Frontiers in Marine Science

Global carbon emissions and associated increase in ocean temperatures are understood to be the main driving force in the degradation of coral reefs. Elevated temperatures impact various life stages of scleractinian corals, from the free-floating planulae of brooding corals to older, sexually viable individuals. With global warming, questions have arisen over whether organismal adaptation will be enough to keep up with the pace of environmental change. Researchers have pursued investigations of whether or not rapid acclimatization, through transgenerational plasticity, can help protect populations until genetic adaptation occurs. Acclimatization in corals has been widely studied in all life stages of corals, with the important exception of recently settled juveniles. In this study, I built upon past research by exposing adult Pocillopora damicornis colonies to elevated (28.5{degrees}C) or ambient (25.5{degrees}C) temperatures and examining the settlement ability and growth of their planulae ex situ. Juveniles from preconditioned parents fared better in higher temperatures compared to their naive counterparts. Lunar timing of planula release between treatments peaked at different times in the lunar cycle. Peak planula release occurred on lunar day 23 for prestressed corals and on lunar day 7 for corals from ambient temperature seawaters. While future projects should follow up on these preliminary trials with in situ experiments to assess this phenomenon in the field, this study represents an important step in understanding how corals may be able to acclimatize and eventually adapt to climate change.

Ocean warming and acidification are among the biggest threats to the persistence of coral reefs. Organismal stress tolerance thresholds are life stage specific, can vary across levels of biological organization, and also depend on natural environmental variability. Here, we exposed the early life stages of Pocillopora acuta in K[a]ne ohe Bay, Hawai i, USA, a common reef-building coral throughout the Pacific, to projected ocean warming and acidification scenarios. We measured ecological, physiological, biomineralization, and molecular responses across the critical transition from larvae to newly settled recruits following 6 days of exposure to diel fluctuations in temperature and pH in Control (26.8-27.9{degrees}C, 7.82-7.96 pHTotal), Mid (28.4-29.5{degrees}C, 7.65-7.79 pHTotal) and High conditions (30.2-31.5{degrees}C, 7.44-7.59 pHTotal). We found that P. acuta early life stages are capable of survival, settlement, and calcification under all scenarios. The High conditions, however, caused a significant reduction in survival and settlement capacity, with changes in the skeletal fiber deposition patterns. In contrast to a limited impact on the expression of biomineralization genes, the dominant transcriptomic response to the High conditions relative to the two other treatments included depressed metabolism, reduced ATP production and increased activity of DNA damage-repair processes. Collectively, our findings indicate that corals living in environments with large diurnal fluctuations in seawater temperature and pH, such as K[a]ne ohe Bay, can tolerate exposure to moderate projected increased temperature and reduced pH. However, under more severe environmental conditions significant negative effects on coral cellular metabolism and overall organismal survival jeopardize species fitness and recruitment.

Understanding how marine species tolerate acidified conditions is critical for predicting biological responses to ocean change. A recent one-year experiment (Long 2026) found that juvenile snow crab (Chionoecetes opilio) maintain growth and molting under acidification (pH 7.8, 7.5), and survival begins to decline only after [~]250 days under severe acidification (pH 7.5). In this companion study, we characterized whole-transcriptome responses after 8 hours and 88 days of exposure to identify molecular mechanisms underlying short-term tolerance and chronic effects of ocean acidification. The immediate transcriptional response involved strong activation of genes associated with mitochondrial metabolism and biogenesis, protein homeostasis, cuticle maintenance, and immune modulation, processes shared between moderate and severe treatments but of greater magnitude under severe acidification. After 88 days, expression patterns diverged, revealing sustained upregulation of stress- and damage-mitigation pathways in the severe treatment (pH 7.5) compared to the moderate treatment (pH 7.8). These findings indicate that crabs in severe acidification are likely to experience chronic OA stress that precedes outward physiological effects, and provides a mechanistic basis for delayed mortality. We further highlight potential early indicators of chronic acidification stress in snow crab, among which a gene likely coding for carbonic anhydrase 7 (CA7, GWK47_031192) appears to be the most promising biomarker. Summary StatementJuvenile snow crabs tolerate ocean acidification through flexible gene expression, but prolonged exposure reveals hidden cellular stress that helps explain delayed mortality.

