Meeting Abstract
Temperature-induced limitations on the capacity of the cardiorespiratory system to transport oxygen from the environment to the tissues have long been regarded as the principal determinant of the upper thermal limits of water-breathing ectotherms. Consequently, the upper thermal niche boundaries of these species are considered highly sensitive to aquatic hypoxia and other environmental stressors that constrain cardiorespiratory performance. However, the generality of this dogma has been questioned as some species have been found to maintain their cardiorespiratory performance at high temperatures. We recently proposed a novel metric for characterizing the oxygen-dependence of the upper thermal limits, and assessing the synergistic effects of environmental stressors on the thermal tolerance of cardiorespiratory performance; the oxygen limit for thermal tolerance (PCTmax), which is the water oxygen tension (PwO2) where an organism’s upper thermal limit (CTmax) starts to decline. Determining PCTmax in a number of tropical, temperate and polar aquatic ectotherms; including black-axil chromis, red drum, lumpfish and antarctic krill, we found that only in the antarctic krill were the upper thermal limits determined by insufficient tissue oxygen supply. In black-axil chromis, red drum and lump fish, the upper thermal limits were maintained over a wide range of water oxygen levels. Aquatic hypoxia should therefore have little impact on the upper thermal niche boundaries of these tropical and temperate species. In light of these findings we propose a framework for characterising the relationship between temperature, oxygen, metabolism and upper thermal niche boundaries of water-breathing ectotherms.
