© 1999 by The Society for Integrative and Comparative Biology
Respiration of Aquatic and Terrestrial Amphibian Embryos1
Department of Enviromental Biology, University of Adelaide Adelaide, 5005, South Australia
Correspondence: 2 E-mail: rseymour{at}zooiogy.adelaide.edu.au
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 Po2, 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. Terrestrial egg masses are more diffusion limited, because gravity and surface tension collapse them, preventing convection between the eggs, and restricting the source for oxygen diffusion. Terrestrial embryos are often larger than their aquatic counterparts and have higher demands for oxygen. Terrestrial conditions have selected for adaptations that reduce respiratory competition between embryos, for example, separating of embryos by large volumes of jelly or reducing the number of eggs in a clutch. The size of foam nests is unlimited, because oxygen for each embryo is supplied directly from the foam.