Abstract Stingrays genus Hypanus currently encompasses nine valid species from the Atlantic and Pacific oceans, though the phylogenetic relationships amongst some of them were based on a single mitochondrial gene and did not involve all putative Hypanus species. To address the monophyly of the genus and its relationship to other Dasyatinae genera, we sequenced the whole mitochondrial genomes of all species that supposedly belong to this genus and representatives of Dasyatinae, Neotrygoninae, and, as an outgroup, Fontitrygon (Urogymninae). Based on phylogenetic analyses, Hypanus is the sister-genus to all other Dasyatinae, and this subfamily is closely-related to Neotrygoninae within the family Dasyatidae. The species F. geijskesi is closely related to H. guttatus rather than to its congeners and should be allocated to Hypanus as H. geijskesi for the genus monophyly. After lineage delimitation analyses, we identified three species complexes composed of H. americanus, H. guttatus, and H. say, with two distinct evolutionary lineages within each, leaving the genus with 13 evolutionary units, of which six are currently under threat and only H. sabinus is of least concern. The urgency in identifying these new lineages lies in the fact they might already be under threat before being formally described.

Abstract A new species of Cyphocharax is described from the rio Juma, which flows across the meridional Amazon basin and is a tributary of the rio Aripuanã in the rio Madeira fluvial system. Features distinguishing the new species from congeners include a longitudinally elongated caudal peduncle blotchextending from the vertical line through the adipose-fin terminus to the base of median caudal-fin rays, the number of scale series along the lateral line and between the lateral line and dorsal-fin origin, morphometric variation, and the patterns of pigmentation along the body. The species was discovered in a lesser-explored region and appears to be an endemic of the rio Aripuanã in the southern Amazon basin.

Abstract Five species of Bryconamericus are known from the upper rio Paraná basin: ‘B.’ aff. iheringii, ‘B.’ coeruleus, B.stramineus, ‘B.’ turiuba and the non-native B. exodon. The new species can be easily distinguished from them by its body depth (27.8–31.8% SL) and the teeth aligned in the outer row of the premaxilla (except ‘B.’ aff. iheringii). The new species is distributed in tributaries to the Piquiri, Ivaí and Tibagi rivers, at altitudes between 498 and 900 m a.s.l. The geographic distribution of the new species is similar to the congener ‘B.’ coeruleus. Other species with similar distribution are Apareiodon vladii and Planaltina kaingang, although they only occur in the Piquiri and Ivaí river basins. Furthermore, this work records the new species with the presence of breeding tubercles. These tubercles are located on the dorsal and lateral portions of the head and on the posterior margin of the scales of mature males; and in females, when present, they are smaller and restricted to the posterior margin of the scales. Comments are made on the presence of breeding tubercles in other groups of fishes, and also on some dimorphic characters present in the new species.

Abstract A new species of the recently discovered miniature tridentine catfish, Rhinotridens, is described from the Amazon basin in Brazil, Colombia, and Peru. It differs from Rhinotridens chromocaudatus, type-species of the genus, in having the caudal fin mostly hyaline, a rictal barbel vestigial or externally absent, a longer anal fin with 22–29 segmented rays, and numerous other features from the skeleton. An osteological account of the new species is provided based on cleared and stained specimens, and traits of its external morphology are revealed through scanning electron microscopy (SEM), which for the first time reveal details of the lip structure of tridentines. The seven morphological characters previously listed as synapomorphies for Rhinotridens are confirmed in the species described herein, corroborating its generic allocation. An additional characteristic from the morphology of the lateral process of the autopalatine is proposed as a novel synapomorphy for the genus.

Abstract Temperature is projected to continue increasing in the upcoming years. The effects of temperature warming in deforested stream populations have not been addressed yet and are a promising area to understand the consequences of increased temperature on fish physiology. Therefore, the aims of this study were to determine the manner in which Bryconops giacopinii from deforested habitat modulates the LDH kinetics in response to warming and whether the antioxidant system is able to withstand thermal stress. We collected individuals from two roadside streams (deforested) and one forested stream and measured the LDH kinetics parameters (Vmax and Km) for pyruvate and lactate, measured the total ROS production, and measured the activity of antioxidant enzymes and the oxidative stress biomarker. Our results showed lower affinity and higher LDH activity for lactate oxidation in road-side populations, suggesting that populations living in high temperatures use lactate as aerobic fuel. Besides, there was an increase in ROS production, and CAT and GSH levels in road-side populations, but not LPO levels, suggesting that B. giacopinni is able to neutralize the ROS production with the antioxidant systems. Our results bring important findings in the adaptation of this specie to a warm environment.

