Changes In Competitive Interactions Between Bulinus (physopsis) Globosus (morelet) And Bulinus Tropicus (krauss) Due To Exposure To Schistosoma Haematobium Miracidia (host-parasite, Parasitology).

Abstract

Interactions between Bulinus (Physopsis) globosus, a snail intermediate host of Schistosoma haematobium, and Bulinus tropicus, a related non-host species with overlapping geographic distribution, were measured by examining changes in fecundity with multiple regression analyses. Changes in interactions between the two snail species were studied when only B. globosus was exposed to S. haematobium miracidia, when only B. tropicus was exposed to the miracidia, and when both species were exposed. When the two snail species were raised together without exposure to the parasite, fecundity of both species was lowered, indicating a competitive interaction. Bulinus globosus fecundity was completely suppressed when the snails were in competition with as few as four Bulinus tropicus snails. Whenever parasitic exposure was involved in the snail interaction experiments, the fecundity of B. globosus increased, indicating that competition between the two species was reduced or masked by snail reactions to the presence of the parasite. The role of density, density-squared, interaction and parasitic exposure variables in the regression analyses of fecundity for each species is discussed. An explanation of the results in terms of chemical factors, possibly produced by the snails, stimulating or inhibiting snail fecundity, is suggested.