CLINAL VARIATION AT AN ESTERASE LOCUS IN LITTORINA-SAXATILIS AND LITTORINA-OBTUSATA

Samples of L. saxatilis (Olivi) and L. obtusata (L.) were taken from populations in an estuary [Nova Scotia, Canada] where selection for certain shell morphologies had previously been established. These samples were examined for allelic variation at a polymorphic esterase locus. The allelic variation in L. saxatilis was correlated with the distance from the mouth of the estuary. Two pairs of alleles in L. saxatilis showed a significant correlation of frequency. The same pattern of variation in L. saxatilis was observed in another estuary. The allelic variation in L. obtusata was similar to that of L. saxatilis, but the correlation of allele frequency was not significant. The variation at the esterase locus of these 2 spp. is probably maintained by selection.