Bidirectional thermal limitations on invertebrate respiration drive habitat compression in response to climate change

Two of the many consequences of climate change are the warming and deoxygenation of Earth’s oceans. This poses a major ecophysiological threat to aquatic ectotherms, as increasing temperature and lower oxygen tensions are traditionally considered to limit aerobic scope (ability to elevate respiration above a resting rate) unidirectionally by raising metabolic rates beyond the capacity of the animal to take up adequate oxygen from its environment. In response to this loss of metabolic habitat, range-shifts within affected species to cooler or more oxygenated waters (e.g. at higher latitudes or greater depth) is often considered a resultant ecophysiological strategy. However, the influence of colder temperatures on the aerobic capacity in aquatic animals remains largely unexplored experimentally. Here we integrate over 400 physiological measurements of hypoxia tolerance across multiple marine invertebrate groups to demonstrate that the temperature effect on aerobic scope of ectotherms can be strongly bidirectional around species-specific thermal optima. Using standard respirometry protocols, we find that absolute oxygen tolerance (measured as the onset of anaerobiosis, or critical oxygen tension) decreases at temperatures both above and below distinct species-specific thermal optima. These data can be explained by decreases to both ventilatory and oxygen diffusion capacity at colder temperatures, leading to a reduction in relative aerobic scope and the ectotherms’ ability to withstand environmental hypoxia, while at high temperatures metabolic rate overcomes relative oxygen supply. These findings have important ecophysiological implications for considering the metabolic habitat loss of marine fauna responding to global warming and deoxygenation as latitudinal and vertical ranges may be much more constrained than previously considered.