| Octocorallia | |
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| Dendronephthya klunzingeri | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Subphylum: | Anthozoa |
| Class: | Octocorallia Haeckel, 1866 |
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Octocorallia (also known asAlcyonaria) is aclass ofAnthozoa comprising over 3,000 species[1] of marine organisms formed of colonialpolyps with 8-fold symmetry. It includes theblue coral,soft corals,sea pens, andgorgonians (sea fans and sea whips) within three orders: Alcyonacea, Helioporacea, and Pennatulacea.[2] These organisms have an internal skeleton secreted by mesoglea and polyps with eight tentacles and eight mesentaries. As with allcnidarians these organisms have a complex life cycle including a motile phase when they are consideredplankton and later characteristicsessile phase.
Octocorals have existed at least since theOrdovician period, as shown byMaurits Lindström's findings in the 1970s.[3] Recent work suggesting that the CambrianPywackia may represent aCambrian octocoral[4] is disputed.[5][6]
Octocorals resemble thestony 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, orpinnules.
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 ofmesogleal tissue, lined by a continuousepidermis 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, calledcoenenchyme, is composed of living tissue secreted by numerous wanderingamoebocytes. Although it is generally soft, in many species it is reinforced with calcareous or horny material.[7]
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 singleciliated groove, orsiphonoglyph, at one side to help control water flow. It opens into a tubularpharynx that projects down into a gastrovascular cavity that occupies the hollow interior. The pharynx is surrounded by eight radial partitions, ormesenteries, that divide the upper part of the gastrovascular cavity into chambers, one of which connects to the hollow space inside each tentacle. Thegonads are located near the base of each mesentery.[7] Octocorals have high phenotypic plasticity from adapting to dynamic environments where temperature, pH, and other parameters are in constant flux[8] and have shown high recruitment rates post-die-off events caused by el nino events.[9]
Bioluminescence is found in 32 genera, a trait estimated to have evolved 540 million years ago, the earliest timing of emergence of bioluminescence in the marine environment.[10]
Octocorallia is considered to bemonophyletic, meaning that 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 separateorders, whereas the current order Alcyonacea was historically represented by four orders: Alcyonacea, Gorgonacea, Stolonifera and Telestacea.
The following taxa are unplaced within Octocorallia according to theWorld Register of Marine Species as of April 2024[update]:[11]
Families:
Genera:
Authority control databases: National |
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