Jenynsia luxata, a new species from northwestern Argentina, is described. This species is diagnosable from all other Jenynsia by the medial processes of left and right pelvic bones relatively reduced and separated from each other. The new species resembles J. multidentata, but it is further distinguished from this species by the absence of a swelling between the urogenital opening and the anterior base of the anal fin in females and details of coloration. Phylogenetic analyses, both under implied and equal weighting, recover the subgenera Plesiojenynsia and Jenynsia as monophyletic units. New information on previously missing characters of Jenynsia maculata is added. These data and phylogenetic characters coded for the new species herein described contribute to a better resolution of the phylogenetic relationships within the subgenus Jenynsia, which is herein supported by additional synapomorphies relative to previous phylogenies.Jenynsia luxata, una nueva especie del noroeste de Argentina, es descripta. Esta especie es diagnosticable por presentar los procesos mediales de los huesos pélvicos izquierdo y derecho relativamente reducidos y separados. La nueva especie se parece a J. multidentata, pero se distingue de ésta por la ausencia de un abultamiento entre la abertura urogenital y la base de la aleta anal en hembras y por detalles en el patrón de coloración. Los análisis filogenéticos, tanto bajo pesos implicados como iguales, recuperan los subgéneros Plesiojenynsia y Jenynsia como unidades monofiléticas. Se aporta nueva información sobre caracteres de J. maculata previamente codificados como entradas faltantes. Esos datos y los caracteres filogenéticos codificados para la nueva especie aquí descripta contribuyen a una mayor resolución de las relaciones filogenéticas dentro del subgénero Jenynsia, que está aquí soportado por sinapomorfías adicionales en relación a las filogenias previas.

Asymmetries in bilateral organisms attract a lot of curiosity given that they are conspicuous departures from the norm. They allow the investigation of the integration at different levels of biological organization. Here we study whether and how behavioral and asymmetrical anatomical traits co-evolved and work together. We ask if asymmetry is determined locally for each trait or at a whole individual level in a species bearing conspicuous asymmetrical genitalia. Asymmetric genitalia evolved in many species; however, in most cases the direction of asymmetry is fixed. Therefore, it has been rarely determined if there is an association between the direction of asymmetry in genitalia and other traits. In onesided livebearer fish of the genus Jenynsia (Cyprinodontiformes, Anablepidae), the anal fin of males is modified into a gonopodium, an intromittent organ that serves to inseminate females. The gonopodium shows a conspicuous asymmetry, with its tip bending either to the left or the right. By surveying 13 natural populations of Jenynsia lineata, we found that both genital morphs are equally common in wild populations. In a series of experiments in a laboratory population, we discovered asymmetry and lateralization for multiple other traits; yet, the degree of integration varied highly among them. Lateralization in exploratory behavior in response to different stimuli was not associated with genital morphology. Interestingly, the direction of genital asymmetry was positively correlated with sidedness of mating preference and the number of neuromasts in the lateral line. This suggests integration of functionally linked asymmetric traits; however, there is no evidence that asymmetry is determined at the whole individual level in our study species.

Oligosarcus itau n. sp. is described. This species is the sister group of the remaining analyzed species of Oligosarcus and this relationship is supported by seven morphological synapomorphies: 1) well developed temporal fossa, 2) absence of a dorsal expansion on the rhinosphenoid, 3) posteroventrally angled articulation between second and third infraorbitals, 4) presence of ectopterygoid teeth on a row, 5) laterally displaced cartilage on the ectopterygoid, 6) presence of bony lamellae bordering laterosensory canal of suprapreopercle, and 7) presence of two pairs of uroneurals. The new species is diagnosed by the following combination of characters: 1) two distinct premaxillary rows of teeth, 2) tricuspidate and pentacuspidate teeth on dentary and posterior premaxillary row, 3) presence of a row of 3–4 tricuspidate to conical ectopterygoid teeth, 4) possession of v-vi,21–23 anal-fin rays, and 5) presence of 41–42 perforated scales on lateral line. Oligosarcus itau n. sp. was previously considered as an Astyanacinus. Astyanacinus moorii (Boulenger), type species of the genus, is included in the Astyanax clade and Astyanacinus platensis Messner is transferred to Oligosarcus.

