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Soft corals

Soft corals

By Ron Shimek

Soft corals, corals and sea anemones are categorized together in the Class Anthozoa of the Phylum Cnidaria, and animals in all of these groups have a lot in common. The basic bottom-dwelling body form in all cnidarians is that of a polyp, essentially just hollow cylinder with a fringe of

A colony of an octocoral colony found on one of the ships in Truk Lagoon. The pinnate branches of the tentacles are quite large and evident.

tentacles around the mouth. All soft coral bodies are derived from the standard polyp, although in some of them the basic shape may be very obscure (Kozloff, 1990; Ruppert and Barnes 1994). The differences between the soft corals as one component of the class Anthozoa, and the stony corals and sea anemones as another, are profound and have significant repercussions with regard to the biology of the animals. From the examination of differences in the basic genetic material of these two groups, it appears that they last shared a common ancestor about 400 million years ago - plenty of time for major differences to develop. As a comparison, the common ancestors of both present-day reptiles and mammals lived about half as long ago.

Soft corals, corals and sea anemones lack the jellyfish or medusa stage characteristic of the other cnidarians. Additionally, instead of having a simple sac-like body with the mouth as an opening at the top, their body wall is turned down into the mouth creating a short tube inside the mouth called the pharynx. Soft corals posses a groove down the length of this pharynx at one end of the slit-like mouth. This groove, called a siphonoglyph, is lined with a dense layer of microscopic, hairlike, beating flagella. These flagella continually pump water into the animal's body; this water current inflates the body, circulates fluids within the body, and provides a volume of water to act as an antagonist to the action of the body musculature.

Diagrammatic representation of a soft coral . The colony surface is shown on the left, while the internal structure is shown on the right. The gonads on the septa are shown in red and the siphonoglyph (in the siphonozooid) is orange. Gastric filaments aid in killing and digesting prey, and fluid circulates through the interconnected gut canals which are lined with gastrodermis. Spicules are shown in blue in the yellow mesoglea.

As with the other cnidarians, the soft coral body wall consists of three layers: an exterior layer of protective epithelial tissue called an epidermis, an interior layer of a digestive epithelial tissue called a gastrodermis, and a fibrous layer that lies between these two tissues. This connective fiber layer, the mesoglea or "middle jelly", generally lacks cells and living components and is composed of proteinaceous collagen fibers (the same material that constitutes tendons and ligaments in mammals - gristle!). Other proteins are also found in the mesoglea. Collagen is non-elastic and not extensible and its presence in the body walls of these animals allows them to resist deformation and maintain their shape. Many of the soft corals are naturally found in shallow water environments that have significant wave action and turbulence. The presence of such a rugged fibrous mesoglea has been shown to be a significant factor in the ability of many sea anemones to live in wave swept areas, and it is probably as critical with the soft corals. (Wainwright and Koehl, 1976; Koehl, 1977).

The polyp portion of the soft-coral gut or gastrovascular cavity is divided by septa - walls of tissue that connect the pharynx to the outside body wall. There are 8 septa in all soft corals, and the if the animal were to be cut in cross-section it would look like a wheel with 8 spokes. The septa are lined with the digestive epithelium and filled with a fibrous mesogleal core. The sides of each septum have muscle bands on them which run parallel to the long axis of the polyp. When these muscles contract, the polyp is withdrawn in to the main body of the colony. The polyps get re-inflated by the water pumped into the animal by the flagella of the siphonoglyphs.

Taxonomically, the soft corals belong to the Subclass ALCYONARIA of the Class ANTHOZOA. These animals are also known by the name Octocorals and the entire group consists of soft corals, sea pens, gorgonians, pipe-organ coral, Tubipora , and the blue coral, Heliopora .

A small colony of Anthelia from Truk Lagoon. Notice the pinnate tentacles.

As the name octocoral implies, these animals are characterized by have having only eight tentacles on each polyp.In addition, these tentacles are pinnate, that is branched in a feather-like pattern, with the branches arising from opposite sides of the main tentacle branch. The relative prominence of these pinnate branches varies a lot; in the star polyps, Pachylavularia viridis , the branches are hardly noticeable, while in some of the Anthelia species, they may be longer than the main branch.

These animals generally have a internal skeleton made of fused or individual calcium carbonate spicules. In those animals such as Dendronephthya klunzingeri have few widely distributed spicules clearly visible in the body wall. Dendronephthya is soft, flexible, and bends in currents, presumably to maximize its suspension feeding capabilities.