Abstract Chromosomal patterns are valuable tools in evolutionary approaches. Despite the remarkable expansion of fish cytogenetic data, they are still highly deficient concerning deep oceanic species, including the Gempylidae snake mackerels. The snake mackerels are important commercial species composed by meso- and bento-pelagic predators with very limited information available about their lifestyle and genetics patterns. This study presents the first chromosomal data of two circumglobal species of this family, Ruvettus pretiosus and Promethichthys prometheus, from the São Pedro and São Paulo Archipelago. Conventional analyses, chromosomal staining with base-specific fluorochromes, and fluorescence in situ hybridization (FISH) for mapping of repetitive DNA classes were used. Both species have 2n = 48 chromosomes, but they highly differ regarding the karyotype formula (FN = 50 and FN = 84). The 18S rDNA/Ag-NOR and the 5S rDNA sites have a syntenic bi-telomeric array in R. pretiosus, but an independent distribution in P. prometheus. The transposable elements are dispersed, while the microsatellites are also clustered in the centromeric and terminal regions of some chromosomes. It is noteworthy that despite the 2n conservation, a marked macro and microstructural diversifications, mainly mediated by pericentric inversions, differentiates the karyotypes of the species, pointing to a particular chromosomal trajectory of the gempylids among marine fish.

Abstract Hypostomus is distributed by Central and South America basins, with diverse species with taxonomic conflicts. This way, the integration of auxiliary techniques contributes to understanding the systematics and phylogeny of the group. Thus, this study aimed to investigate the Hypostomus cochliodon and H. boulengeri from the Onça stream (Paraguai River basin) by allozyme and cytogenetic techniques. Hypostomus boulengeri showed a diploid number of 68 chromosomes (14m+14sm+18st+22a), multiple NOR revealed by Ag-NOR and 18S rDNA FISH, a polymorphism of heterochromatin in acrocentrics and the presence of B microchromosome. Hypostomus cochliodon showed a diploid number of 64 chromosomes (16m+26sm+14st+8a); despite the single NOR, some individuals showed NOR in both telomeres detected by Ag-NOR and 18S rDNA FISH. Isozyme identified two diagnostic loci (Idh-A and Gdh-A) between the two species and multiple loci with unique alleles in H. boulengeri. The genetic variability indicated by the mean heterozygosity (He) was 0.2461 and 0.0309 in H. boulengeri and H. cochliodon,respectively.Thus, this study reports the first cytogenetic data for H. boulengeri and the first isozymatic data for H. boulengeri and H. cochliodon. The two species presented evident cytogenetic and isoenzymatic differences with the obtaining of exclusive genetic markers providing support for future evolutionary studies in the group.

Abstract The upper rio Paraná basin drains the most developed and environmentally degraded region in South America: the Brazilian southeast. While it is one of the most well-known Neotropical fish assemblages, it is also one of the most threatened by anthropic activity. Urbanization, deforestation, dam construction, invasive species, and water pollution not only reduce the living area of species but also alter our perception of the basin and its biotic elements. Such changes are so profound and pervasive that raise uncertainties about the native or non-native status of several species. This paper aims to offer a detailed picture of the native diversity and historical biogeography of the fishes in the upper rio Paraná basin, which we hope will offer a solid foundation for future conservation policies. We update the list of native species, analyze geographical distributions, and identify biogeographical patterns, emphasizing areas requiring recognition as distinct biotas for conservation efforts. Over the past 15 years, more than 100 additional species have been described or recorded, expanding the total to 341 native species belonging to six orders and 30 families, making it the richest river basin in Brazil outside the Amazonian region. Unlike most neotropical basins where Characidae dominates, loricariids make up nearly one-fourth of the total fish diversity in the region. Species and clades concentrate in the central basin, while endemics are confined to peripheral areas due to the complex biogeographical history shared with neighboring basins. Eighteen distinct biogeographical regions are identified, discussing their composition, histories, and conservation implications. The upper rio Paraná basin has 10% of its fish fauna ranked as endangered and present 128 non-native species, including three hybrids. This places it as the major drainage with the largest number of endangered taxa in the neotropics and at the same time the one with the most numerous non-native elements. Existing protected areas are deemed ineffective in preserving diverse assemblages and fail to safeguard the majority of threatened and narrowly-endemic species.