The polyp skeleton
The skeleton of an individual polyp, called the corallite, is a tube that contains vertical plates radiating from the centre. The tube itself is the corallite wall and the plates are the septo-costae. The tubes are joined together by horizontal plates and other structures, collectively called the coenosteum. Some polyps have an additional thin film of skeleton around the wall called the epitheca.
The wall is formed by five skeletal elements which vary in proportion in different coral families and/or genera. These elements are (a) septo-costae (which become thickened within the wall), (b) coenosteum (which forms a sponge-like structure), (c) synapticulae (which are horizontal rods forming a lattice between the septo-costae), (d) sterome (which form a non-porous layer within the wall) and (e) epitheca (which forms a thin non-porous layer on the outside of the wall). The wall is very prominent in some corals, but is inconspicuous in others where individual polyps are indistinct.
The septo-costae are the radial elements of the corallite and are divided (by the wall) into two components: the septa, which are inside the wall and the costae, which are outside the wall. Where the wall is indistinct (as in the Siderastreidae, Agariciidae and colonial fungiids) the septo-costae are single uniform elements. In solitary fungiids the wall is horizontally compressed, with the septa above it and the costae below it. In most corals, the septa are of different lengths and have a cyclical symmetry. They may be in cycles (usually with 6 septa in the 1st cycle, 6 in the 2nd cycle, 12 in the 3rd, 24 in the 4th and so-on if present) or orders (where there is an indeterminate number of septa of each length). In practice, this cyclical arrangement is often unclear. In many corals, but especially in Dendrophylliidae, the cyclical arrangement of septa is embellished into a pattern of fusion called pourtàles plan, where septa or the 4th cycle curve in front of those of the 3rd cycle and fuse. This appears to be a primitive characteristic of the Scleractinia as it sporadically occurs in several families and can also be seen in the earliest fossils. The genus Porites has a unique septal plan which is used extensively in taxonomy.
Septa seldom join at the centre of the corallite (except in the Astrocoeniidae and Pocilloporidae). Instead, their inner margins usually have fine inward-projecting teeth which, in most corals, become intertwined forming a tangle called the columella. In some families, especially the Astrocoeniidae and Pocilloporidae, the columella is pillar- or dome-shaped. In others, especially the Acroporidae, it is usually absent. Many corals have pillar-like projections on the inner margin of some or all of their septa called paliform lobes and these often form a neat circle around the columella called a paliform crown. Some groups of corals have pali instead of paliform lobes. These are the result of the pourtàles plan pattern of septal fusion although the pattern may not be visible in mature corallites.
There are four other parts of the skeleton which are used in general descriptions of corals (apart from being components of corallite walls as noted above): coenosteum, sterome, dissepiments and epitheca. The coenosteum is a general term for porous (not solid) skeletal material situated between the costae of corallites or between one corallite and the next. This is best seen in the Dendrophylliidae where the corallite wall and the skeleton between the corallites consist of a sponge-like matrix of coenosteum. The sterome is a solid sheet which forms the inner lining of (or all of) the corallite wall. This is best seen in families Euphyllidae, Oculinidae and Meandrinidae and gives the skeleton a porcelain-like finish. The dissepiments are thin, blister-like layers of skeleton which form between the corallites and are structurally similar to the sterome. The epitheca is a delicate translucent skeletal layer. It initially occurs as the basal plate deposited by the planula larva on settlement, and thereafter may continue growing to envelop individual corallites. The epitheca is always a distinct skeletal entity which is not covered by living tissue; in some faviids its growth is controlled by tiny polychaete worms to form ‘groove-and-tubercle’ structures.
Some skeletal structures are found only in some corals. Montipora and Porites in particular, have additional skeletal structures which are useful in identification and are explained in the introduction to these genera. Monticules (illustrated) are primarily found in Hydnophora, but may occur in other genera. Ambulacral grooves are seen in a scattering of unrelated species.