|An aquarium picture of an adult male of the Christmas fulu, Haplochormis (Xystichromis) phytophagus in full breeding dress. One of the many colorful Lake Victoria cichlids. Photo by Paul V. Loiselle.|
(This article was originally published in "Cichlid News magazine" Aquatic promotions, Vol. 10. No. 1, January 1998; pp. 30-31. It is here reproduced with the permission of author Ron Coleman).
In a previous column we examined evidence that evolution is occurring at a frantic pace in the Great Lakes of East Africa and in particular Lake Victoria (Coleman, 1997). The questions of why and how this is happening intrigue hobbyists and scientists alike.
In a recent paper (1997) in Science, Ole Seehausen, Jacques van Alphen, and Frans Witte from the University of Leiden (the Netherlands) examined the decline of cichlid diversity in Lake Victoria with an eye toward understanding the mechanisms that have generated and continue to maintain so many species of cichlids. The tragic decline of Lake Victoria cichlid populations is well known among cichlid hobbyists and even the public at large. Increasingly it is regarded as one of humankind's great environmental follies. The introduction of the predatory Nile perch (Lates spp.) thirty years ago has had profound effects on the numbers and diversity of Lake Victoria cichlids, eating many species to (and some beyond) the point of no return (Goldschmidt, 1996). And yet, there appears to be more going on than a predator-run-amuck. Some species of rock-dwelling haplochromines avoid the Nile perch, and yet they too are in decline. Why? Understanding this mystery probes at the roots of cichlid diversity in the Great Lakes.
Seehausen et al. (1997) bring together a number of facts to suggest an answer. First, not all lakes in Africa have explosive radiations of cichlid species. Those that do have distinctly clearer water than those that do not. Second, there are no post-mating reproductive barriers in Lake Victoria cichlids. This means that males and females of different species can and will mate to produce viable hybrid offspring. Hobbyists have become keenly aware of this potential for successful hybridization, and conscientious hobbyists keep species isolated from each other in separate tanks (Rambo, 1997).
But how do these species maintain reproductive isolation in the wild? In Lake Victoria, it appears that coloration is the key. The males are stunning. Each male advertises its species and its quality by intense displays of blues, reds, and yellows in an attempt to be the "chosen one." The females are the choosers. They are drab, in many cases defying distinction among species.
A female selects the best male from those that she sees, based on her full-color vision. So it is mate choice - by females of males - that ensures that a female gets a male of the right species. There is a intriguing correlative pattern in male coloration that offers further insight. This concerns cichlids that live in rocky areas separated from other such areas. In each such area there may be a pair of closely-related species, called sympatric "sibling" species (Seehausen, 1996). Interestingly, males of these species pairs tend to diverge in coloration toward the extremes of the spectrum. If one is predominantly blue, the other tends toward reds or yellows. Could this be to ease the task of discrimination for the females?
Seehausen et al. (1997) conducted a laboratory experiment in which they presented females with a choice of sympatric males. Females consistently preferred conspecific males, i.e., those of the same species as the choosers, over heterospecific males. But this only occurred if the tests took place under full-spectrum light. When the tests were repeated under monochromatic light (that made males of both species look the same), females failed to distinguish between the two types. Clearly females are using color to identify and choose males.
As the human population around Lake Victoria has grown, so too has its effects on the lake. The water has rapidly become cloudier in the last few decades. If female cichlids are using male coloration to identify mates, a decrease in visual acuity may cause difficulties, leading to "wrong" choices. To examine this possibility, Seehausen et al. (1997) performed a number of measurements on the lake and its fishes. They discovered that "intensity" of male coloration, i.e., the degree to which males are "more red" or "more blue," is best explained by the light levels where the males are found. In bright light the researchers found brightly colored fish. In dimmer light - the result of eutrophication from human activities - they found duller-colored fish. This relation held both between and within species.
Further, these color differences were heritable, meaning that when brought into the lab "bright" fish gave rise to "bright" offspring, regardless of light levels. This tells us that it is not something in the water (or the water itself) that causes the color differences directly. Rather, the males appear to have been selected for their particular color expression.
The Lake Victoria pattern is surprising in comparison with other groups of fishes. For example, the bright gold phase of the Midas cichlid (Amphilophus citrinellus) of Nicaragua is most common in more turbid lakes, not in nearby clearer lakes. And the brightest guppies are not found in clear water. In both cases, keen-eyed predators eliminate those fish advertising too boldly. The contradiction between Lake Victoria cichlids and other fishes suggests that mate choice is the driving force in Lake Victoria: the boldly-colored fish are boldly-colored because of female choices, despite the cost of predation to the males.
The question then becomes: how exactly is mate choice affected by reduced light penetration in polluted waters? Seehausen et al. (1997) suggest that females are less able to distinguish between males of different species and might make mistakes. This mixing of gene pools results in a "dulling-down" of the intensity of males colors and a loss of diversity.
It remains to be shown that errors in mate choice are actually the cause of the observed trends; how bad does the water have to get before a female will make mistakes? Careful additional experiments are needed to address this question. However, if these authors' hypothesis is true, pollution in Lake Victoria and other African lakes may be reducing cichlid diversity in a much more insidious manner than the obvious effects of predation by Nile perch.
- Coleman, R. M. 1997. Cichlids and science: how old is Lake Victoria? Cichlid News 6(1):14-15.
- Goldschmidt, T. 1996. Darwin's Dreampond: Drama in Lake Victoria. MIT Press, Cambridge, Massachusetts.
- Rambo, C. 1997. Getting started with Lake Victorian haplochromines. Cichlid News 6(3):22-28.
- Seehausen, 0. 1996. Lake Victoria Rock Cichlids: Taxonomy, Ecology, and Distribution. Verduijn Cichlids, Zevenhuizen, Holland.
- Seehausen, 0, van Alphen, J. J. M., and F. Witte. 1997. Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 227:18081811.