MELANISM IN THE LAND SNAIL HELICELLA-CANDICANS (GASTROPODA, HELICIDAE) AND ITS POSSIBLE ADAPTIVE SIGNIFICANCE

Shell banding polymorphism in 184 local populations of Helicella candi cans (Pfeiffer) from western Czechoslovakia was investigated. The shel ls are white with up to nine dark brown bands, which may fuse. There w as large within- and among-population variation in shell banding. An ' 'index of melanisation,'' indicating proportion of shell surface cover ed with extended or fused bands, revealed geographic patterning of dar k phenotypes. The frequency of dark forms was higher in some areas, du e perhaps to decrease of incident sunshine by fog, clouds or industria l air pollution. High and dense vegetation cover were also associated with melanism. In the laboratory, temperature of irradiated dark shell s increased more rapidly than that of light shells, and the thermal eq uilibrium of the former was higher. The differences were greatest on a white background and with low ambient temperature. In areas of reduce d sunshine, dark individuals may be at an advantage, especially during the autumn breeding period. When exposed to sunshine during summer do rmancy, light forms may also be able to maintain lower body temperatur e than dark forms.