The mutualistic symbiosis between stony corals and unicellular algae of the family Symbiodiniaceae forms the base of coral reef ecosystems. However, anthropogenic stressors, such as rising seawater temperatures, cause a breakdown of the coral-algal symbiosis, so-called coral bleaching, which leads to mass mortalities and a rapid loss of coral reefs. To functionally disassemble the coral-algal symbiosis, corals have been artificially rendered aposymbiotic using temperature stress, DCMU, or menthol. As menthol has proven to be an efficient and gentle bleaching agent, we tested four menthol treatments with six commonly investigated stony coral species. The overarching aim was to establish a broadly efficient bleaching protocol as a guide for future investigations. Menthol-induced bleaching was traced with chlorophyll fluorescence and tissue color analyses over time and confirmed as final symbiont cell density two weeks after the last day of menthol treatment. Here we found that the coral species varied greatly in their menthol bleaching tolerance, underlining the importance of establishing bespoke bleaching approaches. Acropora muricata and Stylophora pistillata were efficiently bleached within two days of menthol exposure with symbiont cell reductions of 94 % to 98 %. For Galaxea fascicularis, Montipora digitata and Porites cylindrica, six days of menthol exposure proved most successful in reducing symbiont density by 92 to 97 %. While these coral species suffered no mortality, fragments of Pocillopora verrucosa died or suffered severe necrosis in most of the protocols, making the tested menthol treatments unsuitable for this species. We demonstrate that repeated menthol treatment at low concentrations renders most coral species aposymbiotic within few days without visual or physiological damage. Our study, therefore, provides a guideline for efficient and customized application of menthol bleaching treatments for future coral symbiosis research.

We test a newly developed instrument prototype which utilizes time-resolved chlorophyll-a fluorescence techniques and fluctuating light to characterize Symbiodiniaceae functional traits across seven different coral species under cultivation as part of ongoing restoration efforts in the Florida Keys. While traditional chlorophyll-a fluorescence techniques only provide a handful of algal biometrics, the system and protocol we have developed generates > 1000 dynamic measurements in a short ([~]11 min) time frame. Resulting high-content algal biometric data revealed distinct phenotypes, which broadly corresponded to clade-level Symbiodiniaceae designations determined using quantitative PCR. Next, algal biometric data from Acropora cervicornis (10 genotypes) and A. palmata (5 genotypes) coral fragments was correlated with bleaching response metrics collected after a two month-long exposure to high temperature. A network analysis identified 1973 correlations (Spearman R > 0.5) between algal biometrics and various bleaching response metrics. These identified biomarkers of thermal stress were then utilized to train a predictive model, and when tested against the same A. cervicornis and A. palmata coral fragments, yielded high correlation (R = 0.92) with measured thermal response (reductions in absorbance by chlorophyll-a). When applied to all seven coral species, the model ranked fragments dominated by Cladocopium or Breviolum symbionts as more bleaching susceptible than corals harboring thermally tolerant symbionts (Durusdinium). While direct testing of bleaching predictions on novel genotypes is still needed, our device and modeling pipeline may help broaden the scalability of existing approaches for determining thermal tolerance in reef corals. Our instrument prototype and analytical pipeline aligns with recent coral restoration assessments that call for the development of novel tools for improving scalability of coral restoration programs.

