Speculating About Species

Left: Tropheus moorii Red Rainbow, from Kala and Right: Tropheus moorii Murago, are they the same species? Photos by Ad Konings.

What species is that? How many species of cichlids have you kept? Is that a new species? For that matter, how many species of cichlids are there? These common questions hinge on one of the most elusive and troublesome concepts in biology, namely, what exactly is a species?

The concept of "what is a species" is on the one hand simple, and yet so fraught with difficulties that it is amazing that we can use the term at all.

Humans have an intuitive concept of species? all the things that look alike are a species. There is a good biological basis for this notion of species. It served our ancestors well, and continues to do so today, to be able to distinguish tigers from deer, venomous snakes from harmless ones, tasty plants from inedible ones, etc. Humans naturally group things that look alike.

This doesn't always work. Consider the mbuna of Lake Malawi. In many of these species the males are distinctive from each other. The females however all look very much alike such that the female of one species looks far more like the female of another species than either resembles the male of its species. Looks can be deceiving. We learn this early in life when we find out that all dogs, despite the great differences in appearance between a sheep dog and a Chihuahua, are actually one species. They all came from the same ancestral stock, and in theory, they can all breed together. So the ability to breed must be key to the notion of species.

The most commonly used species concept in biology is based on breeding. It is called the "biological species concept." In technical terms, it goes as follows (though there are many subtle variants): " a species consists of groups of individuals that can actually or potentially breed with each other and are reproductively isolated from other such groups." This is straightforward enough. If they breed together, they are the same species.

But why is the word "potentially" in the definition? Because not every convict cichlid actually breeds with every other convict cichlid. We recognize that there are practical limits to how much a convict can breed (though convicts certainly try to push those limits!). What we mean is that if we put any male convict with any female convict, in general, they could breed. The problem comes when we realize that our prolific little convict, in the absence of another convict, will also readily breed with a Texas cichlid, a Jack Dempsey, or a host of other species. Does that not constitute "potentially"? But you might argue, a convict would never come in contact with a Texas cichlid in the wild. True, but it might come in contact with another member of the genus Archocentrus, namely A. septemfasciatus. In fact both are found in the Rio Puerto Viejo in Costa Rica where I do field work. They will hybridize in an aquarium in the absence of potential mates of the same species, but they do not appear to hybridize in the wild. The bottom line is that the biological species concept is nice in theory, but it has a few problems on the practical level, as we shall see.

Besides the issue of hybrids in captivity, there are far more pressing problems with the biological species concept, namely that it doesn't work well for museum specimens preserved in formalin, for non-breeding juveniles, or for single specimens in a pet store. So while the biological species concept is the most recognized species concept (and the correct answer to an exam question, should you be asked), we also need more practical approaches to determining what a species is.

In most cases, both hobbyists and scientists revert to what "looks alike." We may do it with highly sophisticated tools ? that lately include molecular techniques and advanced mathematics ? but ultimately we group things that look alike into a species.

But how alike do organisms have to be to be considered the same species? Mammals are relatively simple to work with visually. There aren't that many species of mammals (roughly 4000 or so compared with over 25,000 species of fishes), and humans are attuned to the appearance of mammals. For example, if you look at zebras, you see that not all of them look alike. Some have dark wide stripes, other narrow stripes. Some have stripes on the belly, other don't. With those distinctions we can readily identify several species of zebras. Furthermore, when we watch them in the wild, we see that their behavior supports our notions: the different types live in different localities, and so we call them separate species.

Humans are less attuned to the appearance of fish. It takes substantial time for a person new to cichlids to learn to distinguish the many species in any given group. For example, at first all mbuna look very much alike. No two individuals are exactly alike, but the experienced cichlid keeper will know how to distinguish the variation between two individuals of the same species from variation between species. In some cases this is a matter of extent of a character versus presence/absence. For example, female convicts have orange flecks on the belly. Female convicts differ substantially in the extent of the orange but no other closely-related species has the orange. It is this training and attention to detail that allow the advanced cichlid keeper to readily distinguish one species from another. The key to developing this skill is to look at many different specimens. Unfortunately, the costs of publishing make it impossible for a book or magazine to show twenty different specimens of each species to illustrate the variation. This is an ongoing problem.

