The Relative Efficiency of Gaseous Exchange across the Lungs and Gills of an African Lungfish Protopterus

Of the three genera of lungfishes living today Neoceratodus, the Australian lungfish, is possibly the most primitive. The respiratory physiology of this animal has been extensively studied in recent years (Grigg, 1965 a, b, c; Lenfant, Johansen & Grigg, 1966; Johansen & Lenfant, 1967) and the conclusions these authors draw support the assumptions of the earlier workers in this field (Dean, 1906; Longman, 1928; Spencer, 1891), who maintained that in this animal the lung was an accessory respiratory organ used principally when the animal was in poorly aerated water. The gas exchange occurring over the gills is apparently sufficient to satisfy the requirements of the animal in well-aerated water. Earlier workers on the respiratory physiology of Lepidosiren, the South American lungfish, suggested that in this animal the lungs were the principal respiratory organs (Carter & Beadle, 1930; Cunningham, 1934; Kerr, 1897, 1898). Sawaya (1946) found that the gills accounted for only 2% of the total oxygen uptake, and Johansen & Lenfant (1967) confirmed that the lungs were the principal site for oxygen exchange but thought that some carbon dioxide might be exchanged via the aquatic route. These conclusions show that these two forms are well adapted to their predominant habitat. Neoceratodus is usually found in permanent, well-aerated waters, which only rarely become stagnant (Grigg, 1965 c) and is only a facultative airbreathing form. Lepidosiren, on the other hand, is normally found in marshy waters which are not only often extremely hypoxic and hypercarbic, but are also liable to dry out periodically. Lepidosiren is an obligatory air-breathing animal and can survive the periods of drought by aestivating in a burrow in the mud (Kerr, 1898). The habitat of Protopterus aethiopicus is similar to that of Lepidosiren, and, though it has long been assumed that the lungs were the principal respiratory organ, until very recently little experimental work has been carried out to verify this assumption. Prior to the completion of this manuscript, however, several workers have reported studies which verify this assumption. Jesse et al. (1968) suggest that both lungs and gills are important in respiration in juvenile Protopterus (species not certain) but report no result from adult animals. Johansen & Lenfant (1968), however, demonstrate that the lung is the principal organ in oxygen uptake in Protopterus aethiopicus. In this study experiments to elucidate and quantify the actual efficiency of lung and gill in gaseous exchange will be described and discussed.