A detailed morphometric study has been made of the air-sacs of this air-breathing catfish using whole mounts, light and electron microscopy, of six specimens, body weight 40±2 g. Measurements of surface areas of the gas exchange and non-respiratory surfaces have taken into account foldings of the surface at macro and ultramicroscopic levels. Area of the gas exchange surface was estimated as 23.915cm2 (=0.598cm2/g) which is 67%of the total surface area of the two air-sacs. Significant differences were found in some morphometric parameters which were related to the three antero-posterior regions into which air-sacs were divided. Harmonic mean thickness of the tissue component of the air/blood barrier was estimated for the whole air-sac as 0.342 μm. These and other measurements enabled the diffusing capacity for the air-sacs to be calculated as 0.0638 m1O2/min/mmHg/kg.
These results show that Heteropneustes has an air-breathing organ which is superior to that of Amphipnous cuchia, similar to that of Lepidosiren, but less well developed than that of Protopterus. In addition, Heteropneustes is well adapted to obtain oxygen directly from water by means of its gills and skin as indicated by both morphometric and physiological measurements which also correlate with its life in ponds and streams which are Iiable to dry up.
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