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., and
Acropora 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 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
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,
Xenia caused 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!
References
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..
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