Sunday, November 22, 2009

African Cichlids: evoluton for the home aquarist

I hope to fill my next fish tank with cichlids, the bright and colorful fish native to South and Central Americas (oscars, angelfish, discus) and to African lakes (including the delicious tilapia). Those familiar with Africans will know that each species has special water requirements depending on the lake they originate from. African cichlids, besides being important in the aquarium trade, are understood by biologists to be a prime example of rapid evolution and speciation.

Speciation of large animals (like fish) is often an incredibly slow process, happening on time scales not relevant to the human mind’s perception of the world. Within African lakes, notably, Lake Victoria, Malawi and Tanganyika, the number of cichlid species have exploded in the last few hundred thousand years. There are an estimated 1,600 species of cichlid inhabiting Africa alone. Since these lakes are also ephemeral over geologic time, they evaporate and fill again cyclically, so this rate of speciation is simply astounding. Lake Victoria for example was completely dry 12,000 years ago, and yet is now home to over 300 species of native cichlids. This indicates colonization from surrounding satellite lakes (in flood events perhaps) and rapid speciation. There have been questions raised whether our taxonomic grouping of these fish is correct. That is, whether these are truly different species, or just a variety of morphs of a few species. Genetic evidence supports the former.

One tool biologist have to study speciation is through changes in mitochondrial DNA (mtDNA). Mitochondria organelles within cells have their own unique DNA, from a very early endosymbiosis event where a bacterial cell was engulfed by a cell and co-opted to perform a particular function. mtDNA has a mutation rate higher than the genomic mutation rate in animals so changes can be measured over smaller scales, ideal for trying to determine variation within related species. By comparing the number of changes in a set region of mtDNA, scientists can determine the common ancestry of these species assuming a fixed rate of change of 2.5% every million years. Between the three major African Lakes, it was found is that Lake Victoria and Malawi species were much more similar (1-2 million year common ancestor) than compared to Tanganyika (2-4my). For a comparison, the Victoria and Malawi stock have only a 5% difference in mtDNA, while within South American cichlid stocks, the different is 11%. This low difference supports the hypothesis of very recent speciation. In addition, the mtDNA sequences of Lake Victoria cichlids indicate a monophyletic origin – meaning the founding of the stock was from a single species.

The genetic data also disproves a previously held belief about cichlids. It was found fish within the same lake are more related than to other fish that look alike within a different lake. Similar morphological adaptations in different lakes may be the result of convergent evolution. Convergent evolution is when different species can develop in parallel ways to adapt to their environment; think sharks and dolphins- morphological very similar but genetically unrelated. Speciation and convergence seen in African cichlids may be due to adaption to niche habitats. Within a single lake, there are cichlids that inhabit cavernous rocky shores, open sandy lake bottom, and others even living in discarded shells. Lake environments also have limited resources which put pressure on species to form niche diets as well. While some cichlids are omnivores, many species specialize in grazing on algae, insects on the surface, or some hunting other fish.

African cichlids are a prime example of evolutionary theory in action. Though we may not have much longer to study these wonderful creatures. There are 265 species of cichlid on the ICUN red list, with 109 of these listed as either “endangered” or “critically endangered.” Several cichlid species have been driven to extinction, and others only being kept alive in aquarium. Environmental degradation and introduced species are a serious threat to the population of these fish, particularly in Lake Victoria…a topic I may come back to at a later date.

Johnson, T. C., C. A. Scholz, et al. (1996). "Late pleistocene desiccation of Lake Victoria and rapid evolution of cichlid fishes." Science 273(5278): 1091-1093.

Meyer, A., T. D. Kocher, et al. (1990). "Monophyletic origin of lake victoria cichlid fishes suggested by mitochondrial-dna sequences." Nature 347(6293): 550-553.

images: wikipedia

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