Reef Fish Behavior

If you live in a big, crowded city, you would — believe it or not — have some idea of what it's like to live in a coral reef community. Such is the remarkable similarity between the two environments that naturalists often refer to coral reefs as "cities beneath the sea." The process of living in either situation is not without its difficulties, but on the whole, interaction between individuals or groups of individuals goes on in a remarkably smooth and surprisingly ordered manner.

There are several major reasons why coral reefs act as a magnet for so many species, and each reason demands the development of certain strategies that enable each species to coexist and survive among the multitudes sharing the reefs. Some species rely solely on camouflage and some on concealment, but in the main most are quite happy to live in full view of their closest neighbors. This, as we shall see, requires that a strong territorial instinct be evolved.

Having existed for many millions of years, reef animals have all found little (or big) niches in which to live. Because each niche represents a unique combination of characteristics and qualities, the animals in them largely present no threat to unrelated species owing to the lack of competition for the same resources and energy, which would otherwise be needlessly wasted trying to expel every fish that entered a chosen territory. Thus, many species and individuals can live in perfect harmony within interlinked boundaries.

Problems begin to occur when fish of the same species want to live in close proximity, for each requires access to precisely the same criteria for survival. Such a strain on resources cannot go unchecked if the species is to flourish, and the only resolution is to resort to territoriality backed up not only the threat of physical violence if the situation demands but its actual implementation. It has to be noted that serious physical violence is a relatively unlikely phenomenon — it rarely benefits either party to engage in any more of this behavior than is absolutely necessary.

Although there are numerous minor grounds for the establishment and defence of a territory, the major reasons fall into four main categories: habitat, shelter, food and breeding. These are explained in the accompanying sidebar, "Territorial Criteria."

The link between coloration of the fish and territory is an interesting and vital one. We already know that fish of the same species are the most likely to dispute territorial boundaries.

The famous animal behavioral scientist, Konrad Lorenz, expanded our knowledge in this area considerably. He maintained that coloration acts as a stimulus to trigger a strong territorial response against a member of the same species, whereas other species are largely totally ignored. Further than that, he insisted that all gaudily colored fish defend a territory of some description! This is not to say that dull colored fish do not defend a territory — they often do so quite ferociously.

This may all seem relatively subtle on first consideration, but these studies help us to unravel the larger mysteries as to why coral reef fishes are so brilliantly and diversely colored. For example, Hans Fricke, a student of Lorenz, discovered that a single individual butterflyfish, Chaetodon lineolatus, had established a territory in the Red Sea more than 1600 feet long! Within that territory coexisted many other different species quite peacefully, but C. lineolatus vigorously defended the boundaries against the entry of another of its own kind. Specific coloration and markings enable a positive identification at a tremendous distance, allowing C. lineolatus to speed to the point of intrusion with the certainty that the target was correct, and preventing energy from being expended unnecessarily.

Studies, such as those by Lorenz and Fricke, are useful when we, as aquarists, try to make sense of the behavior of our fish in a captive environment. Central to this is the fact that certain behavioral patterns have been observed to alter radically within the confines of the aquarium — the most important of these being that fish of unrelated species, which would normally coexist peacefully in the wild, suddenly becoming aware of each other and reacting differently.

The form of response may be relatively benign, exceedingly aggressive, somewhere in between or totally unpredictable. Puzzling as this may seem, the clues to these important behavioral changes can be be found by referring back to the list of territorial criteria. Given unlimited space, all species can conform to their own distinctive territorial requirements up to their natural limits. However, within the confines of an aquarium, these requirements may have to be adapted to suit a much smaller environment. Therefore, not only could their own species be seen as a threat, but even similar or unrelated fish as well!

Of course, not all species demand a territory in excess of the space provided within an average aquarium, and these fish make for ideal choices. Hence, to avoid confusion and unnecessary conflict, compatibility charts have been compiled through years of observation and experimentation. As a result, the aquarist need not put his or her fish at unnecessary risk. The most peaceful combination of fish can be assured, eliciting the most natural behavior patterns with a minimum of stress.

Some aquarists are concerned that as some fish change their behavior within the aquarium environment, there could be a moral dilemma at issue here. Do we indeed have the right to alter natural behavioral patterns as a by-product for our own self-gratification? It would be fair to say that most fishkeepers see a problem only if compatibility guidelines are ignored and fish are forced into difficult relationships that could be stressful, or into aquariums that are simply too small to maintain a reasonable size territory, if so required.

Putting what we already know into practice, it's important that the marine aquarist is prepared for certain situations that may arise. For example, a particular species of fish may be described as highly aggressive, but for the moment turns out to be a peaceful juvenile. What we are witnessing here is usually the calm before the storm.

Aggressive fish are usually so because they have a highly developed sense of territoriality. Some species only develop this tendency as they mature — a point in time where they have become powerful enough to defend a space against almost any perceived threat, and a point that can prove unpredictable to say the least. Consequently, a now perfectly peaceful specimen may suddenly become aggressive at a later date, and without warning. Triggerfishes and damselfishes are often very good examples of this type of latent aggressiveness, and their behavior must be constantly monitored.

Even though an established aquarium full of fishes may appear settled and quiet, with no actual sense of order, an invisible territorial pecking order has already been decided. Quite often, a new arrival will instantly disrupt that hierarchy and need to establish a place before things settle down once more. The transition can be made as smooth as possible by choosing completely compatible tankmates.

Even so, new introductions often receive a very hard time before a niche in the hierarchy can be found. Sometimes destroying old boundary markers, such as rockwork arrangements, can help to distract existing tank inhabitants long enough to enable new fish to find their place in the tank's community. However, it is not always a guaranteed procedure, especially when one individual species regards the whole tank and its walls as its territory! In cases such as these, new additions are usually singled out for special attention, which may last for hours, days or even weeks. Fortunately, most aggressors tend to accept an inevitable situation and often refuse to expend copious amounts of energy on a fish that refuses to go away. Thus, the livestock community generally settles down once more.

In common with damselfish, anemonefish are also territorial — and the symbol of their territory is their anemone. Many aquarists keeping anemonefishes without an anemone will see a distinct change in behavior because the fish have nothing to occupy nor defend! Lethargy is a common reaction, as is a distinctly aimless wandering. At the same time, it should be understood that many aquarists have great difficulty keeping anemones alive and thriving. The fish are much easier to keep.

The whole concept of territoriality within the confines of an aquarium raises some interesting, and often uncomfortable, questions. Our responsibility as aquarists is clearly to allow each species to behave as naturally as possible. Failure to do this means we have chosen to ignore their needs, and it is they, not us, that must ultimately pay the price.