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Metabolic rate depression (MRD) has long been proposed as the key metabolic strategy of hypoxic survival, but surprisingly, the effects of changes in hypoxic O 2 tensions ( P w O 2  ) on MRD are largely unexplored. We simultaneously measured the O 2 consumption rate ( Ṁ  O 2  ) and metabolic heat of goldfish using calorespirometry to test the hypothesis that MRD is employed at hypoxic P w O 2   values and initiated just below P  crit , the P w O 2   below which Ṁ  O 2   is forced to progressively decline as the fish oxyconforms to decreasing P w O 2  Specifically, we used closed-chamber and flow-through calorespirometry together with terminal sampling experiments to examine the effects of P w O 2   and time on Ṁ  O 2  , metabolic heat and anaerobic metabolism (lactate and ethanol production). The closed-chamber and flow-through experiments yielded slightly different results. Under closed-chamber conditions with a continually decreasing P w O 2  , goldfish showed a P  crit of 3.0±0.3 kPa and metabolic heat production was only depressed at P w O 2   between 0 and 0.67 kPa. Under flow-through conditions with P w O 2   held at a variety of oxygen tensions for 1 and 4 h, goldfish also initiated MRD between 0 and 0.67 kPa but maintained Ṁ  O 2   to 0.67 kPa, indicating that P  crit is at or below this P w O 2  Anaerobic metabolism was strongly activated at P w O 2   ≤1.3 kPa, but only used within the first hour at 1.3 and 0.67 kPa, as anaerobic end-products did not accumulate between 1 and 4 h exposure. Taken together, it appears that goldfish reserve MRD for near-anoxia, supporting routine metabolic rate at sub- P  crit  P w O 2   values with the help of anaerobic glycolysis in the closed-chamber experiments, and aerobically after an initial (&lt;1 h) activation of anaerobic metabolism in the flow-through experiments, even at 0.67 kPa P w O 2  .

Calorespirometry reveals that goldfish prioritize aerobic metabolism over metabolic rate depression in all but near-anoxic environments