Connectivity and historical demography of the two Mediterranean endemic skates Raja asterias (Delaroche 1809) and Raja polystigma (Regan 1923)

Raja asterias and Raja polystigma are two skates considered endemic of the Mediterranean sea; both species are present along the coasts of the basin, except in the Levantine sea where the occurrence of R. polystigma is not confirmed. Our purpose is to assess their populations connectivity and their historical demographic trends for future conservation plans. We have sequenced the mitochondrial coxI gene in 125 specimens in R. asterias and in 103 specimens in R. polystigma from 7 sampling points, producing two different alignments of 603 bp and 616 bp respectively. The populations connectivity and the demographic histories were investigated by haplotype networks and Bayesian Skyline Plot analyses. The networks showed different patterns in the two species: while in R. asterias three main haplogroups were detected (Algerian, Adriatic and Sardo-Tyrrhenian ones), in R. polystigma the four most frequent haplotypes were detected in at least four populations each, with the exception of the Adriatic sample which showed only one private haplotype. Differences were also observed in the corresponding demographic histories: R. polystigma showed a constant demography in the last 100,000 years, while R. asterias exhibited a sudden expansion dated 70,000 years ago. Our results suggest a good populations connectivity in the R. polystigma species showing, during its evolution, a stable demographic size, while for R. asterias a deep population structuring and a fluctuating demography were detected. On the basis of the distinct present day status and evolutionary histories, our results suggest species-specific conservation plans for these two rajid species.