Pablo A. Scarabotti

Multiple biomarkers responses in Prochilodus lineatus allowed assessing changes in the water quality of Salado River basin (Santa Fe, Argentina)

This field study assessed water quality of Salado River basin by using a set of biomarkers in the fish Prochilodus lineatus. Multiple biomarkers were measured, including morphological indexes (condition factor, liver somatic index), hematological (red and white blood cells) and biochemical (glucose, total protein and cholinesterase activity) parameters. Besides, detoxication and oxidative stress markers (antioxidant enzymes, lipid peroxidation) were measured in liver, gills and kidney. Despite water quality assessment did not show marked differences among sites, biomarkers responses indicate that fish are living under stressful environmental conditions. According to multivariate analysis glucose, glutathione S-transferase activity, lipid peroxidation levels and the count of white blood cells are key biomarkers to contribute to discrimination of sites. So, we suggest use those biomarkers in future monitoring of freshwater aquatic systems.

Dermal lip protuberances associated with aquatic surface respiration in juveniles of the piscivorous characid Salminus brasiliensis (Actinopterygii: Characidae)

Alguns peixes de agua doce da America do Sul podem melhorar o desempenho da respiracao aquatica superficial (RAS) desenvolvendo protuberâncias dermicas labiais na mandibula. Considerava-se que essa adaptacao era limitada aos peixes onivoros ou herbivoros. Este trabalho apresenta os primeiros indicios de que juvenis de um caracideo piscivoro, Salminus brasiliensis, desenvolvem protuberâncias dermicas labiais durante periodos de hipoxia em lagoas de varzea do rio Salado, Argentina. As protuberâncias de S. brasiliensis envolvem porcoes dermicas das duas mandibulas e exibem lobulos laterais ao lado da boca dispostas no plano vertical. Concentracoes de oxigenio dissolvido na agua, inferiores ou iguais a 1,05 mgl-1, foram associadas a um aumento notavel no desenvolvimento das protuberâncias labiais. Nesta especie os lobulos laterais da protuberância podem limitar o acesso de agua a porcao anterior da boca, que esta posicionada mais perto da superficie da agua oxigenada durante a RAS. Finalmente, a RAS, complementada com o desenvolvimento de protuberâncias dermicas labiais, pode ser considerada uma valiosa estrategia para a sobrevivencia em ambientes hipoxicos, mesmo para peixes carnivoros com elevada demanda de oxigenio.

Geographic distribution of the short-tailed river stingray (Potamotrygon brachyura): assessing habitat loss and fishing as threats to the world’s largest obligate freshwater elasmobranch

Freshwater elasmobranchs are threatened but little is known about them. Potamotrygon brachyura is the world’s largest obligate freshwater elasmobranch. This makes it popular among fishermen and suggests a susceptibility to anthropogenic threats. We collected records of P. brachyura (n=70) from media, fishermen and scientific sampling, to estimate its global geographic distribution (using both generalised additive and MaxLike models). Then, we estimated the species’ exposure to habitat modification and fishing pressure, by applying multinomial ordinal models with threat levels as response and the presence or absence of P. brachyura as an independent variable. Distance to coast, depth, water temperature range, salinity range and mean water temperature were the main determinants of the distribution in the Rio de la Plata. This resulted in a narrow coastal distribution. In the Uruguay and Parana River basins, the probability of occurrence was positively associated with flow accumulation, percentage of open water and submerged vegetation, and lowland ecoregions, and negatively to wetness index and altitude. This limited the distribution to large lowland rivers. Approximately 41% of the range of P. brachyura in the Rio de la Plata was subject to high habitat modification. In the Uruguay and Parana River basins, this percentage reached 7%; however, the overlap with high fishing pressure was 59%. For conservation purposes, P. brachyura may function as an umbrella species.

