Colder rotifers grow larger but only in oxygenated waters

Why do colder ectotherms grow more slowly but mature at a larger size? Some researchers have argued that oxygen supply and demand play a crucial role in these processes, but many studies conflated the effects of oxygen and temperature. We studied the body sizes of rotifers (Keratella cochlearis) at different depths in 20 European lakes, taking advantage of gradients in oxygen and temperature during summer, when dense, cool waters sink to low depths and become hypoxic. Rotifers were larger in colder waters, but only in the presence of abundant oxygen. In hypoxic waters, rotifers remained small regardless of temperature. We propose that oxygen supply generates a ceiling for maximal possible body size, especially in environments that elevate metabolic demands. Under this condition, any of several processes-developmental plasticity, genetic divergence, size-dependent mortality, or size-dependent selection of microhabitats-could cause a wider range of body sizes in more oxygenated waters, where the maximal possible size exceeds the adaptive size at any temperature.