Ecologically Relevant Temperature Ramping Rates Enhance the Protective Heat Shock Response in an Intertidal Ectotherm

Thermal stress experiments are essential for understanding organisms' thermal limits and the physiological processes that contribute to establishing those limits. Experiments typically employ either an abrupt transfer to near-lethal temperatures or a gradually increasing thermal exposure. In the current study, we used three populations of the intertidal copepod Tigriopus californicus that are known to differ in upper thermal tolerance to investigate the effects of gradual versus abrupt thermal exposures on survivorship, developmental time, and heat shock protein gene expression. The developmental rate of nauplii was unaffected following the gradual exposure, whereas developmental time slowed by ∼2 d (∼20%) following an abrupt exposure. The gradual exposure also improved survivorship in comparison to the abrupt exposure. Furthermore, the heat shock protein genes hsp70 and hspb1 showed greater upregulation during the gradual thermal exposure compared to the abrupt exposure. Though the differences in response to each thermal regime varied in magnitude among the different populations, the types of responses were very similar (i.e., following the gradual exposure survivorship increased, developmental time showed no effect, and heat shock protein gene upregulation during the exposure increased). Therefore, the enhanced protective effect of the heat shock response during gradual exposures appears to be conserved within the species despite population-level differences in thermal tolerance. Thus, an ecologically relevant thermal exposure likely enables improved cellular protective mechanisms by allowing for an effective and timely heat shock response, which plays a role in mitigating the effects of thermal stress and thereby enhances tolerance to elevated temperatures.