Genetic study of the Atlantic/Mediterranean transition in sea bass (Dicentrarchus labrax)

We report on the genetic differentiation among populations of the common (or European) sea bass (Dicentrarchus labrax) from the North Sea, Britanny, Portugal, Morocco, the Alboran Sea, and the western Mediterranean. Based on allele-frequency variation at six microsatellite loci, a distance tree inferred from Reynold's coancestry coefficient showed that sea bass populations clustered into two distinct groups of populations, an Atlantic group which includes the Alboran Sea east of Gibraltar Strait, and a western Mediterranean group. While no clear geographical pattern emerged within each of these two entities, the sharp transition led us to postulate that the divide may correspond to the Almeria-Oran oceanographic front. This divide was evidenced by a small but highly significant F(ST) value (0.018, P < .001), corresponding at equilibrium to an average effective number of migrants Nm on the order of 14 individuals per generation. We emphasize the idea that the passive retention of larvae on either side of the oceanographic front is not a sufficient explanation for the persistence of this divide.