Abstract
The osmorespiratory compromise hypothesis posits that respiratory epithelial characteristics and physiological regulatory mechanisms which promote gas permeability also increase permeability to ions and water. The hypothesis therefore predicts that physiological responses which increase effective gas permeability will result in increased effective ion and water permeabilities. Though analyses of water and gas effective permeabilities using high temperature have generally supported the hypothesis, water permeability responses to hypoxia remain equivocal and the combination of high temperature and hypoxia untested. We measured diffusive water flux (DWF) and oxygen uptake rate (Ṁo2) in response to acute temperature change, hypoxia, and the combination of high temperature and hypoxia in a hypoxia-tolerant intertidal fish, the tidepool sculpin (Oligocottus maculosus). In support of the osmorespiratory compromise hypothesis, Ṁo2 and DWF increased with temperature. In contrast, DWF decreased with hypoxia at a constant temperature, a result consistent with previously observed decoupling of water and gas effective permeabilities during hypoxia exposure in some hypoxia tolerant fishes. However, DWF levels during simultaneous high temperature and hypoxia exposure were not different from fish exposed to high temperature in normoxia, possibly suggesting a failure of the mechanism responsible for down-regulating DWF in hypoxia. These results, together with time-course analysis of hypoxia exposure and normoxic recovery, suggest that tidepool sculpins actively downregulate effective water permeability in hypoxia but the mechanism fails with multi-stressor exposure. Future investigations of the mechanistic basis of the regulation of gill permeability will be key to understanding the role of this regulatory ability in the persistence of this species in the dynamic intertidal environment.