The systematic status of the lagoon periwinkle, Littorina tenebrosa

Eight samples of Littorina tenebrosa and L. saxatilis (Mollusca: Gastropoda ) from Ireland and Britain, including pairs of each form from two locations in Ireland, were screened for genetic variation at 12 polymorphic enzyme l oci using starch gel electrophoresis. Levels of polymorphism and heterozygo sity were similar in L. tenebrosa and L. saxatilis, apart from a sample of L. tenebrosa from Britain which was less polymorphic than the Irish samples . No alleles were found to be unique to either form. Phylogenetic analysis using UPGMA showed that L. saxatilis and L. tenebrosa populations clustered as a monophyletic group. Nevertheless, the mean genetic distance between p arapatric populations of L. saxatilis and L. tenebrosa ((D) over bar = 0.01 76) was similar to the mean for allopatric populations of either species (( D) over bar = 0.080). This indicates that there is a barrier to gene flow b etween the two forms. Despite this, L. tenebrosa does not merit specific st atus since populations of this snail do not cluster as a distinct group, se parate from L. saxatilis populations.