Hypoxia-seeking behavior, metabolic depression and skeletal muscle function in an amphibious fish out of water

Several animals enter a state of dormancy to survive harsh environmental conditions. During dormancy, metabolic depression can be critical for economizing on limited endogenous energy reserves. We used two isogenic strains (strain 1 and strain 2) of a self-fertilizing amphibious fish (Kryptolebias marmoratus) to test the hypothesis that animals seek hypoxic microhabitats that accentuate metabolic depression during dormancy. Fish were placed in custom experimental choice chambers that maintained an O2 gradient (normoxic to hypoxic) to determine the preferred O2 level of fish out of water. We then measured the O2 consumption rate of water- (control) and air-acclimated (21 days) fish in aerial normoxia, as well as the O2 consumption rate of air-acclimated fish acutely exposed to aerial hypoxia. We then tested the hypothesis that chronic hypoxia acclimation in air would protect endogenous energy reserves and skeletal muscle integrity, thereby maintaining locomotor performance. The hypothesis predicts that K. marmoratus acclimated to aerial hypoxia will deplete energy stores more slowly, demonstrate less skeletal muscle atrophy and have better locomotor performance than fish acclimated to aerial normoxia, presumably owing to hypoxic hypometabolism. We measured the whole-body [glycogen] and lipid content in fish acclimated to water (control), air and aerial hypoxia for 21 days. The cross-sectional area of red and white muscle fibers was also measured, as well as terrestrial locomotor performance (tail-flip jumping).