Genetic differentiation in a pelagic crustacean (Meganyctiphanes norvegica: Euphausiacea) from the North East Atlantic and the Mediterranean Sea

Meganyctiphanes norvegica (M. Sars) is a pelagic crustacean that plays a ke y role in marine food webs of North Atlantic Ocean and marginal seas. We st udied eight population samples collected in the European Atlantic and Medit erranean Sea. By means of single strand conformation polymorphism analysis (SSCP) and direct sequencing, we investigated a segment of 158 base pairs o f the mitochondrial gene coding for the subunit 1 of NADH dehydrogenase. We found 12 sequence variants among the 385 individuals studied. Analysis of molecular variance (AMOVA) showed that 14.75% of the total genetic variabil ity was explained by differences between populations, thus indicating absen ce of panmixia for these populations. Pairwise comparisons revealed three d istinct genetic pools: the first one represented by Cadiz Bay, the second o ne by the Ligurian Sea, and the third one included all the NE Atlantic samp les. We also investigated one population from the Alboran Sea (within the M editerranean basin, east of the Strait of Gibraltar). This population was f ound to be genetically intermediate between the NE Atlantic samples and the Ligurian sample, suggesting that the restriction to the gene flow is not a ssociated with the Strait of Gibraltar, but possibly with the Oran-Almeria oceanographic front. The present work indicates that M. norvegica, although endowed with a high dispersal capacity because of its pelagic habit, can d evelop separate breeding units inside the same oceanic basin (the Atlantic) . Furthermore, the Ligurian sample should be considered as a distinct evolu tionary entity, separated from the Atlantic population.