Cardiorespiratory responses of the facultative air-breathing fish jeju, Hoplerythrinus unitaeniatus (Teleostei, Erythrinidae), exposed to graded ambient hypoxia

The jeju, Hoplerythrinus unitaeniatus, is equipped with a modified part of the swim bladder that allows aerial respiration. On this background, we have evaluated its respiratory and cardiovascular responses to aquatic hypoxia. Its aquatic O 2 uptake ( V˙ O 2 ) was maintained constant down to a critical P O 2 ( P cO 2 ) of 40 mm Hg, below which V˙ O 2 declined linearly with further reductions of P iO 2 . Just below P cO 2 , the ventilatory tidal volume ( V T) increased significantly along with gill ventilation ( V˙ G), while respiratory frequency changed little. Consequently, water convection requirement ( V˙ G/ V˙ O 2 ) increased steeply. The same threshold applied to cardiovascular responses that included reflex bradycardia and elevated arterial blood pressure ( P a). Aerial respiration was initiated at water P O 2 of 44 mm Hg and breathing episodes and time at the surface increased linearly with more severe hypoxia. At the lowest water P O 2 (20 mm Hg), the time spent at the surface accounted for 50% of total time. This response has a character of a temporary emergency behavior that may allow the animal to escape hypoxia.