Recent changes in the distribution of a marine gastropod, Patella rustica, across the Iberian Atlantic coast did not result in diminished genetic diversity or increased connectivity

Aim? The Lusitanian limpet, Patella rustica Linnaeus, 1758, is a rocky shore intertidal mollusc found throughout the Mediterranean and the north-east Atlantic from Mauritania to southern France. A 280-km gap in its distribution in the Iberian north-west was first reported in the early 20th century. Between 2002 and 2005 this gap was bridged, and unusual climatic and hydrographic events that occurred in the late 1990s are hypothesized to have facilitated transport and settlement of colonizers from the southern edge of the gap. In the present work we used genetic data to investigate differentiation between the edges of the former gap, to determine the origin of colonizers and to assess patterns of connectivity across the historical and newly colonized region.Location? Atlantic coast of Iberia (Portugal and north-west Spain) and Biarritz.Methods? The genetic variation of 12 sites, sampled between 2004 and 2006, was assessed through the analysis of five microsatellites (n = 412) and of a 650-bp fragment of the mitochondrial cytochrome c oxidase subunit I gene (COI; n = 163). F-statistics were used to investigate differentiation and patterns of gene flow across the distribution of P. rustica, from southern Portugal to Biarritz. The mitochondrial DNA (mtDNA) mismatch distribution was analysed and the mutation–drift equilibrium tested to investigate putative demographic changes.Results? Levels of differentiation were low and generally non-significant. Comparisons between regions of the historical distribution were all non-significant. For microsatellites, low yet significant genetic differentiation was observed when all three regions were included. Levels of genetic diversity of newly founded populations were similar to those observed within the historical range. Evidence of demographic changes was detected with mtDNA and microsatellites.Main conclusions? The studied region is inhabited by a single population, suggesting that the gap in the distribution of P. rustica was not an effective barrier to gene flow. If it has ever prevented gene flow, it did not last enough time for genetic differentiation to occur. On the other hand, the main currents flowing along the Atlantic coast of Iberia could have been responsible for maintaining connectivity through occasional larval transport across a considerable stretch of inhospitable habitat