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

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 (PwO(2)) on MRD are largely unexplored. We simultaneously measured the O-2 consumption rate (MO2) and metabolic heat of goldfish using calorespirometry to test the hypothesis that MRD is employed at hypoxic PwO(2) values and initiated just below P-crit, the PwO(2) below which MO2 is forced to progressively decline as the fish oxyconforms to decreasing PwO(2). Specifically, we used closed-chamber and flow-through calorespirometry together with terminal sampling experiments to examine the effects of PwO(2) and time on MO2, metabolic heat and anaerobic metabolism (lactate and ethanol production). The closedchamber and flow-through experiments yielded slightly different results. Under closed-chamber conditions with a continually decreasing PwO(2), goldfish showed a Pcrit of 3.0 +/- 0.3 kPa and metabolic heat production was only depressed at PwO(2) between 0 and 0.67 kPa. Under flow-through conditions with PwO(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 MO2 to 0.67 kPa, indicating that Pcrit is at or below this PwO(2). Anaerobic metabolism was strongly activated at PwO(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-Pcrit PwO(2) values with the help of anaerobic glycolysis in the closed-chamber experiments, and aerobically after an initial (<1 h) activation of anaerobic metabolism in the flow-through experiments, even at 0.67 kPa PwO(2).