Osmorespiratory Compromise in Zebrafish (Danio rerio): Effects of Hypoxia and Acute Thermal Stress on Oxygen Consumption, Diffusive Water Flux, and Sodium Net Loss Rates.

The traditional thesis of the osmorespiratory compromise is that low branchial water and ion permeability would be traded off for increased O₂ permeability at times of elevated O₂ demand. However, there is growing evidence of independent regulation of these permeabilities in hypoxia-tolerant fish. Using 0.5-g zebrafish previously maintained under normoxia at 25°C, we investigated responses to acute temperature challenges (15°C or 35°C), acute hypoxia (15 min at 10% or 5% air saturation), as well as longer-term exposures to 10% hypoxia, on O₂ consumption (MO₂), diffusive water flux, and net sodium loss rates. Exposure to 35°C increased, and 15°C decreased all three rates, with diffusive water flux showing the lowest temperature sensitivity, and Na⁺ loss the greatest. Acute 10% and 5% hypoxia increased diffusive water flux and net Na⁺ loss, and it reduced MO₂. All these responses reflected the traditional osmorespiratory compromise. However, during prolonged 10% hypoxia, MO₂ recovered, diffusive water flux decreased below control levels, and Na⁺ loss rate remained elevated, even during posthypoxia recovery. Overall, zebrafish do not fit standard patterns previously seen in either hypoxia-tolerant or -intolerant fish but are clearly able to adjust the effective permeabilities of their gills to O₂, water, and ions independently during acute temperature and hypoxia exposures.