Adult and Larval Dispersal: Complex Population Structures in a Crab Species (Cancer pagurus) with Potentially High Gene Flow

The brown crab, Cancer pagurus, is continuously distributed throughout shallow waters of the NE Atlantic where it is subjected to increasing levels of exploitation. The species life history, with migratory adult females and a prolonged pelagic larval stage (1-3 months), is expected to promote wide scale population connectivity. However, cryptic sub-structuring could have major implications for sustainable fisheries, so microsatellite and mitochondrial DNA (mtDNA) analysis of spatial and temporal samples was employed to investigate population genetic structure across the species range. A number of differentiated regional groupings were detected, with patterns of isolation compatible with known adult migrations and hydrographic restrictions to larval transfer. Levels of genetic variation differed across regions and correlated with harvesting pressures suggesting possible overfishing effects. Within regions distinctive patterns of population structuring were observed.   

In the North Sea landscape analysis of microsatellite variation revealed an apparent barrier to gene flow separating two groups. Differentiation was due to a single locus that exceeded neutral expectations across multiple independent comparisons, indicating divergent selection effects and adaptation that could not be attributed to a specific environmental driver. In contrast, mtDNA analysis revealed a chaotic pattern of microgeographical differentiation. Distinct genetic patterns among males and females were compatible with their respective resident and migratory behaviours. Taking into account crab behaviour and hydrography of the region, the cytonuclear discordance is attributed to female migratory behaviour and larval drift interacting to result in recurrent male-biased gene flow

Within the English Channel chaotic, geographically incongruent population structuring was observed among adult samples. In contrast, genotyping of larvae revealed a clear geographical pattern to population structure. As for the North Sea, data indicated a female dispersal / male-biased gene flow dynamic, however in the Channel there was evidence that rather than due to larval drift this was maintained by return migrations of adult females. Additional support for philopatric behaviour is provided by differentiation of samples within embayments.

This study indicates how multiple biotic and abiotic factors can restrict gene flow in species / systems expected to be characterised by high gene flow. It also highlights how analysis of larval life history stages can be highly informative in elucidating such factors.