Climate change is not only causing steady increases in average global temperatures but also increasing the frequency with which extreme heating events occur. These extreme events may be pivotal in determining the ability of organisms to persist in their current habitats. Thus, it is important to understand how quickly an organism’s heat tolerance can be gained and lost relative to the frequency with which extreme heating events occur in the field. We show that the California mussel, Mytilus californianus—a sessile intertidal species that experiences extreme temperature fluctuations and cannot behaviorally thermoregulate—can quickly (in 24-48 h) acquire improved heat tolerance after exposure to a single sublethal heat-stress bout (2 h at 30 or 35°C) and can maintain this improved tolerance for up to 3 weeks without further exposure to elevated temperatures. This adaptive response improved survival rates by ~75% under extreme heat stress bouts (2 h at 40°C). To interpret these laboratory findings in an ecological context, we evaluated four years of mussel body temperatures recorded in the field. The majority (~64%) of consecutive heat stress bouts were separated by 24-48 h, but several consecutive heat bouts were separated by as much as 22 days. Thus, the ability of M. californianus to maintain improved heat tolerance for up to three weeks after a single sublethal heat-stress bout significantly improves their probability of survival, as ~33% of consecutive heat events are separated by 3-22 days. As a sessile animal, mussels likely evolved the capability to rapidly gain and slowly lose heat tolerance to survive the intermittent, and often unpredictable, heat events in the intertidal zone. This adaptive strategy will likely prove beneficial under the extreme heat events predicted with climate change.

See Methods section of paper for details on how it was analyzed and the overall experimental design.

The analysis code in R is for the survival curve and chi square analyses; you will need to import that dataset "HHStudy_Days1-28_New.csv". The Temp column is a factor for either 25, 30, 35C sublethal temperatures or the control group. "Days_PostHH" indicates how many recovery days were between the sublethal and extreme heat stress bouts. The body mass and shell length are recorded for each individual - however there are some missing data for this-- see Supplmental Info Table 1 for which data are missing. The "LethalHeat_Date" is the date when the extreme heat stress bout was given to the mussels. The "EndDate" is when the mussels died or finished out the 4 week monitoring period. The last column, "status" is whether the mussel survived or died after the Extreme heat stress bout: TRUE = died; FALSE = survived (at 4 week time point they were still alive).