Localities of Mytilus spp. samples along the European coast. The four samples chosen to be representative of each panmictic species patch and used for DNA sequence analysis are indicated by white and black stars for M. edulis and M. galloprovincialis respectively. Additional previously analysed samples  used for the correspondence analysis are indicated by white and black dots for M. edulis and M. galloprovincialis respectively. Sample names and sample sizes (in brackets) are the following: 1, Faro (67); 2, Setubal (28); 3, Biarritz (50); 4, Brouage (30); 5, Boyard (50); 6, Moguériec (24); 7, Morlaix (24); 8, Polzeath (49); 9, Grand-Fort-Philippe (50); 10, Tichwell (27); 11, Cley (32). The three hybrid zones  are indicated by dashed lines: HZ1 between M. galloprovincialis of the Atlantic coast of Iberian Peninsula and M. edulis of the Bay of Biscay, HZ2 between M. edulis of the Bay of Biscay and M. galloprovincialis of Brittany and HZ3 between M. galloprovincialis of Brittany and M. edulis of the North Sea. Patches of M. galloprovincialis are indicated with continuous black lines and patches of M. edulis are indicated by black and white continuous lines.