Mussel acclimatization to high, variable temperatures is lost slowly upon transfer to benign conditions

Climate change is increasing the temperature variability animals face, and thermal acclimatization allows animals to adjust adaptively to this variability. Although the rate of heat acclimatization has received some study, little is known about how long these adaptive changes remain without continuing exposure to heat stress. This study explored the rate at which field acclimatization states are lost when temperature variability is minimized during constant submersion. California mussels (Mytilus californianus) with different acclimatization states were collected from high- and low-zone sites (∼12 versus ∼5°C daily temperature ranges, respectively) and then kept submerged at 15°C for 8 weeks. Each week, the cardiac thermal performance of mussels was measured as a metric of acclimatization state: critical (T_crit) and flatline (T_flat) temperatures were recorded. Over 8 weeks of constant submersion, the mean T_crit of high-zone mussels decreased by 1.07°C from baseline, but low-zone mussels' mean T_crit was unchanged. High- and low-zone mussels' mean maximum heart rate (HR) and resting HR decreased ∼12 and 35%, respectively. T_flat was unchanged in both groups. These data suggest that T_crit and HR are more physiologically plastic in response to the narrowing of an animal's daily temperature range than T_flat is, and that an animal's prior acclimatization state (high versus low) influences the acclimatory capacity of T_crit Approximately 2 months were required for the cardiac thermal performance of the high-zone mussels to reach that of the low-zone mussels, suggesting that acclimatization to high and variable temperatures may persist long enough to enable these animals to cope with intermittent bouts of heat stress.