Thermal plasticity on different timescales, including acclimation/acclimatization and heat hardening response – a rapid adjustment for thermal tolerance after nonlethal thermal stress, can interact to improve the resilience of organisms to thermal stress. However, little is known about physiological mechanisms mediating this interaction. To investigate underpinnings of heat hardening responses after acclimatization in warm seasons, we measured thermal tolerance plasticity, compared transcriptomic and metabolomic changes after heat hardening at 33 or 37oC followed by recovery of 3 h or 24 h in an intertidal bivalve Sinonovacula constricta. Clams showed explicit heat hardening responses after acclimatization in a warm season. The higher inducing temperature (37oC) caused less effective heat hardening effects than the inducing temperature that was closer to the seasonal maximum temperature (33oC). Metabolomic analysis highlighted the elevated contents of glyceropholipids in all heat-hardened clams, which may help to maintain the structure and function of the membrane. Heat shock proteins (HSPs) tended to be up-regulated after heat hardening at 37oC but not at 33oC, indicating that there was no complete dependency of heat hardening effects on up-regulated HSPs. Enhanced energy metabolism and decreased energy reserves were observed after heat hardening at 37oC, suggesting more energy costs during exposure to a higher inducing temperature which may restrict heat hardening effects. These results highlighted the mediating role of membrane lipid metabolism, heat shock responses, and energy costs in the interaction between heat hardening response and seasonal acclimatization, and benefit the mechanistic understanding of evolutionary change and thermal plasticity during global climate change.

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