Coral bleaching, the breakdown of the symbiosis between the coral host and endosymbiotic microalgae, is the main cause of widespread coral reef degradation. Current methods for assessing coral health based on visual appearance, such as the use of color reference cards, are limited by subjective human color perception and low resolution. Digital photography with RGB (Red, Green, Blue) color channel analyses offers a fast, non-invasive, and standardized alternative to estimate physiological parameters. However, the link between coral color and physiological parameters during bleaching may vary depending on the type of stressor. While such approaches are extensively used in plant studies, their application in estimating Symbiodiniaceae cell density and chlorophyll content in corals requires further attention. In this study, we analyzed the correlation between Symbiodiniaceae cell density and chlorophyll content across three coral species (Acropora muricata, Pocillopora verrucosa, and Stylophora pistillata) with 19 color indices derived from the RGB channels currently established as predictors of chlorophyll content in plants. Corals were exposed to three bleaching conditions (acute short-term and chronic long-term heat stress and menthol bleaching) to identify the best color indices for assessing coral health through image analysis. We found that the Red index had the strongest linear correlation with symbiont cell density and chlorophyll content across species (R2 up to 0.97), so that relative changes in this color index can be directly interpreted as corresponding changes in tissue parameters. To train a model that predicts symbiont densities of a distinct sample set using the Red index, we found 10 to 12 samples to be sufficient to achieve an accuracy of > 95 % of the models trained on the full datasets. This research contributes to improved image analysis as a reliable and non-invasive tool for monitoring, by providing guidelines for a systematic use of RGB data to interpret coral health.

The accelerating loss of coral reefs worldwide due to anthropogenic climate change has led to a myriad of studies aimed at understanding the basis of coral resilience to support reef conservation. Here, we integrate physiological measurements with proteomic and metabolomic profiles to examine species-specific responses to increased temperature in three sympatric reef-building corals from the Great Barrier Reef: Acropora hyacinthus, Porites lobata, and Stylophora pistillata. We find species-specific stress response strategies with A. hyacinthus, a thermally sensitive species, exhibiting rapid decline in endosymbiont physiology, coupled with a one-third reduction in protein abundance. In contrast, P. lobata displayed a delayed physiological response to stress and a muted proteome response, suggesting greater resilience. S. pistillata initially showed minor shifts in the proteome followed by colony "bail-out". Overall, we observed markedly different responses in most biochemical pathways in the three coral species. Nonetheless, some known biomarkers of stress, including heat-shock proteins, showed conserved responses to thermal stress with differences in temporal abundance reflecting bleaching resistance. Our results underscore the species-specific nature of coral responses to thermal stress and highlight proteomic signatures associated with symbiosis breakdown, offering mechanistic insights into coral bleaching susceptibility and resilience.

Scleractinian corals form the foundation of coral reefs by secreting skeletons of calcium carbonate. Their intracellular algal symbionts (Symbiodiniaceae) translocate a large proportion of photosynthate to the coral host, which is required to maintain high rates of calcification. Global warming is causing dissociation of coral host and algal symbiont, visibly presented as coral bleaching. Despite decades of study, the precise mechanisms of coral bleaching remain unknown. Separating the thermal stress response of the coral from the algal symbiont is key to understanding bleaching in tropical corals. The facultatively symbiotic northern star coral, Astrangia poculata, naturally occurs as both symbiotic and aposymbiotic (lacking algal symbionts) polyps - sometimes on the same coral colony. Thus, it is possible to separate the heat stress response of the coral host alone from the coral in symbiosis with its symbiont Breviolum psygmophilum. Using replicate symbiotic and aposymbiotic ramets of A. poculata, we conducted a chronic heat stress experiment to increase our understanding of the cellular mechanisms resulting in coral bleaching. Sustained high temperature stress resulted in photosynthetic dysfunction in B. psygmophilum, including a decline in maximum photosynthesis rate, maximum photochemical efficiency, and the absorbance peak of chlorophyll a. Interestingly, the metabolic rates of symbiotic and aposymbiotic corals were differentially impacted. RNAseq analysis revealed more differentially expressed genes between heat-stressed and control aposymbiotic colonies than heat-stressed and control symbiotic colonies. Notably, aposymbiotic colonies increased the expression of inflammation-associated genes such as nitric oxide synthases. Unexpectedly, the largest transcriptional response was observed between heat-stressed and control B. psygmophilum, including genes involved in photosynthesis, response to oxidative stress, and meiosis. Thus, it appears that the algal symbiont suppresses the immune response of the host, potentially increasing the vulnerability of the host to pathogens. The A. poculata-B. psygmophilum symbiosis provides a tractable model system for investigating thermal stress and immune challenge in scleractinian corals.

