Why Are There So Many Kinds Of Reef Animals? by Ronald L. Shimek, Ph. D.

Clichés. Clichés are true statements that get used so often that they lose their real meaning and simply become “filler,” a sort of “literary stuffing” to add words to an essay or conversation. Once a phrase becomes a cliché nobody really thinks about what its words mean anymore. Some clichés enter the common vernacular and even further debase most people’s already pitiful conversational skills. Others of them remain rather localized in their use and owe their existence and persistence to specialized areas of discourse. It really comes as no surprise that our hobby is littered with them. When discussing coral reefs, either as an aquarist or as an ecologist, sigh, one of the most frequently heard clichés relates to the biodiversity of marine life. This truism is that, “The richest coral reefs contain more kinds of animals than any other marine habitats and that they are like the ‘tropical rainforests of the sea.’” This statement is common to the point of seeming to be found in almost all articles that discuss coral reef animals. It is even in this one. Everybody reads it and, probably, by-and-large, everybody accepts it. But, what does it really mean? And what are its implications for the reef aquarist?

Figure 1. Although reef-building corals are the visibly dominant organisms on
a reef, most of the diversity is manifested in other groups of small organisms.

One of the singularly important papers in ecology of the last century was written by the eminent ecologist, G. Evelyn Hutchinson. Hutchinson was an ecologist who taught at Yale University from about 1940 through about the early 1970s. Most ecologists of note in the United States in the last 50 years were either trained by Hutchinson or his students or their students. This includes a LOT of ecologists who have studied and are studying coral reefs; to name a few: Alan Kohn, (a Hutchinson student), Charles Birkeland (a student of a Hutchinson student), Ken Sebens (a student of a Hutchinson student), Gary Vermeij ((a Hutchinson student), and in the reef hobby, Rob Toonen (a grand-student of Hutchinson) and me (also a grand-student). The importance and influence of Hutchinson’s teachings should not be underestimated. Not only did he directly train many of the premier researchers in the science of ecology, he and his students had a way of defining the problems that needed to be addressed so that they could be addressed with the scientific method.

One of his most important papers, in my humble opinion, was titled, “Homage to Santa Rosalia or Why Are There So Many Kinds of Animals?” published in 1959 in the scientific journal, American Naturalist. Unlike many papers that deal with theoretical ecology, this paper is written in clear, lucid prose, and is easily understandable by any reader (actually, a lot of interesting and important theoretical papers in many disciplines are written without recourse to difficult math; Einstein’s 1905 paper wherein he derived the famous equation E = mc² is similarly bereft of complicated math, but in this case the leaps of logic and the ramifications may be just a teeny bit more difficult to deal with). In any case, to rephrase the question Hutchinson asked in his paper, “Why are there so many different types of animals?,” Why are there not just a few “super animals” that dominate each habitat?

To break this question down even further for the coral reef aquarist or biologist, “Why are there so many different types of coral?” The question also devolves further. The richness of the reef may depend on one’s point of view. A biologist may marvel at the magnificent diversity of the many thousands of species of corals, but generally, it appears to the average coral reef aquarist that there really aren’t very many corals at all. There are “large-polyped stony corals” and there are “small-polyped stony corals.” But, often there are just “corals.” Similarly, there are just “bristle worms,” not many thousands of species of polychaete worms. Or sea stars. Or pods. Or…, well, you pick the group; aquarists certainly seem to know that there aren’t that many of them.

This profound – and given the online and print resources available, in many cases, willful - ignorance of the diversity of animals in general and coral reef animals, in particular, has a tremendous affect on our hobby, as it really destroys any chance of the successful husbandry of most of the animals. Hutchinson, in 1959, was able to precisely, and concisely, put his finger on why there are so many different types of animals, and in doing so he laid the groundwork for what would be the key to the successful husbandry and care of many different types of coral reef animals.

To an ecologist there are precise mathematical and statistical measures of diversity. To the lay person, perhaps an aquarist or backyard naturalist, the biodiversity of an area probably, and reasonably, means the number of given species in that area. In this case, then, the question Hutchinson addressed could be rephrased, “How can very similar animal species coexist in a small area?”

