Clustering of mitochondrial cytochrome oxidase I (COI) haplotypes from different European basins. a. Haplotype network based on 925 COI fragments from Phoxinus specimens from various European freshwater basins (colour-coded) of potential introductory resource populations. b. Overview of the European freshwater basins from which haplotypes were included in the haplotype network. The separate haplotype networks for c. clade 10, d. clade 5b, and e. clade 12 include a further breakdown of investigated basins. Numbers identify relevant haplotypes discussed in the results and discussion sections. Clades according to Denys et al. (2020) and Palandačić et al. (2020): clade 5b (P. csikii), clade 9 (P. marsilii), clade 10 (P. phoxinus), clade 12 (P. septimaniae), clade 13 (P. bigerri), clade 16 (P. fayollarum), clade 23 (P. dragarum), clade 11 (P. cf. morella).