DNA barcoding, multilocus phylogeny, and morphometry reveal phenotypic plasticity in the Chinese freshwater mussel (Bivalvia: Unionidae).

Accurate species identification is crucial for developing conservation strategies for freshwater mussels, one of the most imperiled faunas in the world. Traditionally, mussel species description primarily relied on conchological characters. However, shell morphology has great variability, which leads to the complexity of species delimitation. As endemic species to China, was originally described by Heude (1877). and were considered for synonymization of based on shell variants in the early 20th century, which has been long debated due to lack of rigorous molecular analysis. Moreover, great morphological variation caused doubt whether there are cryptic species. In this study, we used a combined phylogenetic and morphometric approach to verify the validity of the synonymization of . The results of molecular species delimitation showed that two molecular operational taxonomic units (MOTUs) were identified in spp., including the lineage and the complex lineage (, , , and ). Phylogenetic analyses revealed that formed a basal monophyletic clade, whose divergence time was relatively recent (4.26 Ma [95% HPD = 1.91-7.22 Ma]), and , , and formed a large polytomy group with very shallow branches. In the previous study, we have demonstrated the validity of . The molecular evidences supported that the complex ( +  + ) was a valid species; and were synonyms of . In addition, three morphospecies (, , and ) were aggregated without clear differentiation based on shell morphometric analysis. We confirmed multiple phenotypes in for species identification and presumed that the phenotypic plasticity was a response to specific habitats. This study clarified the diversity and phylogeny of the group, which is a crucial step for developing new conservation and management strategies for this imperiled group.