Effects of heat acclimation on cardiac function in the intertidal mussel Mytilus californianus: can laboratory-based indices predict survival in the field?

Thermal performance curves are commonly used to investigate the effects of heat acclimation on thermal tolerance and physiological performance. However, recent work indicates that the metrics of these curves heavily depend on experimental design and may be poor predictors of animal survival during heat events in the field. In intertidal mussels, cardiac thermal performance (CTP) tests have been widely used as indicators of animals' acclimation or acclimatization state, providing two indices of thermal responses: critical temperature (T(crit); the temperature above which heart rate abruptly declines) and flatline temperature (T(flat); the temperature where heart rate ceases). Despite the wide use of CTP tests, it remains largely unknown how T(crit) and T(flat) change within a single individual after heat acclimation, and whether changes in these indices can predict altered survival in the field. Here, we addressed these issues by evaluating changes in CTP indices in the same individuals before and after heat acclimation. For control mussels, merely reaching T(crit) was not lethal, whereas remaining at T(crit) for ≥10 min was lethal. Heat acclimation significantly increased T(crit) only in mussels with an initially low T(crit) (<35°C), but improved their survival time above T(crit) by 20 min on average. T(flat) increased by ∼1.6°C with heat acclimation, but it is unlikely that increased T(flat) improves survival in the field. In summary, T(crit) and T(flat) per se may fall short of providing quantitative indices of thermal tolerance in mussels; instead, a combination of T(crit) and tolerance time at temperatures ≥T(crit) better defines changes in thermal tolerance with heat acclimation.