The Cichlid Fishes of Lake Malawi, Africa

Summary of Publication

Kuusipalo, L. 2000. Evolutionary inferences from the scale morphology of Malawian cichlid fishes. Pp. 377-397 in: Rossiter, A., and H. Kawanabe, eds. Ancient lakes: Biodiversity, ecology and evolution. Advances in Ecological Research, volume 31. Academic Press, London.  

The evolution of cichlid fishes in Lake Malawi was studied using a small structural character, fine scale morphology. Because features of the scales are presumed to be selectively neutral and to be independent of a species' niche, similarities in scale structure may give an unbiased view of the relationships among extant species.

The scale morphology of 20 endemic cichlid genera was examined. Several structures with possible value for taxonomic and evolutionary studies were identified, including six types of scale shape and three types of interradial denticle. A further character of use was the spiny area in the exposed part of the scale, which varied from non-existent to over 140 wide.

Phylogenetic relationships revealed from scale characteristics were compared with those obtained from other methods which also utilize selectively neutral characters [mitochondrial DNA (mtDNA) and enzyme structure]. The degree of relatedness of taxa was then investigated in relation to the niche occupied. When the studied species were grouped in a dendrogram according to the similarity of their scales, grouping did not reflect the similarity of niches. Furthermore, electrophoretic and mtDNA studies of this species flock suggest that, although niche is a conservative character, fundamentally new niches have been invaded several times during the evolution of the studied species.

The single basin structure of Lake Malawi offers no clear geographical barriers to facilitate allopatric speciation, but lake sediments indicate this seemingly homogeneous environment to have been historically heterogeneous. Geological sediment cores contain evidence of mass mortalities of fishes, and opportunities for niche switches may have resulted from released competition following local fish kills. The relaxation of competition following such events would have provided opportunities for niche switches, which may have driven some of the speciation in this group.




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