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Stocking the Reef Aquarium: Coral Compatibility

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Stocking the Reef Aquarium: Coral Compatibility

This unedited article first appeared in the Sept/Oct. 1990 issue of Aquarium Fish Magazine

by J. Charles Delbeek B.Ed M.Sc.

   Advertisements for various reef related products are often accompanied by photographs of spectacular aquariums, heavily stocked with corals, sponges and macroalgae. Using these as examples of the "ideal" aquarium, many hobbyists try to duplicate these scenes with limited success. The question then arises as to why? Often the hobbyist feels that he/she is at fault and tends to spend hundreds of dollars on more and/or better equipment. They may also fall prey to the syndrome of changing their approach on a weekly basis depending on which "expert" they last spoke with or whose article they last read. Eventually they become frustrated in being unable to attain the "ideal aquarium" and drop out of the hobby. I can't help but feel at times that they may have been mislead from the very beginning. Perhaps those aquariums didn't really exist, at least not in the long run. You see, it is very difficult to maintain a heavily stocked aquarium for any length of time. This has nothing to do with filtration or water quality, but with something else that I feel has been totally neglected by many authors and hobbyists. It may be that these photos were taken of newly set-up systems and do not represent aquariums much older than 6 months at most. I know that this is not true in all cases but one cannot deny that it is a possibility. It is very easy to produce a really stunning display by simply stocking the aquarium with newly acquired specimens.

   If you have overcome the problems of microalgae, water quality, water movement and lighting you may feel that you have attained the best possible conditions for your animals. Indeed you may have been too successful, for now the animals are growing and spreading over the aquarium, which will eventually bring them into contact with each other. This is where another, more insidious, problem may occur; competition for space. If one thinks of shallow coral reef areas or looks at photographs of these areas, it becomes immediately apparent that space is at a premium; the entire substrate is covered with some form of life. Why does one form not dominate over another? Obviously there must be some sort of controls that allow such a high diversity of life to exist in a limited area. Failure to recognize this fact can lead to a lot of frustration on the part of the hobbyist and can lead to many expensive losses. The purpose of this article is to point out the mechanisms that sessile (stationary) marine organisms can use to hold onto their piece of "real-estate." Competition for space is one of the most important factors limiting populations on marine hard substrata. This is why sessile colonial marine organisms such as anemones, sponges, and soft and hard corals have developed various mechanisms for defending their space and moving into new ones (Sammarco et al, 1983). There are four main mechanisms that such organisms can use: rapid growth to "shade-out" competitors, the development of aggressive structures such as mesenterial (gut) filaments, sweeper tentacles and acrorhagi, and the release of toxic compounds into the water. It does not appear that any one organism utilizes only one of these mechanisms; often they will use a variety of tactics.
Rapid Growth
The growth rates of hard corals are species dependent, with certain species growing much quicker in shallow waters, than others. This is a definite advantage and allows these corals to quickly colonize new areas. A rapid growth rate also allows these species to achieve dominance over other species by overtopping them, thereby reducing the amount of light and water flow they can receive (Huston, 1985). This mechanism has been proposed as an explanation for the dominance of pocilloporid corals in the Pacific and Acropora corals in the Atlantic (Huston, 1985). Presumably this mechanism is of limited importance in a closed system due to the lack of hard corals. However, the hobbyist should not neglect the effects of overtopping and shading caused by large anemones, corals (soft or hard) or macroalgae. This can potentially occur in an aquarium and one should always ensure that a specimen is receiving adequate lighting and water circulation. Allowances should also be made for rapid growth and enough space must be allotted for such when first placing a specimen in the aquarium.

Aggressive Structures

I Mesenterial Filaments

Mesenterial filaments, the coral's digestive organs, can be used quite effectively as aggressive structures. When two hard corals come into contact (either different species or the same) one of them, the aggressor, will extrude mesenterial filaments through the mouth cavity or the body wall, onto the surface of the other, literally digesting it's tissue. This results in a zone of naked skeleton that can then be overgrown (Sebens and Miles, 1988). This zone can be overgrown by the attacking coral or it can be colonized by encrusting organisms, thereby creating a "buffer zone" between the two species (Huston, 1985).

