Is the temperature-size rule mediated by oxygen in aquatic ectotherms?

Temperature is an important environmental factor that influences key traits like body size, growth rate and maturity. Ectotherms reared under high temperatures usually show faster growth, but reach a smaller final size, a phenomenon known as the temperature-size rule (TSR). Oxygen may become a limiting resource at high temperatures, when demand for oxygen is high, especially in water as oxygen uptake is far more challenging under water than in air. Therefore, in aquatic ectotherms, the TSR might very well be mediated by temperature effects on oxygen availability and oxygen demand. To distinguish between the direct effects of temperature and oxygen mediated effects, growth rate and final size were measured in the aquatic ectotherm Asellus aquaticus (Linnaeus, 1758) reared under different temperature and oxygen conditions in a factorial design. Growth could be best described by a modified Von Bertalanffy growth function. Both temperature and oxygen affected age at maturity and growth. Growth responses to temperature were dependent on oxygen conditions (interactive effect of temperature and oxygen). Only under hypoxic conditions, when oxygen was most limiting, did we find a classic TSR. Moreover, when comparing treatments differing in temperature, but where the balance between oxygen demand and supply was similar, high temperature increased both growth rate and final size. Thus effects of oxygen may resolve the life-history puzzle of the TSR in aquatic ectotherms.