Air breathing in the armoured catfish (Hoplosternum littorale) as an adaptation to hypoxic, acidic, and hydrogen sulphide rich waters

Abstract

The armoured catfish (Hoplosternum littorale) from the Amazon River system is a facultative air breather that is tolerant to both acidic and hydrogen sulphide rich waters. Facultative air breathing in fishes is known to be an important strategy for surviving hypoxia, but its importance for surviving in acidic and hydrogen sulphide rich waters has not previously been investigated. Air-breathing frequency in H. littorale increased from 2 to 28 breaths/h as the partial pressure of oxygen [Formula: see text] in the water was reduced from 137 to 105 mmHg (1 mmHg = 133.322 Pa). Further reduction in [Formula: see text] to 55 mmHg resulted in a reduction in air-breathing frequency and depression of the metabolic rate. During exposure to acidic water (pH 2.8, [Formula: see text] = 155 mmHg), air-breathing frequency was 28 breaths/h, and during exposure to hydrogen sulphide in water buffered to pH 5.6 (700 μM, [Formula: see text] = 155 mmHg), air-breathing frequency was 40 breaths/h. In fish denied access to air, 200 μM hydrogen sulphide is lethal. Thus, in the armoured catfish, air breathing may be more important for surviving in hydrogen sulphide rich and acidic waters than for surviving in mild hypoxia.