The identification of bleaching tolerant traits among individual corals is a major focus for many restoration and conservation initiatives but often relies on large scale or high-throughput experimental manipulations which may not be accessible to many front-line restoration practitioners. Here we evaluate a machine learning technique to generate a predictive model which estimates bleaching severity using non-destructive chlorophyll-a fluorescence photophysiological metrics measured with a low-cost and open access bio-optical tool. First, a four-week long thermal bleaching experiment was performed on 156 genotypes of Acropora palmata at a land-based restoration facility. Resulting bleaching responses (percent change in Fv/Fm or Absorbance) significantly differed across the four distinct phenotypes generated via a photophysiology-based dendrogram, indicating strong concordance between fluorescence-based photophysiological metrics and future bleaching severity. Next, these correlations were used to train and then test a Random Forest algorithm-based model using a bootstrap resampling technique. Correlation between predicted and actual bleaching responses in test corals was significant (p < 0.0001) and increased with the number of corals used in model training (Peak average R2 values of 0.42 and 0.33 for Fv/Fm and absorbance, respectively). Strong concordance between photophysiology-based phenotypes and future bleaching severity may provide a highly scalable means for assessing reef corals.

This study examined the long-term impacts of coral bleaching on the reproduction and physiology of Montipora capitata, a dominant reef-building coral in Hawaii. We monitored bleached and non-bleached colonies during and after a natural coral bleaching event in 2014 and analyzed reproductive traits and transcriptomic signatures eight months later. Our study shows that non-bleached and bleached colonies successfully produced gametes. Colonies that bleached had smaller oocytes, and development was slower than in colonies that did not bleach. Corals with different vulnerabilities to bleaching exhibited distinct transcriptomic responses eight months after a bleaching event. Those more prone to bleaching showed suppression of transcripts associated with sperm motility, calcification, and immunity. We found distinct transcriptomic signatures between fringing and patch reefs, suggesting local adaptation and/or acclimatization. To conserve coral reefs and better understand how they will be affected by future heat stress, we need to track which colonies survive and examine how their physiological and reproductive processes are impacted in the short- and long-term. This is critical as consecutive bleaching events become more frequent, and corals have less time to recover. Our study provides valuable molecular and reproductive data that can be used for conservation and management purposes. This information can help us identify signs of coral vulnerability and resilience to bleaching, project how future bleaching events will affect coral reproduction, determine which traits are most at risk, and assess which sites are more likely to be compromised.

Pocillopora acuta is a hermatypic coral with a worldwide distribution and a strong ecological importance. Anthropogenic disturbances and global warming threaten it. Thermal stress can induce coral bleaching, a phenomenon in which the mutualistic symbiosis between the coral polyps host and its endosymbiotic unicellular algae is disrupted, and can lead to the death of entire colonies. Previous works have shown that soma clonal colonies display different levels of survival depending on the environmental conditions they previously faced. Epigenetic mechanisms are good candidates to explain this phenomenon. The clonal nature of a colony and the possibility of generating genetically identical colonies through propagation make corals an attractive model to study the impact of the environment on the epigenome. However, until now, no work had been published on the P. acuta epigenome. One of the main problems is caused by the intracellular location of Symbiodinium, which makes it complicated to isolate coral chromatin free of contamination by endiosymbiotic biological material. Here, (i) we describe a simple method to purify P. acuta chromatin, (ii) we provide the first description of a coral methylome, with a mosaic pattern of cytosine methylation principally in a CpG context (4% of all CpG), and (iii) we show that P. acuta, but not all corals, has an unusual chromatin structure, and displays histone H3 clipping.

