Friday, May 18, 2012

Corals as Colonies – Life-forms

Although some coral genera are solitary, i.e. they form separate individuals (e.g. Cycloseris, p. 24 of the Coral Finder), the majority of scleractinian corals form colonies made up of a number of individuals. These individuals reproduce asexually by budding (see asexual reproduction article for more information), so that over time the colony becomes larger and is composed of more and more individuals (e.g. Acropora, p. 1 of the Coral Finder). The colonial nature of corals has wide ranging implications for coral ecology, biology and identification. One of the main consequences is that colonies may grow into a wide range of shapes or morphologies as new individuals are added and grow. These shapes are termed life forms or growth forms, and can be used as the first step in identifying corals, as certain genera and species grow into specific life forms.

Life form describes the overall shape of the coral, and is synonymous with growth form or morphology. It is important to be aware that there are three sources of confusion when discussing a coral’s life form: 1) many different terms exist for similar life forms, 2) a single colony may exhibit more than one life form; in this instance the predominant life form is generally used, 3) these shapes exist on a continuum rather than being clear cut, separate definitions, so a degree of subjectivity is inherent to the topic.

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A selection of hard and soft corals. The majority of corals live attached to the seafloor.

Corals that are not attached to the sea floor are termed free-living - the genus Zoopilus.

A selection of free-living corals on soft sediment. Clockwise from top: Heteropsammia, Heterocyathus, Cycloseris x2 & Heteropsammia.

Some corals are attached as juveniles and become free-living as adults - the genus Fungia.

Corals may live as a solitary polyp or clone to form a colony of many polyps. Solitary corals can be as large, or larger, than colonies. The genus Ctenactis (left) and Moseleya (right). Note: both of the corals illustrated are also free-living.

Free-living corals can be solitary or colonial - the genus Fungia (left) & Halomitra (right).

Branching corals take many forms - fingers, needles, blades and plates of branchlets - to name a few. Clockwise from top left: Acropora, Seriatopora, Pocillopora & Acropora.

Corals with finger-like branches are termed digitate. Corals with thicker "fingers" form columnar growth forms - the genus Acropora (left) and Isopora (right).

The ecological history of coral colonies produces an infinite variety of forms. The terms we use to describe coral growth forms grade into each other. e.g. the genus Favia (below) can be massive, sub-massive, thick encrusting & thin encrusting.

Meandering corallites produce distinctive colony life forms. Corallites with common walls are meandroid while meandering corallites with separate walls are known as flabello-meandroid. The genus Symphyllia (left) and Euphyllia (right).