Coral Reefs

Coral reefs are facing increasingly devasting impacts from ocean warming and acidification due to anthropogenic climate change. In addition to reducing greenhouse gas emissions, potential solutions have focused either on reducing light stress during heating, or the potential for identifying or engineering "super corals". These studies, however, have tended to focus primarily on the bleaching response of corals, and assume that corals that bleach earlier in a thermal event are more likely to die. Here, we explore how survival, potential bleaching, and coral skeletal growth (as branch extension and densification) varies for conspecifics collected from distinctive reef zones at Heron Island on the Southern Great Barrier Reef. A series of reciprocal transplantation experiments were undertaken using the dominant reef building coral (Acropora formosa) between the highly variable reef flat and the less variable reef slope environments. Coral colonies originating from the reef flat had higher rates of survival and thicker tissues but reduced rates of calcification than conspecifics originating from the reef slope. The energetics of both populations however benefited from greater light intensity offered in the shallows. Reef flat origin corals moved to the lower light intensity of reef slope reduced protein density and calcification rates. For A. formosa, genetic difference, or long-term entrainment to a highly variable environment, appeared to promote coral survival at the expense of calcification. The response divorces coral resilience from carbonate coral reef resilience, a response that was further exacerbated by reductions in irradiance. As we begin to discuss interventions necessitated by the CO2 that has already been released to the atmosphere, we need to prioritise our focus on the properties that maintain valuable carbonate ecosystems. Rapid and dense calcification by corals such as branching Acropora is essential to the ability of carbonate coral reefs to rebound following disturbances events, but may be the first property that is sacrificed to enable coral genet survival under stress.

Caribbean octocorals have not suffered the decades long decline in abundance that has plagued reef-building scleractinian corals. Their success and the formation of octocoral forests has been attributed to their continuing recruitment to reef habitats. Assessing the processes controlling recruitment is essential to understanding the success of octocorals and predicting their future. Benthic grazers on coral reefs can facilitate the growth and recruitment of corals by reducing the abundance of competitive algal turfs and macroalgae or hinder corals through predation of coral tissue and recruits. We assessed the effects of grazing by fishes and the sea urchin Diadema antillarum and mesofaunal predation on octocoral recruitment in a series of manipulative experiments using varying grazer/predator exclusion and inclusion conditions in in situ and ex situ experiments. Exposure to fish and urchin grazing significantly reduced survival and recruitment of single-polyp octocorals, while turf-associated mesofauna did not significantly affect neither recruitment nor survival. We also found a positive relationship between octocoral recruitment and turf algae, a potential related response to the deleterious effect of grazing exposure. These data suggest that grazers and predators mediate the mortality bottleneck characteristic of recruitment. Thus, the declines in the abundance of grazing fishes and urchins throughout the Caribbean may have contributed to the increase in abundance of octocorals in the Caribbean, concurrent with the loss of scleractinians.

Identifying processes that promote coral reef recovery and resilience is crucial as ocean warming becomes more frequent and severe. Sexual reproduction is essential for the replenishment of coral populations and maintenance of genetic diversity; however, the ability for corals to reproduce may be impaired by marine heatwaves that cause coral bleaching. In 2014 and 2015, the Hawaiian Islands experienced coral bleaching with differential bleaching susceptibility in the species Montipora capitata, a dominant reef-building coral in the region. We tested the hypothesis that coral bleaching resistance enhances reproductive capacity and offspring performance by examining the reproductive biology of colonies that bleached and recovered (B) and colonies that did not bleach (NB) in 2015 in the subsequent spawning seasons. The proportion of colonies that spawned was higher in 2016 than in 2017. Regardless of parental bleaching history, we found eggs with higher abnormality and bundles with fewer eggs in 2016 than 2017. While reproductive output was similar between B and NB colonies in 2016, survivorship of offspring that year were significantly influenced by the parental bleaching history (egg donor x sperm donor: B x B, B x NB, NB x B, and NB x NB). Offspring produced by NB egg donors had the highest survivorship, while offspring from previously bleached colonies had the lowest survivorship, highlighting the negative effects of bleaching on parental investment and offspring performance. While sexual reproduction continues in M. capitata post-bleaching, gametes are differentially impacted by recovery time following a bleaching event and by parental bleaching resistance. Our results demonstrate the importance of identifying bleaching resistant individuals during and after heating events. This study further highlights the significance of maternal effects through potential egg provisioning for offspring survivorship and provides a baseline for human-assisted intervention (i.e., selective breeding) to mitigate the effects of climate change on coral reefs.

