EMBRYONIC AND LARVAL RESPIRATION IN THE ARBOREAL FOAM NESTS OF THE AFRICAN FROG CHIROMANTIS-XERAMPELINA

In Zimbabwe, female Chiromantis xerampelina construct spherical foam nests that are suspended above temporary water. The nests average 624 ml in volume and contain 854 eggs. The 1.7mm ova have exceptionally thin jelly capsules and are dispersed in the foamy core of the nest, which is surrounded by a layer of eggless foam. At 25 degrees C, each embryo requires 3.5 days to reach hatching at developmental stage 22, during which it consumes 30 mu l of oxygen. After hatching, each larva remains in the nest for 2 more days and consumes a further 123 mu l of oxygen. The fresh foam contains 77% air, which is sufficient to supply all of the oxygen requirements of the embryos until well after they hatch. Therefore, the size of the egg mass is not Limited by oxygen availability as it is in many other anurans. Oxygen also diffuses into the nest from the atmosphere, but the rate is severely restricted by the wet foam, despite the presence of bubbles. Drying of the outer layer of foam greatly increases its oxygen conductance, but the larvae remain in the inner core of wet foam, where they compete for oxygen at the periphery. With further drying of the nest, the wet foam diminishes in volume and concentrates the larvae at a time when their oxygen demands are approaching the maximum. Oxygen pressures within the wet foam drop below 10kPa and oxygen uptake by the larvae becomes progressively limited, possibly stimulating their emergence from the nest. The delay between hatching and escape from the nest permits the larvae to grow and mature to a stage at which all of the clutch can emerge simultaneously.