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Marine heatwaves are starting to occur several times a decade, yet we do not understand the effect this has on corals across biological scales. This study combines tissue-, organism-, and community-level analyses to investigate the effects of a marine heatwave on reef-building corals. Adjacent conspecific pairs of coral colonies of Montipora capitata and Porites compressa that showed contrasting phenotypic responses (i.e., bleached vs. not bleached) were first identified during a marine heatwave that occurred in 2015 in Kāne’ohe Bay, Hawai‘ i. These conspecific pairs of bleaching-resistant and bleaching-susceptible colonies were sampled for histology and photographed before, during, and after a subsequent marine heatwave that occurred in 2019. Histology samples were quantified for: (i) abundance of mesenterial filaments, (ii) tissue structural integrity, (iii) clarity of epidermis, and (iv) cellular integrity (lack of necrosis/granulation) on a 1–5 scale and averaged for an overall tissue integrity score. Tissue integrity scores revealed a significant decline in overall tissue health during the 2019 heatwave relative to the months prior to the heatwave for individuals of both species, regardless of past bleaching history in 2015 or bleaching severity during the 2019 heatwave. Coral tissue integrity scores were then compared to concurrent colony bleaching severity, which revealed that tissue integrity was significantly correlated with colony bleaching severity and suggests that the stability of the symbiosis is related to host tissue health. Colony partial mortality was also quantified as the cumulative proportion of each colony that appeared dead 2.5 years following the 2019 bleaching event, and tissue integrity during the heatwave was found to be strongly predictive of the extent of partial mortality following the heatwave for M. capitata but not P. compressa, the latter of which suffered little to no mortality. Surprisingly, bleaching severity and partial mortality were not significantly correlated for either species, suggesting that tissue integrity was a better predictor of mortality than bleaching severity in M. capitata. Despite negative effects of heat stress at the tissue- and colony-level, no significant changes in coral cover were detected, indicating resilience at the community level. However, declines in tissue integrity in response to heat stress that are not accompanied by a visible bleaching response may still have long-term consequences for fitness, and this is an important area of future investigation as heat stress is commonly associated with long-term decreases in coral fecundity and growth. Our results suggest that histology is a valuable tool for revealing the harmful effects of marine heatwaves on corals before they are visually evident as bleaching, and may thus improve the predictability of ecosystem changes following climate change-driven heat stress by providing a more comprehensive assessment of coral health. 

Smartphones are ubiquitous in high schools across the US, but students rarely learn about the complex world of elements and materials beneath their shiny exteriors. Educators can bridge the gap between the abstract concept of smartphone elements and the real-world geography of their origin and impact by integrating ArcGIS geospatial software into their curriculum. A team of researchers has been working with high school teachers to infuse geospatial concepts and technologies into their teaching. One project involved a teacher using GIS to revisit his approach to teaching the periodic table: he would have his students investigate the global origins of smartphone components. This approach equipped students with essential knowledge about the materials that power their daily lives and nurtured critical thinking skills and an awareness of the environmental and ethical dimensions of technology consumption. This paper includes a description of the project and how geospatial technology was utilized, as well as a discussion on the implications and future research in this area. 

Climate change threatens symbiotic cnidarians’ survival by causing photosymbiosis breakdown in a process known as bleaching. Direct effects of temperature on cnidarian host physiology remain difficult to describe because heatwaves depress symbiont performance, leading to host stress and starvation. The symbiotic sea anemone Exaiptasia diaphana provides an opportune system to disentangle direct vs. indirect heat effects on the host, since it can survive indefinitely without symbionts. We tested the hypothesis that heat directly impairs cnidarian physiology by comparing symbiotic and aposymbiotic individuals of two laboratory subpopulations of a commonly used clonal strain of E. diaphana, CC7. We exposed anemones to a range of temperatures (ambient, +2°C, +4°C, +6°C) for 15–18 days, then measured their symbiont population densities, autotrophic carbon assimilation and translocation, photosynthesis, respiration, and host intracellular pH (pHi). Symbiotic anemones from the two subpopulations differed in size and symbiont density and exhibited distinct heat stress responses, highlighting the importance of acclimation to different laboratory conditions. Specifically, the cohort with higher initial symbiont densities experienced dose-dependent symbiont loss with increasing temperature and a corresponding decline in host photosynthate accumulation. In contrast, the cohort with lower initial symbiont densities did not lose symbionts or assimilate less photosynthate when heated, similar to the response of aposymbiotic anemones. However, anemone pHi decreased at higher temperatures regardless of cohort, symbiont presence, or photosynthate translocation, indicating that heat consistently disrupts cnidarian acid-base homeostasis independent of symbiotic status or mutualism breakdown. Thus, pH regulation may be a critical vulnerability for cnidarians in a changing climate. 

