Population genomics provides evidence of interspecific hybridisation and population structure in the blue-swimmer crab (Portunus armatus) along the Western Australian coastline

Responsible and sustainable fisheries management requires a comprehensive knowledge of stock structure. Here, we examine population structure in the blue-swimmer crab (Portunus armatus), which supports one of the largest commercial and recreational fisheries within Western Australia (WA). Tissue samples from adult P. armatus were collected from 16 sites along the WA coastline from Albany to the Kimberley, a distance spanning over 3000 km. Adult individuals were also collected from one site in South Australia (SA) and one in the Northern Territory (NT), the latter separated into two morphologically similar species, P. armatus and P. pelagicus, as their ranges are known to overlap in northern Australia. Juvenile crabs were also sampled from five sites within the south-west region of WA to test for evidence of local recruitment. Using a panel of 11,175 single-nucleotide polymorphism (SNP) loci, we found evidence of two genetically distinct clusters in the NT sample and the northernmost WA site in the Kimberley region consistent with two sympatric species. We also found evidence of hybridisation between the two groups where they co-occur. Subsequent analyses on the remaining WA sites and SA, revealed the presence of four genetically distinct populations, represented by sites at SA, Exmouth, Shark Bay and the southwest region of WA. Additional analyses focussing on the south-west of WA exclusively failed to detect any appreciable population structure within this region. Consistent with this pattern, juveniles collected from five sites within south-west region of WA were no more genetically related to other juveniles from the same sampling site, on average, than juveniles collected elsewhere. Our results suggest the south-west region of WA is comprised of a single panmictic population, consistent with the view that populations along this coastline are highly interconnected due to inshore and offshore currents. Future management should consider a more integrated approach for managing blue-swimmer crab fisheries within the south west region of WA and improved monitoring in northern Australia, where two genetically distinct groups are currently fished as a single stock.