Experiencing extremes: how Arapaima gigas (Schinz, 1822) deals with increasing water temperatures

Temperature regulates the life cycle of aquatic organisms. However, in the current climate change scenario, the warming of the planet is putting the resilience of many species at risk, resulting in organic disturbances. In the Amazon, the effects of warming demonstrate the low thermal tolerance of species that are already living near their upper temperature limits. In this study, we evaluated the physiological and biochemical responses of specimens of forty-eight (N = 48) pirarucu juveniles (Arapaima gigas) exposed to increasing levels of water temperature (normal environmental temperature of the period — T_env), 31 °C, 34 °C, and 37 °C) for a period of 96 h, in triplicates (n = 12 fish per temperature). Our main findings show an increase in hematological parameters with a temperature elevation up to 34 °C, followed by suppression at 37 °C. Increases in glucose and lactate levels and reduced triglyceride levels were observed with the elevation in water temperature. Cholesterol levels remained unchanged throughout the experimental period. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) enzymes increased at 34 and 37 °C. Oxidative stress levels were elevated at T_env and were higher at 37 °C when compared to animals at 31 °C. In the liver, the activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx) enzymes increased with the increase in temperature, while the catalase (CAT) enzyme showed reduced activity at 37 °C. Thus, the data from the present study show that the pirarucu is a species that responds to increasing levels of water temperature. However, a temperature of 37 °C seems to offer a greater risk to the species’ ability to cope with the effects of higher temperatures, as it requires greater energy expenditure and induces oxidative stress. Thus, the pirarucu seems to cope better with temperatures that do not exceed 34 °C.