Integrative Taxonomy Reveals a New Species of Freshwater Mussel, Pseudodon septuaginta sp. nov. (Unionidae: Pseudodontini), from the Mekong River Basin in Northeastern Thailand

This study investigated the taxonomy and phylogenetic relationships of the freshwater mussel genus Pseudodon Gould, 1844, with a specific focus on Indochinese populations, using a comprehensive molecular dataset and morphometric analyses. Our findings confirm the valid status of seven among the nine currently recognized extant Pseudodon species, and include the discovery of a new cryptic species, P. septuaginta sp. nov., from the Mekong River Basin in northeastern Thailand. The new species is characterized by a compressed, sub-trigonal shell outline, the presence of a posterior wing, weakly developed pseudocardinal teeth, and obsolete lateral teeth. Phylogenetic analysis revealed this new species as the sister taxon to the adjacent allopatric P. vagulus, which is distributed in Mekong River tributaries in the upper Khorat Plateau, exhibiting a 6.71% uncorrected COI p-distance. Despite some morphological overlap, these two species are distinguishable by pseudocardinal tooth structure and overall shell shape. Furthermore, phylogenetic analyses support a potential Bornean origin for Pseudodon, followed by dispersal into Myanmar and Indochina, with paleo-drainage isolation driving diversification within the Indochinese lineages.