Biomodal Gas Exchange During Variation in Environmental Oxygen and Carbon Dioxide in the Air Breathing Fish Trichogaster Trichopterus

Abstract

Gas exchange in the gourami, Trichogaster trichopterus, an obligate air breather, is achieved both by branchial exchange with water and aerial exchange via labyrinth organs lying within the suprabranchial chamber. Ventilation of the suprabranchial chamber, MO_O2, MCO_CO2, gas exchange ratios of both gills and labyrinth organs, and air convection requirements have been measured under conditions of hypoxia, hyperoxia or hypercapnia in either water or air. In undisturbed fish in control conditions (27 °C), air breathing frequency was 12 breaths/h, gas tidal volume 30 μl/g, total oxygen uptake 5.2 μ.M/g/h and total carbon dioxide excretion 4.1 μM/g/h, indicating a total gas exchange ratio of approximately 0.8. The aerial labyrinth organs accounted for 40% of oxygen uptake but only 15% of carbon dioxide elimination. Hypoxia, in either inspired water or air, stimulated air breathing. Total MO_O2 was continuously maintained at or above control levels by an augmentation of oxygen uptake by the labyrinth during aquatic hypoxia or by the gills during aerial hypoxia. Hypoxia had no effect on MCOl partitioning between air and water. Hypercapnia in water greatly stimulated air breathing. About 60% of total MCO_CO2 then occurred via aerial excretion, a situation unusual among air breathing fish, enabling the overall gas exchange to remain at control levels. Aerial hypercapnia had no effect on air breathing or O₂ partitioning, but resulted in a net aerial CO₂ uptake and a decrease in overall gas exchange ratio. Trichogaster is thus an air breathing fish which is able to maintain a respiratory homeostasis under varying environmental conditions by exploiting whichever respiratory medium at a particular time is the most effective for O₂ uptake and CO₂ elimination.