Thermal tolerance and associated mechanisms are often tested via the critical thermal maximum (CTmax). The agitation temperature is a recently described thermal limit in fishes that has received little mechanistic evaluation. The present study used a temperate elasmobranch fish to test the hypothesis that this thermal tolerance trait is partially set by the onset of declining cardiorespiratory performance and the cellular stress response. Pacific spiny dogfish (Squalus suckleyi) were screened for cardiorespiratory and whole-organism thermal limits to test for associations between thermal performance and tolerance. Then, biochemical markers of secondary stress, aerobic and anaerobic enzyme activities, and molecular markers of cellular stress were determined for various tissues at the agitation temperature and secondary stress markers were determined at CTmax. In dogfish, the agitation temperature was characterised by increased turning activity within experimental chambers and was equal to the temperature at which dogfish exhibited maximum heart rate. Citrate synthase activity increased at the agitation temperature in white muscle relative to unmanipulated dogfish. Furthermore, lactate dehydrogenase activity and accumulated lactate in the plasma and muscle were not affected by acute warming. Cellular stress was apparent in hypothalamus, gill filament and ventricle, denoted by elevated transcript abundance of the stress response gene hsp70 but not the oxygen homeostasis gene hif1α. Conversely, CTmax was characterised by metabolic acidosis driven by anaerobic lactate production, signifying an increased reliance on anaerobic metabolism between the agitation temperature and CTmax. Together, these data provide partial support for our hypothesis, in that cellular stress, but not declining thermal performance, occurred at the agitation temperature.

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