Comparative population genetic structure of marine gastropods (Littorina spp.) with and without pelagic larval dispersal

Population genetic theory predicts that marine animal species with planktonic larvae will have less genetic structure than those with direct development. We compared the genetic structure of four species of littorinid snails - two with planktonic egg capsules that hatch as planktonic larvae and two with benthic egg masses that hatch as crawl-away juveniles. We used DNA sequencing and single stranded conformational polymorphism (SSCP) to assess sequence variation in a 480 bp fragment of the mitochondrial cytochrome b gene and then used an analysis of molecular variance (AMOVA) to estimate Phi (st) for populations from the northeastern Pacific coast. One of the two direct-developing species, Littorina subrotundata, had a moderate amount of population structure (Phi (st) = 0.209) as expected but the other direct-developing species, L. sitkana, was nearly fixed for a single haplotype that made it impossible to precisely estimate Phi (st). One of the two planktonic-developing species, L. scutulata, did not show any significant population structure (Phi (st) = 0.004). In contrast to our expectations, the other planktonic-developing species, L. plena, showed some weak but statistically significant population structure (Phi (st) = 0.052). We discuss how differences in population genetic structure between species with the same type of development may reflect differences in their historical demography.