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The Amazonian oscar is extremely resistant to hypoxia, and tolerance scales with size. Overall, ionoregulatory responses of small (~15 g) and large oscars (~200 g) to hypoxia were qualitatively similar, but the latter were more effective. Large oscars exhibited a rapid reduction in unidirectional Na+ uptake rate at the gills during acute hypoxia (Po2 ~10 mmHg), which intensified with time (7 or 8 h); Na+ efflux rates were also reduced, so net balance was little affected. The inhibitions were virtually immediate (1st h) and preceded a later 60% reduction (at 3 h) in gill Na+-K+-ATPase activity, reflected in a 60% reduction in maximum Na+ uptake capacity without change in affinity (Km) for Nat Upon acute restoration of normoxia, recovery of Na+ uptake was delayed for 1 h. These data suggest that dual mechanisms may be involved (e.g., immediate effects of O2 availability on transporters, channels, or permeability, slower effects of Na+-K+-ATPase regulation). Ammonia excretion appeared to be linked indirectly to Na+ uptake, exhibiting a Michaelis-Menten relationship with external [Na+], but the Km was less than for Na+ uptake. During hypoxia, ammonia excretion fell in a similar manner to Na+ fluxes, with a delayed recovery upon normoxia restoration, but the relationship with [Na+] was blocked. Reductions in ammonia excretion were greater than in urea excretion. Plasma ammonia rose moderately over 3 h hypoxia, suggesting that inhibition of excretion was greater than inhibition of ammonia production. Overall, the oscar maintains excellent homeostasis of ionoregulation and N-balance during severe hypoxia. [ABSTRACT FROM AUTHOR]

The physiological and behavioural responses of two size groups of oscar (Astronotus ocellatus) to hypoxia were studied. The physiological responses were tested by measuring ṀO2 during decreasing environmental oxygen tensions. Larger oscars were better able to maintain oxygen consumption during a decrease in PO2, regulating routine ṀO2 to a significantly lower PO2 threshold (50 mmHg) than smaller oscars (70 mmHg). Previous studies have also demonstrated a longer survival time of large oscars exposed to extreme hypoxia, coupled with a greater anaerobic enzymatic capability. Large oscars began aquatic surface respiration (ASR) at the oxygen tension at which the first significant decrease in ṀO2 was seen (50 mmHg). Interestingly, smaller oscars postponed ASR to around 22 mmHg, well beyond the PO2 at which they switched from oxyregulation to oxyconformation. Additionally, when given the choice between an hypoxic environment containing aquatic macrophyte shelter and an open normoxic environment, small fish showed a greater preference for the hypoxic environment. Thus shelter from predators appears particularly important for juveniles, who may accept a greater physiological compromise in exchange for safety. In response to hypoxia without available shelter, larger fish reduced their level of activity (with the exception of aggressive encounters) to aid metabolic suppression whereas smaller oscars increased their activity, with the potential benefit of finding oxygen-rich areas. [ABSTRACT FROM AUTHOR]

The Rio Negro basin of Amazonia (Brazil) is a hotspot of fish biodiversity that is under threat from copper (Cu) pollution. The very ion-poor blackwaters have a high dissolved organic carbon (DOC) concentration. We investigated the Cu sensitivity of nine Amazonian fish species in their natural blackwaters (Rio Negro). The acute lethal concentration of Cu (96 h LC 50 ) was determined at different dilutions of Rio Negro water (RNW) in ion-poor well water (IPW), ranging from 0 to 100%. The IPW was similar to RNW in pH and ionic composition but deficient in DOC, allowing this parameter to vary 20-fold from 0.4 to 8.3 mg/L in tests. The Biotic Ligand Model (BLM; Windward version 3.41.2.45) was used to model Cu speciation and toxicity over the range of tested water compositions, and to estimate lethal Cu accumulations on the gills (LA 50 ). The modeling predicted a high relative abundance of Cu complexes with DOC in test waters. As these complexes became more abundant with increasing RNW content, a concomitant decrease in free Cu 2+ was observed. In agreement with this modeling, acute Cu toxicity decreased (i.e. 96 h LC 50 values increase) with increasing RNW content. The three most sensitive species (Hemigrammus rhodostomus, Carnegiella strigatta and Hyphessobrycon socolofi) were Characiformes, whereas Corydoras schwartzi (Siluriformes) and Apistogramma agassizii (Cichliformes) were the most tolerant. These sensitivity differences were reflected in the BLM-predicted lethal gill copper accumulation (LA 50 ), which were generally lower in Characiformes than in Cichliformes. Using these newly estimated LA 50 values in the BLM allowed for accurate prediction of acute Cu toxicity in the nine Amazonian fish. Our data emphasize that the BLM approach is a promising tool for assessing Cu risk to Amazonian fish species in blackwater conditions characterized by very low concentrations of major ions but high concentrations of DOC., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Adalberto Luis Val reports financial support was provided by Foundation for Research Support of Amazonas State. Christopher Michael Wood reports financial support was provided by Canada Research Chairs program, and a visiting fellowship from the Science Without Borders Program (CNPq-Brazil). Work in Canada was supported by NSERC (Canada) Discovery grants (RGPIN-2017-03843 to CMW and RGPIN-2019-04400 to AC). Adalberto Luis Val reports financial support was provided by Brazilian National Council for the State Funding Agencies. Adalberto Luis Val reports financial support was provided by Coordination of Higher Education Personnel Improvement., (Copyright © 2023. Published by Elsevier Ltd.)

