Anthozoa
Stony corals, Scleractinia
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Anthozoa Ehrenberg, 1831

Anthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows. Some anthozoans can also reproduce asexually through budding.

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 called zooxanthellae, 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, and the cnidarian gains photosynthetic capability and increased calcium carbonate production in hermatypic corals.^[1]

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.

[edit] Phylogeny

Anthozoa is subdivided into two subclasses Octocorallia and Hexacorallia which form monophyletic groups and generally show 8-way and 6-way symmetry in polyp structure respectively^[3]. Historically Ceriantipatharia was thought to be a separate subclass but the two orders it comprised, Ceriantharia and Antipatharia, are now considered part of Hexacorallia. The extant orders are shown to the right.^[2]

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^[4].

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.^[2]

Giant green anemone, likely Epicystis crucifer, Southern California

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^[5][6]:

• Numidiaphyllida †
• Kilbuchophyllida †
• Heterocorallia †
• Rugosa †
• Heliolitida †
• Tabulata †
• Cothoniida †
• Tabuloconida †

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.

Timeline of the major coral fossil record and developments from 650 m.y.a. to present.[7][8]	edit

[edit] References

[edit] External links

Wikispecies has information related to: Anthozoa

Wikimedia Commons has media related to: Anthozoa

• TSN 51938. Integrated Taxonomic Information System.