RESUMEN
The respiratory organs of the African sharptooth catfish, Clarias gariepinus, were studied to broaden existing understanding of the adaptive stratagems that have evolved for air-breathing in fish. The gills were well-developed and the air-breathing organs (ABOs) comprised labyrinthine organs (LOs), suprabranchial chamber membranes (SBCMs) and gill fans (GFns). Respectively, the gills and the LOs had the highest mass-specific respiratory surface areas of 133.7 and 141.9 mm2 per gram and among the ABOs, with a harmonic mean thickness of the blood-barrier (BGB) of 0.39 µm, the LOs had the thinnest BGB followed by the GFns (0.48 µm) and the SBCMs (0.49 µm): the water-blood barrier of the gills was relatively much thicker (7.93 µm). Vindicating why C. gariepinus is an obligate air-breather, the total mass-specific morphometric (anatomical) diffusing capacity of the ABOs for O2 per unit body weight (W) (Dto2/W) comprised 90.5% of the mean total value for all the respiratory organs. Compared with the East African catfish, Clarias mossambicus, the Dto2/W of the ABOs of C. gariepinus was 5.7 times greater. The difference between the two species of fish may be explained by the physicochemical differences of the aquatic habitats they occupy: the former occupies a seasonal river which dries up during the summer months leaving shallow pools of water in which the O2 concentrations are very low and CO2 very high while the later populates a highly eutrophic dam where the O2 levels greatly fluctuate seasonally.