Fish predation as a factor in the distribution of Lake Malawi gastropods.

Gastropod densities in Lake Malawi, Africa drop dramatically below 4.5 m water depth. Weed beds disappear and the number of molluscivorous cichlids increases below this depth. When molluscivores were excluded from open sand habitat by cages, due to migration and reduced predation, the density of snails increased by 40-60% within a week. Alternatively, when adult Cyrtocara placodon were placed into the cages, snail density equaled the controls. The molluscivores disproportionately consumed snails of the genus Bulinus relative to those of the more heavily armoured genus, Melanoides. Cichlid molluscivores are hypothesized to be responsible for preventing the thin shelled bilharzia vector snail, Bulinus globosus, from successfully invading the open shore areas of Lake Malawi. Lake Malawi may be relatively free of this human disease because of snail predation by cichlids.