Hexacorallia
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.jpg) A stony coral, Acropora latistella
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Hexacorallia (also known as Zoantharia) is a subclass of Anthozoa comprising ~4,300 species of water-based organisms formed of colonial polyps generally with 6-fold symmetry. This includes all of the stony corals, which are vital for coral reef formation, as well as all sea anemones, tube anemones, and zoanthids within six extant orders[1]. They are distinguished from the other subclass of Anthozoa, Octocorallia, in having six or fewer lines of symmetry in their body structure and only single rows of tentacles. These organisms are formed of individual soft polyps which in some live in colonies and can secrete a calcite skeleton. As with all Cnidarians these organisms have a complex life cycle including a motile phase when they are considered plankton and later characteristic sessile phase. It also includes the significant extinct orders of the rugose corals and tabulate corals
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[edit] Phylogeny
Hexacorallia is considered to be monophyletic, that is all contained species are descended from a common ancestor, however it has been suggested that many of the current orders are not. Historically Ceriantharia and Antipatharia were considered to be a separate subclass called Ceriantipatharia, though more recent genetic studies place them in Hexacorallia with Ceriantharia as the oldest, or basal, group.[1]
The subclass includes important coral reef builders the stony corals (Scleractinia), sea anemones (Actiniaria) and related tube-dwelling anemones (Ceriantharia), and zoanthids (Zoanthidea). Antipatharia contains the black corals and Corallimorpharia are similar to anemones. The extant orders are shown below[1]:
| Hexacorallia |
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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 period 570-245 million years ago[2][3]:
- Numidiaphyllida †
- Kilbuchophyllida †
- Heterocorallia †
- Rugosa †
- Heliolitida †
- Tabulata †
- Cothoniida †
- Tabuloconida †
[edit] Hexacorallia or Zoantharia
The terms Hexacorallia and Zoantharia are widely used synonymously. When looking at extant species only, they are equivalent. But some of the extinct species do not show six-fold symmetry. For example, Rugosa † is also called Tetracorallia, for its four-fold symmetry. Therefore Hexacorallia is misleading, and Zoantharia is preferred in paleontological contexts.
[edit] References
- ^ a b c Daly, M.; Brugler, M.P., Cartwright, P., Collins, A.G., Dawson, M.N., Fautin, D.G., France, S.C., McFadden, C.S., Opresko, D.M., Rogriguez, E., Romano, S.L. & Stake, J.L. (21-7-2007). "The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus". Zootaxa 1668: 1-766. ISSN 1175-5326. http://www.nhm.ku.edu/inverts/pdf/daly%20et%20al%20Zootaxa.pdf.
- ^ Oliver, W. A., Jr. (1996). "Origins and relationships of Paleozoic coral groups and the origin of the Scleractinia". in G. D. J. Stanley (ed.). Paleobiology and Biology of Corals. Columbus, Ohio: The Paleontological Society. pp. 107–134.
- ^ Ben Kotrc (2005). "Anthozoa: Subgroups". Fossil Groups. University of Bristol. http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/anthozoa/subgroups.html. Retrieved on 2009-03-23.
[edit] External links
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