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Farlowella is the second richest genus in Loricariinae, broadly distributed in freshwater streams and rivers of South America. In this article, we aimed to expand on the cytogenetic and molecular data available for two allopatric populations of Farlowella hahni. Both populations had diploid chromosome number 58, but with karyotype differences, indicative of chromosomal rearrangements. C-banding showed large heterochromatic blocks at telomeric regions in acrocentric chromosomes in both populations. Fluorescence in situ hybridization (FISH) revealed a single 18S rDNA site in both populations and a single 5S rDNA site for individuals from lower Parana River basin (native region) and multiple 5S rDNA sites for individuals from upper Parana River basin (non-native region). Mitochondrial sequence analyses did not separate the two F. hahni populations. The cytogenetic and molecular data obtained are relevant in a preliminary study and suggested the existence of cryptic diversity and the hypothesis that at least two Farlowella lineages may coexist in the Parana basin.

Brazilian laws allow the catch of some species as ornamental fish; however, in the absence of biological information, such catch can threaten species conservation. Therefore, this study aimed to offer population and reproductive information about Farlowella hahni in streams of the Ivinhema River Basin, Upper Paraná Basin. To accomplish this, a total of 165 F. hahni individuals were analyzed, including 109 females and 48 males, resulting in an important sex ratio difference from 1:1 (χ 2=19.97; p