Forgotten for Decades: Revalidation and Redescription of Raiamas harmandi (Sauvage, 1880) (Cypriniformes: Danionidae) from the Mekong River Basin

The genus Raiamas currently comprises 18 valid species, only 2 of which occur in Asia; the remaining 16 are endemic to Africa. Raiamas harmandi was originally described by Sauvage in 1880 as Bola harmandi, which is distributed in the Great Lakes, Cambodia, the Mekong River Basin. It was considered a synonym of R. guttatus by later researchers. In this study, we examined 49 Raiamas individuals from the Mekong, Irrawaddy, and Salween river basins, recording both meristic counts and morphometric measurements. Based on the morphological evidence, we revised the taxonomy of Raiamas in the Mekong River Basin, confirming R. harmandi as a valid species and providing a comprehensive redescription. Raiamas harmandi can be distinguished from R. guttatus mainly by having more predorsal scales (25–28 vs. 21–23) and a different color pattern on the lateral body. Utilizing a total of 44 aligned COI and Cyt b sequences—including eight newly sequenced individuals of Raiamas from three river basins—we reconstructed its phylogenetic relationships. The analysis strongly supported four R. harmandi individuals from the Mekong River Basin forming a distinct clade, which was the sister to the clade comprising five R. guttatus individuals from the Irrawaddy and Salween river basins. Genetic distances between R. harmandi and R. guttatus ranged from 14.0 to 14.9% for COI and 16.1 to 17.0% for Cyt b. Distributionally, R. harmandi occurs throughout the Mekong River Basin, as evidenced by combined voucher specimens and molecular sequence data.