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

Population genetic theory predicts that marine animal species with plankton ic larvae will have less genetic structure than those with direct developme nt. 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 seq uencing and single stranded conformational polymorphism (SSCP) to assess se quence variation in a 480 bp fragment of the mitochondrial cytochrome b gen e and then used an analysis of molecular variance (AMOVA) to estimate Phi ( st) for populations from the northeastern Pacific coast. One of the two dir ect-developing species, Littorina subrotundata, had a moderate amount of po pulation structure (Phi (st) = 0.209) as expected but the other direct-deve loping species, L. sitkana, was nearly fixed for a single haplotype that ma de it impossible to precisely estimate Phi (st). One of the two planktonic- developing species, L. scutulata, did not show any significant population s tructure (Phi (st) = 0.004). In contrast to our expectations, the other pla nktonic-developing species, L. plena, showed some weak but statistically si gnificant population structure (Phi (st) = 0.052). We discuss how differenc es in population genetic structure between species with the same type of de velopment may reflect differences in their historical demography.