The Life-History, Population-Dynamics and Polymorphism of Cernuella-Virgata (Mollusca, Helicidae)

Abstract

The terrestrial snail, Cernuella virgata, was sampled over 3 years in a permanent pasture and adjacent roadside vegetation at Mt Benson, S.A. C. virgata has an annual life cycle, breeding from autumn to winter. Shell size and the presence of a rib which forms on the inner margin of the shell aperture of large snails during early autumn were used to separate the two cohorts of snails (adults and their young) present during winter. Snails (>6 mm shell diameter) were most abundant in the pasture in spring and in the roadside vegetation in summer. I discuss the importance of dispersal between habitats in contributing to these changes in abundance. C. virgata was aggregated within the pasture at all times of the year, but especially during summer when large proportions of the population (>50%) aestivated on the weed Marrubium vulgare. C. virgata and another abundant snail, Theba pisana, were rarely found together in large numbers in small areas (0.25 m2) within the pasture although both had similar general distributions. Shell sizes and banding patterns of C. virgata varied geographically throughout south-eastern Australia. No correlations between climate and size and banding were found. Size was, however, inversely related to population density. At Mt Benson, unbanded snails were slightly more prevalent in the pasture compared with the roadside vegetation. Shell growth and formation of the marginal rib of C. virgata were advanced by exposure to warm, moist conditions in a glasshouse in summer, but maturation of the albumen gland, and hence reproduction, was not.