Haplotype networks of Cryptocotyle lingua based on cox1 gene sequences (n = 998 sequences) from seven fish species from the eastern English Channel (EEC), Southern Bight of the North Sea (SBNS), eastern North Sea (ENS), central North Sea (CNS), western North Sea (WNS) (A) and from fish hosts (B). Circles indicate haplotypes, their frequencies correspond to the circle sizes. The haplotypes linked by a black line differ by one (no black point) or more (number of black points) substitutions. Colors are consistent with the map in Figure 1 .