A branch of the Cauliflower coral, Dendronephthya klunzingeri . The scattered white calcareous spicules are quite evident in the branches

In other species such as the leather corals, Sarcophyton species, the dense arrangement of spicules in the mesoglea makes for a relatively rigid structure. These colonies are often found in areas with swift water currents or turbulent wave action. Their collagen and spicular arrangement facilitates their survival by allowing slow deformation under extreme currents, but sufficient resistance to the regular currents to allow feeding. Depending on the species, other minerals may be incorporated into the calcium carbonate matrix, and proteinaceous material is often utilized as well. The mineral components of the skeleton can color it, but in most soft corals the spicules are colorless or white.

In addition to an endoskeleton, most soft corals have quite a complicated apparatus of internal plumbing which serves to distribute food and nutrients throughout the colony. All of the guts of all of the polyps connect to this conductive system, and so food can get distributed throughout the entire colony. Obviously, as well, wastes will also be dispersed either out of the colony with excess water, or near enough to the tissue surface so that the material will diffuse out.

For more information on soft coral morphology and biology follow this link, but note that they mistakenly use the term "pennate" (an adjective describing diatom shapes) for "pinnate," the adjective describing octocoral tentacle shapes. Intro to soft corals - pennate

Soft corals are considered to be colonial animals, where the main body of the animal is often derived from a single polyp and with other polyps differentiated to do other functions. Generally the first polyp that forms after the metamorphosis of the larva forms the basic polyp, and other polypoid individuals are budded off of it during growth. The shapes of these polyps can change both with growth and aging of the colony.

Some of the soft corals and sea pens are the most polymorphic of the anthozoans. Typically the leather corals, in the Order Alcyonacea, have several types of polyps. There are tentaculate, feeding individuals (gastrozooids) arranged on the upper surface. These polyps are retractile, and when fully withdrawn, the surface of the colony appears smooth. When the polyps are extended, they may be seen to have eight small tentacles, although in some species the tentacles are represented only by eight small bumps. Often over the colony upper surface are small zooids which lack tentacles. These zooids (siphonozooids) are basically just a mouth with a siphonoglyph in it and function to inflate the colony by pumping water into the common gastrovascular cavity. When the feeding individuals are extended, the siphonozooids appear as small dots between them.

Most tropical soft corals are actively feeding animals(Leversee, 1976; Lasker, 1981; Lasker, et al. 1983; Patterson, 1984, 1991; Sorokin, 1990a,b), but many get supplemental nutrition from zooxanthellae (Dai and Lin, 1993; Dauget, 1992). This facilitates their ease of maintenance, and many species may be kept without too many problems in marine aquaria. Although many species appear to eat many different types of particulate food, some such as the cauliflower corals, Dendronephthya , appear to be mostly herbivorous eating only phytoplankton. As phytoplankton are generally lacking in captive systems, and adding sufficient phytoplankton from culture is either difficult or expensive, captive husbandry of these animals should not be attempted by most hobbyists (Fabricius, et al. 1995) (See the article in the December, 1996 issue of Aquarium Net, Dendronephthya A Seduction of Allusions and Illusions by Eric Borneman ).

A few species, such as some Xenia may have a reduced gut, and not feed; these animals do well under reasonably strong light, although extreme intensities are unnecessary. I have a colony that is growing very well under power compact fluorescent lights and it is subdividing into new colonies at the rate of about one to three new colonies per week. Others, such as sea pens, are suspension-feeding animals that appear to grow in a manner to maximize their water filtering efficiency; zooxanthellae appear to supplement their diet. They feed on small zooplankton which they catch and subdue utilizing their nematocysts.

Soft corals, particularly the leather corals and gorgonians, may be aggressive to their neighbor and in these situations they often use chemical warfare (Sammarco, et al. 1983; Sammarco, et al. 1985). It has recently been demonstrated that many leather corals and other soft corals produce complex organic chemicals that are poisonous to stony corals (Aceret, et al. 1995).

Large leather corals about 1 m (3.3 feet) high photographed at -15 m (50 feet) in Palau.

