Octocorallia, along with Hexacorallia, is one of the two extant classes of Anthozoa. It comprises over 3,000 species of Seawater and Brackish water animals consisting of colonial polyps with 8-fold symmetry, commonly referred informally as "soft corals". It was previously known by the now unaccepted scientific names Alcyonacea and Gorgonacea, both deprecated , and by the also deprecated name of Alcyonaria, in earlier times.
Its only two orders are Malacalcyonacea and Scleralcyonacea, which include corals such as those under the common names of , , and (sea fans and sea whips). These animals have an internal skeleton secreted by their mesoglea, and polyps with typically eight tentacles and eight mesenteries. As is the case with all , their complex life cycle includes a motile, planktonic phase (a larva called planula), and a later characteristic sessile phase.
Octocorals have existed at least since the Ordovician period, as shown by Maurits Lindström's findings in the 1970s. A 2023 work suggested that the Cambrian fossil Pywackia may represent a Cambrian octocoral, and molecular techniques have even pointed to a Precambrian origin for Octocorallia. For instance, a 2021 study built a time-calibrated phylogenetic tree that has placed the origin of Octocorallia in the Ediacaran (578 Mya).
Biology
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 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 coenenchyme, is composed of living tissue secreted by numerous wandering . Although it is generally soft, in many species it is reinforced with calcareous or horny material.
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 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 are located near the base of each mesentery. Octocorals have high phenotypic plasticity from adapting to dynamic environments where temperature, pH, and other parameters are in constant flux and have shown high recruitment rates post-die-off events caused by El Niño events.
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.
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Phylogeny
Unplaced taxa
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