WARKENTIN, KM; GOMEZ-MESTRE, I; Boston University; Boston University: Effects of development, surface exposure, and embryo behavior on oxygen levels in red-eyed treefrog eggs

The red-eyed treefrog, Agalychnis callidryas, deposits gelatinous egg masses on vegetation over water. Although development is highly synchronous, the embryonic period varies by about 30%, depending on exposure to egg predators and pathogens. Embryos develop large external gills, which naturally regress at hatching, rather than at any particular developmental stage. Both induced gill regression and exposure to a hypoxic gas mixture stimulate early hatching, indicating that oxygen stress is a hatching trigger. To characterize the respiratory environment of embryos, and assess effects of development and egg surface exposure on oxygen availability, we used a fiberoptic microprobe to measure PO₂ in the perivitelline fluid within A. callidryas eggs. Perivitelline PO₂ was generally lower later in development, and in eggs with less surface area exposed to air. More surprisingly, there was substantial variation in PO₂ within eggs, both spatially and temporally. We observed strong effects of embryo behavior on perivitelline PO₂ despite the constant ciliary circulation of the perivitelline fluid. This suggests that embryo movements may improve gas exchange by mixing the fluid, and that the position of embryos within their eggs may be important for oxygen uptake. Furthermore, embryos tolerated very low PO₂ levels without hatching. For normally and synchronously developing embryos early in the period of hatching competence (4 d old) we measured perivitelline PO₂ ranging from 0.2 kPa to 12.4 kPa in the approximate center of the perivitelline space. In some cases PO₂ was higher near the egg surface, under portions of the egg capsule exposed to air. Thus A. callidryas embryos might be able to behaviorally avoid low perivitelline PO₂ by strategic positioning of respiratory surfaces within the egg capsule. To trigger hatching, hypoxia may need to be prolonged, severe, or without a micro-spatial refuge.