Respiratory surface areas of an air-breathing siluroid fish Saccobranchus (Heteropneustes) fossilis in relation to body size

The surface area of the gills, air sacs and skin have been measured in specimens of different body size and their relationship to body weight fits the equation: area= aW^b . The slopes ( b ) of the double logarithmic plots are 0.746 (gills), 0.662 (air sacs) and 0.684 (skin). The gills are poorly developed and their average weight specific area is less than figures obtained for sluggish marine fishes. The skin has an area about 70% of the total respiratory surfaces (gills+air sac+skin). Nevertheless the greater thickness of the skin leads to a smaller diffusing capacity of the tissue barrier ( D_t ) as compared with the gills and air sac. The air sac area for each ml of air that it contains is about 10.5 cm² which is much lower than figures obtained for lungs of other air-breathing fish and for tetrapods.