Wednesday, September 20, 2017

Corallite shape

Corallites may take on a wide range of shapes and sizes.  Certain terms will become familiar when learning about corals, and some key ones are discussed below.  It is, however, important to be aware that these classifications are somewhat arbitrary, so please consider the following to be a series of spectra rather than clear cut categories.  Nevertheless familiarity with these terms will be a beneficial tool when identifying corals.

Immersed ↔ Exsert

Corallites that form depressions in the colony surface are described as immersed, whereas those that extend above the colony surface are called exsert.  This is the same term used to describe septa that extend above the corallite, as discussed above.  Whereas some genera may consistently have immersed corallites (e.g. Porites, p. 13 of the Coral Finder), other genera may have species that differ in this characteristic, for example Turbinaria (p. 18 of the Coral Finder) may have either immersed or exsert corallites.  The corallites may also change as they get close to the edges of the colony.

Corallites that are embedded in the skeleton of the colony surface (or coenosteum) are known as immersed corallites - the genus Porites.

When the corallites stand above the colony surface (or coenosteum) they are known as exsert corallites. The genus Turbinaria, shown here, can have both exsert and immersed corallites.

Irregular ↔ Neat

Corallites may be regularly sized producing a neat appearance, or they may grow in an irregular pattern with different sizes, shapes and orientations.  Astreopora provides a classic example of colonies possessing irregular looking corallites (p. 10 of the Coral Finder).  This difference is particularly clear when comparing these photos of Astreopora with those of Diploastrea (p. 11 of the Coral Finder); Diploastrea’s corallites are regularly sized and arranged producing an overall neat appearance to the colony.

Corals having corallites with different sizes, shapes and orientations are referred to as having irregular corallites - the genus Astreopora.

Corallite shape and orientation may vary from irregular (Echinophyllia - left) to neat and regular (Diploastrea - right).

Rounded ↔ Polygonal and Tubular ↔ Conical

The distinctions in corallite shape can be broken down even further.  For example corallites may be rounded or polygonal: Montastrea forms rounded corallites, whereas Leptastrea has more angular corallites, and are therefore described as polygonal (p. 11 and 13 of the Coral Finder, respectively).

Further distinctions can be made regarding corallite shape when discussing exsert corallites.  Rounded corallites may be tubular (e.g. Turbinaria, p.17 of the Coral Finder) or conical (e.g. Astreopora, p. 10 of the Coral Finder).  Similarly polygonal corallites may form cones (e.g. Diploastrea, p. 11 of the Coral Finder).

Corallites may be described as rounded (Montastrea - left) or polygonal (Leptastrea - right).


One last category of corallites that may be separated are those that grow at an angle up from the colony surface.  These corallites are described as inclined and you will read the description ‘inclined towards colony margins’ in a number of genera where the corallites point out towards the edges of the colony (e.g. Mycedium, p. 19 of the Coral Finder).

Sometimes corallites lean over sideways and may be referred to as inclined - the genus Mycedium.