STRUCTURE bar plots. Each bar represents an individual. Individuals are grouped by populations along the x-axis. Along the y-axis is the probability of assignment to a particular population represented by different shades. When analyzing all populations, the Evanno method  determined that the mostly likely number of genetic clusters or populations (k) was 2 (a). The visual representation of these two populations (a) shows that individuals cluster geographically (western North Atlantic versus Mediterranean), and that the Mediterranean populations are genetically distinct from the western North Atlantic populations. At k =3 (b), the two main genetic clusters previously found in the western Atlantic  were recovered, while maintaining a genetically differentiated Mediterranean cluster. When analyzing the Mediterranean populations alone (c) there is no clear genetic structure across the Mediterranean, even though the Evanno method  determined that the most likely number of populations (k) was 4.