Molecular phylogeny and comparative morphology reveal the species validity and systematic position of Lepidodesma (Bivalvia: Unionidae)

Increasingly sophisticated molecular techniques and research tools have greatly advanced the understanding of species diversity and phylogenetic relationships in freshwater mussels. However, malacologists have been puzzled by the taxonomic position and species validity of certain species, particularly those described solely based on conchological characteristics. The genus Lepidodesma is one such group. Here, we integrate shell morphology, soft-body anatomy, and molecular phylogeny to delimit the species validity of Lepidodesma aligera (Heude in Conchyliologie Fluviatile de la Provoince de Nanking 3:17–24, 1877) and Lepidodesma languilati (Heude in Journal De Conchyliologie 22:112–118, 1874). Comparative morphology reveals that L. aligera can be distinguished from L. languilati by its higher post-dorsal wing covered with scattered nodules, as well as differences in the incurrent aperture, excurrent aperture, and gill. Additionally, the average genetic distance based on DNA barcodes (COI) between both species is 4.0%. Integrative taxonomy supports the distinct species of L. aligera, rather than synonymy for L. languilati. Mitochondrial phylogenomic analyses confirm four monophyletic groups (Ambleminae + (Gonideinae + (Unioninae + Parreysiinae))) within the family Unionidae. L. aligera and L. languilati have a well-supported sister-group relationship and form a basal clade splitting from the rest of Unioninae. Therefore, the genus Lepidodesma should be classified as Lepidodesmini in Unioninae. The molecular clock with fossil calibration indicates that Lepidodesma originated in the Early Cretaceous (~121.30 Mya, 95% HPD = 90.37–156.54 Mya) and diverged in the Middle Neogene (~12.94 Mya, 95% HPD = 6.72–22.13 Mya). This study firstly provides anatomical features and molecular data for L. aligera and demonstrates the species validity and the systematic position of Lepidodesma taxa, which enrich our understanding of this rare group and facilitate its management and conservation.