Multiple lineages in freshwater mytiloid bivalves: polyphyly of Sinomytilus and establishment of a new genus from China (Bivalvia, Modiolidae)

The systematics of freshwater mytiloid bivalves has long been confounded by morphological convergence, particularly among East Asian taxa traditionally assigned to Sinomytilus . In this study, an integrative revision of Chinese Sinomytilus sensu lato was conducted, combining shell morphology, anatomical observations, multi-locus phylogenetic analyses, mitogenome-based phylogeny, and molecular dating. Our results demonstrate that Sinomytilus as currently defined is polyphyletic. Sinomytilus swinhoei from the Yangtze and Yellow river basins clusters with Limnoperna species rather than with the type species S. harmandi from Southeast Asia, whereas the small-bodied species from Lake Fuxian forms an independent lineage. Divergence time estimates indicate that the freshwater modiolid clade originated in the middle Jurassic (~173 Ma), coinciding with pronounced sea-level fluctuations around the Aalenian–Bajocian. Accordingly, a new genus is erected, Shenzhousia Zheng, Wang, He & Xiang, gen. nov . (type species: Sinomytilus swinhoei ), and a new subfossil species described, Shanomytilus frendae Wang, Zheng, He & Xiang, sp. nov ., from the Quaternary Luliang Basin. This study clarifies the taxonomy of East Asian freshwater mytiloids and provides new insights into marine–freshwater transitions and the evolutionary significance of ancient lake systems.