Microsatellite genotyping of brown crab Cancer pagurus reveals fine scale selection and 'non-chaotic' genetic patchiness within a high gene flow system

Brown crab Cancer pagurus supports one of the most important fisheries in Europe; however, spatial patterns of connectivity and adaptation are largely unknown and difficult to identify due to the species’ life history, which entails distinct dispersal characteristics during larval and adult life stages. To address this limitation, spatial-temporal genetic structure using 8 microsatellite loci was assessed across the majority of the species’ NE Atlantic distribution. Neutral genetic structuring revealed a background of high gene flow throughout the region, with a superimposed pattern of chaotic genetic patchiness (CGP) linked to stochastic recruitment variability. The CGP was geographically patterned, being prevalent among English Channel samples but absent among North Sea samples, suggesting specific biological (e.g. reproductive ecology) and environmental (seascape) drivers. Such recruitment variability may compromise stock resilience and must be considered within spatial management strategies. Another prominent feature was pronounced differentiation at a single locus for males sampled within a single fjord (Gulmarsfjord) from all other samples, exhibiting the effects of divergent selection. Gulmarsfjord females were genetically similar to all other ‘non-fjord’ samples, and exhibited a comparative level of differentiation at the outlier locus from the Gulmarsfjord males. Due to known dispersal differences between the sexes, the pattern within Gulmarsfjord can be explained by the intermingling of allochthonous females with resident, locally adapted males and demonstrates the occurrence of fine-scale local adaptation in this species. Collectively, the study highlights how considerable intraspecific eco-evolutionary diversification can occur despite high levels of dispersal/gene flow