Weight-specific rates of aquatic oxygen consumption (V โข O 2 , ยตl O 2 g -1 h -1 ) at 23ยฐC were determined for water-breathing (e.g., forcibly submerged) bullfrog tadpoles as functions of stage of development and O 2 tension (PO 2 ). The V
โข O 2 at an O 2 tension near that of air-saturated water (PO 2 ~ 154 mmHg) was independent of stage of development throughout the premetamorphic stages (I-XIX). Aquatic V โข O 2 increased by approximately 24%, relative to the average of the preceding stages, during the first metamorphic stage (XX) and thereafter decreased steadily with developmental stage. The decline in aquatic V โข O 2 resulted in a shift from facultative air-breathing to obligate air-breathing at about stage XXII. Changes with developmental stage of the critical O 2 tension (P c ) corresponded to changes in aquatic V โข O 2 . The P c was low and relatively constant at 29-36 mmHg through stage XVI, started
to increase (to 51 mmHg) during the final premetamorphic stages (XVII-XIX), reached a value near air saturation (159 mmHg) at stage XXII, and was in excess of air saturation for stages XXIII-XXV. The ability to survive continuous submergence paralleled the changes in P c , as tadpoles could survive in air-saturated water without access to air through stages XXII-XXIII, when they drowned. Whole-body lactate concentrations of tadpoles in normoxic water with access to air averaged 0.56 mg/g through stage XX, comparable to that of froglets (stage XXV, 0.67 mg/g) in shallow water. Animals in anoxic water with access to air exhibited an approximate doubling (1.05 mg/g) of lactate concentration for all stages, as did metamorphic stages XXI-XXV in normoxic water with air access. Recently metamorphosed frogs (stage XXV) could survive continuous submergence for up to 200 h without accumulating lactate if the water was hyperoxic (600-700 mmHg), suggesting that cutaneous O 2 exchange in normoxic water is diffusion-limited.
Stages I-XXI in normoxic water breathed air regularly but infrequently (0.4-6.2 surfacings/h), with earlier stages breathing more frequently than later stages. While not required for oxygen uptake during these stages, air-breathing may serve to promote lung development, to prevent lung collapse, or to prevent accumulation of fluid in the lungs. Surfacing rates increased as the PO 2 of the water decreased, but we could discern no clear relationship between the P c for surfacing and developmental stage. Stages XXII-XXV spent most of their time floating.
The changes in aquatic V
โข O 2 , P c , surfacing behavior, and survivorship when forcibly submerged, all suggest that stage XXII is a critical developmental stage during which bullfrog tadpoles switch from a primarily aquatic to a primarily terrestrial mode of existence. When provided with a choice of being in or out of water, early stages abruptly change from a preference for water (e.g., 92.5% of stage XXI) to a preference for land by stage XXII (74%), further indicating the transitional nature of the latter stage.