Translation of environmental variation into physiological performance. (a) Example of a time series of body temperatures measured in a rocky intertidal mussel. When multiplied by a (b) thermal performance curve, (c) performance over time can be estimated. Almost all physiological processes are affected by temperature, and organismal impacts ultimately drive ecological and biogeographic responses. Most organisms are ectothermic (i.e., retain no appreciable metabolic heat) and many have body temperatures that fluctuate with environmental conditions; in some environments, daily ranges of 20°C or more are not uncommon. The relationship between body temperature and ecophysiological metrics or traits such as growth and ability to forage can be described by a thermal performance curve, which is often left-skewed. Such curves (b) can describe not only thermal optima and lethal limits (solid lines) but also sublethal limits (dashed lines) that can be exceeded for short periods but after chronic exposure can lead to mortality. Because the relationship between body temperature (which can differ from environmental temperature) and performance is nonlinear, average performance (blue line in c) cannot be calculated from average temperatures (red lines in a and c) in any eurythermal environment. Notably, by smoothing input data, not only are the potentially lethal effects of extreme temperature neglected but also incorrect estimates of growth and reproduction or the cumulative effects of stress on mortality can ensue.