Figure 1: Variability at the mitochondrial COI locus in Manila clam populations.

(A) Frequencies of the COI clades A, B and C in populations, based on the data obtained previously^33,36 and in this study. The names of the populations sampled for this study are shown in all-capitals. The sizes of the diagrams are proportional to the number of individuals analyzed. Green arrows indicate the dates and directions of the introduction of the species in North America and Europe. Sampling localities were as follows: CHI-N, Qingdao (China), 36°06′N, 120°13′E; CHI-S, Shenzhen (China), 22°30′N, 114°00′E; EUR-1, Po Delta (Italy), 44°51′N, 12°21′E; EUR-2, Ebro Delta (Spain), 40°46′N, 0°45′E; EUR-3, Ria de Arousa (Spain), 42°38′N, 8°45′W; EUR-4, Ria de Vigo (Spain), 42°19′N, 8°37′W; JAP, Aichi Prefecture (Japan), 34°38′N, 136°53′E; NAM-1, Willapa Bay (USA), 46° 33′N, 123°59′W; NAM-2, Hood Canal (USA), 47°38′N, 122°51′W. (B) Phylogenetic network obtained by the median-joining algorithm⁶⁷ showing the evolutionary relationships between the COI sequences found in this study and in previous studies^33,36. The size of each circle is proportional to the total haplotype frequency. The colors of the sectors represent the frequency of the haplotype in each of the four regions studied. Red dots represent hypothetical sequences that have disappeared or have not been sampled. A red star marks a haplotype that appeared in relatively high frequency in Europe only. (C) Pie charts showing the distribution of COI haplotypes in American (boxed) and Japanese populations. Private alleles were pooled in each population. The localities that were the sources of oyster imports in N. America in the 20th century are shown in red dots, and the time periods in which the documented importations took place are given in parentheses. Maps have been obtained from Google Maps. Map data ©2016 Google, SK Telecom, Zenrin . Pie charts were drawn with Microsoft Excel. Maps were modified with Adobe Illustrator 14.0.0 and Adobe Photoshop Extended 11.0.2.