Description of the second scale-developed and eyed Sinocyclocheilus (Cypriniformes: Cyprinidae) species from South Guizhou, China, with evolutionary implications.

The genus Sinocyclocheilus represents the world's most diverse cavefish radiation, typically exhibiting troglomorphic traits such as scale regression and eye degeneration. Notably, Sinocyclocheilus macrolepis from a tributary of the Long-Jiang of the Pearl River retains fully developed scales despite inhabiting epigean streams, a rare trait among its cave-adapted congeners. The Sinocyclocheilus specimens collected from a tributary of the Hongshui-He of the Pearl River in South Guizhou exhibit overall similarities to S. macrolepis with fully developed scales and normal eyes. Phylogenetic analyses using mitochondrial Cyt b and ND4 genes reveal that these specimens represent a novel species, sister to S. macrolepis but with significant genetic divergence, and closely related to Sinocyclocheilus ronganensis. This new species was designated as Sinocyclocheilus pingchowensis in this study. Morphologically, S. pingchowensis differs from S. macrolepis in lateral-line scale counts, head width, gill-opening position and pelvic-fin ray counts. S. pingchowensis along with S. macrolepis and S. ronganensis are herein defined as the S. macrolepis group based on the shared synapomorphies and phylogenetic affinities. The phylogenetic tree suggests that the retention of scales in the species pair of S. pingchowensis and S. macrolepis represents a rare case of evolutionary reversal from cave-adapted degeneration. This phenotypic re-emergence correlates with their re-colonization of surface water, likely reflecting a synergistic adaptation to both photic exposure and occupation of the niche of mid-upper water strata, coinciding with relaxed selection on cave-associated trade-offs. The divergence of S. pingchowensis from its sister species was likely triggered by the river capture event between the two rivers.