Meeting Abstract

S8.5  Thursday, Jan. 6  Hatching plasticity in amphibians: evolution, trade-offs, cues and mechanisms WARKENTIN, K.M.; Boston University, MA kwarken@bu.edu

At least 28 amphibian species from 7 families of frogs and salamanders alter hatching timing in response to conditions affecting egg or larval mortality. Some terrestrially laid or stranded embryos wait to hatch until they are submerged in water. Some embryos laid above water accelerate hatching if they risk egg desiccation; others hatch early if flooded. Embryos can accelerate hatching in response to egg predators and pathogens or delay hatching in response to larval predators; some species do both. Shifts in hatching timing often have obvious benefits. We know less about the trade-offs that favor plasticity, the mechanisms that enable it, and the evolutionary history through which it evolved. Red-eyed treefrogs (Phyllomedusinae) are the best-studied case. Their arboreal embryos hatch rapidly and prematurely to escape from flooding, pathogens, and egg predators, but suffer higher predation in the water than do full-term hatchlings. Delayed regression of embryonic external gills and behavioral orientation in oxygen gradients enable prolonged development in the egg, and hypoxia cues hatching in flooding. Snake-induced hatching is cued by vibrations and embryos integrate at least five vibration properties in a complex risk assessment mechanism. Red-eyed treefrogs show substantial genetic variation for spontaneous hatching timing but essentially none for the onset of hatching competence, suggesting past directional selection and a developmental constraint. All other phyllomedusine treefrogs tested can hatch ~30% prematurely if flooded, avoiding drowning. Most also hatch and escape in snake attacks but two species show little response to snakes. Hatching plasticity thus appears to be basal in phyllomedusines, but responses to different risks, mediated by different environmental cues and sensory modalities, show independent evolutionary changes.