Laboratory and quasi-field studies on interspecific competition between Bulinus globosus and B. tropicus (Gastropoda: Planorbidae)
Competition between Bulinus tropicus and Bulinus globosus was investigated under laboratory and quasi-field conditions. Only the latter species serves as intermediate host for Schistosoma haematobium in Zimbabwe. Growth, egg laying and survival were followed for various densities of the two species in either single-species or mixed-species aquaria, i.e. 2, 4, 6 or 8 snails in single-species aquaria and 2/6, 4/4 and 6/2 B. tropicus / B. globosus in mixed-species aquaria. A separate experiment was done for mixed-species aquaria where the two species were physically separated by a mesh. The results indicated a significant reduction in reproduction by Bulinus globosus in direct competition (unpartitioned aquaria) with B. tropicus. The presence of damaged egg masses (partly consumed) in these aquaria suggests that B. tropicus preyed on B. globosus egg masses. Furthermore, B. tropicus was found to start laying eggs at an earlier age than B. globosus, and its eggs had a shorter hatching time than those of B. globosus. Quasi-field experiments showed that B. globosus populations were slightly less prolific in the presence of Bulinus tropicus although the effect was not statistically significant due to a great variability between ponds and possibly the observation period was too short. Further studies using enclosure experiments are suggested.