Organisms take things from their environment and utilize them. These “things” are termed resources and may include gases such as oxygen or carbon dioxide, dissolved ions such as calcium, foods of various sorts, electromagnetic energy, physical energy such as wave action (this is an odd resource, but see Paine, 1988), or simple space or area. The utilization of one material by an organism is inconsequential to any other organism, provided that the sum of their combined usage does not exceed the supply of the resource.

If, however, a resource is not present in sufficient quantity for all organisms to use it without limit, then the organisms will compete for that resource. Many folks seem to view biological competition similarly to competition in sports. If your team doesn’t make it this year, well, there is always next year. Except for the Cubs, of course! Biological competition is NOT like competition in sports. It kills, and an individual or a species can become just as extinct from competition as it can from disease or predation or an asteroid impact. Consequently, given that there are a lot of animals, a critical question to be asked is: “How similar can any two species be in their needs before they cannot coexist without competition?” Or, perhaps, for coral reef aquarists, the question becomes, “How similar can the needs of any two corals be, before there is not enough of some critical resource to go around?” To meaningfully address this question we have to examine the resources available to corals and coral reef animals, in general.

Some of the generalized factors of water quality in the coral reef environment are essentially the same for all coral reef animals. They all do best in water that ranges around 36 ppt salinity. Likewise, they all do best with water temperatures in the low 80s F. These, after all, are the generalized conditions of all coral reef habitats (Kleypas, et al. 1999). However, once those basic conditions are met, the natural physical requirements of many of the corals may be drastically different and are often quite specialized. Some corals need exceedingly high and turbulent water flow; most acroporids would probably fit into this category. Others need no turbulence whatsoever and do best in areas of bulk laminar flow; most gorgonians would probably fit into this category (Patterson, et al., 1991; Helmuth, et al. 1997; Mills and Sebens, 1997; Sebens, 1997; Ming-Chao et al. 2002). Similar observations may be made about many other groups of coral reef animals. For example, many sponges are adapted for the turbulent areas of a reef crest, but others, particularly tall branching forms, need laminar water flow; these latter animals are found in deeper waters or in areas of consistent constant flow. As it is presently impossible to provide laminar flow in any reef tank, those animals that need that flow and that are adapted to that flow are simply unable to be maintained for any significant length of time given our current state of aquarium technology. The reason for this is likely that they will simply not be able to feed properly, even if the appropriate foods are available, or that they will not have the appropriate current flow regimes for gas exchange or waste removal.

Similar comments may be made with regard to substrate. Both the composition of the substrate and its orientation may be important to the animals living on, and in, it. In natural situations, a limited array of substrates is typically available to any organism. This is because millennia of natural selection have resulted in spawning and larval behavior that ensure that the coral reef animals’ offspring make their way to the appropriate coral reef microhabitats at about the time that such larvae begin to metamorphose from their planktonic phase to their benthic one. When the time for metamorphosis and settlement occurs, the larvae have but a few simple choices to make to end up in the correct microhabitat. Depending upon the animal, habitat choices may be made with regard to a certain surface texture, orientation, illumination, chemical composition or bacterial covering. (The literature on this is truly immense; here are few references to get any interested reader started: Chia and Rice, 1978; Butman, 1987; Fisk and Harriott, 1990; Harrison and Wallace, 1990; Stoner, 1994; Hoegh-Guldberg and Pearse, 1995; Wilson and Harrison, 1998; Slattery et al., 1999). Once any sessile animal has metamorphosed and settled, it is generally in the same habitat for the remainder of its life.

Such niceties as the need for habitat specificity, or current flow, are generally ignored by reef aquarists. In some cases they may not know the appropriate choice; in others, such peculiarities may seem unimportant. And they are, but only to the hobbyists. Habitat choices are important to animals for one specific reason; that being, in one way or another, they ensure that the animal will get an adequate supply of food. In an aquarium, provided that the animal is adequately fed, such niceties as the appropriate habitat or orientation within that habitat may be unimportant, provided that the animal gets the appropriate nutrition.

Hutchinson, in his article, noticed two species of water beetles coexisting in a small basin of fresh water. He wondered just how similar two animals could be to coexist in a small, enclosed environment and discussed this problem in the article. Rephrased, his arguments could be related to reef aquaria without the slightest loss of relevancy. “How similar can two or more species of corals, or any two similar species of coral reef animals, be and coexist?” Before that question can be answered, perhaps an even more basic question needs to be addressed. That is, “How does one measure ’similarity?’” What might appear to us to be two very similar species of corals might be two drastically dissimilar species when it comes to the characteristics that allow them to naturally coexist.

