Commensalistic Feeding Relationships of Three Lake Malaŵi Fish Species

The endemic, haplochromine, cichlid species flock of Lake Malaŵi represents the most diverse assemblage of fishes of any of the world's freshwater lakes. Of particular interest are the interspecific relationships that permit so many species to coexist in this unique ecosystem. We hypothesized that a detailed study of the feeding associations among three sand-dwelling species, Taeniolethrinops praeorbitalis, Cyrtocara moori, and Protomelas annectens, would provide the necessary information needed to determine if these species are effective in harvesting specific food resources. Taeniolethrinops praeorbitalis feeds by plunging its snout into the sand, engulfing a mouthful of sand, retaining prey items, and expelling the sand through its gills. Immediately after T. praeorbitalis plunges into the sand, other species, including C. moori and P. annectens, move closer and begin to forage on the invertebrates suspended in the water column by the disturbance of the substrate or expelled through the gills of T. praeorbitalis. The gap between the gill rakers of T. praeorbitalis is significantly (P < 0.05) wider than that of the other two species; the interraker gaps of C. moori and P. annectens are not significantly (P > 0.05) different from each other. Most P. annectens consumed small Arthropoda and Cladocera (0.3–0.8 mm), whereas most C. moori and T. praeorbitalis consumed chironomids (mean prey size = 4.2 mm). Although there was an association with interraker gap size and prey size when P. annectens and T. praeorbitalis were compared, this relationship was not present when the stomach contents and interraker gap of C. moori was contrasted with those of the other two species. Thus, we concluded that differences in prey size among these three species is a function of interraker gap, prey selectivity, and foraging behavior.