Intraspecific life-history and morphological variations in Lymnaea peregra (Müller) (Gastropoda : Pulmonata) : environmental or genetic variance?

Three neighbouring populations of Lymnaea peregra were studied between January 1985 and December 1986. The populations differed in egg sizes, juvenile growth rates, survival regimes, breeding patterns, and total reproductive efforts. Differences in water temperatures were largely responsible for the annual variation in the timing of the breeding seasons while interpopulation divergence in growth rates, and consequently number of breeding bouts per year, was attributed mainly to varying food availability. Snails at the Don site had lower winter mortalities than the Sheaf and the Rivelin individuals, partly due to the higher winter temperatures at the former site. Moreover, faster current speeds might also be responsible for the higher mortalities at the Sheaf and the Rivelin than the Don site. The lower reproductive efforts exhibited by the Don snails as compared with the other two populations were ascribed to the low food availability, high population density and shortage of oviposition sites at the Don. The Don snails consistently started breeding earlier than the Sheaf and the Rivelin individuals. Mass-culture and controlled breeding experiments revealed that the early-breeding trait of the Don snails was heritable, and probably genetically fixed. It is postulated that the early breeding of the Don snails is an adaptation to exploit a longer breeding season. Quantitative genetical analyses indicated a genetic trade-off between the age at first reproduction and the hatchling size of the Don snails. Similar analyses also showed significant genetic variances for the juvenile growth rates and the hatchling size in the Sheaf and the Rivelin populations respectively. There is some suggestion that the snail populations showed varying reproductive investment per individual offspring as predicted by the Sibly & Calow model. This study shows that L. peregra, though primarily phenotypically plastic, can evolve local adaptations according to specific ecological circumstances.