Integrative Taxonomy Reveals a New Species of Opsariichthys (Teleostei, Cypriniformes) from the Pearl River Basin in Guangxi Zhuang Autonomous Region, China, with Synonymization of O. minutus

The taxonomy of Opsariichthys Bleeker, 1863 in southern China is complicated by conflicting morphological and molecular evidence, particularly the status of O. minutus Nichols, 1926, long treated as a junior synonym of O. bidens Günther, 1873 until its revalidation. To resolve this, we adopted an integrative approach combining D-loop sequencing to anchor specimens to the O. minutus lineage (supplementing scarce GenBank data with vouchers bearing undulating jaw margins), genus-wide cytochrome b (cyt b) phylogenetics to avoid circular reasoning, and detailed comparative morphology. D-loop analyses revealed that all specimens with undulating jaw margins formed two clades: Fujian Province populations clustered with O. bidens, while Guangxi Zhuang Autonomous Region populations formed a distinct monophyletic lineage. Cyt b phylogenetics confirmed Fujian populations shared <2.0% uncorrected p-distances with O. bidens and showed extensive morphological overlap, reaffirming O. minutus as a junior synonym of O. bidens. In contrast, Guangxi populations from the Xijiang River basin (middle and upper Pearl River system, including the Hongshuihe, Guijiang and Liujiang Rivers) formed a distinct monophyletic lineage with 7.4–15.9% uncorrected p-distances from congeners, diagnosed by 49–55 lateral-line scales, two pharyngeal tooth rows, and prominent lower-jaw tubercles arranged in 1–2 rows in breeding males. This lineage is described as a new species of Opsariichthys, with its type locality in the Hongshuihe River. Collectively, our findings not only resolve a long-standing taxonomic puzzle but also underscore the efficacy of integrative taxonomy in uncovering cryptic diversity within the highly biodiverse yet understudied Pearl River basin.