The tambaqui (Colossoma macropomum), migrates annually between whitewater and blackwater rivers of the Amazon. Unlike the whitewater, blackwater is characterized by higher levels of dissolved organic carbon (DOC), including humic acids (HA). Because humic substances impair sensory processes, the current study tested the hypothesis that O 2 and/or CO 2 chemoreception is impeded in blackwater owing to the presence of HA. Thus, the ventilatory responses of tambaqui to hypoxia or hypercapnia were assessed in well water transported from Manaus, local blackwater, and in well water containing HA either extracted from Rio Negro water or obtained commercially (Sigma Aldrich; SA). In well water, tambaqui exhibited typical hyperventilatory responses to hypoxia or hypercapnia. These responses were prevented by simultaneously exposing fish to SA HA (20 mg l -1 ). The negative effects of SA HA on ventilation were prevented when natural DOC (30 mg l -1 ; extracted from Rio Negro water after first removing the endogenous HA fraction) was added concurrently, indicating a protective effect of this non-humic acid DOC fraction. The hyperventilatory responses were unaffected during acute exposure or after acclimation of fish to Rio Negro water. HA extracted from Rio Negro water did not impair the hyperventilatory responses to hypoxia or hypercapnia. This study, while demonstrating a negative effect of SA HA derived from peat (coal) on the control of breathing in tambaqui, failed to reveal any detrimental consequences of HA (derived from the decomposition of a variety of lignin-rich plants) naturally occurring in the blackwaters of the Rio Negro., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Copper oxide nanoparticles (nCuO) are widely used in boat antifouling paints and are released into the environment, potentially inducing toxicity to aquatic organisms. The present study aimed to understand the effects of nCuO and dissolved copper (Cu) on two ornamental Amazon fish species: dwarf cichlid (Apistogramma agassizii) and cardinal tetra (Paracheirodon axelrodi). Fish were exposed to 50% of the LC 50 for nCuO (dwarf cichlid 58.31μgL -1 and cardinal tetra 69.6μgL -1 ) and Cu (dwarf cichlid 20μgL -1 and cardinal tetra 22.9μgL -1 ) for 24, 48, 72 and 96h. Following exposure, aerobic metabolic rate (ṀO 2 ), gill osmoregulatory physiology and mitochondrial function, oxidative stress markers, and morphological damage were evaluated. Our results revealed species specificity in metabolic stress responses. An increase of ṀO 2 was noted in cardinal tetra exposed to Cu, but not nCuO, whereas ṀO 2 in dwarf cichlid showed little change with either treatment. In contrast, mitochondria from dwarf cichlid exhibited increased proton leak and a resulting decrease in respiratory control ratios in response to nCuO and Cu exposure. This uncoupling was directly related to an increase in reactive oxygen species (ROS) levels. Our findings reveal different metabolic responses between these two species in response to nCuO and Cu, which are probably caused by the differences between species natural histories, indicating that different mechanisms of toxic action of the contaminants are associated to differential osmoregulatory strategies among species., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Future climate change scenarios predict threatening outcomes to biodiversity. Available empirical data concerning biological response of freshwater fish to climate change remains scarce. In this study, we investigated the physiological and biochemical responses of two Iberian freshwater fish species (Squalius carolitertii and the endangered S. torgalensis), inhabiting different climatic conditions, to projected future scenarios of warming (+3°C) and acidification (ΔpH=-0.4). Herein, metabolic enzyme activities of glycolytic (citrate synthase - CS, lactate dehydrogenase - LDH) and antioxidant (glutathione S-transferase, catalase and superoxide dismutase) pathways, as well as the heat shock response (HSR) and lipid peroxidation were determined. Our results show that, under current water pH, warming causes differential interspecific changes on LDH activity, increasing and decreasing its activity in S. carolitertii and in S. torgalensis, respectively. Furthermore, the synergistic effect of warming and acidification caused an increase in LDH activity of S. torgalensis, comparing with the warming condition. As for CS activity, acidification significantly decreased its activity in S. carolitertii whereas in S. torgalensis no significant effect was observed. These results suggest that S. carolitertii is more vulnerable to climate change, possibly as the result of its evolutionary acclimatization to milder climatic condition, while S. torgalensis evolved in the warmer Mediterranean climate. However, significant changes in HSR were observed under the combined warming and acidification (S. carolitertii) or under acidification (S. torgalensis). Our results underlie the importance of conducting experimental studies and address species endpoint responses under projected climate change scenarios to improve conservation strategies, and to safeguard endangered freshwater fish., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Roundup Original® (RD) is a glyphosate-based herbicide used to control weeds in agriculture. Contamination of Amazon waters has increased as a consequence of anthropogenic pressure, including the use of herbicides as RD. The central goal of this study was to evaluate the toxic effects of RD on juveniles of tambaqui (Colossoma macropomum). Our findings show that biomarkers in tambaqui are organ specific and dependent on RD concentration. Alterations in gills structural and respiratory epithelium were followed by changes in hematological parameters such as concentration of hemoglobin, particularly in fish exposed to the higher concentration tested (75% of RD LC50 96 h). In addition, both RD concentrations affected the biotransformation process in gills of tambaqui negatively. Instead, liver responses suggest that a production of reactive oxygen species (ROS) occurred in fish exposed to RD, particularly in the animals exposed to 75% RD, as seen by imbalances in biotransformation and antioxidant systems. The increased DNA damage observed in red blood cells of tambaqui exposed to RD is in agreement with this hypothesis. Finally, both tested sub-lethal concentrations of RD markedly inhibited the cholinesterase activity in fish brain. Thus, we can suggest that RD is potentially toxic to tambaqui and possibly to other tropical fish species., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

