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Ocean warming is causing global coral bleaching events to increase in frequency, resulting in widespread coral mortality and disrupting the function of coral reef ecosystems. However, even during mass bleaching events, many corals resist bleaching despite exposure to abnormally high temperatures. While the physiological effects of

Urgent action is needed to prevent the demise of coral reefs as the climate crisis leads to an increasingly warmer and more acidic ocean. Propagating climate change–resistant corals to restore degraded reefs is one promising strategy; however, empirical evidence is needed to determine whether stress resistance is affected by transplantation beyond a coral’s native reef. Here, we assessed the performance of bleaching-resistant individuals of two coral species following reciprocal transplantation between reefs with distinct pH, salinity, dissolved oxygen, sedimentation, and flow dynamics to determine whether heat stress response is altered following coral exposure to novel physicochemical conditions in situ. Critically, transplantation had no influence on coral heat stress responses, indicating that this trait was relatively fixed. In contrast, growth was highly plastic, and native performance was not predictive of performance in the novel environment. Coral metabolic rates and overall fitness were higher at the reef with higher flow, salinity, sedimentation, and diel fluctuations of pH and dissolved oxygen, and did not differ between native and cross-transplanted corals, indicating acclimatization via plasticity within just 3 mo. Conversely, cross-transplants at the second reef had higher fitness than native corals, thus increasing the fitness potential of the recipient population. This experiment was conducted during a nonbleaching year, so the potential benefits to recipient population fitness are likely enhanced during bleaching years. In summary, this study demonstrates that outplanting bleaching-resistant corals is a promising tool for elevating the resistance of coral populations to ocean warming.

18Marine heat waves instigated by anthropogenic climate change are causing increasingly frequent 19 and severe coral bleaching events that often lead to widespread coral mortality. While 20 community-wide increases in coral mortality following bleaching events have been documented 21 on reefs around the world, the ecological consequences for conspecific individual colonies 22 exhibiting contrasting phenotypes during thermal stress (e.g. bleached vs. not bleached) are not 23 well understood. Here we describe the ecological outcomes of colonies of the two dominant reef-24 building coral species in Kāne ohe Bay, Hawai i, Montipora capitata and Porites compressa, 25 that exhibited either a bleaching susceptible phenotype (bleached) or resistant phenotype (non-26 bleached) following the second of two consecutive coral bleaching events in Hawai i in 2015. 27Conspecific pairs of adjacent bleaching susceptible vs. resistant corals were tagged on patch 28 reefs in two regions of Kāne ohe Bay with different seawater residence times and terrestrial 29 influence. The ecological consequences (symbiont recovery and mortality) were monitored for 30 two years following the peak of the bleaching event. Bleaching susceptible corals suffered higher 31 partial mortality than bleaching resistant corals of the same species in the first 6 months 32 following thermal stress. Surprisingly, P. compressa had greater resilience following bleaching 33 (faster pigment recovery and lower post-bleaching mortality) than M. capitata, despite having 34 less resistance to bleaching (higher bleaching prevalence and severity). These differences 35 indicate that bleaching susceptibility of a species is not always a good predictor of mortality 36 following a bleaching event. By tracking the fate of individual colonies of resistant and 37 susceptible phenotypes, contrasting ecological consequences of thermal stress were revealed that 38 were undetectable at the population level. Furthermore, this approach revealed individuals that 39 underwent particularly rapid recovery from mortality, including some colonies over a meter in 40 diameter that recovered all live tissue cover from >60% partial mortality within just one year. 41These coral pairs continue to be maintained and monitored in the field, serving as a "living 42 library" for future investigations on the ecology and physiology of coral bleaching. 43 44 45 46 102 2008; Bahr et al., 2017; Hughes et al., 2017). Kāne ohe Bay, Hawai i, located on the northeast 103 coast of O ahu, is an opportune system for investigating this question of how intraspecific 104 variability in coral responses to thermal bleaching events driven by climate change influence 105 coral survival. The two dominant reef-building coral species in the bay, Montipora capitata and 106Porites compressa, both exhibit differences in thermal performance within and between species 107 during bleaching (Grottoli et al., 2006; Cunning et al., 2016; Wall et al., 2019). Differences in 108 5 symbiont associations and nutritional plasticity a...