Coral reef fish communities can be affected by natural disturbances such as cyclones and coral bleaching. It is not yet understood how long it takes these communities to recover from such extreme events, particularly when they occur repeatedly. To investigate this, we conducted fish surveys repeatedly between 2011 and 2022 at Lizard Island on the Great Barrier Reef in Australia. We focused on two reef sites, Mermaid Cove and Northern Horseshoe, both of which were damaged by a large-scale coral bleaching event in 2016 and 2017, as well as two cyclones that occurred in 2014 and 2015 (the cyclones hit Mermaid Cove but not Northern Horseshoe). Between 2016 and 2017, both reef sites saw a decrease in the total fish abundance of about 68 % and across most functional groups (carnivores, corallivores, herbivores, and omnivores). Despite the two sites showing different decline and recovery patterns, they both showed an improvement in fish abundance and across the majority of functional groups at both sites by 2022. The recovery reached similar numbers as those documented in the fish census data collected before the disturbances occurred. Our findings provide a case study highlighting how fish community resilience can vary on small local scales, with potential recovery if conditions are favourable over several years.

With the increased frequency of marine heatwaves and related coral mass mortality events, it is imperative that we improve our understanding of coral reef recovery processes including how benthic community compositions can change over time. Coral reef benthic communities are influenced by many abiotic factors, but on most reefs local anthropogenic disturbances overshadow these factors thus obscuring their influence. Here, we leverage a dataset from a coral reef with very minimal local anthropogenic disturbance - the uninhabited southern coast of the worlds largest atoll (Kiritimati) - to assess spatial variation in benthic community composition three years after the mass coral mortality event driven by the 2015-2016 El Nino. Across forereef sites ranging from 7 to 22 m, scleractinian coral cover remained very low (6.9 {+/-} 0.4 % SE) while soft coral cover was < 1%. Coral cover was highest at deep sites (18-22 m compared to sites at 7-10 m depth) and exposed locations, where stress-tolerant corals likely made up a larger proportion of the coral community before the mass mortality event. Higher cover of crustose coralline algae at shallow exposed sites, fleshy macroalgae at deep sites, and turf algae at exposed locations were consistent with taxa-specific preferences for light, wave action, and sedimentation. The abundances of the most common genera of juvenile coral (Acropora and Pocillopora) were still low (< 1 juvenile colony per genus per site) and varied only with depth. These findings demonstrate how variation in pre-mortality coral composition can lead to differences in post-mortality benthic communities on low disturbance coral reefs.

The octocoral Paramuricea clavata is an ecosystem architect of the Mediterranean temperate reefs that is currently threatened by episodic mass mortality events related to global warming. Local average thermal regimes nor recent thermal history have been shown to play a significant role in population thermotolerance in this species. The microbiome, however, may play an active role in the thermal stress susceptibility of corals, potentially holding the answer as to why corals show differential sensitivity to heat-stress. To investigate this, the prokaryotic and eukaryotic microbiome of P. clavata collected from around the Mediterranean was characterized before experimental heat-stress to determine if its microbial composition influences the thermal response of the holobiont. We found that the prokaryotic community was not informative in predicting the thermal susceptibility of P. clavata. On the other hand, members of P. clavatas microeukaryotic community were significantly correlated with thermal stress sensitivity. Syndiniales from the Dino-Group I Clade 1 were significantly enriched in thermally resistant corals, while the apicomplexan corallicolids were significantly enriched in thermally susceptible corals. Corallicolids are associated with 70% of coral genera around the world, yet the ecological role of this general anthozoan symbiont has yet to be determined. We hypothesize that P. clavata mortality following heat-stress may be caused by a shift from apparent commensalism to parasitism in the corallicolid-coral host relationship driven by the added stress. Our results show the potential importance of corallicolids and the rest of the microeukaryotic community of corals to understanding thermal stress response in corals and provides a useful tool to guide conservation efforts and future research into coral-associated microeukaryotes.

