Changes in ventilation, metabolism, and behaviour, but not bradycardia, contribute to hypoxia survival in two species of Amazonian armoured catfish

Amazonian armoured catfishes exhibit substantial cardiac hypoxia tolerance, but little is known concerning organismal cardiorespiratory, metabolic, and behavioural responses to low oxygen levels. This study assessed the general mechanisms used by two species of armoured catfish, Glyptoperichthyes gibbceps and Liposarcus pardalis, to survive the frequent periods of hypoxia encountered in the Amazon River. The gill ventilation rate (f(v)) and heart rate (f(h)) were studied under controlled hypoxia in aquaria and under natural hypoxia in a simulated pond. Glyptoperichthyes gibbceps were fitted with radiotelemetry tags and held in field cages to study their habits of depth selection and air breathing. When denied aerial respiration under hypoxia in aquaria, G. gibbceps increased f(v), but neither they nor L. pardalis exhibited alterations in f(h). An increase in f(v) was initially observed in G. gibbceps during pond hypoxia before aerial respiration was initiated and f(v) declined. Glyptoperichthyes gibbceps were hyperglycaemic under normoxia, and extremely large increases in plasma glucose and lactate concentrations were observed under hypoxia. Field studies confirmed their nocturnal behaviour and showed that air breathing increased at night, regardless of dissolved oxygen concentration. Our results show that armoured catfishes preferentially up-regulate f(v) and anaerobic metabolism and exhibit no bradycardia during hypoxia.