Genetic clines across the Atlantic-Mediterranean transition in the harbour crab Liocarcinus depurator

Abstract Environmental gradients in the sea may coincide with phenotypic or genetic gradients resulting from an evolutionary balance between selection and dispersal (i.e. marine clines). The population differentiation of the harbour crab, Liocarcinus depurator, an important by-catch species in the Mediterranean Sea and North-East Atlantic, was assessed here using both genetic and morphometric approaches. A total of 472 specimens were collected along its distribution area, and 17 morphometric landmarks, one mitochondrial gene (COI) and 11 polymorphic microsatellite markers were scored in 350, 287 and 280 individuals, respectively. Morphometric data lacked significant differences, but genetic analyses showed a reduction in gene flow between Atlantic and Mediterranean populations, with a steeper gradient in COI compared to microsatellite markers. Interestingly, nuclear differentiation was due to an outlier locus overlapping with the mtDNA genetic gradient. Such overlapping clines are likely to be maintained by natural selection along the Atlantic-Mediterranean transition area. Our results suggest a scenario of past isolation with local adaptation and secondary contact between the two basins. The process of vicariance may reinforce genetic differentiation at loci maintained by environmental selection even after secondary contact.