Meeting Abstract
Hatching timing is under strong selection and environmentally cued in many species. For eggs in masses, the spatial orientation of hatching may also matter. Agalychnis callidryas lay gelatinous egg masses on leaves over ponds in Central America. Individual eggs are closely packed; most have <50% of their surface exposed to air and strong internal oxygen gradients. Embryos orient their external gills, and heads, towards the air and are thus positioned to escape from the clutch upon hatching. Submerged embryos experience hypoxia, and hatch prematurely to escape, but their first response to flooding is to change position many times, disrupting their outward-facing orientation. Nonetheless, in whole-clutch flooding experiments most embryos hatched correctly; <3% hatched into the jelly, which can be lethal. However, when we flooded individual embryos with similar surface exposure in glass cups, 26% hatched poorly oriented, into the glass. We hypothesized that in flooded clutches embryos use light cues to orient their hatching. To test this, we positioned eggs in close-fitting tubes, with one end illuminated and the other dark, then flooded them and recorded the direction of hatching. Most embryos hatched toward the light, thus using light as a directional cue. To elucidate the role of light in diel hatching patterns of mature embryos, we recorded hatching timing for siblings distributed across three light environments: continuous light, continuous dark, and a natural 12h photoperiod. Continuous light delayed hatching, suggesting light is a hatching inhibitor. Embryos exposed to a photoperiod hatched the most synchronously, soon after dark, suggesting the onset of darkness may stimulate hatching. Overall, our results show that A. callidryas embryos use light cues in at least two contexts, informing both when and where to hatch.
