Thermal acclimation is a key process enabling ectotherms to cope with temperature change. To undergo a successful acclimation response, ectotherms require energy and nutritional building blocks obtained from their diet. However, diet is often overlooked as a factor that can alter acclimation responses. Using a temperate omnivorous fish, opaleye (Girella nigricans), as a model system, we tested the hypotheses that 1) diet can impact the magnitude of thermal acclimation responses and 2) traits vary in their sensitivity to both temperature acclimation and diet. We fed opaleye a simple choice omnivorous diet (ad libitum Artemia sp. and Ulva sp.) or a fixed carnivorous diet (ad libitum Artemia sp.) at two ecologically relevant temperatures (12 and 20°C) and measured a suite of whole animal (growth, sprint speed, metabolism), organ (cardiac thermal tolerance), and cellular-level traits (oxidative stress, glycolytic capacity). When opaleye were offered two diet options compared to one, they had reduced cardiovascular thermal performance and higher standard metabolic rate under conditions representative of the maximal seasonal temperature the population experiences (20&[deg]C). Further, sprint speed and aerobic scope were insensitive to diet and temperature, while growth was highly sensitive to temperature but not diet, and standard metabolic rate and maximum heart rate were sensitive to both diet and temperature. Our results reveal that diet influences thermal performance in trait-specific ways, which could create diet trade-offs for generalist ectotherms living in thermally variable environments. Ectotherms that alter their diet may be able to regulate their performance at different environmental temperatures. --

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