Differences in the shell shape of Lymnaea peregra (müller) (gastropoda: Pulmonata) from lotic and lentic habitats; environmental or genetic variance?

Lymnaea peregra from lotic habitats (running waters) had larger aperture-length/shell-length ratios than contemporaries from lentic sites (standing waters). This pattern was observed in field samples from two separate years (1985 and 1986). Laboratory culture experiments suggested that shell shape could be genetically determined in at least some populations. We hypothesize that lotic snails possess bigger feet, and hence larger apertures than lentic individuals as an adaptation to withstand water current