Hexacoral

Hexacorallia
A stony coral, Acropora latistella

Hexacorallia is a subclass of Anthozoa comprising approximately 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

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 (Zoantharia). Antipatharia contains the black corals and Corallimorpharia are similar to anemones. The extant orders are shown below:[1]

 Hexacorallia 

Ceriantharia




Actiniaria



Antipatharia



Corallimorpharia



Scleractinia



Zoantharia




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 541-242 million years ago:[2][3]

  • Numidiaphyllida †
  • Kilbuchophyllida †
  • Heterocorallia †
  • Rugosa
  • Heliolitida †
  • Tabulata
  • Cothoniida †
  • Tabuloconida †

Hexacorallia or Zoantharia

The terms Hexacorallia and Zoantharia have been used synonymously to avoid confusion with some of the extinct species not showing sixfold symmetry. For example, Rugosa † is also called Tetracorallia, for its fourfold symmetry. Therefore, Hexacorallia might appear misleading, and Zoantharia is sometime preferred in paleontological contexts. However, with Zoantharia being the name used since 1899[4] until recent days [5][6] for the hexacorallian order Zoantharia, the use of the term Hexacorallia is strongly recommended[by whom?] in order to avoid confusions.

References

External links

  • Tree of Life Zoantharia
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