Trait variation in extreme thermal environments under constant and fluctuating temperatures

Climate change is increasingly exposingpopulations to rare and novel environmental conditions. Theory suggests that extreme conditions will exposecryptic phenotypes, with a concomitant increase in trait variation. Although some empirical support for this exists, it is also well established that physiological mechanisms (e.g. heat shock protein expression) change when organisms are exposedto constant versus fluctuating temperatures. To determine the effect of common, rare and novel temperatures on the release of hidden variation, we exposedfathead minnows, Pimephales promelas, to five fluctuating and four constant temperature regimes (constant treatments: 23.5, 25, 28.5 and 31°C; all fluctuating treatments shared a minimum temperature of 22°C at 00.00 and a maximum of 25, 28, 31, 34 or 37°C at 12.00). We measured each individual's length weekly over 60 days, critical thermal maximum(CTmax), five morphometric traits (eye anterior–posterior distance, pelvic finlength, pectoral fin length, pelvic finra...