Classification

The term 'coral' is commonly used for both 'soft' and 'hard' corals and sometimes includes other colonial animals in the phylum Cnidaria (also called Coelenterata). With a sometimes striking resemblance to plants and fungi, corals were initially mistaken for the former. Today the term 'coral' when used without a qualifying term, most commonly refers to hard or skeletonised corals and it is used this way here. The taxonomic relationship between these taxa is indicated in the taxonomic classification scheme below. This classification does not include many extinct orders of skeletonised Cnidaria with uncertain affinities (also see 'Evolution').

Of the orders listed above, the three orders of Hexacorallia (shown in bold) are or were, dominant faunas of coral reefs as well as non-reef habitats. Scleractinia occur in the Mesozoic and Coenozoic; Rugosa and Tabulata occurred in the Palaeozoic (see also Reefs and Evolution). The four octocoral and hydrozoan orders (shown in bold) are all primarily or entirely Coenozoic.

Of the four extant orders, all except the Stylasterina are partly or wholly zooxanthellate (corals which have symbiotic blue-green algae called zooxanthellae in their tissues, see below) and are therefore restricted to sun-lit depths and warm water. Whether or not Rugosa and Tabulata were also zooxanthellate has long been debated without definite outcome.

Palaeozoic corals

Many Cambrian fossils have at times been called 'corals'. The most coral-like of these are small, cup-shaped, mostly solitary organisms with septa. Some have an operculum over the calice opening. In total, seven orders of Palaeozoic corals may be recognised, of which the Tabulata and Rugosa are by far the most important. These two groups have left good fossil records as their skeletons are of the calcitic form of calcium carbonate (limestone) and thus more stable than the aragonite skeletons of Scleractinia. This fossil record has enabled the study and comparison of their morphological structure, diversity and long evolutionary history.

Order Tabulata (Early Ordovician – Permian)

The Tabulata were much less variable than rugose or scleractinian corals. They were all colonial and consisted of slender tube-like corallites 1-3 mm diameter, crossed internally by transverse partitions, the tabulae. Colonies were typically encrusting, flat or massive, but may have also been branching. Individual corallites were in contact, sharing walls, or widely separated. Each corallite had a theca (enveloping sheath) and groups of corallites were enclosed in a sheath-like epitheca. The corallites of some species were connected by fine tubules forming a three-dimensional structure. Where corallites were in close contact, tabulae were usually formed. Where corallites were separated, external horizontal plates (dissepiments) also occurred. Radially-arranged spine-like septa, sometimes forming vertical structures, were present in some species.

Order Rugosa (Mid Ordovician – Permian)

Rugosa had a much greater resemblance to Scleractinia than to the Tabulata. Mature colonies had an array of growth forms comparable to Scleractinia although the majority of taxa were solitary or formed colonies where individual corallites were large and dominant. Rugosa had basic structural components at least superficially similar to Scleractinia, the main difference being in the arrangement of the septa. Tabulae analogous to those of Tabulata were usually very abundant and so was a complex array of dissepiments which appear analogous to those of Scleractinia.

The earliest Rugosa were solitary and of simple construction. Complex colonies had evolved by Late Ordovician and thereafter the few evolutionary trends that are seen are mostly in skeletal detail and most have reversed one or more times. Differences in the pattern of insertion of septa make it unlikely that the Scleractinia arose from the Rugosa although this view is still disputed.

Mesozoic and Coenozoic corals

Order Scleractinia (Middle Triassic to present)

The Scleractinia first gained prominence some 220 mya, well after the end-Permian mass extinction event, and hence like the Tabulata and Rugosa before them, have a long history (see 'Evolution'). Extant Scleractinia have similar numbers of zooxanthellate and azooxanthellate genera and species. These two groups are not taxonomically separate although they are almost all sharply eco-physiologically distinct. Zooxanthellate Scleractinia (sometimes called 'reef-building corals') are mostly colonial, live in shallow sunlit waters and are responsible for the construction of most Mesozoic to present coral reefs. Azooxanthellate Scleractinia are mostly solitary and usually occupy deep water, although there are some deep water colonial species that can form large bioherms tens to hundreds of metres across such as Desmophyllum pertusum (previously Lophelia pertusa) among others, and several common shallow water exceptions. One of the latter is the widespread colonial Indo-Pacific genus Tubastrea - a recent unintentional introduction to the Atlantic, now becoming a widespread 'invasive species' on oil rigs there.

Reef-building corals, of which more than 830 species are detailed on this website, exhibit a diverse range of growth forms and life history strategies. Some species, like many of their deep-water counterparts, are solitary. Others form small to large colonies, the largest of which are many metres across and many hundreds of years in age.

Colonies may simply grow as an encrusting sheet across the underlying substrate, or may develop into a remarkable array of intricate shapes - plates, tables, vases, columns, 'brains', boulders and foliaceous beds to name a few. The range of shapes is both genetically and environmentally determined, the degree of variation differing among species. As with the Tabulata and Rugosa, the fine-scale structure and skeletal elements of the individual corallites that build the colony are important in identification and classification, and are described in detail elsewhere (see, for example, 'Coral structure and growth').

Order Milleporina

Order Stylasterina

The second group of hydrozoans are mostly found only in deep water. All are azooxanthellate. Two genera, Distichopora and Stylaster are commonly found in caves and under overhangs in shallow reef environments.

Distichopora are ornate corals that branch in one plane. They have no cyclosystem; instead, gastropores are aligned along the edge of branches and these are flanked on each side by a row of dactylopores. Reproductive ampullae are clustered towards the ends of side branches.

Stylaster also branches in one plane but branches are fine, tapered and delicate. Gastropores are linked by individual cyclosystems and surrounded by dactylopores. These alternate on the sides of branches, giving the latter a zig-zag pattern. Wart-like reproductive ampullae occur on the sides of older branches. Almost all records are Coenozoic.

Order Helioporacea

Order Alcyonacea