Species delimitation is a permanent issue in systematics. The increasing recognition of geographically isolated populations as independent lineages allowed by new methods of analysis has inflated the species‐populations dilemma, which involves deciding whether to consider separate lineages as different species or structured genetic populations. This is commonly observed between fishes of adjacent river basins, with some lineages being considered allopatric sister species and others considered isolated populations or variants of the same species. Pseudocorynopoma doriae is a characid diagnosed from its single congener by the number of anal‐fin rays and sexually dimorphic characters of males, including distinct fin colouration. The authors found variation in the colour pattern between isolated populations previously identified as P. doriae but no variation in scale or fin‐ray counts. They analysed molecular evidence at the population level and morphological differences related to life history (e.g., colour dimorphism related to inseminating behaviour). The results provide compelling evidence for the recognition of a new species of Pseudocorynopoma despite the lack of discrete differences in meristic data. The recognition of the new species is consistent with biogeographical evidence for the long‐term isolation of the respective river drainages and with differences between the ichthyofaunal communities of these rivers.

link: https://onlinelibrary.wiley.com/doi/10.1111/jfb.14572

FIGURE 2 Live specimens of Pseudocorynopoma, showing the colour pattern and shape of fins in a (a) mature male [UFRGS 22249, 53.6 mm standard length (SL)], (b) immature male (UFRGS 22249, 48.5 mm SL) and (c) mature female (UFRGS 22249, 50.5 mm SL) of Pseudocorynopoma doriae from the rio Uruguay/rio Negro drainages and of a (d) mature male (UFRGS 22112, 52.5 mm SL), (e) immature male (UFRGS 22112, 48.0 mm SL) and (f) mature female (UFRGS 22112, 52.1 mm SL) of Pseudocorynopoma stanleyi from the laguna dos Patos drainage. All specimens represented in scale

FIGURE 7 Pseudocorynopoma stanleyi, holotype. UFRGS 27600, 60.96 mm standard length, male. Brazil, Rio Grande do Sul, Marques de Souza; arroio Tamanduá

FIGURE 5 Haplotypic network for the populations in Pseudocorynopoma doriae inferred based on mtDNA demonstrating the strong genetic structure observed between the two putative species (depicted as cold and warm colours). Each coloured circle indicates a haplotype, with colours indicating the population and size the frequency. Black line ticks indicate the number of mutational steps, and black circles indicate the missing or unsampled steps