Respiratory, Blood, and Heart Enzymatic Adaptations of Sebastolobus alascanus (Scorpaenidae; Teleostei) to the Oxygen Minimum Zone: A Comparative Study

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Abstract
The scorpaenid fishes Sebastolobus alascanus and Scorpaena guttata have similar life styles but differ in their depth distributions: S. guttata lives in shallow water (< 180 m); adult S. alascanus occur predominantly on the upper continental slope (400-1200 m) where the oxygen minimum zone (OMZ) prevails and ambient temperature is much colder. Respiratory properties and the activities of heart-tissue enzymes of these species were compared to determine the effect of different thermal and ambient O2 regimens on metabolism. Measured over the appropriate habitat temperature ranges, the oxygen consumption (VO2) of S. alascanus is two to four times less than that of S. guttata. Correction for differences in habitat temperature accounted for over 50% of this reduction. The depth-related decrease in VO2 for these two benthic fishes is less than that observed for pelagic fishes. The VO2 of S. guttata decreases at O2 concentrations below 1 ml/l, whereas the VO2 of S. alascanus is regulated down to 0.3 ml/l. The ventilation frequency (Vf) of both species increases in progressive hypoxia; but at < 0.5 ml/l, the Vf of S. guttata declines, while that of S. alascanus does not. When measured at the same temperature, pH and CO2, the blood-O2 affinity of S. guttata is significantly lower than that of S. alascanus. The anaerobic/aerobic enzyme activity ratio of pyruvate kinase to citrate synthase, which correlates with the ability of heart tissue to tolerate hypoxia, is significantly higher for S. alascanus than S. guttata. Lactate dehydrogenase (LDH) activity in freshly collected S. alascanus is also significantly above that of specimens acclimated to normoxic water in the laboratory. Only the skeletal muscle isozyme of LDH (LDH-A) is present in the heart of S. alascanus, whereas S. guttata has both LDH-A and heart (LDH-B) isozymes. Data for metabolic rate, critical O2 tension, blood oxygen affinity, and heart metabolic enzyme profiles all show essential adaptations of S. alascanus for life in the OMZ.

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