Does the ability to metabolically downregulate alter the hypoxia tolerance of fishes? A comparative study using...

In this study, the metabolic response and tolerance of cunner (a temperate wrasse species capable of metabolic depression) to graded hypoxia (water O(2) saturation 100-10%) was measured at two temperatures ( approximately 1 and 8 degrees C), and compared with that of the Greenland cod (Gadus ogac). Cunner had significantly lower oxygen consumption (MO(2)) values at both 1 (21.2+/-2.4 mg O(2) kg(-0.83) hr(-1)) and 8 degrees C (31.6+/-1.5 mg O(2) kg(-0.83) hr(-1)) as compared with the Greenland cod (64.0+/-4.3 and 92.5+/-6.3 mg O(2) kg(-0.83) hr(-1), respectively) when measured under normoxia. In addition, this species was considerably more hypoxia tolerant than Greenland cod, as evidenced by an ability to tolerate lower water O(2) saturations and their significantly lower critical oxygen saturation (S(crit)) values (16.9 and 21.4% vs. 40.4 and 54.8% water O(2) saturation at 1 and 8 degrees C, respectively). Surprisingly, the normoxic Q(10) value for cunner MO(2) (1.77) was not indicative of a hypometabolic state at 1 degrees C (based on previous experiments on this species). However, 1 degrees C cunner immediately decreased their MO(2) to a new steady state (44% below normoxic levels) when water O(2) saturation was lowered, and this resulted in a Q(10) of approximately 4.5 at oxygen levels between 80 and 20% saturation. This study provides the first data to suggest that fish capable of metabolic depression have an enhanced hypoxia tolerance, and that the cunner's hypometabolic state at cold temperatures is plastic and dependent on other environmental parameters.