The growth rates of hard corals are species dependent, with certain species growing much quicker 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 over-topping 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 Pocillopora 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 paucity of branching hard corals. However, the aquarist should not forget the effects of over-topping and shading caused by large expanding anemones, corals or macroalgae growths. This can occur in an aquarium. One should always ensure that a specimen is receiving adequate lighting and water circulation. Enough space must be allotted for both expansion and growth when one first places a specimen in the aquarium.
Aggressive Structures Acontia (Mesenterial Filaments)
Mesenterial filaments, or acontia, are part of a coral's digestive organs and 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, can 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). In the aquarium, such damaged areas can become infected by bacteria, attacked by protozoans or colonized by microalgae. We will deal with these problems in greater detail in chapter 10. In most cases, corals of the same species or genus do not cause damage when within touching distance, for example several Euphyllia spp. can usually be placed next to each other without any problems. Smaller polyped corals, such as Acropora spp., have also been observed producing acontia on an almost daily basis (J. Sprung; J.C. Delbeek, pers. obs.). Not only can they cause damage to adjacent corals, but the acontia also serve to keep a microalgae-free zone around, not onlv the base of the colony, but also around
any tips that may have grown onto an adjacent substrate, like the glass of the aquarium.
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 species, but one should at least be aware of the possibility.
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). In some cases contact is not necessary and some species always seem to possess swreepers e.g. Galaxea. These tentacles are usually much longer and thinner than nonnal tentacles and have many more stinging cells (nematocysts) than natural. As a result, their function has changed from one of feeding or light gathering, 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 oi sweeper tentacles are usually associated with stony corals, a recent study has shown that they can also develop in soft corals such as the encrusting Caribbean gorgonian Erytbropodium caribaeorum (Sebens and Miles, 1988). These sweeper tentacles were found to lack pinnules on the tentacles and had bulbous tips 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 stony corals, some of which are regularly kept in aquaria such as Bubble Coral (Plerogyra sinuosa), Anchor Coral (Eupbyllia ancora), Faviaspp., Favites spp. and Galaxea fascicularis. We 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) that deals with aggression in stony and soft corals. This article 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 in stony corals 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), we have placed various Indo-Pacific stony corals in the following aggressive categories. Aggressive: Fungiaspp., Goniopora spp. (nematocysts concentrated in the polyps not in the mesenterial filaments), Galaxea fascicularis and Acropora spp.; Intermediate: Lohopbyllia spp.; Subordinate: Montiporaspp. and Pontes 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). However, in our experience, some species of Acropora can severely damage other corals if brought into contact with them. In the Caribbean, Lang (1973) classified Isopbyllia sinuosa as very aggressive, Montastrea annularis as moderately aggressive and Pontes spp. as weakly aggressive. Interestingly, the initial dominance of Montastrea is due entirely to the action of its mesenterial filaments. 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.
Sweeper tentacles in Galaxea fascicularis. J. Yaiullo and F. Greco.
The length of a single sweeper tentacle is evident in this Galaxea. J.C. Delbeek.
Sweeper tentacles produced by Euphyllia ancora can extend several cm. J.C. Delbeek.
Sweeper tentacles in Montastrea cavernosa. J. Yaiullo and F. Greco.
A sweeper tentacle from a Bubble Coral stings a neighbouring gorgonian. J.C. Delbeek.
In some coral genera the polyps have other mechanisms for aggression involving a method similar to sweeper tentacles. The main advantage of sweeper tentacles is that they allow stationary corals to extend their stinging reach. What about corals that are not stationary? Corals belonging to the family Fungiidae have large, unattached polyps that can move about the substrate freely. Therefore, these corals can frequently come into contact with other corals. In a study of the free-living fungiid coral Fungia scutaria, Chadwick (1988) found that these corals caused extensive damage to other corals in over 94% of encounters. Apparently the mucus of this coral contains a large quantity of nematocysts. By depositing their mucus onto the surface of other corals, they could cause death within 4 days. The mucus transmits the stinging cells, and they have no need of sweeper tentacles.
The use of chemicals to inhibit the growth of one species by another is called allelopathy and is quite common in terrestrial plant ecosystems. It is well known that many soft corals also contain numerous toxic compounds such as sarcophines (isolated from Sarcophyton glaucum, a commonly imported Leather coral), terpenoids, and diterpenoids. Green Star Polyp (Pachyclavularia viridis), contains numerous diterpenoids known as clavirolides-B, C, D and E. Caribbean gorgonians also possess a significant range of chemicals, and many of them are being investigated for medical applications (see Faulkner, 1992). For example Pseudoplexaura spp. release crassin acetate and Eimicea spp. produce eunicin and eupalmerin acetate (Ciereszko and Guillard, 1989). Pseudopterogorgia elizabethi, the Caribbean "Purple Frilly" gorgonian, produces a compound being investigated for use as an anti-inflammatory drug in humans (W. Fenical, pers. comm.). Such chemicals have 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 Leather Coral, Sinularia spp. at concentrations ranging from 1 to 5 ppm (parts per million).