Coral cover on tropical reefs has declined during the last three decades due to the combined effects of climate change, destructive fishing, pollution, and land use change. Drastic reductions in greenhouse gas emissions combined with effective coastal management and conservation strategies are essential to slow this decline. Innovative approaches, such as selective breeding for adaptive traits combined with large-scale sexual propagation, are being developed with the aim of pre-adapting reefs to increased ocean warming. However, there are still major gaps in our understanding of the technical and methodological constraints to producing corals for such restoration interventions. Here we propose a framework for selectively breeding corals and rearing them from eggs to 2.5-year old colonies using the coral Acropora digitifera as a model species. We present methods for choosing colonies for selective crossing, enhancing early survivorship in ex situ and in situ nurseries, and outplanting and monitoring colonies on natal reefs. We used a short-term (7-day) temperature stress assay to select parental colonies based on heat tolerance of excised branches. From six parental colonies, we produced 12 distinct crosses, and compared survivorship and growth of colonies transferred to in situ nurseries or outplanted to the reef at different ages. We demonstrate that selectively breeding and rearing coral colonies is technically feasible at small scales and could be upscaled as part of restorative assisted evolution initiatives. Nonetheless, there are still challenges to overcome before selective breeding can be implemented as a viable conservation tool, especially at the post-settlement and outplanting phases. Although interdisciplinary approaches will be needed to overcome many of the challenges identified in this study, selective breeding has the potential to be a viable tool within reef managers toolbox to support the persistence of selected reefs in the face of climate change.

Reef-building corals are sessile marine organisms that inhabit a wide range of light habitats along depth gradients. As coral biology is often studied in the context of global change and changing temperatures, knowledge gaps persist in our understanding of the molecular and cellular pathways involved in the responses to other factors than temperature, such as light intensity, which decreases exponentially in the water column and gradually changes the environment. To fill this gap, we tested the response of the Hawaiian rice coral Montipora capitata to decreased light intensity in a field experiment in K[a]neohe Bay, Oahu, Hawaii, using Data-Independent Acquisition (DIA) proteomics. There was a significant effect of light intensity on both the coral and zooxanthellae proteomes. In the M. capitata host, 69 proteins differed significantly in abundance between light levels after two years. The 50 proteins identified as significantly more abundant in the control condition were mostly involved in mRNA and RNA processing, pointing toward a positive correlation between metabolic activity, growth rates and increased light levels. The 19 proteins identified as significantly more abundant in the shade treatment were associated with calcium transport and with the structure of key cellular components, such as cell membrane and cytoskeleton. By contrast, zooxanthellae showed only minor changes in protein abundances, with photosynthesis proteins more abundant in the shade treatment and enzymes involved in fatty acid metabolism more abundant in the control treatment. Overall, these findings establish a baseline for our understanding of the cellular and metabolic processes driving Montipora capitatas acclimatization potential to different light intensities.

The symbiosis between corals of the order Scleractinia and dinoflagellates of the family Symbiodiniaceae is sensitive to environmental stress. The oxidative bleaching hypothesis posits that extreme temperatures lead to accumulation of photobiont-derived reactive oxygen species ROS, which exacerbates the coral environmental stress response (ESR). To understand how photosymbiosis modulates coral ESRs, these responses must be explored in hosts in and out of symbiosis. We leveraged the facultatively symbiotic coral Astrangia poculata, which offers an opportunity to uncouple the ESR across its two symbiotic states (symbiotic, aposymbiotic). Colonies of both symbiotic states were exposed to three temperature treatments for 15 days: i) control (static 18{degrees}C), ii) heat challenge (increasing from 18 to 32{degrees}C), and iii) cold challenge (decreasing from 18 to 6{degrees}C) after which host gene expression was profiled. Cold challenged corals elicited widespread differential expression, however, there were no differences between symbiotic states. In contrast, symbiotic colonies exhibited greater gene expression plasticity under heat challenge, including enrichment of cell cycle pathways involved in controlling photobiont growth. Counter to the oxidative bleaching hypothesis, this plasticity did not include signatures of stress, and rather a dampened ESR under heat challenge was observed, suggesting that photobionts reduce the hosts ESR under elevated temperatures in A. poculata.

