Thermal constraints on exercise and metabolic performance do not explain the use of dormancy as an overwintering strategy in the cunner (Tautogolabrus adspersus)

ABSTRACT Winter cold slows ectotherm physiology, potentially constraining activities and ecological opportunities at poleward latitudes. Yet, many fishes are winter-active, facilitated by thermal compensation that improves cold performance. Conversely, winter-dormant fishes (e.g. cunner, Tautogolabrus adspersus) become inactive and non-feeding overwinter. Why are certain fishes winter-dormant? We hypothesized that winter dormancy is an adaptive behavioural response arising in poleward species that tolerate severe, uncompensated constraints of cold on their physiological performance. We predicted that below their dormancy threshold of 7–8°C, exercise and metabolic performance of cunner are greatly decreased, even after acclimation (i.e. shows above-normal, uncompensated thermal sensitivity, Q10>1–3). We measured multiple key performance metrics (e.g. C-start maximum velocity, chase swimming speed, aerobic scope) in cunner after acute exposure to 26–2°C (3°C intervals using 14°C-acclimated fish) or acclimation (5–8 weeks) to 14–2°C (3°C intervals bracketing the dormancy threshold). Performance declined with cooling, and the acute Q10 of all six performance rate metrics was significantly greater below the dormancy threshold temperature (Q10,acute8–2°C=1.5–4.9, mean=3.3) than above (Q10,acute14–8°C=1.1–1.9, mean=1.5), inferring a cold constraint. However, 2°C acclimation (temporally more relevant to seasonal cooling) improved performance, abolishing the acute constraint (Q10,acclimated8–2°C=1.4–3.0, mean=2.0; also cf. Q10,acclimated14–8°C=1.2–2.9, mean=1.7). Thus, dormant cunner show partial cold-compensation of exercise and metabolic performance, similar to winter-active species. However, responsiveness to C-start stimuli was greatly cold-constrained even following acclimation, suggesting dormancy involves sensory limitation. Thermal constraints on metabolic and exercise physiology are not significant drivers of winter dormancy in cunner. In fact, compensatory plasticity at frigid temperatures is retained even in a dormant fish.


Fig. S1
. Thermal sensitivity of cunner performance after acute temperature change (3°C hr -1 using 14°C-acclimated animals) (blue symbols) or acclimation (5-7 weeks) to 14, 11, 8, 5, or 2°C (pink symbols).The winter dormancy threshold temperature of cunner is 7-8°C (Reeve et al., 2022).Each data point represents the Q10 value associated with the change in the mean value of each performance metric (excluding non-rate performance, i.e., exhaustive chase duration, EPOC recovery time, responsiveness to C-start stimuli) over a 3°C interval; e.g., a data point at 2°C is the Q10 value from 5-2°C.Q10 values above the dashed grey line infer a thermal constraint, i.e., greater than the typical thermal sensitivity of metabolism and locomotor performance in fishes (Q10=1-3;Seebacher et al., 2015).There are no acclimated data points >14°C because fish were not acclimated above 14°C.Different groups of cunner were used for each acute or acclimated temperature exposure, hence the use of mean performance at each temperature in calculations.

Journal of Experimental Biology • Supplementary information
Table S4.The Q10 and percent change values for performance metrics (and standard metabolic rate, a cost of basic maintenance) in cunner following acute temperature change (3°C hr -1 using 14°C-acclimated animals) or temperature acclimation (5-7 weeks) above and below the winter dormancy threshold temperature (7-8°C for cunner (Reeve et al., 2022)).Specifically, the Q10 and percent change values were calculated over 6°C intervals above and below the winter dormancy threshold temperature as well as following acute warming of 14°C-acclimated animals to 20°C (i.e., 14-20°C, 14-8°C: normal activity; 8-2°C: dormant).Different groups of cunner were used for each acute or acclimated temperature exposure, so each Q10 and percent change value was calculated using the mean performance of each group (hence, the lack of error estimates Fig. S2.The effect of exhaustive chase exercise on white muscle metabolite contents in cunner

Table S1 .
The effect of exhaustive chase exercise on white muscle metabolite contents in cunner Body masses and total lengths of cunner used in each experiment.
between rest groups within acute or acclimated groups across temperatures, or between chased groups within acute or acclimated groups across temperatures (ANOVA with Tukey HSD posthoc tests, p<0.05).Journal of Experimental Biology: doi:10.1242/jeb.246741:SupplementaryinformationJournal of Experimental Biology • Supplementary informationData are presented as means ± s.e.m.Body mass is presented in g and total length in cm.NA, not applicable.See supporting data file for sample sizes.

Table S2 .
The pre-stimulation variables during the C-start test (Experiment 1) following acute cooling (3°C hr -1 using 14°C-acclimated cunner) or temperature acclimation (5-6 weeks).The variables are distance to the arena wall in body lengths (BL), distance to the stimulus in BL, and angle of the fish relative to the stimulus (°).

Table S3 .
Annular chase duration in seconds (s) and total distance swam in body lengths (BL) by cunner during the annular chase swimming speed test (Experiment 1) following acute cooling (3°C hr -1 using 14°C-acclimated animals) or temperature acclimation (5-6 weeks).