Dietary lipid quality affects temperature-mediated reaction norms of a freshwater key herbivore

Temperature-mediated plasticity in life history traits strongly affects the capability of ectotherms to cope with changing environmental temperatures. We hypothesised that temperature-mediated reaction norms of ectotherms are constrained by the availability of essential dietary lipids, i.e. polyunsaturated fatty acids (PUFA) and sterols, as these lipids are involved in the homeoviscous adaptation of biological membranes to changing temperatures. A life history experiment was conducted in which the freshwater herbivore Daphnia magna was raised at four different temperatures (10, 15, 20, 25°C) with food sources differing in their PUFA and sterol composition. Somatic growth rates increased significantly with increasing temperature, but differences among food sources were obtained only at 10°C at which animals grew better on PUFA-rich diets than on PUFA-deficient diets. PUFA-rich food sources resulted in significantly higher population growth rates at 10°C than PUFA-deficient food, and the optimum temperature for offspring production was clearly shifted towards colder temperatures with an increased availability of dietary PUFA. Supplementation of PUFA-deficient food with single PUFA enabled the production of viable offspring and significantly increased population growth rates at 10°C, indicating that dietary PUFA are crucial for the acclimation to cold temperatures. In contrast, cumulative numbers of viable offspring increased significantly upon cholesterol supplementation at 25°C and the optimum temperature for offspring production was shifted towards warmer temperatures, implying that sterol requirements increase with temperature. In conclusion, essential dietary lipids significantly affect temperature-mediated reaction norms of ectotherms and thus temperature-mediated plasticity in life history traits is subject to strong food quality constraints.