Ports and farms are well‐known primary introduction hot spots for marine non‐indigenous species (NIS). The extent to which these anthropogenic habitats are sustainable sources of propagules and influence the evolution of NIS in natural habitats was examined in the edible seaweed Undaria pinnatifida, native to Asia and introduced to Europe in the 1970s. Following its deliberate introduction 40 years ago along the French coast of the English Channel, this kelp is now found in three contrasting habitat types: farms, marinas and natural rocky reefs. In the light of the continuous spread of this NIS, it is imperative to better understand the processes behind its sustainable establishment in the wild. In addition, developing effective management plans to curtail the spread of U. pinnatifida requires determining how the three types of populations interact with one another. In addition to an analysis using microsatellite markers, we developed, for the first time in a kelp, a ddRAD‐sequencing technique to genotype 738 individuals sampled in 11 rocky reefs, 12 marinas, and two farms located along ca. 1,000 km of coastline. As expected, the RAD‐seq panel showed more power than the microsatellite panel for identifying fine‐grained patterns. However, both panels demonstrated habitat‐specific properties of the study populations. In particular, farms displayed very low genetic diversity and no inbreeding conversely to populations in marinas and natural rocky reefs. In addition, strong, but chaotic regional genetic structure, was revealed, consistent with human‐mediated dispersal (e.g., leisure boating). We also uncovered a tight relationship between populations in rocky reefs and those in nearby marinas, but not with nearby farms, suggesting spillover from marinas into the wild. At last, a temporal survey spanning 20 generations showed that wild populations are now self‐sustaining, albeit there was no evidence for local adaptation to any of the three habitats. These findings highlight that limiting the spread of U. pinnatifida requires efficient management policies that also target marinas.

International audienceThe kelp Saccharina latissima is a species of high ecological and economic importance. We developed a novel set of S. latissima-specific genetic markers that will find applications in conservation biology, biodiversity assessment, and commercial exploitation of this macroalga. Thirty-two expressed sequence tag (EST)-derived microsatellite markers (SSRs) were developed and characterized in this study using publically available EST sequences. Twenty-seven percent of the 7064 analyzed ESTs contained repeat motifs, and polymerase chain reaction (PCR) amplification primers were designed for 96 selected loci. Fifty-one (53 %) of the primer pairs amplified their target loci, of which 32 (33 %) were polymorphic within a sample of 96 S. latissima sporophytes collected from six localities distributed along the European Atlantic coast from Southern Brittany (France) to Spitzbergen (Norway). The 32 loci harbored moderate levels of polymorphism with 2–13 alleles per locus (mean 5.4). The 25 loci that were retained for population genetic analyses revealed substantial genetic differentiation among the European populations (pairwise F ST values ranging from 0.077 to 0.562) that did not follow any pattern of isolation by distance. In addition, within-population genetic diversity was generally low (Hs < 0.323). Two non-mutually exclusive hypotheses were proposed to explain this low diversity pattern: (1) lower variability of the EST-derived microsatellites compared to the random distribution of SSRs developed from genomic DNA since the former are frequently located in coding regions, which are generally less variable, or (2) reduced effective population size of S. latissima. The particularly high genetic differentiation between the French and Scandinavian S. latissima populations is in agreement with the reported ecotypic differentiation, which may reflect an important resource for genetic improvement. The pattern of genetic diversity revealed in this study thus suggests that care should be taken to avoid the transfer of strains between different geographic regions