Hexacorallia is a
class of
Anthozoa
Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, stony corals and soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of ...
comprising approximately 4,300 species of aquatic organisms formed of
polyps, generally with 6-fold symmetry. It includes all of the stony
coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...
s, most of which are
colonial and reef-forming, as well as all
sea anemone
Sea anemones are a group of predatory marine invertebrates of the order Actiniaria. Because of their colourful appearance, they are named after the '' Anemone'', a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria ...
s, and
zoanthids, arranged within five extant
orders.
The hexacorallia are distinguished from another class of Anthozoa,
Octocorallia, in having six or fewer axes of symmetry in their body structure; the tentacles are simple and unbranched and normally number more than eight. These organisms are formed of individual soft polyps which in some species live in colonies and can secrete a
calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scrat ...
skeleton. As with all
Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that ...
ns, these organisms have a complex life cycle including a motile
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruci ...
ic phase and a later characteristic
sessile phase. Hexacorallia also include the significant extinct order of
rugose coral
The rugosa, also called the tetracorallia or horn coral, are an extinct order of solitary and colonial corals that were abundant in Middle Ordovician to Late Permian seas.
Solitary rugosans (e.g., '' Caninia'', '' Lophophyllidium'', '' Neoz ...
s.
Phylogeny
Hexacorallia is considered to be
monophyletic
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...
, 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, Antipatharia was considered to be in a separate class called Ceriantipatharia, though more recent genetic studies place it in Hexacorallia.
The subclass includes important coral reef builders such as the stony corals, sea anemones, and zoanthids. The recognized orders are shown below:
*
Actiniaria - sea anemones
*
Antipatharia
Antipatharians, also known as black corals or thorn corals, are an order of soft deep-water corals. These corals can be recognized by their jet-black or dark brown chitin skeletons, surrounded by the polyps (part of coral that is alive). Antip ...
- black corals
*
Corallimorpharia - corallimorpharians aka "false corals"
* †
Rugosa
The rugosa, also called the tetracorallia or horn coral, are an extinct order of solitary and colonial corals that were abundant in Middle Ordovician to Late Permian seas.
Solitary rugosans (e.g., '' Caninia'', '' Lophophyllidium'', '' Neoz ...
- rugose corals
*
Scleractinia
Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mo ...
- stony corals
* †
Tabulata
Tabulata, commonly known as tabulate corals, are an order of extinct forms of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance ...
- tabular corals
*
Zoantharia - zoanthids
References
External links
*
{{Taxonbar, from=Q132674
Anthozoa
Animal classes