Between individual differences and species-level differences, come differences at the population level. These are some of the most difficult to deal with scientifically. Even if all individuals in one area look more or less alike, with some individual differences, individuals from farther upstream may be longer and more slender, while those near the mouth of the river may be deeper-bodied. These fish never come in contact with each other, so they don't interbreed, but they probably would if you put an individual from one population in the other. But they might not. The differences between populations grade seamlessly into the differences between species. Tropheus moorii illustrates this superbly. .Each of the many "color variants" of T. moorii is found in different locations in Lake Tanganyika, but aside from certain aspects of their color, they look very much alike. Are they separate species? Fishes, and cichlids in particular, make this process of identification even more difficult because they often go through many changes as they grow. Contrast this with birds: birds grow then stop growing. We call this determinate growth. Fish keep growing throughout their lives, even after reaching sexual maturity. We call this indeterminate growth. A field guide to birds often states the size of the adult, and that size is a reliable indicator of the species. One species might be tan with a black eye mark and 20 cm long, while another might be tan with a black eye mark but 25 cm long. That size differential is enough to separate the species. Not so with fish. Given time and food, the 20 cm fish will likely be 25 cm. long one day.

Many cichlids change color as they develop. Tropheus duboisi is an excellent example. The juveniles are black with beautiful white spots. The adults have no such spots. As mentioned earlier, there may also be substantial differences in the appearance of the two sexes, one, both, or neither of which might resemble the juveniles.

Finally, if you can deal with all that, there is the matter of changes in appearance due to behavior. Most noticeable are the changes in color exhibited by Central American or West African cichlids. Many birds change into a breeding plumage but this takes days, weeks, or even months. Many cichlids change appearance on a second-by-second basis. Try putting a small mirror in front of a male of about any Central American cichlid and watch his color change as he confronts the "intruder." But it isn't just color. Some males grow nuchal humps during breeding. We also now know that even such characters as lip morphology and tooth shape can be dramatically altered by diet.

If that wasn't enough, there are even ecological issues, not detectable unless you are in the water with the fish. For example, the Midas cichlid (Amphilophus citrinellus) and the red devil (A. labiatus) look very much alike. They both come from the Great Lakes of Nicaragua where they are found living together. However, when you watch these two species in the wild, you can see that they are in fact living separate lives.

Will things get simpler in the future? Probably not. There are still more cichlids being discovered all the time. For example, Ole Seehausen and his collaborators (1998) recently described fifteen new species of rock-dwelling cichlids from Lake Victoria. Luc de Vos and Lothar Seegers (1998) just described seven new goby cichlids from rivers flowing into Lake Tanganyika. Even for well-studied areas of the New World, we've seen several descriptions of new forms in the past few years (e.g., Loiselle, 1997) with still more to come.

Is all of this academic? Absolutely not. The definition of a species affects everything from where a fish ends up in your local fish show, whether you get Breeder's Award points for having bred a "new" species (as opposed to a novel variety of an established one), to the price of a fish at auction. Each new species offers tremendous insight for science and our understanding of how nature works. Species concepts also severely impact conservation decisions. If a fish is endangered at one location, but then a similar animal is found in an adjacent lake, whether or not it is the same species determines the status of both populations.

The one constant amongst this swirling chaos of competing ideas and concepts is that the family Cichlidae - as with so many of the "big issues" in biology - offers both the greatest challenges and the greatest opportunities for understanding.

(This article was originally published in Cichlid News Magazine, Oct-99 pp. 32-34, It is reproduced here with the permission of author Ron Coleman and Aquatic promotions).