Control of shell shape in Lymnaea Sltagnalis

Abstract

A component of shell shape was measured in two populations of the freshwater gastropod Lymnaea stagnalis, and a highly significant difference noted. Samples of juveniles were then taken from both natural populations, and cultured in the laboratory in tanks containing water from the pond inhabited by one of the natural populations; that is, one of the laboratory populations can be regarded as a transplant, the other as a control. These two cultures were maintained until substantial growth of the juvenile snails had occurred. The shell shapes of the snails were then measured and the two populations found to be almost identical. The difference between this comparison and the one involving the two natural populations was caused entirely by a marked shift in the shell shape distribution of the transplanted population. Thus the large difference in shape observed between the populations in the wild was completely due to direct environmental effects on the phenotype, or any genetic component of the variation was so small as to be undetectable by the method employed. It is noted that this result cautions against acceptance of the evolutionary inferences that are often drawn from studies of phenotypic variation in shell shape where these are unaccompanied by demonstration of an inherited component of the variation. © 1982, The Genetical Society of Great Britain.