Oh, Bartender, I'll Have A Glass Of "Old Aggression" On The Rocks…

As a professor teaching invertebrate zoology, one of the questions I tell my students that they need to answer about every animal they examine is, "How does it feed?" Feeding is the primary motivator for animals. Even though some animals will go without food for a time for reproductive related behaviors, if the animal cannot feed, everything else is immaterial; without food it will die. Differences in the morphology of the feeding apparatus are some of the basic ways in which animals differ, and are reflected in the many and varied body forms that animals take.

People are accustomed to thinking of animals as mobile creatures, moving from place to place. Generally, when they think about these movements, most people will realize the movements are due to the animals' requirements for food. Herbivores move around as they exhaust the tasty plants in any one place, and predators move following their moving prey. But, what about animals that don't have to chase down food? How does the "need to feed" get manifested in changes in their behavior or morphology? Often the changes show up in other aspects of their natural history. Many good examples of that are found in the suspension-feeding animals of shallow water marine environments such as coral reefs.

For animals living in the shallow water marine environments, food is almost never in short supply (a fact that is very important to reef animal husbandry and which has been commented on numerous times in the pages of this magazine - see previous articles (1, 2, 3, 4, 5, 6, 7) by Eric Borneman and myself. The amount of life, of all shapes and sizes, in the planktonic environment is well beyond easy comprehension. Additionally, shallow oceanic waters are always in motion, so this life is also continually being moved around, and it may also be moving of its own accord.

This means that the water in shallow oceanic environments, down to a depth of a few hundred feet or so, is full of small living organisms. Small living organisms are always food for some other organisms, so basically the water in this layer of the oceans is a mobile, continually self-renewed snack bar waiting to be harvested. For an animal that can utilize some method to glean food from the water, the food is never in short supply; each wave, current or eddy brings a new fresh supply of goodies to be eaten.

Not surprisingly, then, natural selection has favored the development of a lot of ways to harvest this food. There are various types of passive or active suspension feeding animals as well as animals that might be considered to be more actively predatory on specific components of the water borne array of foods. Such animals are generally not too selective about their food. As long as it floats or swims, they will try to eat it. Probably because of this general non-selectivity, hobbyists often find that these animals are relatively easy to keep alive, at least for a while. If some relatively nutritious food is placed in the water the animals will feed on it.

Consequently, most animals that reef aquarium hobbyists keep fall into the category of animals that feed directly or indirectly off the plankton. What these suspension-feeding animals don't do, however, is compete for food. Normally, for these kinds of animals, there is always enough food to go around. However, such animals do compete. They compete for the space to live, because having that space guarantees that they will get enough food. The effects of that competition for space are both significant and important for reef aquarists.

Organisms need a lot of different materials for growth and reproduction. When an essential material or required factor is absolutely necessary for the growth or reproductive capacity of organisms, and that material is in short supply, then that particular material is said to be "limiting." Only when a resource is simultaneously limiting for two or more organisms will competition occur. This should be obvious, for if there is more than enough of the resource to go around, nobody will fight over it. Only when one's survival is at stake is it worth it to fight for materials. This is because fighting for materials has its costs, and organisms are in a desperate struggle to minimize costs.

Non-biologists often think that a good definition of evolution is "Survival Of The Fittest," with the emphasis on "Survival." This all harkens back to the old saying, coined by a Victorian biologist enamored with the concept of evolution that, "Nature was red with blood in tooth and claw…" And following this train of thought, the fittest animal is the "survivor." Well, to be sure an animal has to survive to be "fit." But there is more to fitness than simple survival.

Evolutionary or "Darwinian" fitness is not simple survival; rather it is the production of surviving offspring. Within any given context, the "fittest" organism is the one that produces the most offspring that survive to live and reproduce in the next generation. This is why management of "costs" is so important. While reef aquarists are often very familiar with the concept of a budget, they seldom seem aware that their animals are also on a budget.

The budget that our animals are on is not one of cash, but one of energy and materials. Over millennia of evolution, any organisms' physiology has been fine-tuned to utilize the energy and material that is available to it in a most efficient manner. All of this energy and all of the raw materials that an animal gets, it gets as food in one form or another from the environment surrounding it. As far as animals are concerned, energy in the basic sense is sugar, and the raw material for tissue construction is protein. The primary source for both energy and material is feeding.

