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What we did: We used a precise technique called calorespirometry to investigate how a representative fish species (goldfish) combines the use of aerobic, anaerobic and depressed metabolism to maintain energetic balance in hypoxic environments varying in oxygen level and time.
What we found: Goldfish relied almost exclusively on aerobic metabolism in all hypoxic environments despite their exceptional capacities for anaerobic metabolism and metabolic depression. They only employed metabolic depression in anoxic (ie, no oxygen) environments.
What this means: Reserving a reliance on anaerobic metabolism and metabolic depression—both of which are effective, but also inefficient and ultimately harmful—for extreme hypoxia/anoxia is about as effective and sustainable a metabolic strategy of hypoxia tolerance as is possible, one that is well suited to goldfish’s natural environment typified by months-long descents into eventual anoxia. This highlights the importance of effective mechanisms of environmental oxygen extraction to long-term hypoxic survival.