Complex patterns of secondary spread without loss of genetic diversity in invasive populations of the Asian shore crab Hemigrapsus takanoi (Decapoda) along European coasts

Genetic studies of introduced non-native species are a valuable tool to investigate invasion history and pathways, source populations and multiple introductions of alien species, as well as evolutionary genetic changes following establishment in a new environment. We used a set of nine polymorphic microsatellites to analyse the population genetic structure of the introduced Asian shore crab Hemigrapsus takanoi along European coasts. Our dataset covered the complete known European range of the species, including the most recent records from Great Britain and the southwestern Baltic Sea. The results showed a similarly high genetic diversity of H. takanoi throughout Europe, and no indication of genetic bottlenecks during secondary spread, even in the most recently established populations. Analyses for population structure along geographic regions gave support for a separation between the Bay of Seine populations (northern France) and all other populations. Genetic differentiation within the North and Baltic Seas was more subtle and patchy, hinting to potential unrecognised introduction events, dispersal barriers and anthropogenic vector activity. The populations from the Baltic Sea and Great Britain clustered with the Wadden Sea populations, suggesting secondary introductions from the southeastern North Sea as likely invasion pathways. In summary, we suggest that a combination of anthropogenic secondary spread and the species’ reproductive biology have prevented a loss of genetic diversity during its ongoing expansion. We argue that genetic data depicting population status shortly after an introduction event—like the British and Baltic Sea populations of H. takanoi—may provide important baseline data for investigations of genetic changes during establishment and adaptation processes.