A cautionary tale of incomplete evidence: taxonomic confusion in the freshwater mussel Nodularia liuovatus comb. nov. and Pseudocuneopsis heqing stat. rev. (Bivalvia, Unionidae) from China

Precise species identification and classification are prerequisites for assessing and conserving the biodiversity of freshwater mussels (Bivalvia, Unionidae), which are one of the most threatened animal groups in the world. Previous classifications based on shell morphological characteristics have left Lamellidens liuovatus He & Zhuang, 2013 in a state of taxonomic uncertainty at the generic level. Especially recently, based on subjective similarities in shell morphology and the geographic proximity of their type localities, a proposed junior synonym ( Pseudocuneopsis heqing ) has been assigned to this species, and the species has been reclassified under Pseudocuneopsis , further exacerbating taxonomic confusion. This study re-evaluated the species and its synonymous taxon by collecting specimens from the type locality and integrating comparative morphology, geometric morphometrics, and molecular phylogenetic analysis. Morphometric analyses reveal that the shell outlines of Pseudocuneopsis heqing and Pseudocuneopsis liuovatus exhibit significant quantitative differences (MANOVA, p < 0.001). The ML and BI phylogenetic analyses based on multiple gene loci ( COI , 16S rRNA, and 28S rRNA) show that P. liuovatus and P. heqing belong to different lineages, while P. liuovatus is sister to Nodularia hanensis with strong support within the genus Nodularia (PP = 0.99, BS = 98%). Based on the above evidence, we support the restoration of the valid species status of Pseudocuneopsis heqing stat. rev . and the reclassification of Nodularia liuovatus comb. nov . Although the morphological characteristics of N. liuovatus and Nodularia micheloti are extremely similar, the absence of topotypic material or molecular data from the type locality of N. micheloti prevents unequivocal confirmation of their conspecificity. Therefore, we provisionally retain N. liuovatus as a distinct species pending further study.