Climate vulnerability of South American freshwater fish: Thermal tolerance and acclimation.

Freshwater fish are restricted by their physiology to rivers and lakes, and are generally limited in their capacity to disperse across basins. As a result, there is often a close match between the evolutionary history of river basins and their natural history. Thus, the regional landscape and ecological features, such as temperature, have shaped the evolution and adaptation of local fish assemblages. Climate change is expected to affect fish diversity and increase extinction, especially in low latitudes, and it has been suggested that species that inhabit low latitude species are more susceptible since they live close to their maximum thermal limits and have low capacity for acclimation. To understand the mechanisms of variation in thermal tolerance across a broad-scale of South American fishes is fundamental to be able to assess the vulnerability of species and habitat to global warming. Herein, we present the first attempt to analyze the vulnerability of South American freshwater fish species, based on the review of upper thermal limits of 106 species from a broad range of latitudinal habitats. Our findings show that upper thermal limits decrease with latitude, while the thermal safety margin (TSM) increase. Furthermore, the latitude has little effects on the acclimation response ratio, and the TSM decreased with rising temperatures. These data suggest that thermal phenotypic acclimation has low potential for mitigating global warming. These results indicate that South American fish species living in tropical areas are more susceptible to global warming since they are already living close to their maximum habitat temperature.

High Temperature, pH, and Hypoxia Cause Oxidative Stress and Impair the Spermatic Performance of the Amazon Fish Colossoma macropomum.

The control of abiotic parameters is fundamental for fish survival, growth and reproduction. These factors have a direct effect on sperm quality. Thus, this study evaluated the effect of different temperatures (29, 31, 33, and 35°C), pHs (4 and 8), and hypoxia (1 mgO2 L-1) on sperm motility of Colossoma macropomum (tambaqui). The results indicated a longer duration of sperm motility at 29°C (50.1 ± 2.70 s) that progressively decreased when exposed to 35°C (31.2 ± 1.31 s) and hypoxia at pH 4 (27.4 ± 1.42 s) and pH 8 (30.44 ± 1.66 s; p < 0.05), respectively. Sperm oxygen consumption increased in hypoxia at both pH (pH 4 = 61.22; pH 8 = 54.74 pmol s-1). There was an increase in the activity of glutathione-S-transferase (GST) and superoxide dismutase (SOD), as well as in lipid peroxidation levels (LPO) and DNA damage in sperm exposed to higher temperatures and hypoxia. The pH 4 and pH 8 under normoxia did not affect the quality of C. macropomum sperm. These results suggest that water warming and acidification, consequences of climate changes, significantly affect the reproduction of C. macropomum, reducing the quality of spermatozoids during fertilization.

Cellular oxygen consumption, ROS production and ROS defense in two different size-classes of an Amazonian obligate air-breathing fish (Arapaima gigas).

In air-breathing fish a reduction of gill surface area reduces the danger of losing oxygen taken up in the air-breathing organ (ABO) to hypoxic water, but it also reduces the surface area available for ion exchange, so that ion regulation may at least in part be transferred to other organs, like the kidney or the gut. In the air-breathing Arapaima gigas, gill lamellae regress as development proceeds, and starting as a water-breathing embryo Arapaima turns into an obligate air-breathing fish with proceeding development, suggesting that ion regulation is shifted away from the gills as the fish grows. In Arapaima the kidney projects medially into the ABO and thus, probably a unique situation among fishes, is in close contact to the gas of the ABO. We therefore hypothesized that the kidney would be predestined to adopt an increased importance for ion homeostasis, because the elevated ATP turnover connected to ion transport can easily be met by aerobic metabolism based on the excellent oxygen supply directly from the ABO. We also hypothesized that in gill tissue the reduced ion regulatory activity should result in a reduced metabolic activity. High metabolic activity and exposure to high oxygen tensions are connected to the production of reactive oxygen species (ROS), therefore the tissues exposed to these conditions should have a high ROS defense capacity. Using in vitro studies, we assessed metabolic activity and ROS production of gill, kidney and ABO tissue, and determined the activity of ROS degrading enzymes in small (~ 5g, 2-3 weeks old) and larger (~ 670 g, 3-4 months old) A. gigas. Comparing the three tissues revealed that kidney tissue oxygen uptake by far exceeded the uptake measured in gill tissue or ABO. ROS production was particularly high in gill tissue, and all three tissues had a high capacity to degrade ROS. Gill tissue was characterized by high activities of enzymes involved in the glutathione pathway to degrade ROS. By contrast, the tissues of the ABO and in particular the kidney were characterized by high catalase activities, revealing different, tissue-specific strategies in ROS defense in this species. Overall the differences in the activity of cells taken from small and larger fish were not as pronounced as expected, while at the tissue level the metabolic activity of kidney cells by far exceeded the activity of ABO and gill cells.