Relationships between ecologically important traits are increasingly important for the future of coral reefs, which are declining globally due to a variety of stressors. Corals that resist and recover from bleaching will be selected under future climates, which may negatively or positively impact a range of associated traits important for the structure and function of contemporary and future reefs. Using standardized assays, we investigated growth under ambient conditions, bleaching resistance and recovery after bleaching in a model population of 60 Montipora capitata colonies harboring diverse symbiont communities. We found significantly higher trait variance within Durusdinium-dominated colonies, highlighting the interaction of host and symbiont. We also show that symbiont community impacted thermal tolerance and survivorship during the thermal stress assay, but not recovery or growth. There was also a positive relationship between thermal tolerance and change in surface area and thermal tolerance and recovery from equivalent amounts of stress. These results demonstrate limited tradeoffs to thermal tolerance and suggest that bleaching tolerant corals in this model system are suited to recover from stress and maintain high growth rates under ambient conditions, providing insight into the adaptive capacity of thermally tolerant corals in the Anthropocene.

Orbicella faveolata, commonly known as the mountainous star coral, is a dominant reef-building species in the Caribbean, but populations have suffered sharp declines since the 1980s due to repeated bleaching and disease-driven mortality. Prior research has shown that inshore adult O. faveolata populations in the Florida Keys are able to maintain high coral cover and recover from bleaching faster than their offshore counterparts. However, whether this origin-specific variation in thermal resistance is heritable remains unclear. To address this knowledge gap, we produced purebred and hybrid larval crosses from O. faveolata gametes collected at two distinct reefs in the Upper Florida Keys, a nearshore site (Cheeca Rocks, CR) and an offshore site (Horseshoe Reef, HR), in two different years (2019, 2021). We then subjected these aposymbiotic larvae to severe (36 {degrees}C) and moderate (32 {degrees}C) heat challenges to quantify their thermal tolerance. Contrary to our expectation based on patterns of adult thermal tolerance, HR purebred larvae survived better and exhibited gene expression profiles that were less driven by stress response under elevated temperature compared to purebred CR and hybrid larvae. One potential explanation could be compromised reproductive output of CR adult colonies due to repeated summer bleaching events in 2018 and 2019, as gametes originating from CR in 2019 contained less storage lipids than those from HR. These findings provide an important counter-example to the current selective breeding paradigm, that more tolerant parents will yield more tolerant offspring, and highlight the importance of adopting a holistic approach when evaluating larval quality for conservation and restoration purposes.

Natural bleaching events provide an opportunity to examine how local scale environmental variation influences bleaching severity and recovery. During the 2020 marine heatwave, we documented widespread and severe coral bleaching (75 - 98% of coral cover) throughout the Keppel Islands in the Southern inshore Great Barrier Reef. Acropora, Pocillopora and Porites were the most severely affected genera, while Montipora was comparatively less susceptible. Site-specific heat-exposure metrics were not correlated with Acropora bleaching severity, but recovery was faster at sites that experienced lower heat exposure. Despite severe bleaching and exposure to accumulated heat that often results in coral mortality (degree heating weeks [~] 4 - 8), cover remained stable. Approximately 94% of fate-tracked Acropora millepora colonies survived, perhaps owing to reduced irradiance stress from high turbidity, heterotrophic feeding, and large tidal flows that can increase mass transfer. Severe bleaching followed by rapid recovery, and the continuing dominance of Acropora populations in the Keppel Islands is indicative of high resilience. These coral communities have survived an 0.8 {degrees}C increase in average temperatures over the last 150 years. However, recovery following the 2020 bleaching was driven by the easing of thermal stress, which may challenge their recovery potential under further warming. Open Research StatementData are not yet provided but are being compiled. Upon acceptance data will be archived on GitHub.

