The bacterial genus Endozoicomonas is prevalent and abundant in the microbiome of many corals, but the combination of environmental and host factors shaping its distribution remains uncertain. In this study, we used 16S rRNA gene sequencing to examine the distribution of Endozoicomonas across hard (Porites astreoides, Montastraea cavernosa, Agaricia spp.) and soft coral (Muricea spp., Gorgoniidae, Cladiellidae, Malacalcyonacea) microbiomes across a depth gradient from shallow (4.5 m) to upper mesophotic zones (36.6 m) at four sites around the Little Cayman Island, including two marine protected areas. Endozoicomonas was abundant across all coral microbiomes, with soft corals exhibiting near-total dominance (relative abundance range 0.1–99

Caribbean coral reefs have suffered severe declines over the last several decades, leading to local extirpation and functional extinction of the critically endangered species, Acropora cervicornis. In response, coral restoration has become central to conservation efforts, yet genotype-specific stress resilience remains poorly integrated into restoration design, limiting overall success. Here, we evaluated thermal performance among A. cervicornis genotypes from a coral nursery at Little Cayman Island and subsequently monitored their physiological responses in the field during a 16-month common garden outplanting experiment that spanned the 2023 marine heatwave. Thermal performance curves (TPCs) generated under controlled conditions revealed visually distinct profiles among genotypes, but modeled thermal parameters did not differ significantly, suggesting broadly similar thermal ranges. In contrast, field-based physiological and survival data revealed significant genotype-specific differences in bleaching susceptibility, disease prevalence, and mortality. One genotype (OB) exhibited the highest thermal resistance, maintaining photosynthetic efficiency despite substantial endosymbiont loss, but also showed elevated disease susceptibility, suggesting a trade-off between tolerance traits. These results indicate that laboratory-derived TPCs may underestimate ecologically relevant variation in stress tolerance and highlight the need to maintain diverse genotypes in restoration programs. However, the limited genetic diversity within the A. cervicornis population at Little Cayman Island likely constrains its adaptive potential, leaving the future of both restoration success and species persistence in this region uncertain.

Coral reef ecosystems are experiencing significant global declines, with shallow-water environments experiencing the most severe disturbances. Here, we investigated benthic community composition and photo-physiology responses of selected Scleratinian corals along a depth gradient (5–45 m) at three reef sites in the Gulf of Aqaba/Eilat. Using in situ benthic imagery and Fluorescence Induction and Relaxation (FIRe) fluorometer, we quantified functional group cover and assessed photosynthetic performance across the depth gradient. Benthic community structure was strongly depth-dependent, with both live biotic cover and scleractinian coral species diversity peaking at mid-depths alongside significant species turnover across the gradient. Depth-generalist genera ( Stylophora and Porites ) were dominant and exhibited photo-acclimation across the depth gradient, whereas depth-specialists ( Pocillopora and Leptoseris ) displayed depth-restricted distributions. However, despite some traits aligning with light availability, a significant overlap in multidimensional photophysiological space suggests that depth-related patterns in coral abundance are not primarily constrained by limitations in photophysiological acclimation. Instead, the broad photo-acclimation capacity observed across taxa suggests that other ecological processes, such as competition or trophic strategy, may play a greater role in structuring coral assemblages with depth in this location, with potential implications for vertical redistribution under changing environmental conditions.

Coral reefs are increasingly threatened by marine heatwaves, which drive widespread coral bleaching and mortality. Mesophotic coral ecosystems (MCEs) have been proposed as potential thermal refuges due to their greater depth and relative isolation from surface temperature extremes. Yet their resilience to extreme heat events remains uncertain, with location specific conclusions, thus requiring further studies. Here, we investigate the effects of the 2023 marine heatwave in the Cayman Islands, which resulted in prolonged sea surface temperatures exceeding 31 °C and 17.5 DHW with extensive bleaching across shallow coral reefs. Utilizing vertical transect surveys from 10 m to 50 m, we assessed depth-related variations in bleaching prevalence and temperature profiles. Our results indicate a significant decline in bleaching with increasing depth, with a concurrent reduction in temperature. Depth-generalist species exhibited reduced bleaching at greater depths, whereas shallow-water specialists displayed severe bleaching. These findings suggest that while MCEs may provide thermal refuge for some species, their capacity to buffer against climate-driven reef degradation is species-specific. Given the increasing frequency and intensity of marine heatwaves, understanding the role of deeper reef habitats in mitigating coral loss is critical for informing conservation and management strategies. Our study underscores the importance of protecting MCEs as potential thermal refuges while emphasizing the need for continued research on species-specific thermal resilience with depth.

