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Influence of Environmental Po2 on Embryonic Oxygen Consumption, Rate of Development, and Hatching in the Frog Pseudophryne bibroni

Embryos of the frog Pseudophryne bibroni develop on land in depressions underneath objects or vegetation. They hatch only after they have reached Gosner stage 26-27 and are flooded with water. Flooding of the nest reduces Po2 outside the eggs owing to O2 consumption by the embryos. Low Po2 limits the rate of O2 consumption (V̇o2), retards embryonic development, and triggers hatching. In terrestrial embryos at the hatching stage at 12 C, V̇o2 becomes severely limited at Po2 below 14.7 kPa (111 mmHg). In embryos raised in water, rate of development decreases with decreasing Po2 below 38.0 kPa (285 mmHg). In embryos raised on land and then flooded, hatching occurs in less than 12 h when Po2 of the gas above the water is less than 29.9 kPa (224 mmHg). In contrast, hatching of nonflooded embryos on either wet or dry substrate occurs within this time interval only when Po2 is below 5.2 kPa (39 mmHg). This difference in hatching response is apparently due to O2 depletion in the water surrounding the aquatic eggs rather than to differences in water potential of the environment. Thus, hatching is not immediately stimulated within a range of Po2 that limits V̇o2 (5.2-14.7 kPa). Hatching occurs only after protracted flooding, which may help ensure that the hatchling emerges into enough standing water to complete metamorphosis.