Experiment 1: absolute change in critical temperature (T crit ) is significantly related to baseline T crit . Independent of group, the absolute change in mussels' T crit from CTP test 1 to test 2 was dependent on their baseline T crit from test 1 (r 2 =0.60, P<0.0001). Control (CON) in blue and heat-acclimation (HEAT) in red (n=37). Regression model: y is the absolute change in T crit (C) from CTP test 1 to test 2, while the x value is the baseline T crit value (C) from CTP test 1. The solid black line indicates the line of best fit, while the gray shading around the line indicates the 95% confidence intervals. The dotted black line at 0 denotes no change in T crit from CTP test 1 to test 2.

Experiment 1: absolute change in critical temperature (T crit ) is significantly related to baseline T crit . Independent of group, the absolute change in mussels' T crit from CTP test 1 to test 2 was dependent on their baseline T crit from test 1 (r 2 =0.60, P<0.0001). Control (CON) in blue and heat-acclimation (HEAT) in red (n=37). Regression model: y is the absolute change in T crit (C) from CTP test 1 to test 2, while the x value is the baseline T crit value (C) from CTP test 1. The solid black line indicates the line of best fit, while the gray shading around the line indicates the 95% confidence intervals. The dotted black line at 0 denotes no change in T crit from CTP test 1 to test 2.

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Thermal performance curves are commonly used to investigate effects of heat acclimation on thermal tolerance and physiological performance. However, recent work indicates that metrics of these curves heavily depend on experimental design and may be poor predictors of animals’ survival during heat events in the field. In intertidal mussels, cardiac...

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... Fig. 1B for schematic). In one of the trials, for unknown reasons, air temperature in the chamber reached 38.7C for ~2 min; when this slight excessive temperature was noticed, the lid was opened to decrease the temperature back to 38C. As a result of this ~8 min of air temperature >38C, there are two mussels with T crit >38C for this trial ( Fig. ...
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... the lack of significant differences in T crit between the heat-acclimated and control groups, the groups were combined for further analyses (n=37 and was similar for both heat-acclimation and control groups (Fig. 3). Mussels with a lower initial T crit had the largest absolute change in T crit (in ...
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... mussels (35% of the total group: heat-acclimated group = 5 mussels, control group = 8 mussels) exhibited changes in T crit <0.5C (and in some cases negative values); this minimal response occurred only in mussels with an initial T crit ≥35.0C (Fig. ...
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... that died reached their T crit (mean  s.d. = 36.65  1.07C), while none of the survivors reached their T crit during the heat-stress bout. None of the mussels from either group reached their T flat . Despite a plateau in air temperature at 38C for 1 h (after ramping was complete), there was a wide range (~3C) in T crit in the non-survivors (Fig. S3-A). Combining data from Experiments 1 and 2 shows that reaching T crit is not necessarily lethal, but staying at temperatures ≥T crit for more than 10 min is lethal (Fig. ...
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... group reached their T flat . Despite a plateau in air temperature at 38C for 1 h (after ramping was complete), there was a wide range (~3C) in T crit in the non-survivors (Fig. S3-A). Combining data from Experiments 1 and 2 shows that reaching T crit is not necessarily lethal, but staying at temperatures ≥T crit for more than 10 min is lethal (Fig. ...
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... T crit was a sufficient stimulus to induce changes in T crit , and these changes were maintained for at least three weeks even in the absence of additional heat stress. This effect was most evident in mussels with an initially low T crit (<35C), as they experienced the largest increase in T crit after test 1 (Figs. 2 & 3); some animals with a higher initial T crit even saw a slight decrease in T crit at CTP test 2 (Fig. 3). Although these findings are novel in mussels, larger T flat T crit difference). ...
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... changes were maintained for at least three weeks even in the absence of additional heat stress. This effect was most evident in mussels with an initially low T crit (<35C), as they experienced the largest increase in T crit after test 1 (Figs. 2 & 3); some animals with a higher initial T crit even saw a slight decrease in T crit at CTP test 2 (Fig. 3). Although these findings are novel in mussels, larger T flat T crit difference). Perhaps there is a 'ceiling effect' at play, i.e., a maximum T crit that can be induced by heat stress. Further research is required to determine whether maximum T crit is genetically predetermined or established through acclimatization events earlier in ...
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... the responders' and non-responders' T crit in the heat-acclimation group from Experiment 3. In fact, T crit is a good indicator of whether an animal has improved its thermal tolerance only if the animal has an initially low (<35C) T crit , in which case they will likely experience relatively large increases in T crit post-heat acclimation (Fig. ...
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... Fig. 4B). Moreover, studies incorporating continuous monitoring of oxygen consumption during these tests, along with tidal cycle simulation (vs. constant submersion), will also provide further insight into any metabolic changes that might occur with heat acclimation, and how any metabolic changes may be related to changes in T crit and T flat . Fig. 1. Schematics of experimental designs for laboratory experiments (1-3). A (Experiment 1): Mussels (n=58) initially underwent a baseline CTP (test 1), where they were heated until reaching their individual critical temperature (T crit ) and then removed from the heat chamber and placed back into seawater. Three weeks after test 1, surviving mussels (n=37) were split into two groups: control (n=18) or heat ...

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... In particular, we found that individuals reared at 30°C ('heat-selected') as larvae had significantly lower acute heat tolerance as adults (as evidenced by quicker knockdown times) than those reared at 22°C ('control'). This finding appears to contrast with prior empirical and theoretical work in thermal biology finding that exposure to high temperatures at early life-stages leads to acclimation and higher heat tolerance in adulthood for ectotherms (63)(64)(65)(66)(67)(68)(69)(70), including in related Aedes species (71). However, our result may be explained by well-supported mechanisms including variation in adult body size resulting from developmental temperature and/or an accumulation of thermal injury (discussed below). ...
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