Mesophotic coral ecosystems (MCEs)—reefs below 30m depth—represent distinct ecological communities that are under threat from local (e.g., fishing) and global (e.g., climate change) disturbances. However, most MCEs remain unexplored, and their ecological communities are not well characterized. MCEs on remote offshore seamounts are further unexplored and provide the opportunity to assess assembly rules from comparatively less disturbed MCEs, given their remoteness from settlements with high human population densities. Here, we characterize the fish community from the remote offshore seamount of Pickle Bank in the Central Caribbean Sea, exploring differences in the fish community at a 25m depth compared to the mesophotic zone at a 45m depth. We found differences in species composition between the depths, with a significantly higher abundance of fish at mesophotic depths, while species diversity and species richness were significantly higher at the shallow sites. Species from Labridae and Scaridae dominated the biomass at the shallow sites, while species in the family Carangidae dominated biomass in the mesophotic zone. There were also differences in the community composition of trophic guilds between depths, with higher macrocarnivore biomass, macrocarnivore abundance, and omnivore abundance at deep sites compared to shallow sites. Despite the logistical challenges and limitations associated with accessing MCEs on offshore seamounts, these data provide compelling evidence to the growing body of literature documenting MCEs as unique habitats warranting further data collection to obtain a holistic understanding of shallow and mesophotic seamount ecosystems.

Coral reef ecosystems are facing severe degradation due to anthropogenic activities at both local and global scales. In response, extensive restoration efforts are underway, aiming to bolster coral cover and enhance reef fish communities to foster facilitation between fish and corals. This reciprocal relationship is anticipated to improve overall restoration efficacy and enhance coral reef resilience in the face of global warming. Here, we investigate the impact of coral restoration using out-planted Acropora cervicornis colonies attached to raised domes on the associated fish community on the isolated, well-protected reef of Little Cayman Island in the Central Caribbean. Surveys were conducted immediately preceding out-planting, five days later, and 85 days later to capture temporal changes in the fish community. After 85 days of out-planting, there were no changes in fish biomass, abundance, or species richness for the entire fish community. This pattern was consistent for selected fish functional groups. Additionally, no significant differences were observed in the fish community before outplanting, five days after out-planting, or 85 days after out-planting of restoration domes. Our results underscore the limited impact of coral restoration for influencing fish communities in the isolated and highly protected reef of Little Cayman over an 85-day period. Consequently, our findings have implications for using coral restoration as a mechanism to enhance fish populations, particularly in marginally disturbed regions where structural complexity has not been lost. Future restoration programs should therefore incorporate local knowledge of environmental history and restoration needs along with an increased data-driven understanding of the intricate interaction between fish and coral populations to be successful.

Phenotypic variability is the ability of the same species to express different phenotypes under different environmental conditions. Several coral species that exist along a broad depth distribution have been shown to differ in skeletal morphology and nutrient acquisition at different depths, which has been attributed to variability in response to differing levels of light availability. This study examined the phenotypic variability of two common depth generalist corals, Montastraea cavernosa and Porites astreoides, along a gradient from 10 to 50 m in the Cayman Islands, by examining changes in skeletal morphology, photophysiology, symbiont cell density, and chlorophyll concentration. Skeletal features of M. cavernosa were found to increase in size from 10 to 30 m, but returned to smaller sizes from 30 to 50 m, while P. astreoides skeletal features continued to increase in size from 10 to 40 m. No differences were observed in either symbiont density or chlorophyll concentration across depths for either species. However, all photophysiological parameters exhibited significant depth-dependent variations in both species, revealing adaptive strategies to different light environments. These results suggest that both species have high variability in response to depth. Patterns of skeletal morphology and photophysiology, however, suggest that M. cavernosa may be more variable in regulating photosynthetic efficiency compared to P. astreoides, which likely facilitates the broader depth distribution of this species.

