Osmorespiratory compromise in an elasmobranch: oxygen consumption, ventilation and nitrogen metabolism in dogfish sharks (Squalus suckleyi) exposed to hypoxia in different salinities.

Fish face a functional trade-off at the gills between minimizing ion movement and maximizing oxygen uptake - the osmorespiratory compromise, but the extent of this trade-off remains poorly understood in elasmobranchs. Using the Pacific dogfish shark, we assessed the impacts of progressive hypoxia in animals acclimated to 25, 30 and 36 ppt for 4 days at 12 °C. Plasma osmolality increased with water osmolality at 36 ppt (osmoconformation) and decreased at 25 ppt. Plasma urea decreased at 25 ppt, though to a lesser extent than plasma Cl^-, while plasma urea increased to a greater extent than plasma Cl^- at 36 ppt. In normoxia, oxygen consumption rate (MO₂) was elevated by 60% at 36 ppt, and ventilatory index (frequency x amplitude) was elevated by 70%, reflecting increases in both components of ventilation, but these parameters remained unchanged in sharks exposed to 25 ppt. During progressive hypoxia, MO₂ and ventilation exhibited different patterns at the three salinities, but in all three, MO₂ fell linearly below a water PO₂ of ~ 80 Torr (10.7 kPa), indicating oxyconformation. Under hypoxia (45 to 5 Torr; 6.0 to 0.7 kPa) MO₂ was the same at all salinities, while ventilatory amplitude was elevated at both 25 and 30 ppt. At 36 ppt, frequency decreased during hypoxia. Ventilatory index increased during hypoxia only at 30 ppt and not at the other salinities. From these data it is clear that dogfish sharks face an osmorespiratory compromise balancing the needs for urea retention against those of O₂ uptake.