Contrasted patterns of genetic variation in the dogwhelk Nucella lapillus along two putative post-glacial expansion routes

During the last glacial maximum, the dogwhelk Nucella lapillus, like many other rocky shore species, was eradicated from the west Atlantic and largely confined to southern refugia in the east Atlantic. Using microsatellite markers, we studied the geographical distribution of population genetic diversity of N. lapillus along 2 putative post-glacial expansion routes: northward along the east Atlantic shores of Europe, and westward across the Atlantic via Iceland to North America. The patterns of genetic diversity and genetic structure along these 2 routes indicate that the 'trans-Atlantic' expansion involved a stepping-stone migration from Europe to Iceland to North America, with rare long-distance dispersal causing serial bottleneck effects. Along the east Atlantic route, however, long- distance dispersal seems to have occurred more often. These observations, underpinned by hydrography, suggest that rafting could be a major agent of dispersal for marine species lacking planktonic life stages.