Genetic structure and relationships in the snail species complex Littorina arcana Hannaford Ellis, L-compressa Jeffreys and L-saxatilis (Olivi) in the British Isles using SSCPs of cytochrome-b fragments

Snails of the Littorina saxatilis species complex are ubiquitous and important members of hard shore intertidal communities in the North Atlantic. The complex includes L. arcana, L. compressa and L. saxatilis. We investigated species and population structure among these species from six locations in Ireland and Britain using a nonradioactive single strand conformational polymorphism analysis of a 375 base pair fragment of the cytochrome-b gene. Variability was high with 38 haplotypes found in 591 individuals. The most common haplotype in L. arcana and L. compressa was absent from L. saxatilis and the most common haplotype in L. saxatilis was found in low numbers in L. arcana and L. compressa. Haplotypes restricted to L. arcana and L. compressa formed a cluster separate from L. saxatilis haplotypes in maximum parsimony and multidimensional scaling analyses. In analyses examining populations, L. arcana and L. compressa formed a group separate from L. saxatilis. Both the populations in the L. arcana/L. compressa group and in L. saxatilis show similar geographical structuring in that the North England populations were separated from a cluster of Cornwall (South-west England) and Irish populations. We suggested that Pleistocene sea level changes accounted for these congruent genetic patterns. We concluded that L. arcana and L. compressa are more closely related to each other than either is to L. saxatilis and offer this as a resolution to a previous trichotomy.