Population genomics and phylogeography of a benthic coastal shark (Scyliorhinus canicula) using 2b-RAD single nucleotide polymorphisms

The existence of strong genetic structure is expected in species with limited ability to disperse and philopatric behaviour. These life-history traits are found in many small benthic elasmobranchs, such as in the small-spotted catshark (Scyliorhinus canicula). However, no evidence of genetic structure was found across its northeastern Atlantic (NEA) range using traditional molecular markers. Here, fine-scale genetic differentiation was detected between the British Isles and southern Iberia using 2674 single nucleotide polymorphism loci generated using 2b-restriction siteassociated DNA (2b-RAD). Geographical distance and historical demography were two major drivers shaping the distribution of genetic diversity of S. canicula along the NEA. Significant positive spatial autocorrelation of allelic frequencies was detected, with genetic differentiation generally increasing with geographical distance. However, marked genetic divergence of the Celtic Sea and South Portugal collections from their closest neighbours resulted in geographically constrained genetic breaks south of the British Isles and off southwestern Iberia. Historical demographic reconstruction of population pairs across these genetic breaks suggested a scenario of historical isolation before secondary contact, probably related to distinct northern and southern glacial refugia. These results provide new insights into the population structure of S. canicula along the NEA and serve as a reference for benthic elasmobranchs with similar distribution ranges.