The word ‘coral’ is not easily explained. Usually a solid calcareous or horn-like skeleton comes to mind, but then the animals which are called ‘soft corals’ are an exception.
All corals belong to the Phylum Cnidaria, which together with the small Phylum Ctenophora ( ‘comb jellies’) comprise the Coelenterata. The Phylum Cnidaria is characterised by the presence of ‘cnidae’, which are ‘nematocysts’ or stinging cells on the tentacles surrounding the mouth and in many Cnidaria also on other body parts. The Cnidaria are divided into the Class Hydrozoa, the Class Anthozoa, and the Class Scyphozoa. The Scyphozoa are known as free-swimming jelly fish; they do not have a coral skeleton.
Among the Hydrozoa, only two families, which are not directly related to each other, consist of species that form coral skeletons, the Milleporidae ( ‘fire corals’) and Stylasteridae ( ‘lace corals’). Together they are informally called ‘hydrocorals’, a name without phylogenetic significance. Their surface is covered by large ‘gastropores’, which contain single, short feeding tentacles ( ‘gastrozooids’), and smaller ‘dactylopores’, which contain single, hair-like stinging tentacles ( ‘dactylozooids’), respectively. Lace corals or stylasterids may show bright colours; except white, they may be yellow, orange, pink, red, purple or violet. In contrast, the colour of fire corals usually varies between brown and ochre, since the thin outer layer of live tissue covering the skeleton contains zooxanthellae.
The Class Anthozoa is divided into two Subclasses, the Hexacorallia (synonym Zoantharia) and the Octocorallia (synonym Alcyonaria). Among the Hexacorallia, there are four Orders without a skeleton, i.e., the Actiniaria, the Zoanthidea, the Corallimorpharia, and the Ceriantharia), whereas there are two orders with a skeleton, i.e., the Scleractinia (synonym Madreporaria) and the Antipatharia.
Scleractinia, which are popularly known as ‘stony corals’, ‘hard corals’, or ‘true corals’, have a more or less solid calcareous skeleton and polyps that posses multiple sets of six tentacles. The soft tissue of each polyp is supported underneath by a ‘calice’ or ‘corallite’, a part of the skeleton in which a central opening with a spongy columella is surrounded by radiating septa.
Antipatharia are well known as ‘black corals’. Each one has a horny whip-like or branched skeleton, covered by small spines. The 6-tentacled polyps are housed in cup-like protuberances spread over the coral branches.
Octocorallia posses a calcareous skeleton that consists of numerous small particles, which are called ‘sclerites’. This skeleton does not show clearly if the sclerites are scarce and small. If the skeleton of a soft coral consists of sclerites that are large and dense, even soft corals appear to be rigid. A horny skeleton may also be present, as a central axis inside a branch, usually surrounded by ‘sclerites’. Species of Octocorallia posses polyps with eight tentacles. In the octocoral Order Helioporacea (synonym Coenothecalia) the skeleton consists of fused sclerites. Although, the solid skeleton is strikingly blue, in nature specimens of these blue corals can easily be confused with fire corals (Millepora spp.), since it appears brown due to the presence of zooxanthellae in the thin outer layer of soft tissue ( ‘coenenchyme’). In the Order Alcyonacea, the sclerites are inbedded in a fleshy body ( ‘coenenchyme’), whereas in the Order Gorgonacea, there is also a central horn-like axis within the branches. This axis is not clear in all species that belong to genera traditionally classified as gorgonians. Therefore the distinction is artificial and not maintained in modern literature anymore. The previous Order Stolonifera which includes the famous red organpipe coral and several encrusting species is also not recognized as a separate taxon anymore. Together with the Gorgonacea, the Stolonifera have been united with the Alcyonacea. The Order Pennatulacea is still distinct. These corals live semi-burried in soft sediment and the polyps in many species are fused in rows.
Table 1. The present classification of living Coelenterata with the position of taxa that contain only animals with coral skeletons.