Abstract
Corals are threatened worldwide due to rapidly warming oceans associated with anthropogenic climate change. A growing body of work has explored how corals respond to heat stress, however, responses of corals across the full range of their thermal breadth are rarely explored even though winter colds are also known to induce coral stress. Here, we leverage the temperate stony coral, Astrangia poculata, which naturally exhibits a facultative symbiosis with Symbiodiniaceae, to explicitly examine how thermal challenges influence coral hosts in isolation from their symbionts. Aposymbiotic A. poculata were collected from Woods Hole, MA, the northern range limit for this species. Corals were thermally challenged in two independent common garden experiments (Heat challenge: 31°C, 10 days; Cold challenge: 6°C, 16 days) to determine the effects of divergent thermal stressors. Behavioural responses to food stimuli were monitored throughout the thermal challenges and genome-wide gene expression profiling (TagSeq) was used to characterize molecular underpinnings of the coral’s response to stress in its aposymbiotic state. Behaviourally, both thermal challenges induced polyp retraction and colonies failed to respond to food stimuli. Surprisingly, seven times as many genes were differentially expressed under cold challenge heat challenge. Despite a greater magnitude of response under cold challenge, significant similarities in gene expression were detected across the two thermal challenge experiments, with many of the most responsive genes having been previously implicated in coral heat stress response. Given that our data were generated from aposymbiotic colonies, we hypothesize that these genes previously linked to heat stress are more likely indicative of a generalized stress response. Overall this work highlights the unique insights facultatively symbiotic corals offer for deciphering the stress response of the coral host in the absence of bleaching.