Gas exchange with air and water in an air-breathing catfish, Saccobranchus (Heteropneustes) fossilis.

Recent studies on air-breathing fish have shown that different species are adapted in varying degrees for gaseous exchange in both water and air. In the lungfishes these differences are correlated with different degrees of development of the gills, the efficiency of the lung in gas exchange, and the respiratory properties of the blood (Lenfant, Johansen & Grigg, 1966; Johansen & Lenfant, 1967, 1968; Lenfant & Johansen, 1968; Jesse, Shub & Fishman, 1967; McMahon, 1970). A similar situation exists among air-breathing teleosts, but there seems to be an even greater variation in the degree of dependence on water or air-breathing, which is again related to the structure of the gills and air-breathing organs and the normal environments in which they live (Farber & Rahn, 1970; Garey & Rahn, 1970; Hughes & Singh, 1970a, b; Johansen, 1966; Singh & Hughes, 1971). Saccobranchus fossiUs is an air-breathing fish which inhabits pools in tropical Asia. The structure and arrangement of the gills and air-breathing organs were described by Munshi (1962), the air-breathing organ in this case being a posterior diverticulum of the opercular cavity in the form of an air-sac. Studies on the gill respiratory epithelia showed that the gills of Saccobranchus are better adapted for gas exchange than those of other air-breathing teleosts from the same habitat (Munshi & Singh, 1968). Measurements of the air/blood pathway using electron microscopy (Hughes & Munshi, 1972) have shown longer distances than in Anabas (Hughes & Munshi, 1968) and other airbreathing fish from similar habitats. Although inhabiting similar environments, Saccobranchus differs from Anabas by aestivating in the mud when pools dry up in the summer. The purpose of the present study was to investigate the relative roles of the gills and air-breathing organs in gas-exchange under varying environmental conditions. It also throws some light on the mechanism of metabolic adaptation in Saccobranchus under different conditions.