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.

Extreme climatic events are reshaping ecosystems worldwide as individual organisms vary markedly in their ability to withstand these disturbances. Deciphering patterns of persistence on local scales is therefore critical for predicting biodiversity trajectories under intensifying climate extremes. In this study, we examined variation in thermal stress responses among individuals of the coral Stylophora pistillata species complex during a heatwave at Heron Island Reef, Australia. More than half of the focal coral colonies died on the reef, and survival of coral fragments maintained under ex situ common thermal stress conditions was significantly correlated with the survival of their source colony. This demonstrates that survival differences result largely from biological factors rather than differential thermal exposure across reef habitats. Under common garden conditions, we observed striking differences in bleaching severity and survival times among three sympatric cryptic taxa and their highly host-specific symbiont community. Within the most locally common taxon, corals from historically warmer and more seasonally variable reef habitats seem more susceptible to bleaching, contrary to expectations. Together, these results reveal how biological differences among cryptic taxa and among individuals can shape coral responses during a heatwave and advance our understanding of coral vulnerability in a rapidly warming world.

Disease outbreaks have caused mass mortality and threaten the persistence of numerous vulnerable species, including reef-building corals. Spatiotemporal variations in disease prevalence suggest environmental conditions exacerbate disease susceptibility, potentially by compromising host immunity. Understanding how these variations influence disease dynamics and immunity may enable improved prediction of future disease outbreaks. We combined field observations and statistical modeling to identify the environmental drivers of stony coral tissue loss disease (SCTLD) in Florida. We then performed environmental manipulation experiments to confirm the effects of identified factors on immunity in the massive coral Montastraea cavernosa. SCTLD susceptibility was influenced most by the interaction between temperature and chlorophyll-a concentration (nutrients proxy) the month prior to disease survey, as well as the interaction between chlorophyll-a and three-month mean PAR. SCTLD prevalence was highest when temperatures were low (<30 {degrees}C) and chlorophyll-a concentration exceeded [~]6 mg m-3 and/or when PAR and chlorophyll-a concentrations were high. SCTLD severity had a negative relationship with temperature, a colony had a higher probability of dying during the SCTLD outbreak when temperatures stayed below 31.08 {degrees}C, a finding in contrast to previous coral disease studies. In the laboratory, coral fragments were exposed to temperature and nutrient manipulation, then immune challenged. Heat stress largely drove suppression of baseline immunity but increased production of some antioxidants, suggesting host stress. Fragments exposed to moderate ammonium concentration induced the strongest immune responses compared to those grown under high or no ammonium conditions. Combined with modeling results, these findings support long-standing hypotheses that coral disease susceptibility is at least, in part, modulated by environmentally induced changes in immunity. Overall, our results enhance the understanding of the impact of environmental conditions on SCTLD outbreaks and on general coral immunity. They also highlight the possibility of increased disease severity as anthropogenic pressure on marine environments increases.

In many polygynous species, males face stronger intrasexual competition, higher energetic demands, and lower survival than females, especially under resource limitation or environmental stress. Such sex-specific vulnerabilities are expected to intensify with climate change. Yet, in sequentially hermaphroditic systems, where individuals change sex during their lifetime, how sex and sex change shape survival remains largely unexplored. We studied sex-specific survival and growth in the haremic protogynous cleaner wrasse Labroides dimidiatus across eight reefs around Lizard Island, Great Barrier Reef. We tracked a total of 731 adult fish (individually recognizable through marking or idiosyncratic color patterns) over two years. This period included the 2024 El Nino-Southern Oscillation (ENSO), which caused a temporary 1-degree increase in water temperature, severe coral bleaching, and coral mortality at Lizard Island. Contrary to expectations from dioecious systems, terminal-phase males exhibited higher survival than initial-phase females under both normal and in particular ENSO conditions. While male mortality was not affected, female mortality more than doubled during the event, indicating greater physiological or energetic vulnerability. A partial explanation for the overall higher female mortality is their generally faster growth rate, which declined in both sexes during the ENSO event. Our findings challenge existing assumptions of male-biased mortality in polygynous species and highlight that sex and sex change fundamentally shape demographic responses to climate extremes.