For suspension-feeding animals, in most shallow water marine ecosystems, there is sufficient food available at all times to fulfill, the animals' basic needs. These basic needs, in order, are:

In fact, once a basic size is reached so that sufficient resources can get collected and be allocated for reproduction, most animals cease much growth and put all of their excess energy into reproduction. The adult sizes of most marine invertebrates appear to be determined by the tradeoffs necessary to obtain sufficient food to reproduce.

In aquaria, food may often be considered to limit the growth of organisms. Often, hobbyists feed insufficient amounts of food or may only have inappropriate foods available to them. In the real world, as opposed to reef aquaria, food is never in short supply; on a coral reef, no coral, soft coral, or any suspension-feeding animal is likely to be limited by food. All other things being equal, the animals will have enough food to reach an adult size and persist to reproduce without any problems.

Well…. All other things are never equal. The "fly in the ointment" is not lack of food, but lack of space.

For animals feeding from the water column in virtually all marine benthic environments, while food is not limiting, the environment is not infinite. The limiting factor is, simply, the space to occupy whilst you feed, and live. It follows, then, that immobile bottom-dwelling, suspension-feeding, organisms such as corals, soft corals, bryozoans, tunicates, sponges, sea anemones, and some tube worms all compete for space (Benayahu and Loya. 1981; Wellington, 1982; Miller, 1998; Muko, et al, 2001a,b). Interestingly enough, these are just the sorts of animals reef aquarists like to maintain in their systems.

Non-biologists, such as reef hobbyists, often view competition with a sports or business analogy, and often seem to have the viewpoint that "good, healthy competition" is something that their animals should be able to engage in and thrive from it. Well, this is a neat idea; totally, absolutely and 100 percent wrong, but still a neat idea. It would be better to take a look at the agriculture industry. If you want good healthy crops, you protect them - at all reasonable cost - from all competition. The best competition is NO competition. In the natural world, competition kills you just as dead as does predation.

On the most basic level, competition diverts resources that might be used for other aspects of the organism's physiology. These resources include both energy and materials. Competition slows growth, causes injury and slows injury repair, and may stop reproduction altogether. The relative impact of competition is dependent on the type of competition occurring, with the least impact occurring from competition resulting from simple growth forms. Significantly greater impact occurs from competition resulting from more aggressive types of competition.

Figure 1. These two coral heads on a Caribbean reef are fighting to the finish,
and it looks like the lower one (Meandrina meandrites) is winning.

Probably the most basic type of competition is the passive competition seen when one organism grows in such a manner as to intercept a limiting resource. For zooxanthellate organisms such as corals, light, as well as space, may be a limiting resource. Many types of corals have a basic body growth form that allows them to exclude other nearby zooxanthellate organisms. They do this by overgrowing and shading them out. This type of competition is still competition for space, as the net result is that the winner is able to hold on to its turf, but it's considered to be relatively passive. Even though the environment has some effect on the animal's final shape, most of the determining factors are simply the genetics of the organism and these both would occur whether or not a potential competitor was present. Growth forms such as this include tabulate corals such as several species of Acropora. This type of competition may be seen on a small scale in many aquaria. Because most aquarists tend to keep their aquaria on a starvation budget, the contribution of the sugars produced by zooxanthellae to the overall energy budget of the animal may be quite high. At the same time, light intensity in the system may be relatively low. The net result of such conditions is that some zooxanthellate organisms, perhaps some corals, may be on the thin edge of malnutrition. Having one organism, growing over and shading such a coral and subsequently causing its decline or demise, is something that many aquarists have witnessed in their systems. Such competition can be ameliorated in any number of ways, of course. The primary way to prevent such competition from occurring would be to initially place the corals further apart. Work done on the Great Barrier Reef (Endean, et al. 1997) indicates that even small coral heads are seldom closer to one another than about one foot. That would be good spacing in an aquarium as well.

Figure 2. This tabulate Acropora is "shading out" any potential competitors. However, such growth
forms are hydrodynamically unstable, and may break up during storms or even high tidal currents.

Active aggression is a much more serious proposition than is passive aggression as it involves the allocation and expenditure of scarce resources. The detection of a nearby potential competitor is probably the most important sensory input that any sessile animal can process. Generally, if a predator is detected, a sessile animal has few options. It will either get attacked or it won't, but whatever the outcome, it can't flee. Some sessile organisms can produce protective chemicals or structures on demand, and some have defensive behaviors, but generally sessile animals are simply at the mercy of a predator that discovers them (Miles, 1991).

