When and where to hatch? Red-eyed treefrog embryos use light cues in two contexts

Abstract

Hatching timing is under strong selection and environmentally cued in many species.Embryos use multiple sensory modalities to inform hatching timing and many have spontaneous hatching patterns adaptively synchronized to natural cycles. Embryos can also adaptively shift their hatching timing in response to environmental cues indicating immediate threats or opportunities. Such cued shifts in hatching are widespread among amphibians; however, we know little about what, if anything, regulates their spontaneous hatching. Moreover, in addition to selection on hatching timing, embryos may experience benefits or suffer costs due to the spatial orientation of hatching. Amphibian eggs generally lack internal constraints on hatching direction but embryos might, nonetheless, use external cues to inform hatching orientation. The terrestrial embryos of red-eyed treefrogs, Agalychnis callidryas, hatch rapidly and prematurely in response to vibrational cues in egg-predator attacks and hypoxia if flooded. Here we examined A. callidryas' use of light cues in hatching timing and orientation. To assess patterns of spontaneous hatching and the role of light cues in their diel timing, we recorded hatching times for siblings distributed across three light environments: continuous light, continuous dark, and a 12L:12D photoperiod. Under a natural photoperiod, embryos showed a clear diel pattern of synchronous hatching shortly after nightfall. Hatching was desynchronized in both continuous light and continuous darkness. It was also delayed by continuous light, but not accelerated by continuous dark, suggesting the onset of dark serves as a hatching cue. We examined hatching orientation and light as a potential directional cue for flooded embryos. they use light as a directional cue. Our results support that A. callidryas embryos use light cues to inform both when and where to hatch. Both the spatial orientation of hatching and the timing of spontaneous hatching may affect fitness and be informed by cues in a broader range of species than is currently appreciated. TitleWhen and where to hatch? Red-eyed treefrog embryos use light cues in two contexts Short titleRed-eyed treefrog embryos use light cues We also used a test of equal proportions to test 283 for a difference in the incidence of hatching into the jelly or glass between whole-clutch and 284 individual egg flooding experiments, based on the null hypothesis that the proportion of hatching 285 into the glass in individual trials was equal to the observed average incidence of embryos 286 hatching into the jelly in whole-clutch flooding experiments (3.4%). We then tested for non-287 random hatching orientation in the presence of a light cue using a test of equal proportions, based 288 on the null hypothesis that the probability of hatching in each direction was 50%. We used a 289 GLMM to test whether the 'side of insertion into the tube' affected hatching direction, including 290 clutch as a random factor (N = 57 trials for which side of insertion was recorded). For all 291 statisti...

Ontogeny of risk assessment and escape-hatching performance by red-eyed treefrog embryos in two threat contexts

“…They might also behaviorally facilitate rupture by pressing their head against the membrane to increase HGC–membrane contact. Flooded embryos lose the oxygen gradient that helps them orient—and hatch—toward the exposed side of their egg, and in glass cups they also lose directional light cues; this increases the frequency of hatching complications and need for a second rupture site (Güell and Warkentin, 2018; Salazar-Nicholls et al, 2017). Moreover, in both snake and wasp attacks embryos sometimes move—or are pushed—away from their initial rupture site (KMW, observations from video recordings).…”

Multimodal mechanosensing enables treefrog embryos to escape egg-predators

“…We tested the effect of gentamicin on jiggling-induced hatching in two later periods to determine if the dependence of MCH on lateral line function changes developmentally. In A. callidryas, embryos hatch spontaneously from 5-7 d, while younger embryos almost never hatch if undisturbed (Güell and Warkentin, 2018;Hite et al, 2018;Warkentin, 2000;Warkentin et al, 2001). From age 4 to 5 d, embryos become more likely to escape during snake and wasp attacks (Gomez-Mestre and Warkentin, 2007;Warkentin, 1995;Warkentin, 2000) and to hatch in vibration playbacks (Jung et al, 2019;Warkentin et al, In press;Warkentin et al, 2019).…”

“…The embryos of Agalychnis callidryas clearly use cues in multiple sensory modalities, including vibration (Warkentin 2005, hypoxia (Rogge and Warkentin, 2008) and light level (Güell and Warkentin, 2018), to inform hatching. In sensing physical disturbances, these embryos might use one or multiple mechanosensory systems, either to perceive different cue components available in attacks or as potentially redundant or synergistic sensors of the same cue component.…”