Increasingly frequent marine heatwaves are devastating coral reefs. Corals that survive these extreme events must rapidly recover if they are to withstand subsequent events, and long-term survival in the face of rising ocean temperatures may hinge on recovery capacity and acclimatory gains in heat tolerance over an individual’s lifespan. To better understand coral recovery trajectories in the face of successive marine heatwaves, we monitored the responses of bleaching-susceptible and bleaching-resistant individuals of two dominant coral species in Hawai’i,Montipora capitataandPorites compressa, over a decade that included three marine heatwaves. Bleaching-susceptible colonies ofP. compressaexhibited beneficial acclimatization to heat stress (i.e., less bleaching) following repeat heatwaves, becoming indistinguishable from bleaching-resistant conspecifics during the third heatwave. In contrast, bleaching-susceptibleM. capitatarepeatedly bleached during all successive heatwaves and exhibited seasonal bleaching and substantial mortality for up to 3 y following the third heatwave. Encouragingly, bleaching-resistant individuals of both species remained pigmented across the entire time series; however, pigmentation did not necessarily indicate physiological resilience. Specifically,M. capitatadisplayed incremental yet only partial recovery of symbiont density and tissue biomass across both bleaching phenotypes up to 35 mo following the third heatwave as well as considerable partial mortality. Conversely,P. compressaappeared to recover across most physiological metrics within 2 y and experienced little to no mortality. Ultimately, these results indicate that even some visually robust, bleaching-resistant corals can carry the cost of recurring heatwaves over multiple years, leading to divergent recovery trajectories that may erode coral reef resilience in the Anthropocene. 

Increasing ocean temperatures threaten coral reefs globally, but corals residing in habitats that experience high thermal variability are thought to be better adapted to survive climate-induced heat stress. Here, we used long-term ecological observations and in situ temperature data from Heron Island, southern Great Barrier Reef to investigate how temperature dynamics within various thermally variable vs. thermally stable reef habitats change during a marine heatwave and the resulting consequences for coral community survival. During the heatwave, thermally variable habitats experienced larger surges in daily mean and maxima temperatures compared to stable sites, including extreme hourly incursions up to 36.5 °C. The disproportionate increase in heat stress in variable habitats corresponded with greater subsequent declines in hard coral cover, including a three-times greater decline within the thermally variable Reef Flat (70%) and Deep Lagoon (83%) than within thermally stable habitats along sheltered and exposed areas of the reef slope (0.3–19%). Interestingly, the thermally variable Reef Crest experienced comparatively small declines (26%), avoiding the most severe tidal ponding and resultant heat stress likely due to proximity to the open ocean equating to lower seawater residence times, greater mixing, and/or increased flow. These results highlight that variable thermal regimes, and any acclimatization or adaptation to elevated temperatures that may lead to, do not necessarily equate to protection against bleaching and mortality during marine heatwaves. Instead, thermally stable habitats that have greater seawater exchange with the open ocean may offer the most protection to corals during the severe marine heatwaves that accompany a changing climate. 

ABSTRACT Ocean acidification (OA) resulting from anthropogenic CO2 emissions is impairing the reproduction of marine organisms. While parental exposure to OA can protect offspring via carryover effects, this phenomenon is poorly understood in many marine invertebrate taxa. Here, we examined how parental exposure to acidified (pH 7.40) versus ambient (pH 7.72) seawater influenced reproduction and offspring performance across six gametogenic cycles (13 weeks) in the estuarine sea anemone Nematostella vectensis. Females exhibited reproductive plasticity under acidic conditions, releasing significantly fewer but larger eggs compared to ambient females after 4 weeks of exposure, and larger eggs in two of the four following spawning cycles despite recovering fecundity, indicating long-term acclimatization and greater investment in eggs. Males showed no changes in fecundity under acidic conditions but produced a greater percentage of sperm with high mitochondrial membrane potential (MMP; a proxy for elevated motility), which corresponded with higher fertilization rates relative to ambient males. Finally, parental exposure to acidic conditions did not significantly influence offspring development rates, respiration rates, or heat tolerance. Overall, this study demonstrates that parental exposure to acidic conditions impacts gamete production and physiology but not offspring performance in N. vectensis, suggesting that increased investment in individual gametes may promote fitness. 