This is not the case with regard to competitors. Competition for space is generally not a rapid process; it occurs as two animals tend to grow together, and this is often a relatively slow process. This means that the competitors can take the time to marshal their resources for a long or decisive fight. The strategies for fighting often differ, and this difference is reflected in the allocation of resources.

Some animals are masters of the long-term attack. Generally, these tend to be slow growing animals; in the coral world, these would be animals that form massive colonies. Experimental results in the Caribbean indicate that the slow-growing massive corals belonging to the families Mussidae, Meandrinidae, and Faviidae are the most aggressive species (Lang, 1973; Lang, and Chornesky. 1990). Such animals attack their competitors using destructive agents such as mesenterial strands, tentacles and potent chemicals. Their resources seem allocated for the long-haul. They actively attempt to kill and overgrow adjacent competitors, and they can heal injury remarkably well. The flip side of this strategy is that they grow very slowly. Many of their resources appear to be allocated to being aggressive competitors. Probably as a result of this sort of strategy, massive coral colonies are some of the longest lived of animals; some have been shown to be several thousand years old. It is important to realize that natural selection has fashioned these animals not to simply "discourage" any potential competitor, but instead to KILL the potential competitor. And, the more efficient they are at killing it, the longer and more successful their life will be.

Figure 3. These corals were fighting for space in one of my aquaria several years ago. The Acropora,
at the top, was eventually killed. Notice the mesenterial strand extending from the lower colony.

It used to be thought that most stony corals did not primarily compete with chemical means, but evidence has been accumulating for several years that this is a false assumption (Gunthorpe and Cameron 1990; See Borneman, 2002, for a good discussion of this topic). The realization that "so-called" scleractinian corals are not a single evolutionarily defined group, but actually a superficial grouping resulting from several distinct and not-particularly closely related evolutionary lineages (Romano and Palumbi, 1996) helps explain some of the misconceptions. Studies within any subgroup of the scleractinians, probably while not have utility throughout the group, as the group is a false one based on superficial criteria. Consequently, studies showing a lack of chemical competition might be valid for one or a few species, but would not be valid for the group as whole. It is likely that many of these slowly growing, and very long-lived colonies are utilizing a lot of their resources to produce chemicals that are quite potent killers of potential competitors. In nature, the production of these chemical factors would naturally inhibit the settlement and metamorphosis of coral larvae near to the colony, or they might simply kill small coral colonies growing near the larger colony. Such chemical warfare is called allelopathy, and is well known amongst the soft corals, sponges, and tunicates. It is likely an important strategy amongst some of the stony corals as well (Borneman, 2002).

In an aquarium, the absolute production of toxic allelopathic chemicals would likely be quite small. However, such materials tend to be exceptionally toxic and in the confines of a small aquarium, say anything smaller than a couple of thousand gallons, the presence or absence of corals producing such chemicals could really determine the success of the entire system. The magnitude of the effects of allelopathic chemicals is absolutely unpredictable as we have no way to test for or measure them because:

The last factor above may be the most critical. Placement of a new coral fragment next to a more established fragment would be the analogue of settlement and growth of a juvenile coral settling nearby, and would trigger the production of chemicals and this could have disastrous results for the whole system.

Figure 4. Notice the central colony overgrowing the smaller colony to the left. Larger colonies have
more resources to devote to competition and often win their competitive encounters. The lack of
other colonies in this area, about a foot on a side, is probably due to the presence of allelopathic
chemicals from nearby larger coral heads. These chemicals inhibit recruitment into the
area from most larvae.

Other corals compete without chemicals, or utilize additional means of killing their potential competitors. In aquaria, probably the most important of these means are specially developed structures, called "sweeper" tentacles, used in an aggressive manner by many corals such as Galaxea and Euphyllia. Some anemones also utilize similar tentacles (Richardson, et al., 1979; Williams, R. B. 1991; Hidaka et al. 1997; Langmead and Chadwick-Furman. 1999a,b). About twenty or so years ago, the role of these tentacles as aggressive structures had not been elucidated, and they were often presumed to be functional as a means of getting extra food. If you take the time to do some reading in the older sea anemone and coral scientific literature, you may come across the term of "catch tentacle" used to describe these structures as they were thought to serve some special function in "catching" food. Well… they don't catch food, they are used to kill offending encroachers on the space of the coral or anemone, and they can do a pretty good job of it. These specialized tentacles contain a more potent armament of nematocysts, both in types and numbers than do regular tentacles. What I think is interesting about such tentacles is their absolute, and large, size. In many Euphyllia these tentacles may reach 12 to 15 inches in length, and they are more-or-less transitory. The animal will extend them out at will, and this may not be when the hobbyist is watching their tanks. The first time such structures are seen, it may be both enlightening and frightening.

