Hexacorallia

Hexacorallia is a class of Anthozoa.

Other meanings

Both names, i. e. Hexacorallia and Zoantharia, and their English equivalents, also have narrower meanings - see below in the chapter Taxonomy. The name Hexacorallia also has a rare meaning unrelated to the content of this article - it is used as a synonym of the name Ceriantipatharia.

Characteristics

Hexacorals are aquatic organisms formed of polyps, generally with 6-fold symmetry. The number of hexacoral species is approximately 4,300. Extant hexacorals are composed of 6 orders: the tube anemones, zoanthids, sea anemones , corallimorpharians, the black corals and the stony corals. The first four orders are skeleton-less, while the last two orders have a skeleton. Some of the Scleractinia are coral-reef builders.
The Hexacorallia are distinguished from another class of Anthozoa, Octocorallia, in having eight 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 skeleton. As with all Cnidarians, these organisms have a complex life cycle including a motile planktonic phase and a later characteristic sessile phase. Hexacorallia also includes the significant extinct order of rugose corals.
The taxon Hexacorallia is considered to be monophyletic, that is, all contained species are descended from a common ancestor.

Sea anemones

Certain or all skeleton-less hexacorals are called sea anemones in the literature. More specifically, sea anemones can mean one of the following:

Actiniaria only, or
Actiniaria and Corallimorpharia, or
Actiniaria and Zoanthiniaria, or
Actiniaria, Corallimorpharia and Zoanthiniaria, or
Actiniaria, Corallimorpharia, Zoanthiniaria and Ceriantharia.
Taxonomy

Phylogenetic classification

The current system is shown below:
class Hexacorallia :

order Ceriantharia - tube anemones
unnamed clade :
* order Zoanthiniaria – zoanthids
* ?†Coralomorpha - coralomorphs
* order †Tabulaconida
* order †Cothoniida
* order †Tabulata – tabular corals
* order †Rugosa – rugose corals
Hexactiniaria:
* order Actiniaria – sea anemones
* order Corallimorpharia – corallimorpharians
* order †Kilbuchophyllida
* order †Numidiaphyllida
* order Scleractinia – stony corals
incertae sedis:
* order Antipatharia – antipatharians
* family Relicanthidae

The order †Heterocorallia, until recently considered related to or included in Rugosa, is now included in octocorals.
Ceriantharia and Antipatharia were formerly jointly classified in a taxon called Ceriantipatharia, which was considered a separate class outside Hexacorallia.

Skeleton-based classification

Based on the type of their skeleton, the hexacorals are divided into sea anemones, antipatharians and corals. More specifically:

The Ceriantharia, Zoanthiniaria, Actiniaria, Corallimorpharia and Relicanthidae have no skeleton, i. e. they are "soft", and are therefore called sea anemones. Note that the definition of the term sea anemone can vary.
The Antipatharia have a horn-like - and therefore flexible - skeleton.
All the remaining taxa of the above system have a calcium carbonate - and therefore rigid - skeleton, which, in this class, is always compact. They therefore constitute the main group of what is usually called corals. Note that the definition of the term coral can vary, for example octocorals without a compact skeleton and Antipatharia are sometimes also included in the term corals.

In the 19th century, this skeleton-based division into three parts was also the standard formal way of subdividing the taxon hexacorals, i. e. the Hexacorallia consisted of the taxa :

Actiniaria sensu lato , i.e the sea anemones in the broadest sense;
Antipatharia , i. e. the same as today's antipatharians;
Madreporaria sensu lato , i. e. all the rest, meaning all Hexacorallian corals.