Physiological causes and biogeographic consequences of thermal optima in the hypoxia tolerance of marine ectotherms

Abstract

Recent measurements of critical O2 thresholds (‘Pcrit’) in aquatic animals have revealed thermal optima in their hypoxia tolerance. To discern the prevalence, physiological drivers, and biogeographic manifestations of such Pcrit curves, this research investigates experimental and occurrence data using a dynamic model of aquatic water breathers. The model simulates the transfer O2 from ambient water into animal tissues driven by temperature-dependent rates of metabolism, diffusion, and ventilatory and circulatory systems with O2-protein binding. Results show that thermal optima in Pcrit can arise even when all physiological rates increase steadily with temperature. This occurs when O2 supply at low temperatures is limited by a process that is more temperature sensitive than metabolism, and when O2 supply at warmer temperatures is limited by a less sensitive process. Analysis of species respiratory traits suggests this scenario is not uncommon in marine biota, with ventilation and circulation limiting supply under cold conditions and diffusion limiting supply under warm conditions. State-space habitats reveal that species with these physiological traits inhabit lowest O2 waters near the optimal temperature for hypoxia tolerance, and are restricted to higher O2 at temperatures above and below this optimum. These results imply that tolerance to low oxygen can decline under cold and warm conditions and may influence species range limits.