Natural history and molecular evolution of demersal Mediterranean sharks and skates inferred by comparative phylogeographic and demographic analyses

Background. The unique and complex paleoclimatic and paleogeographic eventswhich affected the Mediterranean Sea since late Miocene deeply influenced the distribu-tion and evolution of marine organisms and shaped their genetic structure. Followingthe Messinian salinity crisis and the sea-level fluctuations during the Pleistocene,several Mediterranean marine species developed deep genetic differentiation, and someunderwent rapid radiation. Here, we consider two of the most prioritized groups forconservation in the light of their evolutionary history: sharks and rays (elasmobranchs).This paper deals with a comparative multispecies analysis of phylogeographic structureand historical demography in two pairs of sympatric, phylogenetically- and ecologically-related elasmobranchs, two scyliorhinid catsharks (Galeus melastomus,Scyliorhinuscanicula) and two rajid skates (Raja clavata,Raja miraletus). Sampling and experimentalanalyses were designed to primarily test if the Sicilian Channel can be considered as ef-fective eco-physiological barrier for Mediterranean demersal sympatric elasmobranchs.Methods. The phylogeography and the historical demography of target species wereinferred by analysing the nucleotide variation of three mitochondrial DNA markers (i.e., partial sequence of COI, NADH2 and CR) obtained from a total of 248 individualssampled in the Western and Eastern Mediterranean Sea as well as in the adjacentnortheastern Atlantic Ocean. Phylogeographic analysis was performed by haplotypenetworking and testing spatial genetic differentiation of samples (i.e., analysis of molec-ular variance and of principal components). Demographic history of Mediterraneanpopulations was reconstructed using mismatch distribution and Bayesian Skyline Plotanalyses.Results. No spatial genetic differentiation was identified in either catshark species,while phylogeographic structure of lineages was identified in both skates, withR.miraletusmore structured thanR. clavata. However, such structuring of skate lineageswas not consistent with the separation between Western and Eastern Mediterranean.Sudden demographic expansions occurred synchronously during the upper Pleistocene(40,000–60,000 years ago) in both skates andG. melastomus, likely related to optimalenvironmental conditions. In contrast,S. caniculaexperienced a slow and constantincrease in population size over the last 350,000 years.Discussion. The comparative analysis of phylogeographic and historical demographicpatterns for the Mediterranean populations of these elasmobranchs reveals that histor-ical phylogeographic breaks have not had a large impact on their microevolution. Wehypothesize that interactions between environmental and ecological/physiological traitsmay have been the driving force in the microevolution of these demersal elasmobranchspecies in the Mediterranean rather than oceanographic barriers.