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 (f(R)) monitored and were subjected to normoxic water (PwO(2) = 140 mmHg) without becoming hyperactive for 30 h. During this period, all fish survived without displaying evidences of hyperactivity and maintained unchanged f(R), confirming that this species is a facultative air-breather. Its aquatic O-2 uptake ((V) over dot O-2) was maintained constant down to a critical PO2 (PcO2) of 60 mmHg, below which (V) over dot O-2 declined linearly with further reductions of inspired O-2 tension (PiO(2)). Just above the PcO2 the ventilatory tidal volume (V-T) increased significantly along with gill ventilation ((V) over dot G), while fR changed little. Consequently, the water convection requirement (V) over dot G/(V) over dot O-2) increased steeply. This threshold applied to a cardiac response that included reflex bradycardia. AB was initiated at PiO(2) = 140 mmHg (normoxia) and air-breathing episodes increased linearly with more severe hypoxia, being significantly higher at PiO(2) tensions below the PcO2. Air-breathing episodes were accompanied by bradycardia pre air-breath, to tachycardia post air-breath.