This study determined the respiratory responses to progressive hypoxia in oscar, an extremely hypoxia-tolerant Amazonian cichlid. Oscar depressed oxygen consumption rates (MO2), beginning at a critical O2 tension (Pcrit) of 46Torr, to only 14% of normoxic rates at 10Torr. Total ventilation (Vw) increased up to 4-fold, entirely due to a rise in ventilatory stroke volume (no change in ventilatory frequency), and water convection requirement (Vw/MO2) increased substantially (up to 15-fold). Gill O2 extraction fell steadily, from 60% down to 40%. Although O2 transfer factor (an index of gill O2 diffusion capacity) increased transiently in moderate hypoxia, it decreased at 10Torr, which may have caused the increased expired-arterial PO2 difference. Venous PO2 was always very low (< or =7Torr). Anaerobic metabolism made a significant contribution to ATP supply, indicated by a 3-fold increase in plasma lactate that resulted in an uncompensated metabolic acidosis. Respiration of isolated gill cells was not inhibited until below 5Torr; because gill water PO2 always exceeded this value, hypoxic ion flux arrest in oscars [Wood et al., Am. J. Physiol. Reg. Integr. Comp. Physiol. 292, R2048-R2058, 2007] is probably not caused by O2 limitation in ionocytes. We conclude that metabolic depression and tolerance of anaerobic bi-products, rather than a superior capacity for O2 supply, allow oscar to thrive in extreme hypoxia in the Amazon.

The Amazonian oscar is extremely resistant to hypoxia, and tolerance scales with size. Overall, ionoregulatory responses of small ( approximately 15 g) and large oscars ( approximately 200 g) to hypoxia were qualitatively similar, but the latter were more effective. Large oscars exhibited a rapid reduction in unidirectional Na(+) uptake rate at the gills during acute hypoxia (Po(2) approximately 10 mmHg), which intensified with time (7 or 8 h); Na(+) efflux rates were also reduced, so net balance was little affected. The inhibitions were virtually immediate (1st h) and preceded a later 60% reduction (at 3 h) in gill Na(+)-K(+)-ATPase activity, reflected in a 60% reduction in maximum Na(+) uptake capacity without change in affinity (Km) for Na(+). Upon acute restoration of normoxia, recovery of Na(+) uptake was delayed for 1 h. These data suggest that dual mechanisms may be involved (e.g., immediate effects of O(2) availability on transporters, channels, or permeability, slower effects of Na(+)-K(+)-ATPase regulation). Ammonia excretion appeared to be linked indirectly to Na(+) uptake, exhibiting a Michaelis-Menten relationship with external [Na(+)], but the Km was less than for Na(+) uptake. During hypoxia, ammonia excretion fell in a similar manner to Na(+) fluxes, with a delayed recovery upon normoxia restoration, but the relationship with [Na(+)] was blocked. Reductions in ammonia excretion were greater than in urea excretion. Plasma ammonia rose moderately over 3 h hypoxia, suggesting that inhibition of excretion was greater than inhibition of ammonia production. Overall, the oscar maintains excellent homeostasis of ionoregulation and N-balance during severe hypoxia.

The contribution of alterations in mitochondrial K(ATP) channel activity and the sarcoplasmic reticulum (SR) to anaerobic cardiac function in the anoxia tolerant armored catfish Liposarcus pardalis were assessed. K(ATP) channels contribute to hypoxic cardioprotection in mammals, but little is known of their action in more hypoxia tolerant animals. Anoxia resulted in a decrease in force in isometrically contracting ventricle strips to approximately 40% of the pre-anoxic level. This was maintained for at least 2 h. Upon reoxygenation, hearts recovered to the same level as control preparations. Treatment with 5-hydroxydecanoic acid (5HD), a specific mitochondrial K(ATP) blocker significantly increased force in preparations during anoxia and caused hypercontracture at reoxygenation. Ryanodine, a specific inhibitor of SR function, significantly increased force loss in ventricle preparations under anoxia. Results show that mitochondrial K(ATP) channel activity and SR function are important in anaerobic and post-anaerobic contractility in armored catfish heart.