The rapid loss of reef-building corals owing to ocean warming is driving the development of interventions such as coral propagation and restoration, selective breeding and assisted gene flow. Many of these interventions target naturally heat-tolerant individuals to boost climate resilience, but the challenges of quickly and reliably quantifying heat tolerance and identifying thermotolerant individuals have hampered implementation. Here, we used coral bleaching automated stress systems to perform rapid, standardized heat tolerance assays on 229 colonies of Acropora cervicornis across six coral nurseries spanning Florida's Coral Reef, USA. Analysis of heat stress dose–response curves for each colony revealed a broad range in thermal tolerance among individuals (approx. 2.5°C range in F v /F m ED50), with highly reproducible rankings across independent tests ( r = 0.76). Most phenotypic variation occurred within nurseries rather than between them, pointing to a potentially dominant role of fixed genetic effects in setting thermal tolerance and widespread distribution of tolerant individuals throughout the population. The identification of tolerant individuals provides immediately actionable information to optimize nursery and restoration programmes for Florida's threatened staghorn corals. This work further provides a blueprint for future efforts to identify and source thermally tolerant corals for conservation interventions worldwide.

Global climate change is altering coral reef ecosystems. Notably, marine heatwaves are producing widespread coral bleaching events that are increasing in frequency, with projections for annual bleaching events on reefs worldwide by mid‐century. Responses of corals to elevated seawater temperatures are modulated by abiotic factors (e.g. environmental regimes) and dominant Symbiodiniaceae endosymbionts that can shift coral traits and contribute to physiological legacy effects on future response trajectories. It is critical, therefore, to characterize shifting physiological and cellular states driven by these factors and evaluate their influence on in situ bleaching (and recovery) events. We use back‐to‐back bleaching events (2014, 2015) in Hawai'i to characterize the cellular and organismal phenotypes of Montipora capitata corals dominated by heat‐sensitive Cladocopium or heat‐tolerant Durusdinium Symbiodiniaceae at two reef sites. Despite fewer degree heating weeks in the first‐bleaching event relative to the second (7 vs. 10), M. capitata bleaching severity was greater [bleached cover: ~70% (2014) vs. 50% (2015)] and environmental history (site effects) on coral phenotypes were more pronounced. Symbiodiniaceae affected bleaching responses, but immunity and antioxidant activity was similar in all corals, despite differences in bleaching phenotypes. We demonstrate that repeat bleaching triggers cellular responses that shift holobiont multivariate phenotypes. These perturbed multivariate phenotypes constitute physiological legacies, which set corals on trajectories (positive and/or negative) that influence future coral performance. Collectively, our data support the need for greater tracking of stress response in a multivariate context to better understand the biology and ecology of corals in the Anthropocene. A free Plain Language Summary can be found within the Supporting Information of this article.

Phys. Rev. D

Global climate change is altering coral reef ecosystems. Notably, marine heat waves are producing widespread coral bleaching events with increased frequency, with projections for annual bleaching events on many reefs by mid-century. The response of corals to elevated seawater temperatures is, however, influenced by a combination of environmental legacies, the biology of the host, and the resident symbiont community, each of which have the capacity to modulate resistance and resilience to environmental stress. It is critical, therefore, to evaluate the potential for shifting physiological and molecular baselines driven by these factors in back-toback in situ bleaching (and recovery) events given the potential for mortality of corals in response to episodic bleaching episodes and increased ocean warming. Here, we use the regional bleaching events of 2014 and 2015 in the Hawaiian Islands and subsequent recovery periods to test the hypothesis that coral multivariate responses (physiotypes) differed in back-to-back bleaching events, modulated by both environmental histories and symbiotic partnerships (Symbiodiniaceaea). Bleaching severity was greater in the first-bleaching event, and concomitantly, environmental history effects were more pronounced. Melanin, an immune cytotoxic response, provided an initial defense during the first-bleaching event and primed antioxidant activity, which peaked in the second-bleaching event. While magnitude of bleaching differed, immune response patterns were shared among corals harboring heat-sensitive and heattolerant Symbiodiniaceae. This supports a pattern of increased constitutive immunity in corals during repeat bleaching events, demonstrated by greater specialized enzymes (catalase, superoxide dismutase, peroxidase) and attenuated melanin synthesis. This study demonstrates bleaching events trigger cumulative and interactive physiotypes driven by environmental legacy and cellular memory. Quantifying baseline and altered coral physiotypes (multivariate