THE TRANSITION TO AIR BREATHING IN FISHES IV. IMPACT OF BRANCHIAL SPECIALIZATIONS FOR AIR BREATHING ON THE AQUATIC RESPIRATORY MECHANISMS AND VENTILATORY COSTS OF THE SWAMP EEL SYNBRANCHUS MARMORATUS

The gills, adjacent buccopharyngeal epithelium, and skin of the swamp eel Synbranchus marmoratus (Bloch) function for both aerial and aquatic respiration. Aquatic cutaneous O2 uptake occurs continuously at rates that, while dependent upon aquatic O ^ tension (PwO2), are in direct proportion to body surface area. Branchial aquatic O2 uptake takes place during intermittent ventilation which occurs in proportion to body mass. Because of reductions in the body surface area to volume ratio that occur with growth, cutaneous oxygen uptake comprises a larger per-centage of the total oxygen uptake of small fish and, to compensate, large fish ventilate more. The mass exponent for total rate of oxygen uptake (VOz) (0-894 ± 0-145) is within the range predicted from the contributions of cu-taneous VQ2 (mass exponent 0-651 ± 0-167) and the number of minutes each hour that branchial ventilation occurs (0-378 ± 0-105). Hyperoxia increases cutaneous VOz and reduces branchial ventilation. Total VQ2 was also reduced in hyperoxia and calculations relating this to the reduction in ventilation time yield ventilatory cost estimates that increase with body size and that are high compared to those of othe