They often produce small amounts of these chemicals, but their chemical production accelerates when they are in the presence of stony corals(Pawlik and Fenical, 1992). Such chemicals may accumulate in reef aquaria and affect other animals. Additionally, many of them seem to secrete antifouling agents, either to prevent diatom and bacterial growth or to inhibit settling by other marine organisms (Kim 1994). Although soft corals, leather corals, gorgonians and many other alcyonarians, are permanently attached to hard substrates, a few others primarily sea pens and their relatives are found in unconsolidated sediments and many of them are quite mobile. They may move up and down in the sediments many times each day or remain immobile for long periods of time and then become quite mobile for a short time. Some of them, such as the Northeastern Pacific sea pen, Ptilosarcus gurneyi, have been observed to "fly" or "balloon" through the water over the substrate. This species and some others appear to be able to generate hydrodynamic lift when water flows over its "leaves". Even the more sessile ones may move, however, by differential growth and probably some sort of slow creeping(La Barre, and Coll. 1982). Colonies of Xenia are quite mobile, albeit on a slow time scale; mine seem to be able to move at the rate of about a centimeter a week.

For more information about sea pens follow this link.

As with all animals that are basically sessile, soft corals can be considered to mounds of flesh waiting to become somebody’s meal (Griffith, 1994). There are many predators upon them. The best studied examples of soft coral predation are the sea pen beds of the North Eastern Pacific and some gorgonians in the Caribbean (Harvell, et al. 1993). Sea pens are soft-sediment dwelling soft corals that are often found in great beds. In these situations, they act as a food resource for many different types of predators. In subtidal areas around Seattle, Washington, the sea pens are preyed upon by five species of nudibranchs and four different sea stars (Birkeland, 1974).

Some Caribbean gorgonians are preyed upon by the Flamingo Tongue snails, Cyphoma gibbosum , and one species of fireworm, while others are eaten by file fish (Vreeland, and Lasker. 1989). Often these soft corals have chemical defenses against some predators (Coll, et al. 1982; Targett, et al. 1983; Van-Alstyne, and Paul. 1992; Harvell, et al. 1993).

A Caribbean gorgonian showing the snails, Cyphoma gibbosum , eating the flesh of the branches and a file fish which was plucking polyps off the branches.

Leather corals seem to be eaten by a number of different animals, but probably the most insidious from the aspect of the marine aquarist is the snail, Rapa rapa (Kohn, 1983). These snails can bore there way into a leather coral leaving only a small hole to mark their presence. Once inside they have the potential of completely eating the whole leather coral, but do it from the inside. The leather coral will appear to be fine, until one day it starts to disintegrate. If the hobbyist examine the coral basically all they find is a hollow shell and a rather large, about walnut-sized, snail in the cavity. In these cases, the colony may regenerate from the tissue fragments, but whole colony is no longer viable.

These potential predators aside, many soft-corals are amongst the easiest of animals for reef-aquarists to maintain. Coupled with their pleasing shapes and often striking colors, they are a good choice for marine aquaria. Their colonial morphology facilitates asexual reproduction, and many of the animals actively subdivide and reproduce in our systems. Others may be purposely split to provide many daughter colonies. They are good animals for both beginning and advanced reef aquarists.

For some other photographs of soft corals follow this link.

For an extensive bibliography of soft coral literature follow this link:

Aceret, T. L., P. W. Sammarco and J. C. Coll. 1995. Toxic effects of alcyonacean diterpenes on scleractinian corals. Journal of Experimental Marine Biology and Ecology. 188:63-78.

Birkeland, C. 1974. Interactions between a sea pen and seven of its predators. Ecological Monographs. 44:211-232.

Coll, J. C., S. L. Barre, P. W. Sammarco, W. T. Williams and G. J. Bakus. 1982. Chemical defenses in soft corals (Coelenterata: Octocorallia) of the Great Barrier Reef: A study of comparative toxicities. Marine Ecology Progress Series. 8:271-278.

Dai, C. F. and M. C. Lin. 1993. The effect of flow on feeding of three gorgonians from southern Taiwan. Journal of Experimental Marine Biology and Ecology. 173:57-69.

Dauget, J. M. 1992. Effects of change in colony orientation on the morphology of Isis hippuris Linne, 1758 (Gorgonacea): Preliminary study. Bulletin De La Societe Zoologique De France. 117:375-382.

Fabricius, K. E., Y. Benayahu and A. Genin. 1995. Herbivory in asymbiotic soft corals. Science. 268:90-92.