The distribution of symbiotic scleractinian corals is driven, in part, by light availability, as host energy demands are partially met through translocation of photosynthate. Physiological plasticity in response to environmental conditions, such as light, enables the expansion of resilient phenotypes in the face of changing environmental conditions. Here we compared the physiology, morphology, and taxonomy of the host and endosymbionts of individual Madracis pharensis corals exposed to dramatically different light conditions based on colony orientation on the surface of a shipwreck at 30 m depth in the Bay of Haifa, Israel. We found significant differences in symbiont species consortia, photophysiology, and stable isotopes, suggesting that these corals can adjust multiple aspects of host and symbiont physiology in response to light availability. These results highlight the potential of corals to switch to a predominantly heterotrophic diet when light availability and/or symbiont densities are too low to sustain sufficient photosynthesis, which may provide resilience for corals in the face of climate change.

Mesophotic Coral Ecosystems (MCEs) represent unique ecological habitats that range from 30 to 150 m deep, harbouring phylogenetically distinct species and offering refuge for many taxa during times of environmental stress. Yet owing to inaccessibility of ecosystems at these depths, most MCEs remain unexplored, with quantifications of ecological communities in these habitats lacking across many regions. Here, using open- and closed-circuit technical diving, we quantified benthic and fish community composition at four mesophotic reef sites (45 m depth) in Grand Cayman. We show significant differences in benthic community composition over a small spatial scale driven by disparate coverage of sponges, crustose coralline algae, and sand/rubble, yet consistent patterns of macroalgal dominance representing >50% coverage at each site and low hard coral cover at an average of 2.4%. Reef fish species richness, biomass, and density was consistent across sites, however the relative contribution of individual species to community composition differed significantly. Macrocarnivores were found to be the dominant contributors to biomass, with invertivores the most speciose, and omnivores and planktivores at the highest densities, consistent with previous descriptions of mesophotic fish assemblages in other regions. Similarly, the low hard coral cover and high macroalgae and sponge cover of the benthic communities also appear ecologically similar to several described mesophotic reefs yet is not uniform across the Caribbean. The ecological organisation of Grand Cayman’s MCEs may result from a variety of factors such as isolation from other major land masses, geology, local geography, and anthropogenic activity at both the local and global scale and highlight the importance of continued exploration and documentation of MCE communities.

Invasive lionfish ( Pterois miles and P. volitans ) populations are now pervasive across the Caribbean and Western Atlantic. These generalist mesopredators have the potential to negatively impact reef fish populations throughout their invaded range. Targeted removal by culling has been shown as an effective way to manage shallow-water lionfish densities, however, lionfish are found across a broad depth range, with reproductive populations found at mesophotic depth. This study aimed to determine the efficacy of short-term culling of mesophotic lionfish populations and its impact on resident reef fish populations. Surveys of lionfish and reef fish populations were conducted monthly at three mesophotic reef sites in Bermuda from July to December 2017, with subsequent surveys six and 12 months later. During each survey, all lionfish were removed, measured, and sexed. Lionfish density was found to decrease significantly over the recurrent culling period, with the rate of recolonization declining as density declined, resulting in continued reductions with each subsequent culling. The percent of females also declined during recurrent culling, suggesting that culling may further reduce population stability through impacts to reproductive potential. Reef fish community composition changed throughout the study, with shifts in dominant species and trophic groups despite stable species richness and diversity. However, fluctuations in lionfish density as well as reef fish density and biomass reflected patterns of seasonality indicating that impacts of the physical environment may mask the influence of lionfish culling. Overall, these data suggest that culling of invasive lionfish on mesophotic reefs should target high density sites and seasonal aggregations in order to effectively contribute to management strategies.

