Mismatch distributions of the observed haplotypes for each species. The histograms are the observed frequencies of pairwise divergences and the line refers to the expectation under the sudden population expansion model. All COI sequences of S. japonica were divided into two putative groups: northern group and southern group, according to the phylogenetic analyses (See the results in the phylogenetic analysis). Reference: Cellana toreuma, Dong et al. ; Sargassum horneri, Hu et al. ; Atrina pectinata, Liu et al.