The Cichlid Fishes of Lake Malawi, Africa

Abstract of Doctoral Dissertation

Peter Nicholson Reinthal
Duke University, 1987
Adviser: John G. Lundberg  

Morphology, ecology, and behavior of a group of the rock-dwelling cichlid fishes (Cichlidae: Perciformes) from Lake Malawi, Africa  

The morphology, ecology, and behavior in a subset of a monophyletic assemblage of rock-dwelling cichlid fishes for Lake Malawi, Africa were analyzed. These were studied to determine mechanisms of ecological coexistence in this high diversity community, the behavioral and ecological correlates of species differentiation, and morphological divergence in relation to ecology and phylogeny. Neurocrania were examined for shape and size variation. Principal components and regression analyses were performed on a set of 24 measures of the neurocrania of 86 individuals from eleven species. The results suggest effective morphological discrimination between species and that most structural differences were the result of shape changes based on size. However, the vomerine region showed distinct shape changes independent of size. Vomer shape was correlated with foraging behavior and feeding habits, and may be important in permitting partitioning of resources.

Length of the gastrointestinal tract, relative to standard length, reflected differences in diet and habitat in 16 species. Species feeding primarily in sediment-rich environments have longer intestines than those feeding on more animal material or in sediment-free environments or both. Coiling patterns of the digestive tracts were found to be similiar throughout the group and were not considered informative as phylogenetic characters.

Stomach analyses of twelve species of herbivorous cichlids show partitioning of algal resources. Dietary differences ranged from species (e.g. Pseudotropheus tropheops "orange-chest") feeding primarily on blue-green algae of the genus Calothrix to species (e.g. P. zebra) feeding primarily on diatoms (especially Melosira sp.). Dietary differentiation allows evolution toward further levels of specialization through feeding on different of the same trophic level (e.g. algae).

A survey of the periphyton with transects indicated that the algal resources were not homogeneous but spatially patchy and variable. Through caging experiments, algal growth was found to be decreased by grazing. Feeding experiments showed that fish differentiated between various algal resources and stomach contents were not necessarily indicative of food preferences.

These data corroborate a hypothesis of natural selection for differentiation of trophic morphology through resource limitations. The results support an argument against interspecific differences being stochastic in nature, i.e., the sole consequence of genetic sampling associated with a speciation event. Behavior, morphology, and ecology are important correlates in speciation and permit fine-scale diversification in the use of resources.




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