Résumé
Ports and farms are well-known primary introduction hotspots 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 found in three contrasting habitat
types: farms, marinas, and natural rocky reefs. In 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 microsatellites, 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 2 farms located
along ca. 1000 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 spill-over from marinas into the wild. Finally, a temporal
survey (20 generations) showed that wild populations are self-sustaining,
without local adaptation to any of the three habitats. These findings
highlight that limiting the spread of U. pinnatifida requires management
policies that also target marinas.