High acute thermal tolerance does not prevent physiological impairment and spatial thermal vulnerability in the...

Climate change is intensifying heatwaves in tropical freshwaters, reducing thermal safety margins and threatening ectotherms such as the Amazonian prawn Macrobrachium amazonicum, an ecologically and socioeconomically important species. We tested whether acute warming reduces thermal safety margins and induces sublethal physiological impairment. CTMax was measured in a separate group of individuals, acute thermal safety margins were estimated, and prawns were exposed to environmental temperature, 31 °C, and 35 °C for 24 or 48 h to assess mitochondrial respiration, antioxidant defenses, and gill histopathology. M. amazonicum exhibited a high acute CTMax of 42.6 °C; however, thermal safety margins declined to 0.6-3.6 °C under heatwave scenarios. Warming stimulated respiratory complexes, especially Complex IV, suggesting compensatory activation of aerobic metabolism, but also increased proton leak and reduced respiratory control ratio, indicating impaired mitochondrial coupling. GPx activity declined after 48 h, while gill damage increased at 35 °C. Multivariate analysis showed that only 35 °C induced a coordinated physiological shift. Overall, M. amazonicum can enhance respiratory capacity under acute heat, but at the cost of mitochondrial efficiency and gill integrity. Future projections indicated contraction of thermally safe areas, raising concerns for fisheries and aquaculture.