Abstract
Species within the Superorder Cladocera have long been considered cosmopolitan organisms since their first descriptions. However, taxonomic studies integrating morphological and molecular data have revealed numerous geographically isolated species complexes. In this study, we investigated the genetic diversity within the genus Diaphanosoma using a fragment of the mitochondrial gene cytochrome c oxidase subunit I ( COI ), together with morphological descriptions and characterization of climatic niche breadth. Our results revealed a distinct clade of species-level divergence, which is here described as Diaphanosoma tohai sp. nov., based on comparisons between Neotropical populations and those of Diaphanosoma birgei from the Nearctic region. Molecular and taxonomic analyses revealed high phylogenetic complexity within the group, clustering Diaphanosoma tohai sp. nov. with D. cf. heberti and separating a cryptic lineage of North American D. birgei into a distinct clade. Morphological features, such as the hook on the posterior margin of the valve, and the comparison of the climatic niche breadths support these genetic differences. Our results expand current knowledge of Neotropical biodiversity and provide new insights into the non-cosmopolitan nature of Diaphanosoma species and the persistent issue of high morphological stasis within the group.