II Acrorhagi

Acrorhagi are specialized structures that were first recognized in coldwater species of the anemone family Actiniidae. They consist of inflated sacs that protrude from below the tentacles and are loaded with stinging cells. When they make contact with another anemone they leave behind a layer of tissue that results in localized tissue death of the intruder (Sebens and Miles, 1988). It is not clear whether these structures appear in tropical specimens but you should at least be aware of the possibility.

III Sweeper Tentacles

Sweeper tentacles are specialized tentacles that appear on polyps after several weeks of contact with other organisms (Hidaka and Yamazato, 1984; Sebens and Miles, 1988). These tentacles are usually much longer and thinner than normal tentacles and have many more stinging cells (nematocysts) than normal. As a result their function has changed from one of feeding to one of defense or aggression. Elongated polyps of some corals such as Goniopora, can also be used as "sweeper polyps" for aggressive purposes (Sheppard, 1979). Although the production of sweeper tentacles are usually associated with hard corals, a recent study has shown that they can also develop in soft corals (Octocorallia) such as the encrusting Caribbean gorgonian Erythropodium caribaeorum (Sebens and Miles, 1988). These sweeper tentacles were found to lack pinnules on the tentacles and had bulbous tips that were loaded with nematocysts (Sebens and Miles, 1988). Such specialized tentacles form only along the edge of the colony that is in contact with another coral, encrusting algae, or by nematocyst discharge (Hidaka and Yamazato, 1984).

  Ates (1989) provides a listing of aggressive hard corals, some of which are regularly kept in aquaria such as Bubble Coral (Plerogyra sinuosa), Anchor Coral (Euphyllia ancora), Favia sp., Favites sp. and Galaxea sp.. I urge you to read this article, it is an excellent discussion of the phenomenon of coral aggression from a European hobbyist's point of view. Another excellent reference is an article written by Mike Paletta (1990) which deals with aggression in hard and soft corals and provides a listing of those species commonly found in home aquaria, arranged in order from most to least aggressive.

  Various authors have tried to quantify the aggressive capabilities of corals and to rank them in order of aggression. Thomason and Brown (1986) found that there was a direct relationship between aggressive proficiency and the number of nematocysts per polyp and/or mesenterial filament. Interestingly, it is the number of nematocysts per structure that is important, not the size or number of the polyps and mesenterial filaments. Combining their findings with those of Sheppard (1979), various Indo-Pacific corals can be placed in the following aggressive categories: Aggressive: Fungia spp. (Mushroom Corals), Goniopora spp. (nematocysts concentrated in the polyps not in the mesenterial filaments), Galaxea spp., andAcropora spp. Intermediate: Lobophyllia spp. Subordinate: Montipora spp. and Porites spp. (Thomason and Brown, 1986). Although Acropora spp. are classified as being aggressive it is generally believed that they rely more on overtopping and asexual reproduction by fragmentation to compete for space. The small size of Acropora spp. nematocysts supports this hypothesis (Thomason and Brown, 1986). In the Caribbean, Lang (1973) classified Isophyllia sinuosa as very aggressive, Montastrea annularis as moderately aggressive and Porites spp. as weakly aggressive. Interestingly, the initial dominance of _Montastrea_ is due entirely to the action of its mesenterial filaments. However, Bak et al. (1982) found that as the length of the aggressive encounter increased, the subordinate coral was able to develop sweeper tentacles and reverse the interaction in its favour.