My first experience with sweeper tentacles came shortly after I got my first Euphyllia, about 15 years ago. I was familiar with such structures, in the abstract; I had read about them, but had never seen them "in action." I brought this beautiful coral home from the store, and set it up in my tank. All was well for a while. A couple of days later, however, I came home from work, grabbed a cool can of suds from the fridge and sat down in front of my tank. I then got to watch as this long, thin, structure rose out of my coral and quite deliberately started to be moved around the coral. It was truly fascinating. Even more fascinating was the response of the animals that it touched. The tentacle gently touched the mantle of a Tridacna that was near the coral, and at each point of contact, a white spot appeared. Within an hour, each of those spots was a hole through the mantle. Where the tentacle touched some zoanthid polyps, those polyps simply contracted, and never opened again. Where it touched another coral, well, that was even more interesting, because it seemed to elicit swelling and inflation responses, but it didn't seem to kill it, at least not after the first contact. It didn't get a second chance. After finishing my can of foamy liquid (one must have the proper priorities), I rearranged my tank so that my coral didn't have the opportunity to "reach out and touch someone."

Competitive aggression in aquaria, then, may have three main manifestations; one is specific, relatively short-ranged, and determined by how far the animal can reach to do its damage. The second is more system wide, and the effects are determined by the potency of the allelopathic chemicals and their abundance. The final method would be by much slower processes such as physical overgrowth.

The responses to these attacks can vary, but at the very least, the detoxification of the chemicals takes energy away from the recipient, and this is presuming that the chemicals can be detoxified. Repair of the injuries caused by sweeper tentacles, and other aggressive structures such as mesenterial filaments, is more straight forward, but it takes both structural materials such as proteins and food energy.

Judging by looking back through the various "Tanks of the Month," I feel that it is probably fruitless to suggest that most hobbyists should try to avoid competition in their systems. Many of these systems are "Poster Tanks" for competition in action, and this kind of tank seems to be continually thought of as "the way to go." Nonetheless, such tanks containing many Bonsai'd versions of corals placed cheek-to-jowl (if corals had cheeks or jowls) basically looks to this scientist like an experiment in competition run amok. The one saving grace of many of these systems is that some of the corals in them, such as many of the Acropora, often do not seem to be particularly good or effective competitors using chemicals or stinging defenses. In nature, many of them seem to fill the ecological niches of "weeds;" that is, they come into a system, grow up fast and reproduce, and then they get replaced by more successful, but slower growing animals which outcompete them for space. Basically such corals are adapted to living in situations where the environment is in a continual state of disruption, which in nature is largely accomplished by storms. These corals thrive in the continual disruption of reef aquaria, and as they are not particularly accomplished competitors, they persist where more long-lived and better competitors would be severely suffering from competitive claustrophobia.

Many hobbyists seem to choose these potentially poorly competitive corals for their systems. Such judicious or, more likely, accidental choices of animals that are probably relatively poor competitors are a simple, and unconscious means of maintaining the competitive pressures in reef aquaria at a low level. Many of the better competitors, such as some of the Euphyllia species, have the reputation of being difficult to keep, probably because of their competitive natures. Additionally, as almost all hobbyists like to see rapid growth, they tend to choose corals that grow rapidly. The slower growing and, coincidentally, prolific producers of allelopathic chemicals are simply not chosen by many hobbyists. If they are chosen, however, such corals may, in time, encounter situations in the close-packed environment of a hobbyist's tank that stimulate the production of poisons in quantities that allow their accumulation in our systems. In such situations, these corals are likely to end up poisoning themselves as well as the rest of the system. The relative ease by which many of the smaller mouthed corals may be kept may simply be a reflection of their weedy, non-competitive nature, which aquarists have adapted to their benefit.