O_LIGlobal climate change is altering coral reef ecosystems. Notably, marine heat waves are producing widespread coral bleaching events that are increasing in frequency, with projections for annual bleaching events on reefs worldwide by mid-century. C_LIO_LIThe response of corals to elevated seawater temperatures can be modulated by abiotic factors at site of origin and dominant endosymbiont type, which can result in a shift in multiple coral traits and drive physiological legacy effects that influence the trajectory of reef corals under subsequent thermal stress events. It is critical, therefore, to evaluate the potential for shifting physiological and cellular baselines driven by these factors in in situ bleaching (and recovery) events. Here, we use the back-to-back regional bleaching events of 2014 and 2015 in the Hawaiian Islands and subsequent recovery periods to test the hypothesis that coral multivariate trait space (here termed physiotype, sensu (Van Straalen, 2003) shift in multiple bleaching events, modulated by both environmental histories and symbiotic partnerships (Symbiodiniaceae). C_LIO_LIDespite fewer degree heating weeks in the first-bleaching event relative to the second (7 vs. 10), bleaching severity in a dominant reef building coral on Hawaiian reefs, Montipora capitata, was greater (~70% vs. 50% bleached cover) and differences due to environmental history (reef site) were more pronounced. Melanin, an immune cytotoxic response, provided an initial defense during the first event, potentially priming antioxidant activity, which peaked in the second-bleaching event (i.e., a legacy effect). While magnitude of bleaching differed, immune response patterns were shared among corals harboring heat-sensitive and heat-tolerant Symbiodiniaceae. This supports a pattern of increased constitutive immunity in corals resulting from repeat bleaching events, with greater specialized enzymes (catalase, peroxidase, superoxide dismutase) and attenuated melanin synthesis. C_LIO_LIThis study demonstrates bleaching events have implications for reef corals beyond shaping their ecological assemblages. These events can change the magnitude and/or identity of response variables contributing to physiotype, thus generating physiological legacies carried over into the future. Quantifying baseline coral physiotypes and tracking their shifts will be critical to understanding and forecasting the effects of increased bleaching frequency on coral biology and ecology in the Anthropocene. C_LI

Massive corals, like some Porites species, are key reef-builders and provide critical habitat on coral reefs. Porites often exhibit high tolerance to stressors, like warming from climate change, although the drivers of these patterns are unclear. Using high-throughput sequencing, we examined whether associations with Symbiodiniaceae underpin this tolerance. Symbiodiniaceae diversity and community composition within two massive species (Porites lutea and P. lobata), whose identities were verified through morphometric examinations, were examined across 200 km of Ningaloo World Heritage Reef, Australia. Contrary to previous studies showing low diversity of high-fidelity symbionts, we found significant variability within Cladocopium, which differed significantly across host species, reef sites, and temperature metrics. Locations with broader temperature ranges and a greater number of extreme accumulated heat stress events (measured as [&ge;] 8 Degree Heating Weeks) exhibited significantly higher prevalence and abundance of thermotolerant Cladocopium C116. Community structure also varied significantly by the number of DHW events per location, where a low level of disturbance (1-3 events) resulted in a relatively lower community diversity compared to sites with 3-6 and 6-9 events. The southernmost and coolest site (in terms of Maximum Monthly Mean) harboured the greatest Symbiodiniaceae community diversity, whereas the northernmost and warmest site harboured the greatest Symbiodiniaceae sequence diversity. These lines of evidence suggest that Porites may hold greater capacity to vary, and by extension modify, their symbiont community diversity than previously thought. This flexibility could contribute to the genus Porites long-term evolutionary success and relative resilience to global marine heatwaves.

