Oxygen, temperature and the body size of aquatic animals
Body size is intimately tied to oxygen budgets through size related changes in oxygen requirements and respiratory surface area. Respiratory constraints may be stronger for aquatic ectotherms that rely on underwater gas exchange than for those that breathe air: the diffusion of oxygen is orders of magnitude lower in water than in air, while the higher density and viscosity of water greatly elevate the cost of breathing. Hence, oxygen is usually deemed central to explain organisms of gigantic proportions inhabiting cold polar waters with large quantities of dissolved oxygen (polar gigantism). Likewise, (aquatic) giants inhabited the world at a time of a hyperoxic prehistoric atmosphere (Palaeozoic gigantism). Temperature also exerts strong influences on oxygen supply and demand. The increase in metabolism with increasing temperature can give rise to oxygen limitation and the associated heat tolerance in some aquatic ectotherms. Lethal temperatures found in the lab are often well above the temperatures that animals encounter in their natural environment. Therefore, knowledge on sub-lethal effects of warming is needed. Analyses of extensive field data show that oxygen limitation may also be important at realistic, sub-lethal temperatures for aquatic insects: Hypoxia and warming clearly amplified each other’s impact. The concordance between laboratory results and field data suggests that oxygen limitation also restricts species abundance in the field at temperatures well below upper lethal limits, possibly because oxygen becomes limiting for growth and reproduction in warmer water.
Temperature also influences body size and thermal effects could be mediated by oxygen. Temperature affects the availability of oxygen in water and the cost of breathing by changing the viscosity. The consequences of such changes are dependent on body size. This may help explain why size clines along temperature and latitudinal gradients are much more pronounced in aquatic ectotherms, why gigantism is especially prevalent in aquatic ectotherms, and why mass-scaling exponents often change with temperature. As body size is a major driver of how ecosystems function, understanding how body size is tied to energy budgets in aquatic ectotherms will greatly increase our ability to predict the consequences of global warming.
Host: Gregor Kalinkat