Anthozoa

Anthozoa is one of the three subphyla of Cnidaria, along with Medusozoa and Endocnidozoa. It includes sessile marine invertebrates and invertebrates of brackish water, such as sea anemones, stony corals, soft corals and sea pens. Almost all adult anthozoans are attached to the seabed, while their larvae can disperse as plankton. The basic unit of the adult is the polyp, an individual animal consisting of a cylindrical column topped by a disc with a central mouth surrounded by tentacles. Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual. Colonies of stony corals are strengthened by mainly aragonite and other materials, and can take various massive, plate-like, bushy or leafy forms.
Members of Anthozoa possess cnidocytes, a feature shared among other cnidarians such as the jellyfish, box jellies and parasitic Myxozoa and Polypodiozoa. The two classes of Anthozoa are class Hexacorallia, with members that have six-fold symmetry such as stony corals, sea anemones, tube anemones and zoanthids, and class Octocorallia, with members that have eight-fold symmetry, such as soft corals, gorgonians, and sea pansies. Some additional species are also included as incertae sedis until their exact taxonomic position can be ascertained.
Anthozoans are carnivores, catching prey with their tentacles. Many species supplement their energy needs by making use of photosynthetic single-celled algae that live within their tissues. These species live in shallow water and many of them are hermatypic. Other species lack the zooxanthellae and, having no need for well-lit areas, typically live in deep-water locations.
Unlike other members of phylum Cnidaria, anthozoans do not have a medusa stage in their development. Instead, they release sperm and egg cells into the water. After fertilisation, the planula larvae form part of the plankton. When fully developed, the larvae settle on the seabed and attach to the substrate, undergoing metamorphosis into polyps. Some anthozoans can also reproduce asexually through budding or by breaking in pieces.

Diversity

The name "Anthozoa" comes from the Greek words άνθος and ζώα, hence ανθόζωα = "flower animals", a reference to the floral appearance of their perennial polyp stage.
Anthozoans live exclusively in marine and brackish water. They include sea anemones, stony corals, soft corals, sea pens, sea fans and sea pansies. Anthozoa is the largest taxon of cnidarians; over six thousand solitary and colonial species have been described. They range in size from small individuals less than half a centimetre across to large colonies a metre or more in diameter. They include species with a wide range of colours and forms that build and enhance reef systems. Although reefs and shallow water environments exhibit a great array of species, there are in fact more species of coral living in deep water than in shallow, and many taxa have shifted during their evolutionary history from shallow to deep water and vice versa.

Phylogeny

In 1995, genetic studies by J. E. N. Veron and a group of scientists that analyzed the ribosomal DNA of many species, found that the order Ceriantipatharia, now deprecated and replaced by its constituent orders Ceriantharia and Antipatharia,
was "the most representative of the ancestral Anthozoa".
After Ceriantipatharia became deprecated, newer phylogenetic studies determined that Ceriantharia is not the basal clade of Anthozoa, but of Hexacorallia, and that Antipatharia and the remaining orders of Hexacorallia are its descendants.
Anthozoa is a monophyletic clade within Cnidaria. A comprehensive phylogenetic study from 2021, has indicated that its two extant subclades, Hexacorallia and Octocorallia, are also monophyletic sisters.
The same study has inferred the independent appearance of key characters in different lineages of subphylum Anthozoa. The reconstructed ancestor of Anthozoa is supposed to have been a solitary polyp with bilateral symmetry, lacking a skeleton and photosymbiosis, already present in the Tonian period when the two classes of Anthozoa, Hexacorallia and Octocorallia, have probably diverged.
Octocorallia gained colonial growth and calcified elements early on. The acquisition of photosymbiosis occurred many times independently in its different subclades.
In the case of Hexacorallia, its early diverging order, Ceriantharia, retained the ancestral characteristics of being solitary, and lacking a skeleton and photosymbiosis. Zoantharia was the first order to gain colonial growth. Antipatharia and Scleractinia gained colonial growth independently from other orders. From the Devonian on, all subclades of Hexacorallia acquired photosymbiosis independently, with the exception of Ceriantharia and Relicanthus that lack it completely. A similar independent origin for radial symmetry happened to orders Actiniaria, Relicanthus, Scleractinia and Corallimorpharia.

Cladogram of Anthozoa according to molecular phylogenetics

studies have determined that the most likely phylogeny of the extant clades of Anthozoa can be represented by the following cladogram:

Taxonomy

The taxonomy of the subphylum Anthozoa is being continuously updated and modified, based on molecular biology, molecular phylogenetics, and other methods. Its most recent version can be found in the World Register of Marine Species. It subdivides Anthozoa into two monophyletic, extant classes: Hexacorallia and Octocorallia, generally showing different symmetry of polyp structure, and two extinct, fossil classes: Rugosa † and Tabulata †.
Hexacorallia includes coral reef builders, sea anemones, and zoanthids.
Octocorallia comprises the sea pens, soft corals, and blue coral. Sea whips and sea fans, known as gorgonians, are part of Alcyonacea and historically were divided into separate orders.