Abstract Marine heatwaves are occurring more frequently as climate change intensifies, resulting in global mass coral bleaching events several times per decade. Despite the time between marine heatwaves decreasing, there is evidence that reef-building corals can develop increased bleaching resistance across repetitive marine heatwaves. This phenomenon of acclimatization via environmental memory may be an important strategy to ensure coral persistence; however, we still understand very little about the apparent acclimatization or, conversely, sensitization (i.e., stress accumulation or weakening) of reef-building corals to consecutive heatwaves and its implications for the trajectory and resilience of coral reefs. Here, we highlight that not only will some corals become stress hardened via marine heatwaves, but many other individuals will suffer sensitization during repeat heatwaves that further exacerbates their stress response during repeat events and depresses fitness. Under current and predicted climate change, it is necessary to gain a better understanding of the acclimatization vs. sensitization trajectories of different species and individuals on the reef, as well as identify whether changes in bleaching susceptibility relates to physiological acclimatization, trade-offs with other biological processes, and ultimately coral persistence in the Anthropocene. 

Ocean acidification is a growing threat to coral growth and the accretion of coral reef ecosystems. Corals inhabiting environments that already endure extreme diel pCO 2 fluctuations, however, may represent acidification-resilient populations capable of persisting on future reefs. Here, we examined the impact of pCO 2 variability on the reef-building coral Pocillopora damicornis originating from reefs with contrasting environmental histories (variable reef flat versus stable reef slope) following reciprocal exposure to stable (218 ± 9) or variable (911 ± 31) diel pCO 2 amplitude (μtam) in aquaria over eight weeks. Endosymbiont density, photosynthesis and net calcification rates differed between origins but not treatment, whereas primary calcification (extension) was affected by both origin and acclimatization to novel pCO 2 conditions. At the cellular level, corals from the variable reef flat exhibited less intracellular pH (pHi) acidosis and faster pHi recovery rates in response to experimental acidification stress (pH 7.40) than corals originating from the stable reef slope, suggesting environmental memory gained from lifelong exposure to pCO 2 variability led to an improved ability to regulate acid–base homeostasis. These results highlight the role of cellular processes in maintaining acidification resilience and suggest that prior exposure to pCO 2 variability may promote more acidification-resilient coral populations in a changing climate. 

Abstract 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 bleaching have been well documented, the consequences of heat stress for bleaching‐resistant individuals are not well understood. In addition, much remains to be learned about how heat stress affects cellular‐level processes that may be overlooked at the organismal level, yet are crucial for coral performance in the short term and ecological success over the long term. Here we compared the physiological and cellular responses of bleaching‐resistant and bleaching‐susceptible corals throughout the 2019 marine heatwave in Hawai'i, a repeat bleaching event that occurred 4 years after the previous regional event. Relative bleaching susceptibility within species was consistent between the two bleaching events, yet corals of both resistant and susceptible phenotypes exhibited pronounced metabolic depression during the heatwave. At the cellular level, bleaching‐susceptible corals had lower intracellular pH than bleaching‐resistant corals at the peak of bleaching for both symbiont‐hosting and symbiont‐free cells, indicating greater disruption of acid–base homeostasis in bleaching‐susceptible individuals. Notably, cells from both phenotypes were unable to compensate for experimentally induced cellular acidosis, indicating that acid–base regulation was significantly impaired at the cellular level even in bleaching‐resistant corals and in cells containing symbionts. Thermal disturbances may thus have substantial ecological consequences, as even small reallocations in energy budgets to maintain homeostasis during stress can negatively affect fitness. These results suggest concern is warranted for corals coping with ocean acidification alongside ocean warming, as the feedback between temperature stress and acid–base regulation may further exacerbate the physiological effects of climate change.