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Phylum Cnidaria > Class Anthozoa > Subclass Zoantharia/Hexacorallia > Order Sclerectinia
Hard corals: some common shapes & textures
Order Scleractinia
updated Nov 11
Identifying coral species is tricky and requires experience and expertise. Many different aspects of the coral are considered in working towards an identity. These include the overall colony shape, the shape and arrangement of the corallite, the structure of the corallite (scroll to bottom of this page for more), especially the small structures (which often require microscopic examination of a dead specimen - it's hard to see these structures in a live coral as the tissue covers them), in some the shape of the polyp too. The geographical location and depth at which the coral was found also helps.

It is very difficult to accurately identify corals to species from just a photograph.

Can we tell the species of corals from their overall shapes? The overall shape of a colony can be affected by the stresses it faces, e.g., waves, damage, disease. Thus, the same species of hard coral may take on different shapes depending on where the colony grows, e.g., shallow water, deeper water, exposed to waves or sheltered waters. There are also geographical variations.

Also, a single colony of hard coral may have a combination of shapes. Some, for example, may form an encrusting layer with some portions of the colony rising up in columns or branches.

Some common shapes of hard coral colonies include the following described below:
Corals can take on a wide variety of shape.
Raffles Lighthouse, Jul 06
Massive: solid skeleton and similar in shape in all directions, i.e., usually shaped like a boulder which may be dome-shaped, spherical or hemi-spherical. Such shapes are usually found in shallow and mid-depth water and are resistant to, but don't depend on, high water flows. Such colonies grow slowly (1-3cm a year) and generally live for a long time.

Not all boulder-shaped colonies are solid. Some are made up of branching corallites. The hollow, branching insides of the colony may be hidden by fleshy tissues or expanded tentacles. These colonies are more fragile than colonies made of solid skeletons.

Encrusting or crustose: a layer stuck to a hard surface, often growing to follow the contours of the surface. Many corals have an encrusting base from which other growth forms emerge, e.g., columns or branches.

Branching (arborsecent) : forming branches with regularly spaced side branches so that the colony looks like a bush or a tree. Such colonies grow in areas exposed to high wave action and surge. They usually grow more quickly.
Some branching colonies form a platform of tightly packed branches that looks like a table.

Columns (Columnar): forming thick unbranched columns usually from a common solid base (i.e., thicker than branching corals, usually with blunt tips as opposed to sharper tips of branching forms). Such shapes are usually found in mid-depth water sheltered from extreme water flow.

Plate-like (Laminar): forming a thin horizontal plate, sometimes in tiers or terraces of plates. Some may be folded into ruffles or have columns or knobs arising from the plates.
Some plate-like colonies can form a vase-like or cone shape. Such shapes are usually found in sheltered, mid-water depths.

Foliage-like (Folicaeous): forming thin plates that are often folded into shapes that resemble leaves or petals of a rose, or a head of lettuce. Such shapes are usually found in areas with turbulent water flow.

The surface patterns on hard corals are a result of the shape of the corallites and the way the corallites grow.

Some common surface patterns on hard coral colonies include:
Plocoid: each corallite is distinct and has its own walls

Phaceloid: each corallite is distinct and has its own walls. Each corallite is long and tubular resulting in corallites that look like trumpets. These long corallites may be arranged to result in a spherical-looking colony, especially when the polyp tentacles are extended and obscure the corallites.

Ceroid: each corallite is distinct but the walls between neighbouring corallites are fused into one.

Meandroid: Corallites form long valleys and there are no distinct polyps walls are shared.

Flabelloid: Corallites from short valleys with separate walls.

Flabello-meandroid: Corallites form long valleys with separate walls.

Other surface patterns on hard corals may result from patterns on the skeleton between the corallites (coenosteum). Other identification details include the shape of polyps and tissues covering the skeleton.

*Species are difficult to positively identify without close examination.
On this website, they are grouped by external features for convenience of display.

Parts of a corallite
Reproduced from Corals of the World Australian Institute of Marine Science,
Veron, JEN and Stafford-Smith, MG (2011)

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 pourtales 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 pourtales 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.


  • Veron, Jen. 2000. Corals of the World Australian Institute of Marine Science, Australia. 3 volumes.
  • Borneman, Eric H. 2001. Aquarium Corals: Selection, Husbandry and Natural History T.F. H Publications. 464 pp
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