The armoured catfish, Ancistrus chagresi, is a facultative air breather and uses its stomach as an air-breathing organ (ABO). Comparisons of control fish and fish that had become acclimated to hypoxia and air breathing for 14–21 days were carried out to assess the effects of this treatment on bimodal (aerial and aquatic) gas exchange capacity. Hypoxia acclimation elicits physiological and biochemical changes that enable A. chagresi to increase O2 utilization both by its gills in hypoxic water and by its ABO. Compared with control fish, hypoxia-acclimated Ancistrus have a higher blood--O2 affinity and more haemoglobin (Hb) and can maintain a higher aquatic oxygen consumption rate (VO2) in hypoxic (Pw, Ow, O2, = 5–20 mmHg) water. They also have a 25% larger ABO volume, are able to hold each air breath longer, and can reduce ABO O2 partial pressure to a lower level. In both groups, respiratory CO2-release occurs primarily through the gills. An air breath instantly causes tachycardia and a reduction in the frequency and amplitude of branchial ventilation. Their lower cardiac and gill ventilation rates in hypoxia and during air breathing suggest that hypoxia-acclimated fish are more adapted for hypoxia than are control fish. During the period an air breath is held in the ABO, hypoxia-acclimated fish exhibit more coordinated phase shifts in gill ventilation and cardiac rates. These may favour an initial phase of efficient aerial O2 uptake from the ABO and transport through the body followed by a period of aquatic CO2 release from the gills.

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