In most corals, the over-all appearance of a colony is not a direct outcome of the way its corallites multiply. However, in the Family Faviidae, the type of budding may determine the type of colony that results. In this family, the terms used to describe both budding (the formation of corallites) and growth form are usually the same. (For example, the term ‘meandroid’ may be used to describe both the type of budding and the type of colony.)
In most corals, there is a clear distinction between what is an individual and what is a colony. This is not always so, as seen in the Family Fungiidae, where there is a continual gradation between solitary individuals (with a single mouth) and colonies (with many mouths), as exemplified by the sequence Cycloseris ? Fungia ? Ctenactis ? Herpolitha ? Polyphyllia. In this sequence, Cycloseris and (usually) Fungia exist only as solitary individuals with a single mouth while Polyphyllia forms colonies with many mouths. A single specimen of Ctenactis or Herpolitha could be considered a solitary individual with many mouths or a colony of individuals, each with a single mouth. Likewise, in some corals there may not be a clear distinction between what is an individual and what is a row of individuals. This is best seen in Families Faviidae and Mussidae, where there is a continual gradation between colonies composed of distinct polyps (corallites) to colonies where individuals are recognisable only by the existence of mouths and/or columella centres, to colonies where there is no sign of individuality.
All corals that form colonies do so by a process of budding, where the parent polyp divides itself into two or more daughter polyps (intratentacular budding), or daughter corallites form on the side of the parent colony (extratentacular budding), or polyps lose their identity, as seen in colonies with valleys. Some colonies have both intra- and extra-tentacular buds.
If the corallites of a colony all have their own walls they are called plocoid or phaceloid, depending on how elongate they are. If they share common walls they are called meandroid or cerioid, depending on whether or not they form valleys. If they are meandroid and have their own walls they are termed flabello-meandroid.
These growth forms confer several constraints on corallite replication and growth. Plocoid and phaceloid colonies can have both intratentacular and extratentacular budding, while cerioid colonies can only have intratentacular budding. Plocoid, cerioid and meandroid colonies have integrated corallites or valleys, while adjacent corallites or valleys of phaceloid and flabello-meandroid colonies may have little or no connecting tissue. The latter may compete for space and other resources, with the result that some parts of colonies overgrow other parts.
Some colonies combine two growth forms. Euphyllia and Lobophyllia colonies may be phaceloid toward the colony centre where lack of space constrains valley formation and be flabello-meandroid at the periphery where there are no such constraints. Similarly, Symphyllia colonies may have both meandroid and flabello-meandroid areas; Favia colonies may have both plocoid and meandroid areas; Favites and Goniastrea colonies may be both plocoid and cerioid. There are also many intermediate forms between plocoid and fully phaceloid and (very commonly) between cerioid and fully meandroid colonies.
A variety of other types of colony formation are found in corals, but these are uncommon.
The most common terms used to describe growth form are ordinary descriptive words. Massive means solid and similar in shape in all dimensions. Encrusting means adhering to the substrate. Branching means forming branches. Arborescent means tree-like. Columnar means forming columns. Laminar means plate-like. Explanate means forming solid sheets. Other terms are used with particular groups of corals; all are explained in the glossary. However, there are so many different shapes of corals that such descriptive terms can be misleading and carry less meaning than illustrations.
A common modification of all descriptive terms is the addition of the prefix sub to the term (e.g. submassive, subcerioid, sub-equal), meaning ‘less than’ or ‘not quite’.
The much-studied coral Porites forms large hemispherical colonies which typically grow (radially) at a rate of around 1 cm per year as determined by x-rays of thin slices. Some more heavily calcified colonies of other corals grow at slower rates than this, although most are faster. Staghorn Acropora readily grows (linearly) up to about 30 cm per year. Plate-forming Acropora also grows (in diameter) up to about 30 cm per year.