Temporal range: 570–0Ma Late Ediacaran to Recent
Anthozoa is a class within the phylum Cnidaria. 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. More than 6,100 species have been described.
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. The gonads 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 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 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 closely resemble anemones in structure, although they are generally much smaller. Stony coral are found in most seas.
|Phylogeny of Anthozoa|
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. 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.
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:
- 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.
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|Wikimedia Commons has media related to Anthozoa.|
- Data related to Anthozoa at Wikispecies
- "Anthozoa Ehrenberg, 1834". Integrated Taxonomic Information System.