Morphological analysis
Principle component analysis (PCA) of 15 morphological characters (Supplementary Fig. 1) extracted from 100 randomly-chosen individuals (n = 50 from the northern lineage, and n = 50 from the southern lineage) were examined (Fig. 4). The first two components (PC1 and PC2) accounted for 78.81% of the total variation. Ordination of PC1 and PC2 scores of 100 individuals delimited by cox1 lineage indicated that the two lineages mostly overlap along both axes. Therefore, we conclude that these shell morphological characters do not differ between the two mitochondrial lineages, despite their substantial genetic divergence.