Oxygen drives skeletal muscle remodeling in an amphibious fish out of water.

Skeletal muscle remodeling in response to terrestrial acclimation improves the locomotor performance of some amphibious fishes on land, but the cue for this remodeling is unknown. We tested the hypothesis that muscle remodeling in the amphibious Kryptolebias marmoratus on land is driven by higher O2 availability in atmospheric air, and the alternative hypothesis that remodeling is induced by a different environmental or physiological condition that fish experience on land. Fish were acclimated to 28 days of air, or to aquatic hyperoxia, hypercapnia, hypoxia, elevated temperature or fasting conditions. Air, fasting and hyperoxic conditions increased (>25%) the size of oxidative fibers in K. marmoratus while hypoxia had the reverse effect (23% decrease). Surprisingly, hyperoxia acclimation also resulted in a transformation of the musculature to include large bands of oxidative-like muscle. Our results show that K. marmoratus is highly responsive to environmental O2 levels and capitalizes on O2-rich opportunities to enhance O2 utilization by skeletal muscle.