Classification according to the World Register of Marine Species (WoRMS)

†= extinct

phylum Cnidaria
* subphylum Anthozoa
** class Hexacorallia
*** order Actiniaria
**** suborder Anenthemonae
**** suborder Enthemonae
**** suborder Helenmonae
*** order Antipatharia
*** order Ceriantharia
*** order Corallimorpharia
*** order Hexanthiniaria †
*** order Kilbuchophyllida †
*** order Scleractinia
*** order Zoantharia
** class Octocorallia
*** order Malacalcyonacea
*** order Scleralcyonacea
** class Rugosa †
*** order Heterocorallia †
*** order Pterocorallia †
*** order Stauriida †
*** order Stereolasmatina †
** class Tabulata †
*** order Auloporida †
*** order Favositida †
*** order Halysitida †
*** order Heliolitida †
*** order Lichenariida †
*** order Sarcinulida †
*** order Syringoporida †
*** order Tetradiida
Examples of some major anthozoan taxa

Anatomy

The basic body form of an anthozoan is the polyp. This consists of a tubular column topped by a flattened area, the oral disc, with a central mouth; a whorl of tentacles surrounds the mouth. In solitary individuals, the base of the polyp is the foot or pedal disc, which adheres to the substrate, while in colonial polyps, the base links to other polyps in the colony.
The mouth leads into a tubular pharynx which descends for some distance into the body before opening into the coelenteron, otherwise known as the gastrovascular cavity, that occupies the interior of the body. Internal tensions pull the mouth into a slit-shape, and the ends of the slit lead into two grooves in the pharynx wall called siphonoglyphs. The coelenteron is subdivided by a number of vertical partitions, known as mesenteries or septa. Some of these extend from the body wall as far as the pharynx and are known as "complete septa" while others do not extend so far and are "incomplete". The septa also attach to the oral and pedal discs.
The body wall consists of an epidermal layer, a jellylike mesogloea layer and an inner gastrodermis; the septa are infoldings of the body wall and consist of a layer of mesogloea sandwiched between two layers of gastrodermis. In some taxa, sphincter muscles in the mesogloea close over the oral disc and act to keep the polyp fully retracted. The tentacles contain extensions of the coelenteron and have sheets of longitudinal muscles in their walls. The oral disc has radial muscles in the epidermis, but most of the muscles in the column are gastrodermal, and include strong retractor muscles beside the septa. The number and arrangement of the septa, as well as the arrangement of these retractor muscles, are important in anthozoan classification.
The tentacles are armed with nematocysts, venom-containing cells which can be fired harpoon-fashion to snare and subdue prey. These need to be replaced after firing, a process that takes about forty-eight hours. Some sea anemones have a circle of acrorhagi outside the tentacles; these long projections are armed with nematocysts and act as weapons. Another form of weapon is the similarly armed acontia which can be extruded through apertures in the column wall. Some stony corals employ nematocyst-laden "sweeper tentacles" as a defence against the intrusion of other individuals.
Many anthozoans are colonial and consist of multiple polyps with a common origin joined by living material. The simplest arrangement is where a stolon runs along the substrate in a two dimensional lattice with polyps budding off at intervals. Alternatively, polyps may bud off from a sheet of living tissue, the coenosarc, which joins the polyps and anchors the colony to the substrate. The coenosarc may consist of a thin membrane from which the polyps project, as in most stony corals, or a thick fleshy mass in which the polyps are immersed apart from their oral discs, as in the soft corals.
The skeleton of a stony coral in the order Scleractinia is secreted by the epidermis of the lower part of the polyp; this forms a corallite, a cup-shaped hollow made of calcium carbonate, in which the polyp sits. In colonial corals, following growth of the polyp by budding, new corallites are formed, with the surface of the skeleton being covered by a layer of coenosarc. These colonies adopt a range of massive, branching, leaf-like and encrusting forms. Soft corals in the subclass Octocorallia are also colonial and have a skeleton formed of mesogloeal tissue, often reinforced with calcareous spicules or horny material, and some have rod-like supports internally. Other anthozoans, such as sea anemones, are naked; these rely on a hydrostatic skeleton for support. Some of these species have a sticky epidermis to which sand grains and shell fragments adhere, and zoanthids incorporate these substances into their mesogloea.