Respiration of Aquatic and Terrestrial Amphibian Embryos1

SYNOPSIS. Respiratory constraints on the structure of single eggs and egg masses have affected the mode of amphibian reproduction in water and in air. Aquatic eggs generally require less oxygen, develop faster, and hatch earlier, but these characteristics are related to small ovum size. A comparison of two species of aquatic and terrestrial breeding frogs with similarly sized ova shows no differences in hatching stage, maximum rate of oxygen uptake, oxygen conductance of the egg capsule, or Po2 difference across the capsule. However, the aquatic species develops about 2.4 times faster and tolerates lower environmental Pu2, suggesting adaptation for development in ephemeral water. Modelling of diffusive oxygen transport into a single aquatic egg shows that a large amount of jelly (or a boundary layer) around the capsule may not greatly restrict gas exchange, if the inner radius of the capsule is large. However, gelatinous egg masses that contain other embryos that compete for oxygen are therefore limited in size, unless the eggs are ventilated by convection of water among them. Aquatic egg are often suspended in masses above the substrate, promoting oxygen movement into the mass from all directions