Triplophysa lucicauda, a New Species of Tibetan Stone Loach (Nemacheilidae) from Shigatse City, Xizang

Triplophysa is an important component of high-altitude freshwater fish fauna on the Qinghai Tibet Plateau and adjacent regions, and morphological overlap among closely related species complicates species identification. In this study, we conducted an integrative taxonomic investigation of a Triplophysa population collected from the Peng Chu River in Shigatse, Tibet, China, combining morphological examination, morphometric analysis, and mitochondrial cytochrome b (Cyt b) sequence data, and describe it as a new species, Triplophysa lucicauda sp. nov. The new species is characterized by a unique combination of morphological traits, including a conspicuous bright yellow circular spot on the dorsal surface of the posterior caudal peduncle, as well as distinctive fin positions, fin-ray counts, barbel morphology, and proportional measurements. Based on 34 proportional characters, we analyzed the new species alongside T. microps and T. brevicauda. The first three principal components cumulatively explained 60.75% of the total morphological variation, and canonical variate analysis revealed discriminant structure among species. Both MANOVA and PERMANOVA detected significant overall differences, and all pairwise comparisons remained significant after Benjamini–Hochberg correction; the overall PERMANOVA yielded R2 = 0.2598 and p = 0.0012, whereas the test of multivariate dispersion detected no significant among-group differences (p = 0.6536). The overall classification accuracy of leave-one-out cross-validation with a complete processing pipeline was 72.22%, with six of eight specimens of the new species correctly classified. Bayesian phylogenetic analysis of Cyt b sequences showed that the three sequenced specimens of the new species formed a monophyletic clade with maximum posterior probability support (BPP = 1.000), and the minimum K2P genetic distance between the new species and the comparative lineages was 5.13%. The diagnostic characters, overall morphological differentiation, and molecular lineage evidence jointly support the species validity of T. lucicauda. The discovery of this new species enriches the known diversity of the genus Triplophysa in the Himalayan region.