In March 2020, the world went into lockdown to curb the spread of the novel coronavirus (SARS-CoV-2), with immediate impacts on wildlife across ecosystems. The strict 2-year long lockdown in Grand Cayman provided an unprecedented opportunity to assess how the ‘human confinement experiment’ influenced the community composition of reef fish. Using a suite of multivariate statistics, our findings revealed a stark increase in reef fish biomass during the 2 years of lockdown, especially among herbivores, including parrotfish, with drastic increases in juvenile parrotfishes identified. Additionally, when comparing baseline data of the community from 2018 to the 2 years during lockdown, over a three-fold significant increase in mean reef fish biomass was observed, with a clear shift in community composition. Our findings provide unique insights into the resilience of reef fish communities when local anthropogenic stressors are removed for an unprecedented length of time. Given the functional role of herbivores including parrotfish, our results suggest that reductions in human water-based activities have positive implications for coral reef ecosystems and should be considered in future management strategies.

Abstract The ecological distribution of coral species from shallow to mesophotic reefs is dependent on light, which varies drastically among local environments. Current definitions of mesophotic coral ecosystems primarily rely on a 30‐m recreational SCUBA boundary to define the upper limits of the community; however, this boundary does not consider local conditions and physiological adaptions of coral species. Using in situ benthic imagery and chlorophyll fluorescence measurements, we examined species distribution and community similarity, as well as photoacclimatization of two common depth‐generalist species (Montastraea cavernosa and Porites astreoides) across shallow to mesophotic reef zones at Little Cayman Island. Photoquadrat image analysis revealed a significant shift in coral species assemblages between 25 and 35 m, which was accompanied by a 30% drop in available surface light, suggesting light is a key driver of coral community composition. Patterns of photoacclimatization across depths differed significantly between the two coral species, with available surface light and the quantum yield of photochemistry in photosystem II found to be significant determinants of each species' abundance. These results provide valuable baseline data on coral community composition across a broad depth gradient in Little Cayman that can contribute to a growing body of evidence to set an upper boundary of mesophotic reefs based on light availability and photoacclimatization potential of depth‐generalist species.

Reef‐building corals respond to the temporal integration of both pulse events (i.e., heat waves) and press thermal history (i.e., local environment) via physiological changes, with ecological consequences. We used a “press‐pulse‐press” experimental framework to expose the brooding coral Porites astreoides to various thermal histories to understand the physiological response of temporal dynamics within and across generations. We collected adult colonies from two reefs (outer Rim reef and inner Patch reef) in Bermuda with naturally contrasting thermal regimes as our initial “press” scenario, followed by a 21‐day ex situ “pulse” thermal stress of 30.4°C during larval brooding, and a “press” year‐long adult reciprocal transplant between the original sites. Higher endosymbiont density and holobiont protein was found in corals originating from the lower thermal variability site (Rim) compared to the higher thermal variability site (Patch). The thermal pulse event drove significant declines in photosynthesis, endosymbiont density, and chlorophyll a , with bleaching phenotype convergence for adults from both histories. Following the reciprocal transplant, photosynthesis was higher in previously heated corals, indicating recovery from the thermal pulse. The effect of origin (initial press) modulated the response to transplant site for endosymbiont density and chlorophyll a , suggesting contrasting acclimation strategies. Higher respiration and photosynthetic rates were found in corals originating from the Rim site, indicating greater energy available for reproduction, supported by larger larvae released from Rim corals post‐transplantation. Notably, parental exposure to the pulse thermal event resulted in increased offspring plasticity when parents were transplanted to foreign sites, highlighting the legacy of the pulse event and the importance of the environment during recovery in contributing to cross‐generational or developmental plasticity. Together, these findings provide novel insight into the role of historical disturbance events in driving differential outcomes within and across generations, which is of critical importance in forecasting reef futures.

