Gene pools in rockpools: coastal <i>Ochthebius</i> beetles across the western Mediterranean

Abstract Supratidal rockpools, ecotones between land and sea, are fragmented habitats with extreme environmental fluctuations, housing unique species like Ochthebius beetles. These beetles, which exhibit broad geographical ranges and cryptic lineages in the western Palaearctic, serve as a model for studying population genetics. This study investigates the population genetic diversity and structure of three co-occurring species in the western Mediterranean, and the impact of geographic and environmental distances, coastal features, and marine barriers on gene flow in two of them. The study reveals low genetic diversity in all three species, with similar observed heterozygosity values. Ochthebius quadricollis exhibited higher diversity and notable population variations, being the only species with marked genetic structure. For this species, AMOVA indicated significant genetic variation among the predefined groups, which was higher than within groups. We found an association between genetic differentiation and geographical distances but only for O. quadricollis. Our research highlights great connectivity and gene flow among supratidal rockpool populations of aquatic beetles, probably due to multiple passive dispersal pathways operating at different spatial scales. The three species exhibit varying degrees of genetic differentiation along the Iberian coast, confirming the genetic patterns observed in previous studies with less precise markers and larger scales. However, some discrepancies were noted, especially regarding the role of the Ibiza Channel as a barrier to genetic flow in O. quadricollis, where isolation by distance appears to be crucial. While marine currents were initially hypothesized to influence connectivity, our results suggest that their contemporary role in shaping recent gene flow is limited compared to that of geographic distance.