Density-mediated metabolic rate in <i>Daphnia magna</i> interacted with temperature

Zooplankton change metabolic function as a physiological response to crowding. Yet it is unclear whether zooplankton physiological responses also change according to species and measurement conditions, and whether the physiological responses to crowding are also changed by temperature. Further investigations are essential to unravel the intricate interplay between crowding-induced stress, energy allocation strategies, and behavioral adaptations. In this study, we determined the metabolic rates of cladoceran <i>Daphnia magna</i> in three different densities (1, 10, and 20 individuals per 50 ml) at two temperatures (10 and 20 °C) using a highly accurate optical oxygen meter. We aimed to clarify how crowding and temperature together affect metabolic rate. At both temperatures, metabolic rates of <i>D. magna</i> in the two crowded treatments were significantly lower than those in a single treatment, and as temperature decreased, so the rates of metabolic decrease slowed. <i>Q</i>₁₀ values varied from 1.63 for the 20 individuals per 50 ml treatment, to 2.29 for the 1 individual per 50 ml treatment. This suggests that the slowing down of metabolic rates due to crowding is slightly less pronounced at lower temperatures and may be linked to reduced food consumption.