Oxygen availability and oxygen delivery, but not oxidative stress shape heat tolerance in diploid and triploid zebrafish larvae

The oxygen- and capacity-limited thermal tolerance (OCLTT) hypothesis predicts that limits to oxygen availability or delivery set upper thermal boundaries. We tested this by manipulating oxygen concentration and delivery capacity (comparing diploid and triploid zebrafish larvae, which differ in cell size and surface:volume ratio) and measuring survival under different intensities of heat stress (i.e. different temperatures) and under different exposure durations. We predicted that hypoxia would reduce, and hyperoxia improve, heat tolerance, and that triploids would show reduced tolerance in all oxygen tensions due to lower oxygen delivery capacity, especially under hypoxia. Our experiments broadly supported these predictions. Oxygen modulated heat tolerance in both diploids and triploids, more so in triploids. Thus, both oxygen availability and delivery capacity appear to set high temperature limits, at least during longer exposures. Finally, we tested whether reactive oxygen species (ROS) and oxidative damage drive heat injury. Hypoxia doubled ROS production regardless of temperature or ploidy, but neither antioxidant capacity nor protein damage differed across temperature, ploidy or oxygen treatments, suggesting oxidative stress does not underlie the oxygen effects on heat tolerance. In conclusion, oxygen-based mechanisms differentially shape heat tolerance depending on stress duration and intensity. Low oxygen availability generally constrains tolerance, oxygen delivery capacity becomes critical under high demand for oxygen and low oxygen availability, and oxidative stress does not appear to play a primary role.