Data from: 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 O2 tensions (PwO2) on MRD are largely unexplored. We
simultaneously measured the O2 consumption rate (ṀO2) and metabolic heat
of goldfish using calorespirometry to test the hypothesis that MRD is
employed at hypoxic PwO2s and initiated just below Pcrit, the PwO2 below
which ṀO2 is forced to progressively decline as the fish oxyconforms to
decreasing PwO2. Specifically, we used closed-chamber and flow-through
calorespirometry together with terminal sampling experiments to examine
the effects of PwO2 and time on ṀO2, 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 PwO2, goldfish
showed a Pcrit of 3.0±0.3 kPa and metabolic heat production was only
depressed at PwO2 between 0 and 0.67 kPa. Under flow-through conditions
with PwO2 held at a variety of oxygen tensions for 1 and 4 h, goldfish
also initiated MRD between 0 and 0.67 kPa but maintained ṀO2 to 0.67 kPa,
indicating that Pcrit is at or below this PwO2. Anaerobic metabolism was
strongly activated at PwO2 ≤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 PwO2s 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 PwO2.