Lack of spatial genetic variation in the edible crab (Cancer pagurus) in the Kattegat-Skagerrak area

The stock structure of the edible crab (Cancer pagurus L.) in the Kattegat and Skagerrak was investigated using eight microsatellite DNA loci. Replicate samples, collected 4-6 years apart, were derived form the Kattegat (Grove Bank, 57 degrees N) and the Skagerrak (Lunneviken, 59 degrees N), plus a geographical outgroup sample from the Norwegian Sea (Midsund, 62 degrees N). Genetic differentiation among samples, estimated as global F(ST) = 0.002, was significant (p = 0.03) when the statistical test was based on allele frequencies, but not when based on genotype frequencies. Moreover, all single- and multilocus pairwise tests between samples were non-significant. An analysis of molecular variance, AMOVA, did not reveal significant differentiation between spatial (Kattegat vs. Skagerrak) or temporal (2001/2002 vs. 2006/2007) groups of samples. Power analysis suggested that the loci and sample sizes employed conferred a power of > 90% of detecting even low (true F(ST) = 0.0002) levels of population structure. Low spatial and temporal genetic structure might be explained by either or both of (i) high levels of contemporary gene flow in the area attributable to adult migration or larval dispersal or both factors taken together, and (ii) patterns of historical gene flow persisting among recently founded large populations.