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Predicting the sensitivity of reef-building corals to disturbance, including bleaching, requires an understanding of physiological responses to stressors, which may be limited by destructive sampling and the capacity of common methodologies to characterize early life history stages. We developed a new methodology using laser scanning confocal microscopy (LSCM) to measure and track the physiological condition of corals. In a thermal stress experiment, we used LSCM to track coral condition during bleaching in adults and juveniles of two species, Montipora capitata and Pocillopora acuta. Depth of fluorescence in coral tissues provides a proxy measure of tissue thickness, whereas Symbiodiniaceae population fluorescence relates to both population density and chlorophyll a content. In response to thermal stress, there were significant shifts in tissue thickness and Symbiodiniaceae fluorescence with differences between life stages. This method is particularly well suited for detecting shifts in physiological condition of living corals in laboratory studies, especially in small juvenile colonies.

Reef restoration via direct outplanting of sexually propagated juvenile corals is a key strategy in preserving coral reef ecosystem function in the face of global and local stressors (e.g. ocean warming). To advance our capacity to scale and maximize the efficiency of restoration initiatives, we examined how abiotic conditions (i.e. larval rearing temperature, substrate condition, light intensity, and flow rate) interact to enhance post-settlement survival and growth of sexually propagated juvenile Montipora capitata. Larvae were reared at 3 temperatures (high: 28.9°C, ambient: 27.2°C, low: 24.5°C) for 72 h during larval development, and were subsequently settled on aragonite plugs conditioned in seawater (1 or 10 wk) and raised in different light and flow regimes. These juvenile corals underwent a natural bleaching event in Kāne‘ohe Bay, O‘ahu, Hawai‘i (USA), in summer 2019, allowing us to opportunistically measure bleaching response in addition to survivorship and growth. This study demonstrates how leveraging light and flow can increase the survivorship and growth of juvenile M. capitata. In contrast, larval preconditioning and substrate conditioning had little overall effect on survivorship, growth, or bleaching response. Importantly, there was no optimal combination of abiotic conditions that maximized survival and growth in addition to bleaching tolerances. This study highlights the ability to tailor sexual reproduction for specific restoration goals by addressing knowledge gaps and incorporating practices that could improve resilience in propagated stocks.

Reef‐building corals form nutritional symbioses with endosymbiotic dinoflagellates (Symbiodiniaceae), a relationship that facilitates the ecological success of coral reefs. These symbionts are mostly acquired anew each generation from the environment during early life stages (“horizontal transmission”). Symbiodiniaceae species exhibit trait variation that directly impacts the health and performance of the coral host under ocean warming. Here, we test the capacity for larvae of a horizontally transmitting coral, Acropora tenuis, to establish symbioses with Symbiodiniaceae species in four genera that have varying thermal thresholds (the common symbiont genera, Cladocopium and Durusdinium, and the less common Fugacium and Gerakladium). Over a 2‐week period in January 2018, a series of both no‐choice and four‐way choice experiments were conducted at three temperatures (27, 30, and 31°C). Symbiont acquisition success and cell proliferation were measured in individual larvae. Larvae successfully acquired and maintained symbionts of all four genera in no‐choice experiments, and >80% of larvae were infected with at least three genera when offered a four‐way choice. Unexpectedly, Gerakladium symbionts increased in dominance over time, and at high temperatures outcompeted Durusdinium, which is regarded as thermally tolerant. Although Fugacium displayed the highest thermal tolerance in culture and reached similar cell densities to the other three symbionts at 31°C, it remained a background symbiont in choice experiments, suggesting host preference for other symbiont species. Larval survivorship at 1 week was highest in larvae associated with Gerakladium and Fugacium symbionts at 27 and 30°C, however at 31°C, mortality was similar for all treatments. We hypothesize that symbionts that are currently rare in corals (e.g., Gerakladium) may become more common and widespread in early life stages under climate warming. Uptake of such symbionts may function as a survival strategy in the wild, and has implications for reef restoration practices that use sexually produced coral stock.