Effects of water oxygen on cardiac and metabolic responses in native and invasive green crabs (Carcinus maenas) exposed to acute warming

The green crab ( Carcinus maenas ) is an extremely successful invasive species, known for its tolerance to environmental perturbations. While the thermal limits and hypoxia tolerance of green crabs have been well studied, the potential benefit of an oxygen surplus during acute thermal stress remains unclear. We use a multi-stressor approach to evaluate the oxygen dependence of upper thermal limits in green crabs captured from the Swedish North Sea and the Canadian Pacific. We use measurements of oxygen consumption ( Ṁ O 2 ) to investigate how environmental dissolved oxygen (50%, 100% and 150% air saturation) influences upper thermal limits of green crabs from capture sites on two continents. Under normoxia, green crabs exhibited an increase in Ṁ O 2 from the 16 °C acclimation temperature to a peak Ṁ O 2 at 34.8 ± 1.4 °C in Swedish crabs and 34.6 ± 0.4 °C in Canadian crabs, followed by an abrupt decline and collapse at their upper thermal limits. Corresponding heart rate (HR) measurements in Swedish green crabs mirrored the patterns observed in Ṁ O 2 . Swedish and Canadian green crabs also responded similarly to environmental oxygen, exhibiting consistent reductions in upper thermal limits (the point of metabolic collapse) in hypoxia and negligible improvement between normoxia and hyperoxia. Thus, we propose that while green crab thermal tolerance is oxygen limited in hypoxia, at normoxia and above, thermal tolerance is instead dependent on the collapse of other vital physiological functions. These results contrast previous suggestions of oxygen and capacity limited thermal tolerance in green crabs and other crustaceans in normoxia.