Isolation by distance and low connectivity in the peppery furrow shell Scrobicularia plana (Bivalvia)

Scrobicularia plana da Costa, 1778, a commercially important bivalve species in southern Europe, is commonly found along the NE Atlantic and Mediterranean coasts. Like other intertidal mollusk species, it has a wide distributional range and high potential for larval dispersal. However, S. plana has a patchier distribution than most co-distributed soft sediment bivalves of the intertidal, which could lead to lower interpopulation connectivity and stronger population structure. We surveyed 18 locations from throughout the species' range to determine overall population structure, phylogeographic distribution and historical demography. We sequenced a portion of the mitochondrial cytochrome-c-oxidase I gene (COI) for 423 individuals. Three population clusters (Trondheim, Atlantic and Pisa) were identified on the basis of pairwise FSTs. Demographic parameters were analysed in a coalescence framework. Strong differentiation was found between most Atlantic locations and the single Mediterranean location (Pisa). Among Atlantic locations, differentiation was weak and non-significant, though significant isolation-by-distance was detected. A star-shaped phylogeny with mostly 1-step mutations was found. Although 65 haplo-types were detected, 50 were private. The higher diversity observed in southern Europe, Brittany and Norway was consistent with glacial refugia. Population expansion occurred recently with the oldest split, which was between all Atlantic groups and the Mediterranean group, taking place 0.3 to 1.1 million years ago (Myr). Negative values for neutrality tests and the star-shaped haplotype network were also indicative of recent population expansion, although such a pattern can also be the result of a selective sweep. An isolation-by-distance effect and absence of migration reveal low interpopulation connectivity, which is likely reinforced by the species' patchy spatial distribution.