Cepaea nemoralison clifftops in south-west England

Abstract

Samples of the helicid snailCepaea nemoraliswere taken from the sea cliffs of south-west England. Patterns in the frequencies of the shell colour and banding polymorphisms around the coast are described. An increase in five-banded morph frequency from north- to south-facing coasts, at the expense of mid-banded, has been shown to occur around other westerly-facing peninsulas. The findings are compatible with the hypothesis that visual selection, as a force affecting morph frequencies, is overridden by climatic selection on the more exposed cliffs of north Cornwall. The factors influencing shell colour polymorphisms are not evident from this survey. Two areas with unusual morph frequencies, in north Cornwall, were chosen for more intensive study. The first area has a high mid-banded morph frequency associated with the workings of an old tin and copper mine, and a high five-banded frequency inland from this; nearby coastal populations are characterized by a high unbanded morph frequency. An hypothesis to explain the situation, in terms of invasion from inland of the mine workings when mining ceased, is put forward. The second area, a length of clifftop with unusually high five-banded morph frequency, is described. It seems likely that the presence of a contiguous duneland population, with the high five-banded frequency typical of duneland populations, affects morph frequencies on the clifftop. Further evidence to support this view is provided by the unusual distribution of the snail populations on the clifftop. In this area, clifftop populations ofC. nemoralisare confined to sheltered habitats, and are absent from cliff-slopes, a habitat on which they are common elsewhere along the coast.