Résumé
Macroalgae can represent a significant component of biological introductions in coastal areas. One such example is the Asian kelp Undaria pinnatifida, which has been both intentionally and unintentionally introduced throughout the world. The alga was first introduced in the Bay of St-Malo (English Channel, France) for cultivation in the early 1980s, where it subsequently escaped and established a number of spontaneous satellite populations in the wild. We here document the present-day distribution of U. pinnatifida in this bay based on a grid sampling design. The alga was observed in most of the surveyed sites (84%). To test if this broad distribution of U. pinnatifida could be at all associated with the annual supply of propagules released from the local farm, we evaluated population genetic diversity and connectivity throughout the bay using nine microsatellite loci. We detected limited gene flow that could not be attributed to any isolation-by-distance mechanism, or to habitat type. In further support of a lack of direct connection between farms and satellite populations, spontaneous populations in the open bay were strongly differentiated from farmed populations. Future studies should investigate environmental disturbance, including commercial and recreational nautical activities, which may promote further spread of this pioneer alga.