OXYGEN TRANSPORT IN EGG MASSES OF THE AMPHIBIANS RANA SYLVATICA AND AMBYSTOMA MACULATUM: CONVECTION, DIFFUSION AND OXYGEN PRODUCTION BY ALGAE

Many amphibians lay their eggs in gelatinous masses up to 10­20 cm in diameter, posing problems for diffusive oxygen delivery. Oxygen may also be provided by water convection between eggs or by oxygen production by endogenous algae. We studied egg masses of two local amphibians, Rana sylvatica and Ambystoma maculatum, to estimate the importance of each of these processes. We injected dye to check for water channels, measured oxygen partial pressures within egg masses to determine the influence of external water convection and lighting, measured oxygen consumption and production in darkness and light and calculated expected gradients through egg masses with a cylindrical, homogeneous egg mass model. Rana sylvatica had relatively loose egg masses with water channels between the eggs; water convection was important for oxygen delivery. Ambystoma maculatum had firm egg masses with no spaces in the jelly between eggs; thus, there was no opportunity for convective oxygen delivery. The egg masses were cohabited by Oophila ambystomatis, a green alga found specifically in association with amphibian egg masses. Oxygen delivery in A. maculatum was by diffusion and by local production by the algal symbiont. Analysis of a cylindrical egg mass model and measurement of oxygen gradients through egg masses indicated that diffusion alone was not adequate to deliver sufficient O2 to the innermost embryos at late developmental stages. In the light, however, egg masses had a net oxygen production and became hyperoxic. Over the course of a day with a 14 h:10 h light:dark cycle, the innermost embryos were alternately exposed to hyperoxia and near anoxia.