Tachysurus wuyueensis, new species, is described from the Qiantang-Jiang Basin, situated in Suichang County, Zhejiang Province and Xiuning County, Anhui Province, southeast China. The coastal basin drains into the East China Sea. The new species belongs to the T. pratti-T. truncatus group within the genus Tachysurus defined by having a smooth anterior margin of the pectoral-fin spine, short maxillary barbels not extending beyond the base of the pectoral-fin spine, short dorsal spine not exceeding two thirds of head length and an emarginated caudal fin. This new species is distinct from all other species of this group, Tachysurus pratti (Gunther, 1892), T. truncatus (Regan 1913), T. gracilis (Li, Chen & Chan, 2005) and T. brachyrhabdion (Cheng, Ishihara & Zhang, 2008), in having a shorter prepelvic body (length 40.0–46.4% of SL vs. 45.8–54.8%). It further differs from T. pratti, T. truncatus and T. gracilis in having more vertebrae (45–47 vs. 37–44) and more anal fin rays (21–25 vs. 14–20), from T. brachyrhabdion in having a more slender body (depth 10.1–13.5% of HL vs. 13.1–17.6%). Molecular phylogeny, based on the mitochondrial cytochrome b (cyt. b) gene confirms the validity of T. wuyueensis and the T. pratti-T. truncatus group. Furthermore, this study addresses the diagnostic traits distinguishing the T. pratti-T. truncatus group from the T. tenuis-T. crassilabris group which have historically been treated as a single species group due to morphological similarities.

The Tachysurus argentivittatus group represents the smallest-bodied lineage within Tachysurus with adults not exceeding 50 mm in SL, and is widely distributed across East Asia. Owing to the pronounced morphological conservatism, species delimitation within this group has long been challenging. Previous studies recognized two known species with this species group, T. argentivittatus sensu stricto restricted to the Pearl River and T. mica distributed from Yangtze to Amur Rivers, and also indicated that the Hainan Island population represents an undescribed lineage. In this study, geographic sampling was expanded to include populations from central-northern Vietnam and coastal region of Chinese mainland, all located along the northern margin of the East Asian tropics. Using an integrative framework, the validity of T. longispinalis in the Red River basin is confirmed. Furthermore, this species forms part of a species complex, comprising four reciprocally monophyletic lineages. Within this complex, two allopatric lineages from Hainan Island and the Moyang River (in Guangdong Province of China) exhibit consistent genetic divergence and stable morphological diagnosis, and are described herein as T. lativittatus sp. nov. and T. stenomaculatus sp. nov. respectively. The central Vietnam lineage shows shallow genetic divergence and limited morphological differentiation from T. longispinalis and is provisionally treated as T. aff. longispinalis. Diversification within the complex occurred from the late Pliocene to the middle Pleistocene, coinciding with two major climatic transitions linked to the onset and intensification of Northern Hemisphere glaciation since approximately 3 Ma, when repeated sea-level declines likely facilitated dispersal of ancestral populations and promoted subsequent divergence. Moreover, our findings refine the morphological boundaries of the T. argentivittatus group.

The northern margin of the East Asian tropics harbors exceptional endemism, yet its freshwater fish fauna remains poorly characterized. Tachysurus virgatus, long considered a single widespread species in this region, exemplifies this taxonomic gap. Through integrative taxonomic analyses combining morphological and molecular evidence, we uncovered remarkable cryptic diversity within this species. Phylogenetic reconstructions reveal that true T. virgatus is restricted to the Wanquan River basin on Hainan Island, whereas nine additional lineages from other drainages represent previously unrecognized species described in this study. Together with two known congeners (T. spilotus and T. kyphus), these form the monophyletic T. virgatus species group for which we provide morphological diagnoses. Pronounced incongruence between morphological and molecular divergence characterizes this group, underscoring the necessity of integrative approaches for species delimitation in morphologically conserved lineages. Moreover, distinct habitat specialization patterns were identified within the group, with species exhibiting adaptations to three ecological niches: mainstem river, montane streams, and generalist habitats occupying both. Correlations between habitat preferences and functional morphology were preliminarily discussed. Using our revised taxonomic framework, we evaluated the conservation status of all species under IUCN Red List criteria, with several newly described taxa warranting threatened status due to extremely limited distributions. This study highlights the pervasive cryptic diversity of freshwater fishes in the north margin of East Asian tropics and emphasizes the critical importance of rigorous taxonomic resolution to guide effective conservation in this biogeographically complex region.