In various transplant experiments using the soft corals Lobophytum paucifloriim (Devil's Hand), Sinularia pavida and Xenia sp. (Pulsating Polyps), Sammarco et al. (1983) documented the effects on the stony corals Pavona cactus and Porites 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 stony corals, and the susceptibility of the hard corals, was entirely species specific. In some cases, inter-colony distances of 30 cm (12 in.) resulted in stunted growth and death of the stony coral, while another species of soft coral only caused damage when in direct contact with the stony coral. For example, Pontes andrewsi was the most sensitive hard coral, reacting to two of the three soft corals used, even at distances of 10 cm (4 in.). However, a soft coral that can cause damage by contact does not necessarily cause damage by releasing compounds into the water. For example, Xenia caused extensive damage to both hard corals tested, only when in contact with them. If a soft coral (e.g. Lobopbytum pauciflonim) could cause damage from a distance, it could also cause damage upon contact.
Some soft corals (e.g. Sarcopbyton, gorgonians, Heliopora) shed a waxy film periodically to rid their surface of attached algae and detritus. This film can be toxic to some stony corals if it should rest j on them. Tissue damage and death in areas where the film rests on a stony coral is possible (J. Sprung, per. obs.), though regrowth of the tissue is rapid when the film is removed.
Through the use of toxic compounds, soft corals are able to compete with stony corals for space by stunting their growth or actually killing portions of a colony and overgrowing them. However, hard corals are not without their own defenses, and they can cause serious damage to soft corals as well. You will become an expert on aggressive interactions between different corals mostly by trial and error.
In the species descriptions of corals, chapter 13, we give more detailed information on coral aggression and placement in the aquarium. The information provided here, and by Ates (1989) and Paletta (1990), provide you with a starting point. Watch the inhabitants in an aquarium closely, and note the reactions to the introduction of a new specimen.
The study of toxic compounds from corals is a relatively new one. Subsequently, their effects in a closed system clo 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, it is unknown if the toxic
compounds produced in an aquarium will accumulate or break down in time. Can they be removed bv chemical filtration and
protein skimming, 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.
When placing a coral that you suspect may belong to an aggressive genus, you should make sure that there is adequate space between it and other specimens. Generally, 10-15 cm (5-8 in.) is enough, but don't forget that some species can expand many times their normal size, and this should be taken into consideration (i.e. measure from the expanded polyp, not just from the base of the skeleton). Also, if the coral develops sweeper tentacles, they may reach much further than polyps. Sweeper tentacles can be removed by pinching the tentacle off using your fingers or a scissors, though new ones will replace them (see Chapter 10). Soft corals often deflate at night, and the collapsed branches may rest on adjacent pieces of coral. This is especially common in soft corals of the genus SinulariaiD. Maier, pers. comm.). Please do not hesitate to move a coral that you believe is being injured. Once damaged, they become more susceptible to disease, parasites and a 1 gal encroachment.
The final variable to consider is growth. One must allow adequate room to grow, especially for encrusting soft corals such as Xenia spp., Anthelia spp. (Waving Hand Polyps), Pachyclavularia sp. (Star Polyps) and zoanthicl anemones. It may become necessary to trim and separate specimens that have grown together, especially if they begin to develop aggressive interactions. This is a great way to propagate your specimens for exchange with other aquarists, to trade cuttings for something new, or to help stock another aquarium. Stony corals such as Acropora and Euphyllia will also grow amazingly fast. Acropora is easily managed by cutting branches like pruning a bush. Fleshy polyped corals like Euphyllia or Catalapbyllia are not so easily pruned, and generally must be re-positioned or moved to a larger aquarium when they grow too big. Understand that in a successful aquarium, the day quickly arrives when trimming and culling your invertebrates becomes a necessity. We describe propagation techniques for stony corals in chapter 13.
This photo appears to be a scene from a reef in Fiji, but was actually taken in the basement of a house on an island off the coast of Norway! It shows the top view of Alf Jacob Nilsen's reef aquarium. Just how dense can the corals be allowed to grow'?' With fast growing branched Acropora, Pocillopora, and Montipora shown here one must be careful to prune the growth often to prevent overshading and stinging of neighboring colonies. A.J. Nilsen.
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