Toxic Compounds

It has been well known that many soft corals contain numerous toxic compounds such as terpenoids and sarcophine (isolated from Sarcophyton glaucum, a commonly imported Leather coral). These chemicals proved to be very efficient in deterring predators but recent evidence suggests that they are also released into surrounding waters as a mechanism for maintaining living space (Sheppard, 1979). Coll et al (1982) isolated toxic terpenes from seawater surrounding several soft corals including the common aquarium Finger or Tree Coral, Sinularia spp. The use of chemicals to inhibit the growth of one species by another is called allelopathy and is quite common in terrestrial ecosystems.

  In various transplant experiments using the soft corals Lobophytum pauciflorum (Devil's Hand), Sinularia pavida (Tree Coral?) and Xenia sp. (Pulsating Polyps), Sammarco et al documented the effects on the hard corals Pavona cactus andPorites andrewsi when the soft corals were brought close to the colonies or in direct contact with them. They concluded that the effects of soft corals on hard corals, and the susceptibility of the hard corals, was entirely species specific. In some cases, intercolony distances of 30 cm resulted in stunted growth and death of the hard coral, while another species of soft coral only caused damage when in direct contact with the hard coral. For example, Porites andrewsi was the most sensitive hard coral, reacting to two of the three soft corals used, even at distances of 10 cm. However, a soft coral that can cause damage by contact cannot necessarily cause damage by releasing compounds into the water. For example, Xeniacaused extensive damage to both hard corals tested only when in contact with them. On the other hand, if a soft coral (e.g. Lobophytum pauciflorum) could cause damage from a distance it could also cause damage upon contact (Sammarco et al, 1983).

  Through the use of toxic compounds, soft corals are able to compete with hard corals for space by stunting their growth or actually killing portions of a colony and overgrowing it. However, as we have seen, hard corals are not without their own defenses and serious damage can occur to the soft coral too.

Planning the Aquarium

Hopefully you have now gained a better understanding of some of the mechanisms that can occur on a reef. Its now time to look at how this relates to our aquariums. When one sets out to stock an aquarium, you must take into account a number of variables such as lighting and water movement. Unfortunately, this article has now given you another to take into consideration: will coral X get along with coral Y next to it? This is not an easy question to answer as it involves mostly trial and error. The information provided here, and by Ates (1989) and Paletta (1990), should provide you with a starting point. The key is to closely watch the inhabitants in an aquarium and their reactions to the introduction of a new specimen. As an example I offer you a story related to me by John Burleson of J.P. Burleson, Inc. It seems shortly after adding a colony of Xenia to his 150 gallon display tank, he found that his prefilter began clogging at a greater rate than normal. After many weeks of carefully removing selected pieces and then returning them to the tank he found that if he removed either the new Xenia colony or his Tridacna derasa clam, the clogging stopped. Apparently, the clam would produce huge amounts of mucus in response to the presence of the Xenia in the aquarium, which would clog his prefilter material! The Xenia must have been producing something that the clam found objectionable.

  The issue of toxic compounds in corals is a relatively new one. Subsequently, their effects in a closed system do not appear to have been given much consideration in the operation and planning of an aquarium. Paletta (1990) suggests that soft corals not be placed upstream of other specimens but in a crowded aquarium, this is difficult to do. Also, I am unaware if the toxic compounds produced in an aquarium will accumulate or might break down in time. Can they be removed by chemical filtration or do they just build-up in the water? This is definitely an area of concern especially when it comes to keeping a highly diverse population and/or delicate specimens. Perhaps this might explain why certain corals are difficult to keep despite acceptable water quality. For example Goniopora are generally recognized around the world as difficult to keep alive for more than a year in a closed system. However, they are maintained quite easily in an open system at the Waikiki Aquarium (Sprung, personal communication). One possible explanation may be that, since they are closely related to the toxin sensitive Porites sp., they are adversely affected by the toxic effluent of other corals that might build up in a closed system. This would not be a problem in an open system as they would be constantly flushed out.

