Pattern of genetic isolation in the crab Pachygrapsus marmoratus within the Tuscan Archipelago (Mediterranean Sea)

In the present study the benthic crab Pachygrapsus marmoratus was used to assess the connectivity level within the Tuscan Archipelago, an area in between the Ligurian and Tyrrhenian Seas, Italy. The Archipelago comprises seven islands lying inside the largest Marine Protected Area of the Mediterranean Sea. We genotyped a total of approximately 230 individuals, from eight populations (four protected and four unprotected), at eight microsatellites. Overall, our results showed a clear partitioning of genetic variation within this area: all populations were separated from each other, except the two southernmost islands. This genetic subdivision could be due to a “sweepstakes reproductive effect” (i.e. only a small proportion of the available gene pool successfully contributes to the replenishment of each population). Otherwise, the lack of a pattern of isolation by distance coupled with a low percentage of individuals assigned to its own population seemed to indicate that retention of larvae near the parent population is unlikely in promoting the recorded genetic structure. However, we also recorded evidence of past demographic events in all populations, that may have played a pivotal role in shaping the recorded pattern of intraspecific differentiation. Finally, we did not record any difference in the level of genetic variation in populations that were protected (i.e. experiencing less human pressure) compared to unprotected populations; this could suggest that, from a genetic point of view, the effects of protection have not yet been revealed