Cardiorespiratory responses to hypoxia in the African catfish, Clarias gariepinus (Burchell 1822), an air-breathing fish
The African catfish, Clarias gariepinus, possesses a pair of suprabranchial chambers located in the dorsal-posterior part of the branchial cavity having extensions from the upper parts of the second and fourth gill arches, forming the arborescent organs. This structure is an air-breathing organ (ABO) and allows aerial breathing (AB). We evaluated its cardiorespiratory responses to aquatic hypoxia. To determine the mode of air-breathing (obligate or accessory), fish had the respiratory frequency (fR) monitored and were subjected to normoxic water (PwO2 = 140 mmHg) without becoming hyperactive for 30 h. During this period, all fish survived without displaying evidences of hyperactivity and maintained unchanged fR, confirming that this species is a facultative air-breather. Its aquatic O2 uptake ($$ \dot{V}{\text{O}}_{2} $$) was maintained constant down to a critical PO2 (PcO2) of 60 mmHg, below which $$ \dot{V}{\text{O}}_{2} $$ declined linearly with further reductions of inspired O2 tension (PiO2). Just above the PcO2 the ventilatory tidal volume (VT) increased significantly along with gill ventilation ($$ \dot{V}_{\text{G}} $$), while fR changed little. Consequently, the water convection requirement $$ \left( {\dot{V}_{\text{G}} /\dot{V}{\text{O}}_{2} } \right) $$ increased steeply. This threshold applied to a cardiac response that included reflex bradycardia. AB was initiated at PiO2 = 140 mmHg (normoxia) and air-breathing episodes increased linearly with more severe hypoxia, being significantly higher at PiO2 tensions below the PcO2. Air-breathing episodes were accompanied by bradycardia pre air-breath, to tachycardia post air-breath.