These are exciting times for cichlid enthusiasts. All sorts of new fishes are appearing in the hobby even as old favorites are being rediscovered. Research on cichlids is also increasing as more and more scientists discover the relevance and utility (not to mention the joy and wonders) of this fascinating family of fishes. In the course of my own research on parental care and egg size, I frequently encounter interesting articles on the evolution, behavior, and reproduction of cichlids. Unfortunately most of these studies are published in rather "dry" scientific journals, which many hobbyists don't have access to. My aim in this article (and future installments of the series) is to share some of these exciting discoveries with readers of Cichlid News. My goal will be to try to capture and convey the essence of the research. If your interest is piqued such that you want to check out further details, full citations to the papers discussed will be included for your reference purposes.
Lake Victoria is a hot topic in both the cichlid hobby and scientific circles. The diversity of species once found in the lake, the human interference that has all but wiped them out, and attempts at saving those species that remain are all topics you may have seen covered in past issues of Cichlid News (see for example McAndrews, 1992; Rambo, 1993; Loiselle, 1996, 1996) and elsewhere.
We've known for some time that Lake Victoria is relatively young, especially when compared to Lake Malawi or Lake Tanganyika which are millions of years old. Yet Lake Victoria contains a staggering diversity of cichlid fishes, second only to that of Lake Malawi. A recent study by Thomas Johnson of the University of Minnesota emphasizes just how remarkable these cichlids are. In the less than provocatively-titled article "Late Pleistocene dessication of Lake Victoria and rapid evolution of cichlid fishes," Johnson and his coauthors estimate that Lake Victoria was bone dry roughly 12,400 years ago! As we all know when a favorite cichlid "goes over the top" and ends up on the fishroom floor for a few days, cichlids do not do well without water. This can only mean that the 300+ endemic species of haplochromines found in Lake Victoria must have speciated and diverged in this tremendously short period of time.
To put this in perspective, early man was well on his way 12,000 years ago. For example, paintings by humans on cave walls dating over 20,000 years old have been documented. Cichlids of Lake Victoria were (and still are) evolving and speciating literally as we humans watched!
How did Johnson et al. (1996) prove this point? Lake Victoria presently has a maximum depth of only 69 meters. Previous work had shown that the lake was at least 65 meters deeper 15,000 to 17,000 years ago, but the question remained: did small lakes or ponds remain within the basin when the overall level of the main lake dropped? If so, groups of cichlids may have survived (and diverged) in these "refugia" and then spread out to repopulate the larger lake when water levels again rose.
To test this idea, the researchers started with seismic reflection tests which allowed them to map out the topography of landforms over the bottom of the lake. After locating the deepest regions, they obtained core samples from the bottom in these areas. They found that the geography of the lake bottom is relatively uniform such that the deepest spot is not separated from other parts of the lake by ridges or other topographic relief, meaning that "if this site was dry, then the entire lake basin was dry," according to the authors. When they looked at the cores from the deeper parts of the lake, they found evidence that these locations were dry and had terrestrial grasses growing on them only 12,400 years ago. Cichlids can't live on a dry, grassy plain; therefore we can only conclude that this fabulous diversity is the product of evolution over just the past 12,000 years.
The "drying-out" data complete a picture suggested by Axel Meyer and his coworkers at the State University of New York, who have hypothesized that all the haplochromines of Lake Victoria are derived from a single species (Meyer et al., 1990). They used molecular techniques involving mitochondrial DNA to show that despite the huge variation in the morphology and ecology of these fishes (i.e., what they look like, what they eat, and how they behave), the entire group contains less genetic variation than does the human species. Furthermore, this work shows that while there may be species in Lake Victoria and Lake Malawi playing similar ecological roles, they have "converged" on these roles independently, rather than by common ancestry. In other words, rock-dwelling cichlids of Lake Victoria are more closely related to other cichlids in their own lake than they are to rock-dwellers of Lake Malawi.
Just as we learn that Lake Victoria is younger than we ever imagined possible, we now have evidence that the Amazon basin is older than previously thought. Using similar techniques as those outlined above, P. A. Colinvaux (of the Smithsonian Tropical Research Institute in Panama) and his team examined core samples from a lake in western Brazil. They found no evidence of grasses at this locality for at least the past 40,000 years. Instead, the area has been wet enough to support tropical rain-forest for this entire period, discounting the notion that the Amazon was broken up into small, isolated refugia during the last ice age. As regards the cichlids of the Amazon, this finding suggests that they may represent older lineages than we previously envisioned that have had access to large interconnected expanses of water for long periods of time.
(This article was originally published in Cichlid News magazine, Aquatic promotions, Vol. 6. No. 1, January 1997; pp. 14-15. It is here reproduced with the permission of author Ron Coleman).
- Colinvaux, P. A & P.E. de Oliveira, J.E. Moreno, M.C. Miller, M.B. Bush. 1996. "A long pollen record from lowland Amazonia: forest and cooling in glacial times". Science. (n. 274), pp. 85-88 (crc08162)
- Johnson, Thomas C & C.A. Scholz, M.R. Talbot, K. Kelts, R.D. Ricketts, G. Ngobi, K. Beuning, I.Ssemmanda & J.W. McGill. 1996. "Late Pleistocene desiccation of Lake Victoria and rapid evolution of cichlid fishes". Science. v. 273(n. 5278), pp. 1091-1093. DOI: 10.1126/science.273.5278.1091 (crc05000) (résumé)
- Loiselle, Paul V. 1996. "Fulu of the Yala Swamp - Part 1: Overview of the Fishes". Cichlid News Magazine (crc00491)
- Loiselle, Paul V. 1996. "Fulu of the Yala Swamp - Part II: Husbandry and Conservation". Cichlid News Magazine (crc00497)
- McAndrews, R. 1992. "Lake Victoria: A Great Lake In Ecological Flux". Cichlid News Magazine (crc00404)
- Meyer, Axel & T. D. Kocher, P. Basasibwaki & A. C. Wilson. 1990. "Monophyletic origin of Lake Victoria cichlid fishes suggested by mitochondrial DNA sequences". Nature. v.347, pp. 550-553. DOI: 10.1038/347550a0 (crc02555) (résumé)
- Rambo, Charles. 1993. "Report on the Lake Victoria Cichlid Conservation Symposium". Cichlid News Magazine (crc00419)
© Copyright 1997 Ron Coleman, all rights reserved
Coleman, Ron. (février 19, 1998). "How Old Is Lake Victoria?". Cichlid Room Companion. Consulté le mars 05, 2021, de: https://cichlidae.com/article.php?id=84&lang=fr.