Meeting Abstract
Agalychnis callidryas embryos develop in gelatinous egg masses on plants above ponds, into which tadpoles fall upon hatching. If young clutches are flooded or fall into ponds, development slows and embryos often die. However, from halfway through normal embryonic development flooded embryos can hatch to escape. To quantify ontogenetic changes in the response to flooding, we submerged egg clutches of 5 ages in normoxic water for 1 h. Only 13% of embryos hatched at the onset of competence, at 3.8 d, and hatching increased to 97% at 6.5 d, near the peak of spontaneous hatching. Hatching speed and synchrony also increased developmentally. Higher response proportion and speed might reflect higher oxygen demand, such that flooding imposes a greater metabolic cost on more developed embryos. To assess developmental changes in metabolism, unconstrained by gas exchange across the egg capsule, we used closed system respirometry to measure oxygen consumption of individual newly hatched A. callidryas tadpoles at 4 ages across the plastic hatching period. We estimated Pcrit, the level below which oxygen limits metabolism, in two ways, fitting a broken-stick regression to metabolic rates across oxygen levels and noting the oxygen level at which tadpoles suddenly increased activity and surfacing attempts. Then we calculated unconstrained metabolic rates above Pcrit. Both Pcrit and unconstrained metabolic rate increased developmentally but showed substantial overlap across and variation within ages. Thus metabolic needs may contribute relatively little to the striking ontogenetic change in flooding-induced hatching. In supra-lethal hypoxia, embryos near the onset of hatching competence may tolerate a period of metabolic suppression in order to hatch at a more developed stage.
