The Effect of Water Conditioned by Either Helisoma duryi or Biomphalaria camerunensis on the Growth and Reproduction of Juvenile B. camerunensis (Pulmonata: Planorbidae)

Abstract

The effect of conditioning of water by either H. duryi (Wetherby) [a potential biological control agent of schistosomiasis vectors] or B. camerunensis (Boettger) at various population densities on the growth and egg laying of juvenile B. camerunensis was investigated. The experimental system was balanced aquaria where water was conditioned for 4 wk before introduction of assay B. camerunensis into screened cages. No growth inhibition of assay snails in H. duryi conditioned water was indicated for an initial period of 3 wk; a growth promotion was indicated in conspecific conditioned water. During the next 3 wk a growth inhibition of assay snails was found in H. duryi conditioned water; there was no effect in conspecific conditioned water. Assay snails from H. duryi conditioned water had a higher egg production than snails from B. camerunensis conditioned water. Comparing growth of assay snails with the growth of the conditioning snails (as biomass production) a positive correlation was found when considering only aquaria where growth of the conditioning snails was less than 150 mg; a negative correlation was found for aquaria with a growth of conditioning snails more than 96 mg. Most aquaria with B. camerunensis conditioning were found in the 1st range while most aquaria with H. duryi conditioning were found in the 2nd range. Egg laying (as biomass) was negatively correlated with the final Ca concentration and conductivity, and positively correlated with the growth of the conditioning snails. Egg production of assay snails as a fraction of the total production was higher in H. duryi than in conspecific conditioned water. The possible explanations for these effects are discussed. There is no evidence for the presence of inhibitors of pheromone nature secreted by H. duryi.