of the species also harbour a type of algae, dinoflagellates calledzooxanthellae, in a symbiotic relationship; the reef building corals known as hermatypic corals rely on this symbiotic relationship particularly. The zooxanthellae benefit by using nitrogenous waste and carbon dioxide produced by the host or , and the cnidarian gains photosynthetic capability and increased calcium carbonate production in hermatypic corals.
Anemones and certain species of coral live in isolation, however most corals form colonies of genetically identical polyps; these polyps closely resemble anemonies in structure, although are generally considerably smaller. Most kinds of stony coral live in all parts of the underwater world.
Anthozoa is subdivided into two subclasses Octocorallia and Hexacorallia which form monophyleticgroups and generally show 8-way and 6-way symmetry in polyp structure respectively. Historically Ceriantipatharia was thought to be a separate subclass but the two orders it comprised, Ceriantharia andAntipatharia, are now considered part of Hexacorallia. The extant orders are shown to the right.
Hexacorallia includes important coral reef builders the stony corals (Scleractinia), sea anemones (Actiniaria) and related tube-dwelling anemones (Ceriantharia), and zoanthids (Zoantharia). Genetic studies of ribosomal DNA has shown Ceriantharia to be a monophyletic group and the oldest, or basal, order among them.
Octocorallia comprises the sea pens (Pennatulacea), soft corals (Alcyonacea), and blue coral (Helioporacea). Sea whips and sea fans, known as gorgonians, are part of Alcyonacea and historically were divided into separate orders.
A number of extinct orders of corals have been classified as their calcium skeleton forms a prolific fossil record. These are generally thought to be close to the ancestors of modern Scleractinians and existed during the Paleozoic era 570–245 million years ago:
These are all corals and correspond to the fossil record time line shown below; they comprise the majority of the fossils of Anthozoa due to their calcareous skeleton being preserved.
