Octocorallia

Octocorallia Temporal range: Cambrian–Recent PreЄ Є O S D C P T J K Pg N
Dendronephtya klunzingeri
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Anthozoa
Order:	Octocorallia Haeckel, 1866
Families
Alcyonacea Helioporacea Pennatulacea

Octocorallia (also known as Alcyonaria) is a subclass of Anthozoa comprising ~3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea fans and sea whips) within three orders: Alcyonacea, Helioporacea, and Pennatulacea.^[1] These organisms have an internal skeleton secreted by mesoglea and polyps with eight tentacles and eight mesentaries. 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.

Biology

Octocorals resemble the stony corals in general appearance and in the size of their polyps, but lack the distinctive stony skeleton. Also unlike the stony corals, each polyp has only eight tentacles, each of which is feather-like in shape, with numerous side-branches, or pinnules.

Octocorals are colonial organisms, with numerous tiny polyps embedded in a soft matrix that forms the visible structure of the colony. The matrix is composed of mesogleal tissue, lined by a continuous epidermis and perforated by numerous tiny channels. The channels interconnect the gastrovascular cavities of the polyps, allowing water and nutrients to flow freely between all the members of the colony. The skeletal material, called coenchyme, is composed of living tissue secreted by numerous wandering amoebocytes. Although it is generally soft, in many species it is reinforced with calcarous or horny material.^[2]

The polyp is largely embedded within the colonial skeleton, with only the uppermost surface, including the tentacles and mouth, projecting about the surface. The mouth is slit-like, with a single ciliated groove, or siphonoglyph, at one side to help control water flow. It opens into a tubular pharynx that projects down into a gastrovascular cavity that occupies the hollow interior. The pharynx is surrounded by eight radial partitions, or mesenteries, that divide the upper part of the gastrovascular cavity into chambers, one of which connects to the hollow space inside each tentacle. The gonads are located near the base of each mesentery.^[2]

Phylogeny

Octocorallia is considered to be monophyletic, that is all contained species are descended from a common ancestor, but the relationships between subdivisions are not well known. The sea pens (Pennatulacea) and blue coral (Helioporacea) continue to be assigned separate orders, whereas the current order Alcyonacea was historically represented by four orders: Alcyonacea, Gorgonacea, Stolonifera, Telestacea.

The current phylogeny is shown below,^[1] where Calcaxonia and Holaxonia are groups of gorgonians and the four remaining divisions of Alcyonacea are merely distinguished for convenience.

Full corallum of Tubipora musica.

Octocorallia	Alcyonacea  Calcaxonia Holaxonia Alcyoniina Protoalcyonaria Scleraxona Stolonifera Helioporacea Pennatulacea

References

1. ^ ^{a b} 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. (2007-07-21). "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. 
2. ^ ^{a b} Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 164–169. ISBN 0-03-056747-5. 

External links

• "Octocorallia". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=52004.