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Freshwater bivalves (Bivalvia, Unionida, Unionidae) are among the most threatened groups of animals in the world. Taxonomic uncertainties pose a particular challenge due to significant intraspecific variation and apparent plasticity in shell morphology among these species, impeding efficient conservation planning and management. The species Unio szechenyii that is endemic to China has long been regarded as a synonym for Cuneopsis heudei due to the high convergence in shell morphology. The recently validated species Middendorffinaia mongolica is currently restricted to the Amur Basin and Primorye in eastern Russia. This species is classified under the tribe Middendorffinaiini of the subfamily Unioninae within the family Unionidae based on multilocus phylogeny. However, the lack of comprehensive taxonomic information and sufficient molecular phylogenetic data casts uncertainty on the validity and taxonomic status of these two species. We aim to reassess the validity of Unio szechenyii and confirm the taxonomic status of Middendorffinaia mongolica , a new record for China, by integrating taxonomy with robust mitochondrial phylogenomics. We support the validity of Unio szechenyii and propose a recombination as Cuneopsis szechenyii comb. nov. by integrating shell morphometry, soft-body anatomy and molecular evidence. By determination of the mitochondrial genome of Cuneopsis szechenyii , Cuneopsis kiangsiensis , Middendorffinaia mongolica and Pseudocuneopsis wuana , we elucidate intrageneric relationships within the genera Cuneopsis and Pseudocuneopsis. In addition, based on the updated consensus taxonomic framework including tribes and subtribes in Unioninae, mitochondrial phylogenomics strongly support the transfer of Middendorffinaia mongolica from the tribe Middendorffinaiini to the tribe Unionini and classification under the subtribe Middendorffinaiina stat. rev. ZooBank: The maternal mitochondrial genomes of Unio szechenyii , Cuneopsis kiangsiensis , Middendorffinaia mongolica and Pseudocuneopsis wuana are sequenced. The validity of Unio szechenyii , previously considered a synonym of Cuneopsis heudei , is supported by the integration of conchometry, soft-body anatomy and molecular evidence, and a recombination as Cuneopsis szechenyii comb. nov. is proposed. The intrageneric relationships within the genera Cuneopsis and Pseudocuneopsis are elucidated based on the mitochondrial phylogenomics. Middendorffinaia mongolica , a new record for China, is proposed for transfer from the tribe Middendorffinaiini to the tribe Unionini and classification under the subtribe Middendorffinaiina stat. rev. (Image credit: Ruiwen Wu.) [ABSTRACT FROM AUTHOR]

Accurate identification and precise classification of freshwater mussel species that are among the most threatened freshwater taxa in the world, play a crucial role in informing conservation and management efforts for these organisms. However, due to the variability in shell morphology, relying solely on shell characteristics for species taxonomy poses significant challenges, thereby impeding effective conservation planning and management. The freshwater mussel genus Ptychorhynchus Simpson, 1900 is one such group in need of study. We integrate molecular phylogeny, shell morphology and soft-body anatomy to examine the classification of Ptychorhynchus denserugata (Haas, 1910) and Ptychorhynchus resupinatus (von Martens, 1902). The COI barcoding data support the clustering of P. denserugata and Nodularia douglasiae within a single clade, and P. denserugata shares the diagnostic feature of the genus Nodularia , i.e. knobs or bumps on the inner mantle surface in the excurrent aperture. Therefore, by integrating molecular data and anatomical characteristics, we confirm that the nominal species P. denserugata syn. nov. is a new synonym for N. douglasiae. The multi-locus (COI + ND1 + 16S rRNA + 18S rRNA + 28S rRNA) phylogeny and mitochondrial phylogenomics support the transfer of P. resupinatus from Ptychorhynchus to the newly elevated genus Cosmopseudodon stat. rev., as Cosmopseudodon resupinatus stat. rev. that is still considered the designated type species. We also describe a new species based on integrative taxonomy, i.e. Cosmopseudodon wenshanensis sp. nov. The comprehensive understanding of the taxonomy and diversity of the revised Cosmopseudodon species, and shell heteromorphism of N. douglasiae (= P. denserugata syn. nov.), will serve as a crucial foundation for further scientific assessment and conservation strategies pertaining to these taxa. ZooBank: Ptychorhynchus resupinatus is transferred from Ptychorhynchus to a newly elevated genus Cosmopseudodon stat. rev., as Cosmopseudodon resupinatus stat. rev. Ptychorhynchus denserugata syn. nov. is confirmed as a new synonym for Nodularia douglasiae. A new species, Cosmopseudodon wenshanensis sp. nov., is described based on integrative taxonomy. (Image credit: Ruiwen Wu.) [ABSTRACT FROM AUTHOR]