Two new genera and species of Cyprinodontiformes from the Miocene of Tucumán, Argentina, are described: Tucmanableps cionei and Sachajenynsia pacha. Both exhibit sexual dimorphism, indicated by the modification of the male anal fin into an intromittent organ; tricuspid teeth in the jaws; abdominal pelvic fins and other features that demonstrate a relationship with the family Anablepidae. Males of Sachajenynsia pacha gen. et sp. nov. present modifications of the anal fin, herein interpreted as an incipient gonopodium, whereas the anal fin of Tucmanableps cionei gen. et sp. nov. is modified into a complex gonopodium with a median distal plate, which is unique among the Anablepidae. In extant genera of the family, modification of the anal fin into an intromittent organ is related to internal fertilization; this condition is inferred for S. pacha but was clearly the reproductive mode of T. cionei. In our phylogenetic analysis, S. pacha is recovered as the sister group of all anablepids with evident internal fertilization, while T. cionei forms a clade with Anableps species. The occurrence of two new anablepid genera showing distinct and unique morphologies suggests that the fossil diversity of the Anablepidae may be higher than the extant one. Also, the new taxa were found in open lake deposits with coastal and inner lake sediments of the Río Salí Formation associated with the Miocene Entrerrian–Paranense Marine Ingression, supporting the idea that coastal environments and brackish waters are related to the generic diversification of the Anablepidae, as was suggested for the species of Jenynsia.

A new species assigned to the genus Trichomycterus from the area of the waterfalls of Tabay stream, Paraná River basin, Misiones, Argentina, is described. Trichomycterus ytororo sp. nov. is distinguished from all other species in the genus by the presence of 31–35 dorsal procurrent caudal-fin rays and the combination of some external characters such as: coloration, number of pectoral–fin rays and pores of the laterosensory canals. The new taxon belongs to a presumably monophyletic group of species composed of T. crassicaudatus, T. igobi, and T. stawiarski based on the presence of 24 or more thickly ossified and rigid procurrent caudal-fin rays with a slender distal tip extending along the tips of at least ten neural spines.

Jenynsia tucumana new species is described from the upper Río Salí basin, province of Tucumán, northwestern Argentina. The new species is diagnosed by the possession of a row of dark markings ranging from dots to small vertical stripes from the tip of adpressed pectoral fin to the posterior margin of the hypural. Also, the new species has a mandibular canal pore W and a symmetrical fifth anal-fin ray in adult males; whereas the females lack a urogenital swelling. According to a phylogenetic reanalysis of the genus, the new species is sister to most species of the subgenus Jenynsia, except for J. onca and possibly J. sanctaecatarinae.

Altitudinal gradients represent an appropriate system to assess whether there is a relationship between richness patterns, environmental variables, and the ecological processes that determine the species type and number inhabiting a given area. In mountain streams freshwater fishes, the most prevalent relationship is a monotonic decrease in species richness with elevation. The objective of this study was to evaluate four hypotheses that can explain the negative relationship between local fish species richness and altitude, 1) the hypothesis of decreasing energy availability, 2) the hypothesis of increasing climate severity, 3) the hypothesis of habitat diversity, and 4) the hypothesis of isolation by physical severity of the environment. Fish and macro-invertebrates were collected following standard methods from 83 sites (between 200-4 000 meters) of two river basins in the Bolivian Amazon. The first hypothesis was tested by analyzing relationships between the density of macro-invertebrates, the richness of invertivorous fish species and altitude; while the second and third hypotheses were assessed by a multiple regression analysis (GLM) between fish species richness and several local and regional factors. Besides, assemblage dissimilarity between sites along the altitudinal gradient was analyzed using βsim and βness indices. Fish richness decreases linearly with increasing altitude. The density of macro-invertebrates tends to increase at higher altitudes, contrary to invertivorous fish species richness, suggesting that energy availability is not a limiting factor for fish species colonization. The GLM explained 86 % of the variation in fish species richness, with a significant contribution of water temperature, maximum slope in the river mainstem, and stream width. There is a higher species turnover (βsim) between sites at low elevation. Inversely, βness shows higher values in the upper parts, corresponding to change in assemblages mainly due to species loss. Taken together, these results suggest that climatic and physical severities create strong barriers to colonization, further explaining the decrease in fish richness along the altitudinal gradient.

A new species of Astyanax (Characiformes, Characidae) from the upper rio Bermejo basin,