Multilocus phylogeography of Patella caerulea (Linnaeus, 1758) reveals contrasting connectivity patterns across the Eastern-Western Mediterranean transition

Aim: The Mediterranean Sea is a biodiversity hotspot, hosting a wide variety of habitats and organisms resulting from its complex past history and present-day processes. Although several biogeographical areas are defined within the Mediterranean Sea, a comprehensive description of the main connectivity patterns among these areas is still lacking. In the present work, we describe the main features structuring the past and present genetic diversity and connectivity in a shallow subtidal limpet across the Mediterranean Sea. Location: Mediterranean Sea. Methods: The genetic diversity and structure of the common and widespread limpet Patella caerulea were analysed in 32 locations across the Mediterranean Sea using two mitochondrial markers (COI and 16S) and five nuclear microsatellite loci. Demographic history and phylogenetic analyses were carried out using mitochondrial markers, and assignment tests were performed on nuclear markers. Results: Two main evolutionary units were detected according to both mitochondrial markers. The most significant genetic differentiation was located between the southern tip of Sicily and the Calabria Peninsula. However, the full transition between the Western and Eastern basins is broadly located in the area between the Siculo-Tunisian Strait and the eastern Ionian Sea. Highly variable nuclear markers revealed further complexity related to current connectivity patterns, showing a genetic structure in the Sicily Channel and the Tyrrhenian-Ligurian transition and gene flow across the Messina Strait. The three markers showed a consistent pattern of isolation by distance only in the Eastern basin. Main conclusions: Complex connectivity patterns in the central Mediterranean confirm that this area represents a genetic diversity hotspot for limpets, reflecting the interplay of the past and present processes. For this crucial area, we propose to use the term "transition area," which may better reflect its complexity and so avoid further debate on the location of a single "genetic barrier."