Meeting Abstract
Arboreal embryos of Agalychnis callidryas hatch prematurely to escape danger, but early hatching increases larval mortality. We examined the ontogeny of embryo responses to hypoxia and physical disturbance, which cue hatching during flooding and predator attacks. Hypoxia-cued hatching began before disturbance-cued hatching, prior to the appearance of large hatching gland cells that mediate rapid escape in attacks, or a vestibuloocular reflex that indicates otic mechanoreception. Using scanning electron microscopy and in situ hybridization probing for hatching enzyme mRNA, we found a second hatching gland cell type that appears earlier in development; these likely mediate the first hypoxia-cued hatching but largely disappear before spontaneous hatching. We video-recorded hypoxia-cued hatching of embryos at different stages to examine decisions and performance. After flooding, embryos first changed position many times, using movements like those used to orient in oxygen gradients in eggs in air. With development, this oxygen-seeking behavior decreased, consistent with the lower risk of mortality outside the egg. Embryos then performed hatching-specific movements. Development improved hatching speed and success; less developed embryos made smaller holes in the membrane, suffered greater body compression squeezing out, and their movements were less effective in propelling them from the capsule. Capsules collapsed around embryos that did not immediately enter their initial hole, allowing fluid to leak. More developed embryos digested additional holes in the membrane to escape, but younger ones sometimes remained trapped.