  When placing a coral which you suspect may belong to an aggressive genus, you should make sure that there is adequate spacing between it and other specimens. Generally, 10-15 cm should be enough but don't forget that many species can expand to many times their normal size during the coarse of the day and this should be taken into consideration i.e. don't measure only from the base! Also, if the coral develops sweeper tentacles, they may reach much further than normal polyps. Another variable that some aquarists may not consider is that many corals change their shape at night too! What you may have thought was a safe distance during the day might not be during the night due to expansion or deflation of the coral, this is especially common in soft corals of the genus Sinularia (D. Maier, personal communication). Please do not hesitate in moving a coral that you believe is suffering damage, once damaged they become more susceptible to disease, parasites and algae.

  The final variable to consider is growth. Ensure that the specimen has adequate space to grow, especially encrusting forms such as Xenia sp., Anthelia sp. (Waving Hand Polyps), Star Polyps (Clavalaria sp.) and zoanthids. It may become necessary to trim and separate specimens that have grown together, especially if they begin to develop aggressive interactions such that one begins to suffer. This is a great way to propagate your specimens and trade them in for something new or to help stock another aquarium. Believe me, in a successful aquarium, the day will come when trimming and culling your invertebrates will become as necessary as trimming Caulerpa! Well that brings us near the end of this series on the workings of reef systems. I hope that you have enjoyed the articles and that both novice and advanced hobbyists learned something new. As I stated at the beginning of the series, the aim of these articles was to get you thinking and not to be a how-to series. I hope that I achieved this in some way and that you have a better understanding of what a reef aquarium is and the tremendous educational potential that they harbour. The more people who see and learn how to use these systems properly, the more they will appreciate the complexity and fragility of a living coral reef. Perhaps then, there will be more of an outcry against the destruction of these ecosystems through indiscriminant fishing and curio collecting practices. Now that we have spent that last few months looking at how a reef system works, I thought it would be fitting to close out this series by taking a look at, what I consider to be, some successful aquariums. Yes folks, they do exist and I'll prove it to you, next issue I'll show you some tanks that will literally make your eyes pop out!


Ates, R. 1989. Aggressive behaviour in corals. Freshwater and Marine Aquarium 12(8):104-105,107,110,112.

Bak, R.P.M., Brouns, J.J.W.M. and F.M.L. Heys 1982. Complexity of coral interaction: influence of time, location and epifauna. Mar. Biol. 69:215-220.

Coll, J.C., Bowden, B.F., Tapiolas, D.M. and W.C. Dunlap 1982. In situ isolation of of allelochemicals released from soft corals (Coelenterata: Octocorallia): a totally submersible sampling apparatus. J. Exp. Mar. Biol. Ecol. 60:293-299.

Hidaki, M. and K. Yamazato 1984. Intraspecific interactions in a scleractinian coral, Galaxea fascicularis: Induced formation of sweeper tentacles. Coral Reefs 3:77-85.

Huston, M.A. 1985. Patterns of species diversity on coral reefs. Ann. Rev. Ecol. Syst. 16:149-177.

Lang, J. 1973. Interspecific aggression by scleractinian corals. 2: Why the race is not only to the swift. Bull. Mar. Sci. 23:260- 279.

Paletta, M. 1990. Coral aggression in reef aquaria. SeaScope 7 (Winter):1-2.

Sammarco, P.W., Coll, J.C., La Barre, S. and B. Willis 1983. Competitive strategies of soft corals (Coelenterata: Octocorallia): allelopathic effects on selected scleractinian corals. Coral Reefs 2:173-178.

Sebens, K.P. and J.S. Miles 1988. Sweeper tentacles in a gorgonian octocoral: morphological modifications for interference competition. Biol. Bull. 175:378-387.

Sheppard, C.R.C. 1979. Interspecific aggression between reef corals with reference to their distribution. Mar. Ecol. Prog. Ser. 1:237-247.

Thomason, J.C. and B.E. Brown 1986. The cnidom: an index of aggressive proficiency in scleractinian corals. Coral Reefs 5:93-101..

Used by permission.  Many thanks to Charlies and Linda Raabe for their support.

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