The factors that generally separate similar species in nature are often related to how they feed. Of all the activities an animal must do, obtaining nutrition is the most important one. If an organism cannot obtain food, everything else is immaterial; it dies. In natural situations, if too many species of animals are specialized to eat the same food in any given habitat, one or more, or all, of them will likely perish. The discussion of how similar organisms can be to one another and coexist, and under what conditions that coexistence is stable, has intrigued ecologists for 40 years or more, and is nowhere near being completely elucidated. Robert MacArthur (another Hutchinson student) and Richard Levins really initiated the concept of “limiting similarity” which states that two or more species cannot exist in a stable assemblage unless they differ in some finite manner in the way in which they use resources that are limiting to each of them (MacArthur and Levins, 1967). Numerous studies have supported this concept in one way or another, and often it can be shown that some critical structure, such as beak sizes in seed foraging birds, differ in closely-related animals by discrete, finite, and constant amounts.

In the shallow-water marine environment many animals, such as corals, get much, or most, of their necessary nutrition from the waters surrounding them. Such animals are suspension-feeding organisms that depend on materials from the water for the proteins and structural materials necessary to build tissues and skeletons. Many coral reef animals also have zooxanthellae, but while these symbionts provide necessary sugars and some other organic compounds, it is the other foods such as particulate organic material, zooplankton, bacterioplankton, and dissolved organic matter that allow these animals to grow and thrive (Hamner, et al. 1988; Sorokin, 1991; Heidelberg, et al. 1997; Johnson, 1997; Mills and Sebens, 1997; Sebens, et al. 1997, 1998; Anthony, 1999; Heidelberg, et al. 2004). As these potential food sources are very important to the animals, natural selection often has acted on the animals to facilitate their collection of such materials.

The point of the above discussion is that the diversity of coral reef animals is real, it is manifested by the variety of different animals and it is maintained by the array of different microhabitats and ways of resource utilization. Among corals, that diversity is generally, but not always, manifested in their ways of collecting food (Borneman, 2002-2003). These morphological differences in the ways in which they capture their nutrients are really pretty obvious, and important to those reef aquarists astute enough to realize that “form follows function.” Close examination of the animals’ morphology, coupled with a bit of cogitation, should allow some real advances in husbandry of these animals.

Some of the changes that will be needed are obvious. As aquarists we feed our tanks a number of things. Few of them are even remotely related to natural foods. Obviously, this will affect our ability to keep these animals alive. Cnidarians, in general, capture food by the use of stinging capsules (nematocysts) in their tentacles. There are close to 30 different types of nematocysts described on a gross visual level, and undoubtedly other subtypes will be differentiated by their microstructures or perhaps by the use of chemical signatures. Each of these types is related to a different sort of food or a different method of capturing prey. Examination of the nematocysts, however, is generally beyond the scope of most aquarists. What the aquarist may see, however, is that corals differ significantly and strikingly in the size, shape, and number of tentacles that they possess. The tentacles are the basic “trophic” or “food-gathering” structures of cnidarians, and differences in them reflect either differences in food collected or differences in habitat if they are collecting the same food.

Food collection in these sessile animals is not just a matter of waiting until the food bumps the tentacles and then grabbing it. The animals will actively change their shape and orientation with relation to specific water flow patterns to maximize food collection. Some foods will be collected, but interestingly enough, others will be ignored.

Additionally, until about 30 years ago, it was assumed that most nematocysts discharged passively (see, for example, Meglitsch, 1972). In other words, if something struck a tentacle, it would automatically trigger nematocyst discharge. We now know this is not the case; it appears that all cells bearing nematocysts are enervated and that, depending on the signals coming from the nerves, the nematocysts can either be primed for, or inhibited from, firing (Harrison and Westfall, 1991). In the language of a cnidarian physiologist, this means that the item striking the tentacle must elicit the appropriate neuronal discharge. In the language of more normal people (cnidarian physiologists are an odd bunch…) it means that the item striking the tentacle must “taste” right. Only then are the nematocysts fired.

