Production of Eggs and Young Snails by Adult Theba-Pisana (Muller) and Cernuella-Virgata (Da Costa) (Mollusca, Helicidae) in Laboratory Cultures and Field Populations

Abstract

The fecundities of two terrestrial helicid snails, Theba pisana and Cernuella virgata, were measured in laboratory cultures. The breeding seasons of both species extended from autumn to spring, with most eggs and largest clutches being laid in autumn. Significant positive correlations between shell size and total number of eggs produced, numbers of clutches and clutch sizes were demonstrated for both species, but these correlations were generally weak and not consistently significant for snails collected from different habitats. In field populations in pastures in South Australia, the numbers of juvenile T. pisana and C. virgata (> 6 mm in shell diameter) produced per adult were inversely related to the densities of adults at the start of the breeding season. Possible reasons for this variation in production are discussed. Previous studies with other species of helicid snails have suggested that crowding can reduce the sizes of adults and hence their production of young. Such an explanation is inadequate in this case.