Variation in a Colony of the Snail Cepaea nemoralis (L.)

Abstract

An abundant linear colony of this polymorphic helicid land-snail, living along 2 miles (3.2 km) of a very uniform artificial bank near Cambridge, was sampled at intervals of 220 yd (201 m), over 6000 snails being collected. Many different shell color and banding pattern phenotypes showed wide fluctuations in frequency along the bank apparently not related to any vegetation differences, which were minimal, nor to selective predation or other environmental factors. Samples were analysed into 3 age-groups based on growth and wear of shells, of approximately 1/2 - 2 years, 2-3 years, and over 3 years old, and certain phenotypes were found to be disproportionately represented in older age-groups. This is attributed to a differential survival of 2 - 5% per year. Since survival trend for any particular phenotype was directed the same way whether or not it occurred commonly in a sample, the often large differences along bank cannot be due to local differences in trends of survival. Marking-recapture experiments show a high degree of "population viscosity" in this colony, with an estimated standard deviation of displacement for individual snails of only 6 yd (5.5 m) per year. Adult mortality was about 50% per year. Population is now too abundant (mean density for whole colony estimated at 2.9 adults/sq yd) for observed local genetic variation to be produced by genetic drift. Numbers were much reduced by flooding 5 years before present work began, probably to levels at which the Sewall Wright effect could have operated. This may then provide an example of founder or bottleneck effect at a time of reduced numbers, rather than of continuing genetic drift. Given slow rates of dispersal, and the lack of differential environmental selection, results of this have carried over into a period of high numbers, leading to local variation, non-adaptive in origin, in a population which, although large and continuous, is not at all panmictic.