As a bit of an aside, one often sees reference to “weak” nematocysts in some soft corals such as Dendronephthya species, implying that these nematocysts are not used to capture prey, because they are too “weak” to do so (Fabricius, et al. 1995). Therefore, the supposition is that these corals may not feed, or may feed on things that don’t need to be captured with nematocysts. A legitimate question, then, is, “Why do these animals have nematocysts at all?” These items are proteinaceous secretions of cells and as such are quite “expensive” to produce. Natural selection doesn’t tolerate wastage. Animals that waste resources are rapidly replaced by animals that don’t. The more relevant pair of questions, perhaps, are: “What, exactly, are the “weak” nematocysts used for? And, are they really “weak” when actively used against a particular class or type of prey item?” I think in these cases, the bottom line is that the animals’ diets are very difficult to study and are poorly known. Perhaps, the term “weak” in this context should be more appropriately applied to the hypotheses and explanations rather than to the nematocysts.

In a reef tank, nematocyst discharge may be noticed by some simple observations. Certain food items may impact the tentacles of a coral or sea anemone and roll right off. No nematocyst discharge in this case. Other foods may trigger these capsules to discharge resulting in the item sticking to the tentacles, after which it may or may not be eaten. No nematocyst discharge will mean that the food item is either unacceptable or that the animal is not able to eat it for some reason, and for most cnidarians this may be the “litmus test” of initial acceptability for any given food. To trigger nematocyst discharge, the material has to, at least, have an appropriate chemical signature.

However, determining a truly acceptable food is a difficult proposition. We have no way of providing the items that constitute the normal foods of most of our corals. Not the least of the problems in this regard is that the diets of very few stony corals have been adequately studied, so it is quite difficult for a hobbyist to get information on what to feed them. Secondarily, we have no way of even obtaining many of these foods. It is impossible to find a bag of reef food containing the larvacean tunicates, pteropod snails, planktonic copepods and invertebrate larvae that have been demonstrated to be the food of some reef organisms (Hamner, et al. 1988; Sorokin, 1991; Heidelberg, et al. 1997; Johnson, 1997; Mills and Sebens, 1997; Sebens, et al. 1997, 1998; Anthony, 1999; Heidelberg, et al. 2004). Finally, and possibly most critically, we know very little about these animals’ absolute nutritional requirements.

Additionally, two closely-related animals may respond very differently in the aquarium or in nature to the same food. Two anemones that I worked on in a temperate region, Urticina columbiana and U. piscivora, are quite similar in gross morphology. They are large animals about the same size and shape and they have large tentacles. Even though they are sometimes found in the same general habitat, they eat quite different prey. As the name piscivora indicates, that species eats fishes. Urticina columbiana, on the other hand, appears to eat jellyfish. Although the diets of Indo-Pacific host anemones have not been adequately investigated, it is likely that the difference in tentacle structure, as well as the differences in “adhesiveness” seen in these species, indicate different food preferences; however, good quantitative data from natural populations are lacking.

Figure 4. The temperate anemones discussed above. Although they appear to be quite similar, their diets differ substantially. Left: Urticina piscivora eats fishes. Right: Urticina columbiana has been found with its gut stuffed with jellyfish. Individuals of this species reach huge sizes, by the way, over 1 m wide, and often are more massive than any Indo-Pacific host anemone.

Similarly in corals, the differences seen within even genera ??that have similarly sized polyps likely reflect either different food preferences, or different ways of catching the same foods. The differing ways of catching similar foods are likely reflected in differences in microhabitat water flow regimes. The fact that acroporid corals dominate the reef crest areas but are often relatively less abundant in deeper areas or areas less subject to turbulent flows, indicates that their overall morphology, including colony shape, polyp shape, and tentacle structure, are well adapted to those turbulent areas. In many regards, this makes them “preadapted” for the “standard” reef tanks.

