Mechanisms of enhanced cardiorespiratory performance under hyperoxia differ with exposure duration in yellowtail kingfish

Hyperoxia has been shown to expand the aerobic capacity of some fishes,
although there have been very few studies examining the underlying
mechanisms and how they vary across different exposure durations. Here, we
investigated cardiorespiratory function of yellowtail kingfish (Seriola
lalandi) acutely (~20 hours) and chronically (3-5 weeks) acclimated to
hyperoxia (~200 % air saturation). Our results show that aerobic
performance of kingfish is limited in normoxia and increases with
environmental hyperoxia. Aerobic scope was elevated in both hyperoxia
treatments driven by a ~33% increase in maximum O2 uptake (MO2max),
although the mechanisms differed across treatments. Fish acutely
transferred to hyperoxia primarily elevated tissue O2 extraction, while
increased stroke volume-mediated maximum cardiac output was the main
driving factor in chronically acclimated fish. Still an improved O2
delivery to the heart in chronic hyperoxia was not the only explanatory
factor as such. Here, maximum cardiac output only increased in chronic
hyperoxia compared to normoxia when plastic ventricular growth occurred,
as increased stroke volume was partly enabled by an ~8-12% larger relative
ventricular mass. Our findings suggest that hyperoxia may be used
long-term to boost cardiorespiratory function potentially rendering fish
more resilient to metabolically challenging events and stages in their
life-cycle.