Abstract
Crepidula fornicata, one of the most invasive marine sessile invertebrates in Europe, has traits commonly associated with a weak population genetic structure (high population connectivity): wide environmental tolerance, high fecundity, and a long planktonic larval stage (21 d). In order to determine the scale at which effective dispersal occurs in this potentially highly dispersive species, we designed a hierarchical sampling protocol and analyzed genetic variation at 4 microsatellite loci in 17 populations from 6 French bays situated in the English Channel and at the entrance of the English Channel (Iroise Sea). A strong regional pattern with populations clustered into 3 main groups (Iroise Sea, western and eastern English Channel) was revealed, as was significant isolation by distance along the Channel coasts. Conversely, a high degree of genetic connectivity was observed within the study bays. These estimates of effective dispersal were in agreement with outputs of a 2D larval-dispersal model based on local hydrodynamic conditions, suggesting that connectivity between populations of C. fornicata is influenced by hydrodynamic feature. This study suggests the existence of barriers to gene flow of C. fornicata over moderate distances (ca. 100 km).