Griffith, J. K. 1994. Predation of soft corals (Octocorallia: Alcyonacea) on the Great Barrier Reef, Australia. Australian Journal of Marine and Freshwater Research. 45:1281-1284.

Harvell, C. D., W. Fenical, V. Roussis, J. L. Ruesink, C. C. Griggs and C. H. Greene. 1993. Local and geographic variation in the defensive chemistry of a West Indian gorgonian coral ( Briareum asbestinum ). Marine Ecology Progress Series. 93:165-173.

Kim, K. 1994. Antimicrobial activity in gorgonian corals (Coelenterata, Octocorallia). Coral Reefs. 13:75-80.

Koehl, M. A. R. 1977. Water flow and the morphology of zoanthid colonies. Proceedings of the Third International Coral Reef Symposium. 1:437-444.

Kohn, A. J. 1983. Feeding biology of Gastropods. In: Wilbur, K. M. Ed. Physiology (2). Academic Press. New York. pp. 1-63.

Kozloff, E. N. 1990. Invertebrates . Saunders College Publishing. Philadelphia. 866 pp.

La Barre, S. and J. C. Coll. 1982. Movement in soft-corals: An interaction between Nephthea brassica (Coelenterata:Octocorallia) and Acropora hyacinthus (Coelenterata:Scleractinia). Marine Biology. 72:119-124.

Lasker, H. R. 1981. A comparison of the particulate feeding abilities of three species of gorgonian soft coral. Marine Ecology Progress Series. 5:61-67.

Lasker, H. R., M. D. Gottfried and M. A. Coffroth. 1983. Effects of depth on the feeding capabilities of two octocorals. Marine Biology. 73:73-78.

Leversee, G. 1976. Flow and feeding in fan-shaped colonies of the gorgonian coral, Leptogorgia . Biological Bulletin. 151:344-356.

Patterson, M. R. 1984. Patterns of whole colony prey capture in the octocoral, Alcyonium siderium . Biological Bulletin. 167:613-629.

Patterson, M. 1991. Passive suspension feeding by an octocoral in plankton patches: empirical test of a mathematical model. Biological Bulletin. 180:81-92.

Pawlik, J. R. and W. Fenical. 1992. Chemical defense of Pterogorgia anceps , a Caribbean gorgonian coral. Marine Ecology Progress Series. 87:183-188.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology . Saunders College Publishing. Philadelphia. 1056 pp.

Sammarco, P. W., J. C. Coll, S. L. Barre and B. Willis. 1983. Competitive strategies of soft corals (Coelenterata: Octocorallia). Allelopathic effects on selected scleractinian corals. Coral Reefs. 1:173-178.

Sammarco, P. W., J. C. Coll and S. L. Barre. 1985. Competitive strategies of soft corals (Coelenterata: Octocorallia). II. Variable defensive responses and susceptibility to scleractinian corals. Journal of Experimental Marine Biology and Ecology. 91:199-215.

Sorokin, Y. I. 1990a. Plankton in the reef ecosystems. In: Dubinsky, Z. Ed. Coral reefs. Elsevier. Amsterdam. pp. 291-327.

Sorokin, Y. I. 1990b. Aspects of trophic relations, productivity, and energy balance in coral-reef ecosystems. In: Dubinsky, Z. Ed. Coral reefs. Elsevier. Amsterdam. pp. 401-410.

Targett, N. M., S. S. Bishop, O. J. McConnell and J. A. Yoder. 1983. Antifouling agents against the benthic marine diatom, Navicula salinicola. Homarine from the gorgonians Leptogorgia virgulata and L . setacea and analogs. Journal of Chemical Ecology. 9:817-829.

Van-Alstyne, K. L. and V. J. Paul. 1992. Chemical and structural defenses in the sea fan Gorgonia ventalina : Effects against generalist and specialist predators. Coral Reefs. 11:155-159.

Vreeland, H. V. and H. R. Lasker. 1989. Selective feeding of the polychaete Hermodice carunculata Pallas on Caribbean gorgonians. Journal of Experimental Marine Biology and Ecology. 129:265-277.

Wainwright, S. A. and M. A. R. Koehl. 1976. The nature of flow and the reaction of benthic cnidaria to it. In: G. O. Mackie. Ed. Coelenterate Ecology and Behavior. Plenum Press. New York. pp. 5-21.

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Last modified 2006-11-19 02:11