Acute temperature effects on metabolic rate, ventilation, diffusive water exchange, osmoregulation, and acid–base status in the Pacific hagfish (Eptatretus stoutii)
The Pacific hagfish (Eptatretus stoutii) is a representative of the most basal extant craniates, and is a marine osmoconformer with an extremely low metabolic rate (MO2 = 475 µmol O2/kg/h at 12 °C). We investigated potential physiological trade-offs associated with compensatory changes in gill ventilation and perfusion when 12 °C-acclimated hagfish were acutely exposed to 7 °C or 17 °C, as reflected in diffusive unidirectional water flux ($${J_{{{\text{H}}_2}{\text{O}}}}$$JH2O, measured with tritiated water: 3H2O), net ammonia flux (Jamm), and plasma ion and acid–base status. $${J_{{{\text{H}}_2}{\text{O}}}}$$JH2O was high (~ 1.4 L/kg/h at 12 °C) in comparison to marine teleosts and elasmobranchs. MO2 increased linearly with temperature (R2 = 0.991), and was more sensitive (Q10 = 3.22) in the 12–7 °C range than either Jamm (1.86) or $${J_{{{\text{H}}_2}{\text{O}}}}$$JH2O (1.35), but the pattern reversed from 12 to 17 °C (Q10s: MO2 = 2.77, Jamm = 2.88, $${J_{{{\text{H}}_2}{\text{O}}}}$$JH2O = 4.01). Heart rate, ventilatory index (a proxy for total ventilation), and coughing frequency also increased but with different patterns. At 17 °C, plasma [Ca2+] and [Mg2+] decreased, although osmolality increased, associated with elevations in plasma [Na+] and [Cl−]. Blood pH and PCO2 were unaffected by acute temperature changes while [HCO3−] increased. Hyperoxia (PO2 > 300 Torr) attenuated the increase in $${J_{{{\text{H}}_2}{\text{O}}}}$$JH2O at 17 °C, did not affect Jamm, and had diverse effects on plasma ion and acid–base status. Our results suggest a clear osmorespiratory compromise occurring for the diffusive water fluxes as a result of acute temperature changes in this osmoconformer.