Cryptic species, mitochondrial phylogenomics and historical biogeography in the endemic genus Schistodesmus (Bivalvia, Unionidae) from China.

Accurate identification of species and distributions is essential for developing effective conservation and recovery strategies for threatened taxa. Owing to the extreme shell variation and evolutionary convergence, defining and classifying species based solely on morphology can be a challenging and ambiguous process. The freshwater mussel genus Schistodesmus (Bivalvia, Unionidae), which is endemic to China, has traditionally been considered to comprise only two species: Schistodesmus lampreyanus and Schistodesmus spinosus. In this study, an extensive collection of samples from China was conducted and an integrative taxonomic approach, including shell morphology, soft-body anatomy, molecular systematics and biogeography, was employed to investigate the genus Schistodesmus. The results reveal three cryptic species: Schistodesmus luqiaoensis sp. nov. , Schistodesmus tongpenensis sp. nov. and Schistodesmus xinyuensis sp. nov. , and well-supported phylogenetic relationships are reconstructed based on mitochondrial genomic data. This discovery increases the number of species in the genus to five, thereby substantially improving our understanding of its diversity and systematic relationships. The molecular clock analysis and biogeographical reconstruction based on fossil-calibrated dating indicate that the ancestor of the genus originated in the paleo-Yangtze River Basin during the Eocene (c. 50.74 Ma), with species diversification beginning c. 18.68 Ma in the Early Neogene. The uplift of the Qinghai–Tibet Plateau, along with associated paleoclimate changes and local river capture events, significantly facilitated the diversification process of this group. This study not only reconstructs the species diversity framework of Schistodesmus but also highlights the significance of integrated molecular technologies in addressing morphological convergences and promoting effective species conservation. ZooBank: An integrative taxonomic approach, including morphological and molecular data, revealed three new cryptic species: Schistodesmus luqiaoensis , Schistodesmus tongpenensis , and Schistodesmus xinyuensis. Maternal mitochondrial genomes of the first two species and Schistodesmus lampreyanus were sequenced, and interspecific relationships elucidated based on mitochondrial phylogenomics. Molecular clock analysis and biogeographical reconstruction based on fossil-calibrated dating indicated that the ancestor of Schistodesmus originated in the paleo-Yangtze River Basin during the Eocene (c. 50.74 Ma) and began diversifying c. 18.68 Ma in the Neogene. (Photography by Kaiyu Hou.) [ABSTRACT FROM AUTHOR]