Coral reefs are iconic examples of climate change impacts because climate-induced heat stress causes the breakdown of the coral-algal symbiosis leading to a spectacular loss of color, termed coral bleaching. To examine the fine-scale dynamics of this process, we re-sampled 600 individually marked Montipora capitata colonies from across K[a]ne ohe Bay, Hawaii and compared the algal symbiont composition before and after the 2019 bleaching event. The relative proportion of the heat-tolerant symbiont Durusdinium in corals increased in most parts of the bay following the bleaching event. Despite this widespread increase in abundance of Durusdinium, the overall algal symbiont community composition was largely unchanged, and hydrodynamically defined regions of the bay retained their distinct pre-bleaching compositions. Furthermore, depth and temperature variability were the most significant drivers of Symbiodiniaceae community composition by site regardless of bleaching intensity or change in relative proportion of Durusdinium. Our results suggest that the plasticity of symbiont composition in corals may be constrained to adaptively match the long-term environmental conditions surrounding the holobiont, independent of an individual corals stress and bleaching response.

We employed Fast Repetition Rate fluorometry for high-resolution mapping of marine phytoplankton photophysiology and primary productivity in the Lancaster Sound and Barrow Strait regions of the Canadian Arctic Archipelago in the summer of 2019. Continuous ship-board analysis of chlorophyll a variable fluorescence demonstrated relatively low photochemical efficiency over most of the cruise-track, with the exception of localized regions within Barrow Strait where there was increased vertical mixing and proximity to land-based nutrient sources. Along the full transect, we observed strong non-photochemical quenching of chlorophyll fluorescence, with relaxation times longer than the 5-minute period used for dark acclimation. Such long-term quenching effects complicate continuous underway acquisition of fluorescence amplitude-based estimates of photosynthetic electron transport rates, which rely on dark acclimation of samples. As an alternative, we employed a new algorithm to derive electron transport rates based on analysis of fluorescence relaxation kinetics, which does not require dark acclimation. Direct comparison of kinetics- and amplitude-based electron transport rate measurements demonstrated kinetic-based estimates were, on average, 2-fold higher than amplitude-based values. The magnitude of decoupling between the two electron transport rate estimates increased in association with photophysiological diagnostics of nutrient stress. Discrepancies between electron transport rate estimates likely resulted from the use of different photophysiological parameters to derive the kinetics- and amplitude-based algorithms, and choice of numerical model used to fit variable fluorescence curves and analyze fluorescence kinetics under actinic light. Our results highlight environmental and methodological influences on fluorescence-based productivity estimates, and prompt discussion of best-practices for future underway fluorescence-based efforts to monitor phytoplankton photosynthesis.

Corals and dinoflagellate algae form a unique mutualistic symbiosis that provides the energetic and structural foundation for shallow coral reef ecosystems. Despite the long success of this partnership in oligotrophic seas, coral reefs are in decline due to increasing threats from rising seawater temperatures and disease, both of which can lead to bleaching and mortality. In order to better understand the mechanisms that underpin this mutualism, it may be necessary to dismantle the coral-algal symbiosis. Previous studies have experimentally bleached corals using thermal stress, photosynthetic inhibitors (DCMU), and menthol. We compared lab-induced bleaching of staghorn coral Acropora cervicornis by menthol treatment to traditional thermal stress. The larger aim was to adapt existing bleaching protocols to this important coral species used in restoration as a guide for future studies. Bleaching in corals treated with menthol or exposed to elevated temperature stress (31{degrees}C) was monitored by measuring photosynthetic activity determined by Fv/Fm using pulse-amplitude modulated (PAM) fluorescence. Corals were also monitored for symbiont density and overall health using the CoralWatch Coral Health Chart card throughout the experiment. We found that A. cervicornis bleached in response to both menthol treatment and thermal stress, but menthol treatment was more effective at reducing algal symbiont photosynthetic capacity (Fv/Fm) without negatively affecting the health of the coral. Our results indicate that menthol treatment at 0.38 mM rendered staghorn coral aposymbiotic within fourteen days without any visual or physiological damage to the coral. This study provides a simple and effective menthol-bleaching treatment protocol for future studies on staghorn coral.

