Cytogenetics and DNA barcode in Hoplias gr. malabaricus (Characiformes, Erythrinidae) reveals correlation between karyomorphs and valid species

The Hoplias malabaricus group comprises seven described species, including seven karyomorphs for H. malabaricus . However, the relationship between morphological and chromosomal data within the group remains unresolved. Most studies on the Hoplias malabaricus group from the Iguazu River ecoregion have primarily focused on its upper and middle sections, while the lower Iguazu River remains unstudied. Therefore, in this study, we investigated a population of H. malabaricus from the Capanema’s River (lower Iguazu River basin), Brazil, using cytogenetic techniques and DNA barcode. Through these analyses, we identified the karyomorphs A and D living in sympatry in the lower Iguazu River basin. The karyomorph A, identified as native to the Iguazu River basin, had two cytotypes, designated as A1 and A2, which are differentiated by C-positive heterochromatin and rDNA distribution patterns. Conversely, this is the first record of karyomorph D for the Iguazu River basin, which was previously thought to be endemic to the Upper Paraná River basin and may therefore represent an introduced karyomorph. Interestingly, the cytotype A2 is very similar (same haplogroup) to sequences of Hoplias argentinensis , while the karyomorph D from the lower Iguazu River basin is similar to sequences identified as Hoplias misionera . These findings provided new insights into the diversity of Hoplias gr. malabaricus from the Neotropical region, achieved through cytogenetic and DNA barcode integration. The results also highlight the increased conservation risk for native species in the Iguazu River ecoregion, particularly considering the ecological position occupied by trahiras within the trophic levels. Graphical abstract A sample of the Hoplias malabaricus group was collected in the Iguazu River basin, municipality of Capanema, near Argentina. The specimens were subjected to cytogenetic and DNA barcode analysis. Two karyomorphs were found, A and D. Karyomorph A is possibly native to the Iguaçu River basin, while karyomorph D is considered non-native, indicating a possible introduction.