The key mechanisms associated with the survival of Gulf toadfish, Opsanus beta, when in severe hypoxia

Florida's shallow seagrass beds experience daily diurnal fluctuations in environmental oxygen, leading to hypoxic episodes (PO₂ < 2 mg L^-1, ~6 kPa) that have increased in frequency and intensity in recent decades. The native Gulf toadfish (Opsanus beta) have been shown to employ a suite of adaptive cardiovascular, ventilatory and metabolic adjustments to survive moderate hypoxia, but little is known of the mechanisms used to survive severe hypoxia. The objective of this study was to characterize the acute response of Gulf toadfish to severe hypoxia (<2 kPa). We hypothesized that Gulf toadfish would respond to severe hypoxia by reducing energy expenditure while switching to anaerobic metabolism. Cannulated adult Gulf toadfish were exposed to either control conditions (normoxia for 4 h) or hypoxia (PO₂ ~0.4 kPa for 3 h followed by 1 h recovery). Heart rate, pulse pressure, caudal arterial blood pressure, ventilation frequency and ventilation amplitude were monitored and blood samples were taken from both groups and analysed for pH, glucose, lactate and other endpoints. Hypoxia-exposed fish experienced a statistically significant 70% decrease in heart rate and a 32% decrease in ventilation frequency during hypoxia exposure. Moreover, plasma glucose concentrations increased 5-fold and plasma lactate concentrations increased 6-fold, with a minimal reduction in pH. Our findings support our hypothesis that Gulf toadfish are highly hypoxia tolerant and survive severe hypoxia by reducing energy expenditure and switching to anaerobic metabolism. It is important to understand how the Gulf toadfish deal with severe hypoxia due to the increasing prevalence of severe hypoxia in its natural environment.