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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. 1 1 2 2 3 3 4 4 5 5. The basic wall components of corals. 1 The wall of this Acanthastrea is primarily composed of thickened septo-costae. This is best seen in families Faviidae and Mussidae, also some Caryophylliidae. 2 The wall of this Duncanopsammia is primarily composed of sponge-like coenosteum. This is best seen in families Dendrophylliidae and Poritidae (except Alveopora). 3 The wall of this Pavona is primarily composed of horizontal rods of synapticulae. This is best seen in families Siderastreidae, Agariciidae and Fungiidae. 4 The wall of this Echinophyllia is primarily composed of sterome. This is best seen in the Euphyllidae, Oculinidae, Meandrinidae and Pectiniidae. 5 The wall of this Conotrochus is partly composed of epitheca. This mostly occurs in azooxanthellate corals including the Flabellidae and some Caryophylliidae. Other major families may have two equally dominant wall components: the Pocilloporidae and Acroporidae have walls of mixtures of thickened septo-costae and coenosteum; most Caryophylliidae have walls of mixtures of thickened septo-costae and epitheca. Diagrammatic representation of the basic skeletal elements of corals. Diagrammatic representation of the basic skeletal elements of corals.

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. (a) Normal cycles of septa, (b) pourtàles plan. Numbers indicate cycles. (a) Normal cycles of septa, (b) pourtàles plan. Numbers indicate cycles.

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. 1 1 2 2. The appearance of the columella and paliform lobes. 1 A Scolymia showing the typical appearance of a columella composed of a tangle of spines from the inner margins of septa. 2 A Goniastrea with paliform lobes forming a neat crown.

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. 1 1 2 2 3 3 4 4 5 5. 1 Corallite walls and the skeleton between the corallites of this Turbinaria skeleton consists of a sponge-like matrix, the coenosteum. 2 The smooth skeleton between the septa of this Catalaphyllia skeleton is the sterome. 3 The fine blisters of skeletal material between the corallites of this Galaxea are the dissepiments. 4 The skeletal layer covering the outside of this Trachyphyllia is the epitheca. Drawing: Geoff Kelley 5 Fine skeletal structures between the corallites of this Montastrea are called ‘groove-and-tubercle’ structures and are composed of epitheca.

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. left left right right. The formation of monticules. Hydnophora colonies with sections of wall of variable length (left) intergrade with other colonies with walls as short as they are wide (right). The star-like structures that result are the monticules. The ribs down the sides of the monticules are costae. The grooves running along the tops of the walls are ambulacral grooves. They represent a tendency toward flabello-meandroid structure in meandroid colonies. The grooves running along the tops of the walls are ambulacral grooves. They represent a tendency toward flabello-meandroid structure in meandroid colonies.

J.E.N. Veron