Crush-resistance of soft-sediment gastropods of Lake Malaŵi: Implications for prey selection by Molluscivorous fishes

An increase in human urinary schistosome transmission in southern Lake Malaŵi has been suggested to be associated with decreased density of molluscivorous fishes due to illegal seine-net fishing from the shore. In addition, the increased density of snails (Melanoides spp.) through the invasion of an Asian morph could have changed the predators' prey choice. At Chembe village, the intermediate host snail of urinary schistosomes, Bulinus nyassanus, constitutes <5% of the total gastropod fauna. This study was designed to compare crushing resistance of the intermediate host snails Bulinus globosus and B. nyassanus with that of Melanoides tuberculata, which dominates the gastropod fauna. A crush value index (CVI) as an indicator of potential prey value was expressed as the ratio of benefit (weight of snail tissue) to cost (crush resistance of snail shell). Bulinus globosus had the highest CVI. Using shell height as measure of snail size indicated that B. nyassanus had higher CVI than M. tuberculata within the size range of snails consumed by Trematocranus placodon, one of the molluscivore fishes. This may be one of the reasons that B. nyassanus is a preferred prey of T. placodon. In spite of this preference, the reduced population of T. placodon has not been able to control the population of B. nyassanus because of its apparent opportunistic feeding on large numbers of M. tuberculata.