Big Bank Shoals of the Timor Sea
An environmental resource atlas

FILTER-FEEDING ECOSYSTEMS

Biology and ecology of gorgonians

Sea fans and sea whips are part of the group called gorgonians and are classified as Gorgonacea, an order of the subclass Octocorallia (=Alcyonaria). They are close relatives of soft corals (Order Alcyonacea) on the evolutionary tree (see Figure 23. Relationships within the Class Anthozoa, p44). Soft corals are the most conspicuous and abundant octocorals of the Indo-Pacific region. In contrast to the Caribbean where the Gorgonacea dominate with about 1,200 species of gorgonian known.

General biology

Gorgonian sea fan (L. De Vantier).

The general biology, polyp morphology and defensive chemistry of gorgonians is similar to that of soft corals, p48. The growth form of gorgonian colonies is, however, quite distinctive. The colony is supported by an axial skeleton from which side branches arise. These contain the polyps.

The skeleton may be made of sclerites, like other octocorals, but is frequently a combination of sclerite pieces and a horny but flexible core of protein called gorgonin. The tendons of vertebrates are composed of a similar protein called collagen but the axes of gorgonians have been shown to have twice the tensile strength of tendons (Jeyasuria and Lewis, 1987). In sea whips the branches are minute and the colony has the appearance of a single rod, whereas sea fans are highly branched and often interconnected, forming colonies that are bushy or grow in a single plane to form a fan shape (Figure 33).

Figure 33. Examples of the order Gorgonacea. a: gorgonian fan (Melithaeidae), b: sea whip (Juncella sp), c: Plexaura sp, d: sea fan (Isis sp) (K. Fabricius).

Feeding

Gorgonians inhabit a wide variety of depths, but the majority are found in deep water. A few shallow water species possess symbiotic zooxanthellae and benefit from their photosynthetic production of energetic compounds, in the same manner as soft and hard corals. However, the most important mode of feeding amongst the gorgonians is through filtering plankton from the surrounding water.

Gorgonians are commonly found in benthic habitats that are subject to persistent currents. To sieve enough food in a given time, filter-feeding animals require a large volume of water to pass through their tentacles. Gorgonian colonies, particularly the sea fan types, thrive in positions that are swept by relatively vigorous currents.

Colonies may vary their growth form and orientation depending upon the type and direction of local currents. When currents are turbulent and come from variable directions, the growth form is typically bushy. A region that is subject to strong and highly uni-directional currents will tend to support colonies with concave fans. The fans are positioned so that the ‘dish’ faces towards the oncoming current.

Gorgonian polyps feeding. (K. Fabricius)

The flat fan, characteristic of the majority of gorgonians, is often found where the currents are uni- or bi-directional. Different morphologies have developed to maximise the amount of plankton entering the sieves of the polyps. This is achieved by reducing the amount in which one part of the colony ‘shades’ another part of the colony from the current.

Gorgonians must avoid being damaged by currents but, at the same time, must remain erect in order to feed effectively. These two requirements have produced a compromise between rigidity and flexibility, with colonies typically having the elasticity of stiff rubber. When currents threaten to damage a colony, the colonies have the ability to bend and then spring back into their original position.

Reproduction

Asexual propagation is frequently the dominant reproductive mode. This serves to produce large populations from a single, founding colony as a result of the settling and growing of sexually produced larvae. Sexual reproduction follows the pattern characteristic of all anthozoans, with both internal and external larval development having been recorded (for sexual reproduction of hard coral and soft coral, page 46). The fertilised egg develops into a planula larvae which settles and differentiates to produce a polyp. This then multiplies into a colony. Subsequent asexual reproduction can be a very successful means of rapidly establishing numerous colonies.

At a site in the Caribbean, 94 percent of gorgonian colonies were found to be the result of asexual reproduction from fragments. These had broken from existing colonies and reattached and grown (Lasker, 1984). It was concluded that constrictions of the branches from the colonies had developed and increased the likelihood of breaking and fragmentation. The sea whip Junceella fragilis takes fragment formation one step further. The polyps and coenchyme tissue are reabsorbed at a point near the tip of the colony. Eventually, the gorgonin core breaks and a fragment falls to the bottom. These daughter colonies reattach and grow (Walker and Bull, 1983).

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Last updated - 1 September 98

Copyright ©1996-1998 Australian Institute of Marine Science

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