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 pirarucu ( Arapaima gigas ) exposed to increasing levels of water temperature (normal environmental temperature of the period (T _env ), 31 ºC, 34 ºC and 37 ºC) for a period of 96 h, in triplicate (N = 3). Our main findings show an increase in hematological parameters with a temperature elevation up to 34 ºC, followed by suppression at 37 ºC. 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. ALT and AST enzymes increased at 34 and 37 ºC. Oxidative stress levels were elevated at T _env , and were higher at 37 ºC when compared to animals at 31 ºC. In the liver, the activities of SOD and GPx enzymes increased with the increase in temperature, while the CAT enzyme showed reduced activity at 37 ºC. 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 ºC 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 ºC.