Astyanax fishes are among the most important food-web components of South America rivers. In the Iguaçu River basin, the Astyanax genus is represented mainly by endemic species. For millions of years, that hydrographic basin has been geographically isolated from the Paraná River basin by the Iguaçu Falls. Recently, a species from the Upper Paraná River basin identified as Astyanax bimaculatus was revised and described as a new species named Astyanax altiparanae Garutti & Britski, 2000. Fauna endemism and geographic isolation triggered interest in investigations to evaluate the identification and genetic relatedness among two A. altiparanae populations from the Upper Paraná River basin and the population identified as A. bimaculatus in the Iguaçu River, upstream from the Iguaçu Falls. Mitochondrial DNA sequences and RAPD markers revealed high genetic diversity within each population, as well as low genetic distance, high gene flow, and high mitochondrial DNA similarity among all three populations. In conjunction with morphological similarities, these results demonstrated that the population presently known as Astyanax bimaculatus in the Iguaçu River should actually be stated as Astyanax altiparanae. Furthermore, it could be inferred that the A. altiparanae population is not endemic and most likely it was recently introduced in the Iguaçu River basin, maintaining the ancestral genetic identity.

Invasive and native populations of the Amazonian fishes 'peacock bass' Cichla monoculus and of a not yet described species 'blue tucunare´' here referred as Cichla sp. 'Azul' were analysed for genetic diversity using the hypervariable domain of the mitochondrial DNA (mtDNA) control region plus steady diagnostic random amplified polymorphic DNA loci. There is no detailed historical record of the introduction of Cichla species into the Upper Parana´River basin, where they became invasive and a potential threat to local ichthyofauna. Genetic diversity among invasive populations confirmed the hypothesis of multiple introductions in this hydrographic basin. Moreover, a large and previously unknown population of natural fertile hybrids between C. cf. monoculus and Cichla sp. 'Azul' was identified in the Itaipu hydroelectric reservoir and in the floodplain of the Upper Parana´River. Crossbred morphotypes were similar to C. cf. monoculus, but their morphological identification was not unequivocal. This hybrid population was characterized by high genetic diversity and it was composed of hybrids possessing concurrently nuclear DNA fragments specific for C. cf. monoculus as well as fragments specific for Cichla sp. 'Azul'. The nuclear DNA markers indicated that reproductive isolation between C. cf. monoculus and Cichla sp. 'Azul' has broken down in the new environment, and mtDNA sequences revealed that both species can be the female donor in the interspecific crosses. The data presented herein are potentially useful for future taxonomic, genetic and evolutionary studies in the complex Cichla group, for monitoring of invasive populations, and for further development of ecological guidelines.