Temperature drives physiological changes on three timescales: acute, acclimatory and evolutionary. Acutely, passive temperature effects often dictate an expected two-fold increase in metabolic processes for every 10°C increase (Q₁₀). Yet for acclimatory or evolutionary time scales, selection often mitigates these acute effects. This selection also should concomitantly reduce interindividual variation. However, the individual variation in physiological trait thermal sensitivity is not well characterized. Here we quantified physiological responses to a 16°C temperature difference across nine thermally distinct Fundulus heteroclitus populations. Traits included whole animal metabolism (WAM), critical thermal maximum (CTmax), and substrate-specific cardiac metabolism measured in approximately 350 individuals. These traits exhibit high variation among both individuals and populations that depends on acclimation temperature. Thermal sensitivity or Q₁₀ variation is unexpected and ranges from 0.6 to 5.4 for WAM. Thus, with a 16°C temperature increase, some individuals have the same or lower metabolic rates while others have metabolic rates almost seven-fold higher (Q₁₀ = 5.4). Furthermore, a significant portion of this variation is related to habitat temperature, such that warmer populations have a significantly lower Q₁₀ for WAM and CTmax than colder populations. These data support a novel hypothesis: individual variation in thermal sensitivity reflects different physiological strategies to respond to environmental temperature variation and provides the potential for many different adaptive responses to temperature.

For temperature data, Hobo Data loggers were deployed over August 2018 at the listed sites. For each location, loggers collected one measure every 5 minutes. Time data was matched with the high tide times per day and the mean minimum high tide temperatures were extracted using HOBOware software. 

For whole animal metabolism, data was collected using a PreSens oxygen meter, with PreSens output file that was processed as the lowest tenth percent value in a distribution of metabolic rates.

For CTmax, the temperature at which fish lost equilibrium was recorded.

For cardiac metabolism, data was collected using a PreSens oxygen meter with PreSens output file. These files were processed to extract the slope of the last 3 minutes of oxygen consumption.

Full methods can be found in the associated manuscript.

Abbreviations:

MMT = mean minimum temperature

Temp = acclimation temperature (12 ot 28)

aug2018_temp = habitat temperature

pop = population

mass = mass in grams (g)

length = length in mm

tenyr_mean_temp = mean temperature across 10 years extracted from closest NOAA site

pair = warm/cold populations close together with significant temperature difference

mt_haplotype = likely mitochondrial haplotype based on location and geographic break (not confirmed molecularly)

temp_var = variance in ten year NOAA temperature data

accl_order = order the fish were acclimated for measurement