Changing respiratory importance of gills, lungs and skin during metamorphosis in the bullfrog Rana catesbeiana.

Oxygen uptake (MO2) and carbon dioxide excretion (MCO2) by the skin, lungs and gills (if present) of Rana catesbeiana have been measured at 20 degrees C during 4 developmental stages - strictly water breathing tadpoles, air breathing tadpoles, post-metamorphic bullfrogs and 4-year-old adult bullfrogs. In aquatic tadpoles, branchial performance is comparable to that of teleost fishes, but a large skin area to body mass ratio, particularly for the tail, plus a thin and highly vascularized skin, presumably facilitates a large (60% of total MO2) cutaneous O2 uptake. As development proceeds, MO2 by the gills decreases and the lungs assume importance in O2 uptake, but the skin remains the major organ of O2 uptake until metamorphosis is nearly complete. Immediately after metamorphosis, O2 uptake by the lung is elevated to 80% of total MO2. Carbon dioxide excretion in both aquatic and air breathing tadpoles was also achieved mostly by the skin (60% of total MCO2, R = 0.9). The lungs of air breathing tadpoles excreted less than 2% of total MCO2, rising to a maximum of only 20% (R = 0.2) even in adult bullfrogs. The considerable importance of the skin to CO2 excretion thus rises even further with the degeneration of the gills at metamorphosis, with R for the skin rising from 0.8 before metamorphosis to 7.5 in adults. Thus, large adjustments in skin and lung gas exchange occur as the larval gills slowly degenerate, and lung ventilation is initiated and increased. Aquatic O2 uptake is rapidly superseded by the uptake of O2 from the air, while CO2 excretion largely remains a function of the aquatic respiratory surfaces throughout the life cycle of the bullfrog.