The change in state of Caribbean coral reefs over the last 40 years has been characterized by phase shifts from scleractinian coral cover to macroalgal cover, the loss of structural complexity and a decline in biodiversity. Not only do scientists want to understand these changes, but also predict the future of coral reefs and their capacity for resilience. In particular, the loss of herbivory, due to declines in parrotfish and the sea urchin Diadema antillarum, has been implicated in many studies as a proximate cause of the coral to macroalgal phase shift. However, reports of the particular role of these putative herbivores have varied, with some studies claiming a causal role for parrotfish, others for Diadema and still others suggesting no such relationships. Often these studies just examined one response measure of coral reef biodiversity. In this paper, I report the relationship between parrotfish and Diadema to many metrics of reef organization surveyed simultaneously in the same transects in reefs outside and within the Marine Protected Area (MPA) of Grand Cayman, an island that has been affected by increasing tourism over the last 30 years. The magnitudes of the various measures of reef diversity reported here are consistent with those reported elsewhere. The relationships among those measures are consistent with those reported in some prior studies and inconsistent with others, reflecting the variation in responses documented in prior studies. The presence of sea urchins was associated with survey sites having higher levels of coral cover, lower levels of macroalgae cover, and lower densities of parrotfish than survey sites without sea urchins. Moreover, parrotfish abundance was associated with a decrease in coral cover and little relationship to macroalgae cover. Neither coral cover nor macroalgae cover was different in sites within the MPA compared to sites outside the MPA. I argue that the combination of site-specific local stressors and their interaction with global stressors makes it unlikely that any one island or even regional reef system could serve as an exemplar for Caribbean-wide reef degradation. Moreover, it is difficult to assess the potential for reef resilience in the face of the ongoing assaults from increasing tourism pressures and global climate change.

Understanding the ontogeny and reproductive biology of reef-building organisms can shed light on patterns of population biology and community structure. This knowledge is particularly important for Caribbean octocorals, which seem to be more resilient to long-term environmental change than scleractinian corals and provide some of the same ecological services. We monitored the development of the black sea rod Plexaura homomalla, a common, widely distributed octocoral on shallow Caribbean reefs, from eggs to 3-polyp colonies over the course of 73 days. In aquaria on St John, U.S. Virgin Islands, gametes were released in spawning events three to six days after the July full moon. Cleavage started 3 hours after fertilization and was holoblastic, equal, and radial. Embryos were positively buoyant until becoming planulae. Planulae were competent after 4 days. Symbiodiniaceae began infecting polyps at around 8 days post fertilization. Development was typical for Caribbean octocorals, except for the occurrence of a novel form of asexual reproduction in octocorals: polyembryony. Fragmentation of embryos during development may represent a temporally varied tradeoff between number and size of propagules, in which large eggs have higher fertilization rates followed by polyembryony, which maximizes the number of surviving recruits by generating more, albeit smaller, larvae. Polyembryony may contribute to the success of some gorgonians on Caribbean reefs as other anthozoans are in decline.

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 are rarely explored. In addition, it is difficult to disentangle the role of symbiosis on the stress response of obligately symbiotic coral hosts. Here, we leverage aposymbiotic colonies of the facultatively symbiotic coral, Astrangia poculata, which lives naturally with and without its algal symbiont, to examine how broad thermal challenges influence coral hosts. 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 behavioral responses to food stimuli and genome-wide gene expression profiling (TagSeq) were performed. Both thermal challenges elicited significant reductions in polyp extension. However, five times as many genes were differentially expressed under cold challenge compared to heat challenge. Despite more genes responding to cold challenge, there was significant overlap in which genes were differentially expressed across thermal challenges. These convergently responding genes (CRGs) were associated with downregulation of motor functions and nematocysts while others were consistent with stress responses previously identified in tropical corals. The fact that these responses were observed in aposymbiotic colonies highlights that many genes previously implicated in stress responses in symbiotic species may simply represent the corals stress response in or out of symbiosis.

