A novel freshwater‐adapted genus revealed: Integrative taxonomy and biogeography of freshwater razor clams (Bivalvia: Pharidae) in East Asia

Abstract The India‐Eurasia continental collision and the subsequent uplift of the Qinghai‐Tibet Plateau have profoundly influenced the diversification and endemism of aquatic organisms across Asia. Notably, the freshwater razor clam genus Novaculina , previously the sole known freshwater member of the family Pharidae, originally comprised four species distributed across Asia. Among these, the protected Chinese endangered species N. chinensis , with fused syphons and unclear taxonomic status, has drawn conservation and scientific focus. The recent reclassification of N. siamensis into the new genus Cenonovaculina highlighted the fused syphons of N. chinensis as a shared morphological trait; however, further investigation is needed to determine whether N. chinensis should be recognised as a distinct genus. Moreover, the phylogenetic relationships and biogeographic history of Novaculina sensu lato and its allied genera remain elusive, particularly given the limited molecular data available for the endangered Chinese species N. chinensis . In this study, we analysed newly collected specimens of N. chinensis from the Yangtze and Pearl River Basins, compiling genetic datasets that include three genes (COI, 16S rRNA, and 28S rRNA) as well as whole mitochondrial genome data. High‐throughput sequencing revealed no mitogenome heteroplasmy in somatic and male gonadal tissues, confirming the absence of doubly uniparental inheritance in N. chinensis . Our phylogenetic analyses revealed that Novaculina chinensis does not cluster with either Novaculina congeners or Cenonovaculina . Instead, these results robustly support the polyphyletic nature of Novaculina s.l. , identifying N. chinensis as a relic marine‐derived freshwater lineage. We therefore erect Uninovaculina gen. nov . for N . chinensis . Our results also indicate that the subfamily Pharellinae originated from a saline habitat approximately 77.2 million years ago (95% HPD = 65.3–91.2). These findings highlight the critical influence of geological and climatic changes on the distribution of freshwater razor clams in Asia, underscoring the complex interplay between these factors and adaptive evolutionary processes.