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

Arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, hatch prematurely in response to hypoxia when flooded and to mechanosensory cues in snake attacks, but hatching later improves tadpole survival. We studied ontogenetic changes in risk assessment and hatching performance of embryos in response to flooding and physical disturbance. We hypothesized that risk assessment decreases as hatchling survival improves and hatching performance increases as embryos develop. Because snakes eat faster than embryos asphyxiate, we hypothesized that embryos decide to hatch sooner and hatch faster in response to mechanosensory cues. We video-recorded individual embryos hatching in response to each cue type, then compared the incidence and timing of a series of events and behaviors from cue onset to complete hatching across ages and stimuli. Latency from cue to hatching decreased developmentally in both contexts and was shorter with mechanosensory cues, but the elements contributing to those changes differed. Hypoxia assessment involved position changes, which decreased developmentally along with assessment time. Mechanosensory cue assessment occurred more rapidly, without movement, and decreased with age. The first stages of hatching, membrane rupture and head emergence, were surprisingly age independent but faster with mechanosensory cues, congruent with greater effort under more immediate risk. In contrast, body emergence and compression showed ontogenetic improvement consistent with morphological constraints but no cue effect. Both appropriate timing and effective performance of hatching are necessary for continued development. Different stages of the process vary with development and environmental context, suggesting combinations of adaptive context- and stage-dependent behavior, cue-related constraints on information acquisition, and ontogenetic constraints on elements of performance.

Characterizing environmental stress responses of aposymbiotic Astrangia poculata to divergent thermal challenges.

Anthropogenic climate change threatens corals globally and both high and low temperatures are known to induce coral bleaching. However, coral stress responses across wide thermal breadths remain understudied. Disentangling the role of symbiosis on the stress response in obligately symbiotic corals is challenging because this response is inherently coupled with nutritional stress. Here, we leverage aposymbiotic colonies of the facultatively symbiotic coral, Astrangia poculata, which lives naturally with and without its algal symbionts, to examine how broad thermal challenges influence coral hosts in the absence of symbiosis. A. poculata were collected from their northern range limit and thermally challenged in two independent 16-day common garden experiments (heat and cold challenge) and behavioural responses to food stimuli and genome-wide gene expression profiling (TagSeq) were performed. Both thermal challenges elicited significant reductions in polyp extension. However, there were five times as many differentially expressed genes (DEGs) under cold challenge compared to heat challenge. Despite an overall stronger response to cold challenge, there was significant overlap in DEGs between thermal challenges. We contrasted these responses to a previously identified module of genes associated with the environmental stress response (ESR) in tropical reef-building corals. Cold challenged corals exhibited a pattern consistent with more severe stressors while the heat challenge response was consistent with lower intensity stressors. Given that these responses were observed in aposymbiotic colonies, many genes previously implicated in ESRs in tropical symbiotic species may represent the coral host's stress response in or out of symbiosis.