The marine heatwave in the summer of 2023 was the most severe on record for Floridas Coral Reef, with unprecedented water temperatures and cumulative thermal stress precipitating near 100% coral bleaching levels. An existing SCTLD coral fate-tracking program assessed over 4200 coral colonies across five offshore and four inshore reef sites approximately every two months, allowing for analyses of bleaching-related mortality and diseases during and after the marine heatwave. Across the vast majority of assessed corals, including multiple sites and species, there was no partial or full mortality as a result of the 2023 bleaching event. The two sites that did experience substantial bleaching-related mortality were those experiencing the highest levels of cumulative thermal stress. However, the substantial acute mortality at one of them occurred at relatively low levels of cumulative stress, suggesting death was the result of exceeding thermal maxima. At the two sites with notable mortality, 43% and 24% of all monitored corals died, but mortality varied among species. Brain corals fared worse than boulder corals, with Pseudodiploria strigosa the most heavily impacted species. The health status of corals before the bleaching event had little impact on whether they experienced disease or bleaching-related mortality during the event. At three sites, we observed unusual lesions on Orbicella faveolata colonies shortly after color returned to the corals; the lesions were only observed for a few months but on some colonies led to substantial tissue loss. Though not part of the coral monitoring program, we also observed substantial losses and local extinctions of Acroporid corals at most sites, as well as probable local extinctions of octocorals at three of the four inshore reefs. Though most reef-building corals came through the 2023 event with no mortality, continually rising temperatures are likely to make these temperature regimes more common and widespread. We encourage future research on what the unusual O. faveolata lesions are, and why the brain and boulder corals fared differently at highly-impacted sites. Our results also provide perspective on how restoration strategies, particularly those focused on species likely to die under current and future climate regimes, should consider shifting focus to species likely to survive. Finally, these results highlight the importance of this type of monitoring, with a focus on fate-tracking individuals through disturbance events, including a large number of individuals of multiple species across a geographic range and multiple habitats.

The symbiotic relationship between coral and its endosymbiotic algae, Symbiodiniaceae, greatly influences the hosts potential to withstand environmental stress. To date, the effects of climate change on this relationship has primarily focused on adult corals. Uncovering the effects of environmental stress on the establishment and development of this symbiosis in early life stages is critical for predicting how corals may respond to climate change. To determine the impacts of future climate projections on the establishment of symbionts in juvenile corals, ITS2 amplicon sequencing of single coral juveniles was applied to Goniastrea retiformis and Acropora millepora before and after exposure to three climate conditions of varying temperature and pCO2 levels (current and RCP8.5 in 2050 and 2100). Compared to ambient conditions, juvenile corals experienced shuffling in the relative abundance of Cladocopium (C1m, reduction) to Durusdinium (D1 and D1a, increase) over time. We calculated a novel risk metric incorporating functional redundancy and likelihood of impact on host physiology to identify the loss of D1a as a low risk to the coral compared to the loss of "higher risk" taxa like D1 and C1m. Although the increase in stress tolerant Durusdinium under future warming was encouraging for A. millepora, by 2100, G. retiformis communities displayed signs of symbiosis de-regulation, suggesting this acclimatory mechanism may have species-specific thresholds. These results emphasize the need for understanding of long-term effects of climate change induced stress on coral juveniles and their potential for increased acclimation to heat tolerance through changes in symbiosis. Originality StatementHere we assessed changes in the uptake and establishment of Symbiodiniaceae in the early lifehistory stages of two coral species under future climate scenarios. Our study represents the first such assessment of future climate change projections (increased temperature and pCO2) influencing Symbiodiniaceae acquisition and specifically shows a community structure dominated by the stress tolerant genus Durusdinium. We also develop a novel risk metric that includes taxonomic function and redundancy to estimate the impact of symbiont taxa changes on coral physiology. Through the risk metric, we relate the stress-induced changes in symbiont community structure to the likelihood of functional loss to better understand the extent to which these changes may lead to a decrease in coral health.