Integrative approach reveals hidden diversity in the Brachionus (Rotifera, Monogononta)

The phylum Rotifera has many species complexes that require integrative analyses to resolve their taxonomic identity. Brachionus angularis comprises different morphotypes. In Aguascalientes, Mexico, we observed two of these morphotypes coexisting. To explore their species identity, we applied an integrative approach that combined taxonomical, ecological, and molecular data. Our results demonstrate taxonomic differentiation in the lengths of females, males, and diapausing eggs between the two morphotypes. We also observed distinct hatching patterns and clearly different ornamentation in diapausing eggs between the two morphotypes. Genetic variation between one monoclonal culture of each morphotype revealed distances of 18.8% for mitochondrial COI and 13.73% for nuclear ITS1 regions. These marked, concordant divergences in mitochondrial and nuclear markers across the closely related Brachionus species, together with diagnostic morphological and ecological differences, support the recognition of the two morphotypes as distinct species. Accordingly, we describe Brachionus agsensis sp. nov. and Brachionus hydrocalidensis sp. nov. as new species.