The aerobic physiology of the air-breathing blue gourami, Trichogaster trichopterus, necessitates behavioural regulation of breath-hold limits duringhypoxic stress and predatory challenge

Physiological characteristics of the blood oxygen transport system and musc le metabolism indicate a high dependence on aerobic pathways in the blue go urami, Trichogaster trichopterus. Haemoglobin concentration and haematocrit were modest and the blood oxygen affinity (P-50 = 2.31 kPa at pH 7.4 and 2 8 degreesC) and its sensitivity to pH (Bohr factor, Phi = -0.34) favour oxy gen unloading at a relatively high oxygen pressure (PO2). The intracellular buffering capacity (44.0 slykes) and lactate dehydrogenase (LDH) activity (154.3 iu g(-1)) do not support exceptional anaerobic capabilities. Air-bre athing frequency in the blue gourami is expected to increase when aquatic o xygen tensions decline. Under threat of predation, however, this behaviour must be modified at a potential cost to aerobic metabolism. We therefore te sted the hypothesis that metabolic responses to predatory challenge and aqu atic hypoxia are subject to behavioural modulation. Computer-generated visu al stimuli consistently reduced air-breathing frequency at 19.95, 6.65 and 3.33 kPa PO2. Bi-directional rates of spontaneous activity were similarly r educed. The metabolic cost of this behaviour was estimated and positively c orrelated with PO2 but not with visual stimulation thus indicating down-reg ulation of spontaneous activity rather than breath-holding behaviour. Neith er PO2 nor visual stimulation resulted in significant change to muscle lact ate and ATP concentrations and confirm that aerobic breath-hold limits were maintained following behavioural modulation of metabolic demands.