The decline of important reef building corals has motivated the development of habitat suitability models used to identify optimal locations for coral restoration. In the Florida Keys habitat suitability models incorporate coarse spatial data sampled over large areas, resulting in recommended outplant sites at distant locations, making it logistically difficult and expensive to access and regularly monitor. Restoration efforts to date show that outplanting success can vary widely within a limited space, necessitating improved predictive abilities of coral outplant success at high spatial resolutions within a restoration site. With the advent of Structure-from-Motion image reconstruction, fine-scale, site specific, digital terrain models can be created to support habitat suitability model development. In this study, generalized linear mixed models used extracted seafloor terrain attributes and environmental variables to identify within site locations of high Acropora cervicornis growth and healthy coral cover of long-term outplants. Percent healthy coral cover significantly decreased after two years of outplantation. The submodel of corals exclusively less than two years old was unable to identify environmental conditions associated with higher healthy cover. For all corals, outplant recommendations for higher healthy cover are in deeper waters, away from the coast, in less rough terrain, and closer to the reef edge. Model results for growth support these recommended outplant sites, in addition to concave locations near high slope relief. Finally, our results also indicate that marine heat waves, but especially marine cold waves negatively correspond with coral growth, and high wind events positively correspond with coral growth. These model results provide a basis for further endeavors in modeling endangered organismal success, which are vulnerable to minute differences in local environmental conditions.

Laboratory experiments have long been used to guide predictions of organismal stress in response to our rapidly changing climate. However, the ability to simulate real world conditions in the laboratory can be a major barrier to prediction accuracy, creating obstacles to efforts informing ecosystem conservation and management. Capitalizing on an extensive experimental literature of coral bleaching physiology, we performed a systematic review of the literature and assembled a database to identify the methods being used to measure coral bleaching in heating experiments and assess how closely heating experiments resembled marine heatwaves (MHWs) on coral reefs. Observations of the maximum photochemical yield of Photosystem II (FV/FM), though not a direct measure of bleaching, vastly outnumbered Symbiodiniaceae density and chlorophyll (g cm-2, pg cell-1) observations in the available literature, indicating the widespread misuse of FV/FM as a proxy for coral bleaching. Laboratory studies in our database used significantly higher maximum temperatures, degree heating times ([~] 1.7 x) and heating rates ([~] 7.3 x), and significantly shorter durations ([~] 1.5 x), than MHWs on coral reefs. We then asked whether exposure differences between lab and reef altered the relationship between coral bleaching and heating metrics using the example of hormesis, the biphasic dose response wherein low to moderate doses elicit some benefit, while high doses are deleterious. We fit curves on the data both with and without ecologically relevant heating metrics and found hormetic curves in some response variables were altered with the exclusion of exposures that fell outside of the bounds of MHWs on coral reefs. Differences between lab exposures and real-world MHWs were large enough to alter the relationships, indicating a high likelihood of prediction error. We recommend laboratory-based studies of coral bleaching use ecologically relevant exposures to improve our predictions of the coral physiological response to our rapidly warming oceans.

Reef-building corals owe their evolutionary success to their symbiosis with unicellular algae (Symbiodiniaceae). However, increasingly frequent heat waves lead to coral mass-bleaching events and pose a serious threat to the survival of reef ecosystems. Despite significant efforts, a mechanistic understanding of coral-algal symbiosis functioning, what leads to its breakdown and what can prevent it, remains incomplete. The main obstacles are low amenability of corals to experimental handling and, owing to its obligatory nature, the difficulties of manipulating the coral-algal association. Indeed, many studies on the symbiotic partnership are conducted on other cnidarian model organisms and their results may therefore not be fully transferable to tropical reef-building corals. Here, we identify the tropical stony coral species Galaxea fascicularis as a novel candidate coral model system. Individual polyps of this species can be separated, enabling highly replicated genotype studies, and are well suited to experimental investigation of the symbiosis as they can be easily and effectively rid of their algal symbionts (bleached). We show that bleached adult individuals can reestablish symbiosis with non-native symbionts, and we report the completion of the gametogenic cycle ex-situ, with the successful spawning in aquaria over multiple years. These achievements help overcome several of the major limitations to direct research on corals and highlight the potential of G. fascicularis as an important new model system for investigations of symbiosis functioning and manipulation.