Food chain length in a large floodplain river: planktonic or benthic reliance as a limiting factor

Food chain length (FCL) is a key integrative variable describing ecosystem functioning. The aim of the present study was to test the hypothesis that the relative importance of planktonic and benthic energy pathways is a major factor affecting FCL in the Middle Parana River. Samples were obtained from in eight waterbodies, measuring chlorophyll-a concentrations and the abundance of benthic invertebrates and the trophic position of top predators by stable isotope analysis. There was no evidence that resource availability, disturbances or ecosystem size limited FCL. Similarly, the body size and trophic position of predators were not correlated. However, the relative abundance of planktonic and benthic resources was correlated with FCL. In addition, stable isotopes analysis showed that the benthic reliance of top predators is correlated with their trophic position. The results of the present study indicate that because the major benthic primary consumer is a large fish (Prochilodus lineatus), the size structure of individual food chains is an important factor determining FCL. Whereas in floodplain rivers large detritivorous fishes are targets of commercial fishing, overfishing in the Middle Parana River could be expected to increase FCL, the opposite effect to that seen in marine environments.

Climatic seasonality, hydrological variability, and geomorphology shape fish assemblage structure in a subtropical floodplain

We applied a multiscale approach to understand the patterns of longitudinal, lateral, and temporal variability in fish assemblage structure of a large floodplain river and explored their relationships with climatic seasonality, hydrological variability, and floodplain geomorphology. We studied the fish assemblage structure of 35 connected lakes along a 600-km section of the Paraná River over 5 y. Longitudinal changes in fish assemblage structure were associated with a latitudinal climatic gradient, whereas temporal changes were associated with climatic seasonality and hydrological variability. Longitudinal and seasonal changes in assemblage structure were probably based on different thermal preferences of species that migrate seasonally along the main river axis. Extraordinary floods in summer promoted downstream dispersal of several species, which increased similarity between downstream and upstream regions. Thermal seasonality of this subtropical system was a strong factor driving temporal changes in assemblage structure, and the effects of the hydrological regime largely depended on flood pulses coincident with periods of warmer temperatures. Fish assemblages varied secondarily along a lateral gradient represented by types of geomorphic units distributed in the floodplain. The geomorphic units generated patchy environmental heterogeneity at the landscape scale that resulted in discontinuous patterns of fish assemblage structure. Assemblage composition was more similar among lakes belonging to similar geomorphic units but situated at geographically distant regions than among lakes belonging to different geomorphic units situated in the same region. Therefore, patterns in fish assemblage variation at regional and landscape scales appear to be determined largely by the dispersal and selection of suitable habitats by fish across the hierarchy of spatial and temporal scales.

Phenotypic plasticity associated to environmental hypoxia in the neotropical serrasalmid Piaractus mesopotamicus (Holmberg, 1887) (Characiformes: Serrasalmidae)

Muchos peces caracidos sudamericanos desarrollan protuberancias dermicas reversibles en las mandibulas para optimizar la respiracion acuatica superficial (RAS) durante la hipoxia. Actualmente, aspectos basicos de esta adaptacion permanecen desconocidos, principalmente debido a la escasez de estudios experimentales. En experiencias de laboratorio, determinamos el tiempo necesario para el desarrollo y la reversion completos de estas estructuras en Piaractus mesopotamicus , y estudiamos comparativamente respuestas conductuales, morfologicas, y respiratorias en un gradiente de concentracion de oxigeno disuelto (OD). Los cambios morfologicos durante la hipoxia consistieron en protuberancias dermicas del labio inferior, el borde anterior del maxilar, y el borde distal de la valvula opercular, incrementando el numero conocido de estructuras modificadas. Estas se desarrollaron completamente en menos de 6 horas y se revirtieron en menos de 3 horas. La mayoria de los rasgos observados siguieron una curva de respuesta logistica, con valores criticos entre 0,90 y 2,70 mgL-1 de OD. La frecuencia respiratoria y el desarrollo de la valvula opercular presentaron valores criticos similares ubicados por encima del nivel de tolerancia al OD, mientras que la RAS y las protuberancias dermicas mandibulares presentaron criticos por debajo de dicho nivel. Estas observaciones apoyan la relacion funcional existente entre estos grupos de rasgos conductuales y morfologicos. Este estudio demuestra que esta especie puede modificar reversiblemente porciones del sistema respiratorio para optimizar las respuestas a la hipoxia.