Heterotrophy has been shown to mitigate coral–algal dysbiosis (coral bleaching) under heat challenge, but the molecular mechanisms underlying this phenomenon remain largely unexplored. Here, we quantified coral physiology and gene expression of fragments from 13 genotypes of symbiotic Oculina arbuscula after a 28-d feeding experiment under (1) fed, ambient (24 °C); (2) unfed, ambient; (3) fed, heated (ramp to 33 °C); and (4) unfed, heated treatments. We monitored algal photosynthetic efficiency throughout the experiment, and after 28 d, profiled coral and algal carbohydrate and protein reserves, coral gene expression, algal cell densities, and chlorophyll-a and chlorophyll-c2 pigments. Contrary to previous findings, heterotrophy did little to mitigate the impacts of temperature, and we observed few significant differences in physiology between fed and unfed corals under heat challenge. Our results suggest the duration and intensity of starvation and thermal challenge play meaningful roles in coral energetics and stress response; future work exploring these thresholds and how they may impact coral responses under changing climate is urgently needed. Gene expression patterns under heat challenge in fed and unfed corals showed gene ontology enrichment patterns consistent with classic signatures of the environmental stress response. While gene expression differences between fed and unfed corals under heat challenge were subtle: Unfed, heated corals uniquely upregulated genes associated with cell cycle functions, an indication that starvation may induce the previously described, milder “type B” coral stress response. Future studies interested in disentangling the influence of heterotrophy on coral bleaching would benefit from leveraging the facultative species studied here, but using the coral in its symbiotic and aposymbiotic states.
Broadcast-spawning scleractinian corals undergo mass spawning annually, the timing of which is determined by a series of environmental and chemical cues. Flower Garden Banks reefs (FGB, NW Gulf of Mexico) have high coral cover compared to the rest of the Tropical Western Atlantic and exhibit highly synchronous coral spawning, although spawning observations have been largely limited to shallower depths (17-24 m). Using a remotely operated vehicle (ROV), we report observations of mesophotic (39-42 m) broadcast spawning of three major reef-building coral species at East FGB. We observed spawning of 20 Montastraea cavernosa colonies, an order of magnitude more than has been previously reported for this population at mesophotic depths. Spawning times of the three target species (M. cavernosa, Orbicella franksi, and Pseudodiploria strigosa) ranged from 45 min prior to sunset up to 1 h, 33 min after sunset, consistent with shallow conspecifics. We place these observations in the context of other spawning reports of the same species at the FGB and throughout the Tropical Western Atlantic. In addition, the ROV was directed to collect eight genetically unique fragments of gravid Orbicella faveolata and employed a novel gamete collection device designed to capture gametes from actively spawning colonies in situ. These data and collection techniques highlight the benefits of using ROV technology for coral-spawning research, particularly for mesophotic coral reef ecosystems, where gamete release occurs across time ranges exceeding limits imposed by recreational diving. Finally, our observations of overlap in the timing of spawning along the shallow to mesophotic continuum at the FGB suggest-at least for the corals monitored here-that the potential for cross fertilization across depths exists.
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