Specializations of shape and habitats are seen in other corals as well and are often related to habitat variables (Anthony, 2000; Anthony and Fabricius, 2000). Often reef aquarists absolutely ignore these specializations, and this undoubtedly contributes to the absurd mortalities seen in corals such as the elegance corals, Catalaphyllia jardinei and the various Goniopora species. Elegance corals are sand or mud dwelling animals often found in very turbid waters. They live buried in the sediments with their inflated polyps laying over the sediment surfaces. They are not adapted to be kept in turbulent or high current tanks, and especially if they are placed on or in rocks where their delicate tissues can be abraded against the rocks by the continuous variations in water flow. Similarly, Goniopora species are found in turbid, nutrient laden waters of coral reef lagoons and backwaters (Veron, 1986, 2000b; Peach, 1996; Borneman, 2001). They have been shown to eat all sorts of things ranging from various larger crab larvae to phytoplankton (Peach, 1996). They also are not adapted to the vigorous turbulent flow of a reef crest tank, but rather live in the gentle, more laminar current regimes of lagoonal areas.

Neither Goniopora nor Catalaphyllia species do well in standard reef tanks; nor should they be expected to. The conditions in these tanks are quite unlike their normal and natural habitats. Yet, hobbyists continually purchase both species– often repeatedly – as they try to find the magic set of inappropriate conditions to keep them alive under the abnormal conditions of a tank more-or-less designed to keep reef crest corals alive and more-or-less designed to kill these particular corals.

Back to the cliché, again. Coral reefs are like the “tropical rain forests of the sea.” Such reefs have a phenomenal diversity of animals. That diversity is manifested in the total array of animals, not just the corals, found on the reefs. The corals, however, are the causative agent for that diversity as they create the array of microhabitats necessary to allow the myriad of animal species each to survive in its own set of specific conditions. The real reason both rain forests and reefs are diverse is that both contain an impressive array of microhabitats, each with its own specific set of organisms. However, reef aquarists seem hell-bent on ignoring all the causes of this diversity. Rather, they set up a tank that attempts to mimic a very narrow range of conditions, and then they try to grow animals from all sorts of different habitats in this one specific environment. This is rather like taking a clear-cut tropical rain forest filled with, and managed for, optimal growth of a single population of banana plants and then trying to grow arboreal orchids, termites, balsa wood, and jaguars in it along with the bananas in a stable arrangement. It won’t work. The wonder is, of course, that anybody expects it to.

It seems that it is time for those in the hobby to recognize that one size doesn’t fit all. This state of affairs also occurred long ago in the freshwater aquarium hobby when it became evident that, for example, African cichlids and guppies really didn’t have the same sorts of requirements. Why it should take so long to become apparent in the reef aquarium hobby is a puzzlement. Coral reef animals are intrinsically no more delicate or harder to keep than are any other aquatic animals. They just need to be given the appropriate conditions. Unfortunately, vendors and hobbyists alike often ignore the appropriate conditions for their survival in their quest for the unusual, beautiful or entertaining.

The concept of having a biotope or specific microhabitat tank is not new. Nor is the concept of providing animals with the appropriate foods or microhabitats. Somehow, however, there is a gap between the realization that this can be done, and the realization that this should be done. In the mean time many thousands of animals get imported only to perish under inappropriate conditions. The problem is one of education, I suppose. Ignorance is a disease easily cured by the acquisition of knowledge. The knowledge for the care of many of these animals is easily available for the reading. It may be as well, as I alluded to earlier in this essay, that most reef aquarists are simply unaware of the differences between animal species within a group, and that they perceive, for example, all so-called “sps” corals, or “pods” as being roughly equivalent. Perhaps the problem is that many aquarists are unaware that small differences in conditions may have significant effects on the survival of their animals. If so, I think it may be a very long time, indeed, before this hobby reaches a state where more than a relatively few small polyped coral morphotypes are maintained, cultured and grown. Given the effects documented by Eric Borneman, and others, on the distribution and abundances of many of the common corals and coral reef animals by harvesting them for the aquarium trade, it will be interesting to see how many of the species hobbyists treasure will fare economically. Will they become so rare that commercial harvesting is impossible, or will they become biologically extinct in the interim? This is a pessimistic view, and perhaps it will be shown to be incorrect as aquarists become more enlightened and attuned to the needs of their animals. However, it is worth remembering the statement, “The difference between an optimist and a pessimist is that the optimist believes we live in the best of all possible worlds, and that the pessimist fears that the optimist is correct.” Unfortunately, I think that change is very unlikely.

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