A Comparative Study of Three Species of Bithyniidae (Mollusca: Prosobranchia): Parafossarulus Manchouricus, Bithynia Tentaculata and B. (Gabbia) Misella.

Abstract

Three species of freshwater snails of the family Bithyniidae, Bithynia tentaculata, B. (Gabbia) misella and Parafossarulus manchouricus, were compared in regard to various biological characteristics and a satisfactory culture method was devised for laboratory rearing of all three species. These snails are iteroparous and lay eggs once a year. P. manchouricus grew to adult size in about 54 days after hatching, and laid eggs 150-156 days after hatching. The bithyniid protoconch is nearly smooth, except for small, low spiral wrinkles, a sculpture which is quite different from that of the Hydrobiidae, a family in which the bithyniids are sometimes placed. Two different chromosome numbers were observed in the three bithyniid species: N = 17 in B. tentaculata and P. manchouricus and n = 18 in B. (G.) misella. There were morphological differences in the karyotypes of the three species, as might be expected, but also differences in the karyotypes of European and American strains of B. tentaculata, which was not expected. This may confirm a pre-European-settlement origin for some of the American B. tentaculata. Allozyme variations between the allopatric populations of P. manchouricus, B. tentaculata and B. (G.) misella were observed in glutamate-oxaloacetate transaminase, alkaline phosphatase-1, acid phosphatase-2, alanine aminopeptidase, esterase-4 and esterase-5 activities. The glutamate-oxaloacetate transaminase b and ing patterns were species-specific in B. tentaculata and population-specific in the Chongpyung (Korea) strain of P. manchouricus. Variability in the latter species may be related to genetics of susceptibility to Clonorchis infection. The electromorphs for esterase-4 activity also showed population-specific patterns for the Portugal, German and Michigan strains of B. tentaculata. The allozymes encoded by alkaline phosphatase-2 and alkaline phosphatase-3 loci in P. manchouricus were only associated with female snails. This is the first sex-related allozyme reported for mollusks. Laboratory-bred P. manchouricus were susceptible to Clonorchis sinensis, with an overall infection rate of 15%. However, B. tentaculata and B. (G.) misella snails were refractory to infection with C. sinensis, and therefore do not need to be emphasized in snail surveys for clonorchiasis.