As the devastating impacts of global climate change and local anthropogenic stressors on shallow-water coral reefs are expected to rise, mesophotic coral ecosystems have increasingly been regarded as potential lifeboats for coral survival, providing a source of propagules to replenish shallower reefs. Yet, there is still limited knowledge of the capacity for coral larvae to adjust to light intensities that change with depth. This study elucidates the mechanisms underlying plasticity during early life stages of the coral Porites astreoides that enable survival across broad depth gradients. We examined physiological and morphological variations in larvae from shallow (8–10 m) and mesophotic (45 m) reefs in Bermuda, and evaluated differences in survival, settlement patterns and size among recruits depending on light conditions using a reciprocal ex situ transplantation experiment. Larvae released from mesophotic adults were found to have significantly lower respiration rates and were significantly larger than those derived from shallow adults, indicating higher content of energetic resources and suggesting a greater dispersal potential for mesophotic larvae compared to their shallow counterparts. Additionally, larvae released from mesophotic adults experienced higher settlement success and larger initial spat size compared to larvae from shallow adults, demonstrating a potential connection between parental origin, offspring quality, and recruitment success. Although both shallow and mesophotic larvae exhibited the capacity to survive and settle under reciprocal light conditions, all larvae had higher survival under mesophotic light conditions regardless of parental origin, suggesting that conditions experienced under low light may enable longer larval life, further extending the dispersal period. These results indicate that larvae from mesophotic Porites astreoides colonies are likely capable of reseeding shallow reefs in Bermuda, thereby supporting the Deep Reef Refugia Hypothesis.

Coral recruitment represents a key element for coral reef persistence and resilience in the face of environmental disturbances. Studying coral recruitment patterns is fundamental for assessing reef health and implementing appropriate management strategies in an era of climate change. The FluorIS system has been developed to acquire high resolution, wide field-of-view (FOV) in situ images of coral recruits fluorescence and has proven successful in shallow reef environments. However, up to now, its applicability to mesophotic coral ecosystems remains unknown due to the complexity of the system and the limited time available when working at mesophotic depth. In this study we optimized the FluorIS system by utilizing a single infrared-converted camera instead of the bulkier regular dual-camera system, substantially reducing the system complexity and significantly decreasing the acquisition time to an average of 10 s for a set of 3 images. Moreover, the speed-FluorIS system is much more economical, decreasing the cost of the full set-up by roughly 40% compared to the original dual-camera system. We tested the utility of the speed-FluorIS by surveying coral recruits across shallow and mesophotic reefs of the Red Sea (Gulf of Eilat) and Bermuda, two of the most northerly reefs in the world with markedly different substrate and topography, and demonstrate that the modified system enables fast imaging of fluorescence to study coral recruitment patterns over a broader range of depths and reef topographies than previous fluorescence methods. Our single-camera system represents a valuable, non-invasive and rapid underwater tool which will help standardize surveys and long-term monitoring of coral recruits, contributing to our understanding of these vital and delicate early life stages of corals.

Invasive lionfish (Pterois volitans and P. miles) are now ubiquitous throughout the Caribbean and Western Atlantic on shallow and deep reefs. Recent surveys in Bermuda have revealed dense aggregations of lionfish on mesophotic reefs (60 m depth), yet these densities are not pervasive across reefs at this depth. Using diver-led visual surveys of mesophotic reef sites, this study examines how variations in potential ecological drivers may affect lionfish distribution. Significant correlations of lionfish densities were found with prey fish density and prey fish biomass, where sites with higher abundances of prey fishes have greater densities of lionfish. Furthermore, higher densities of lionfish also correlated significantly with higher juvenile Paranthias furcifer biomass, a preferred prey type for lionfish. Prey fish diversity, on the other hand, was not related to lionfish density, nor did prey fish community composition differ in a way that reflected lionfish distributions. The influence of seawater temperature was found to have the strongest effect on lionfish distribution, where higher lionfish densities were found at sites with lower bottom temperature. However, temperature co-varied with prey fish density, prey fish biomass, and P. furcifer biomass, implying that physical parameters of the environment (i.e., temperature) likely influence ecological parameters (i.e., prey fish abundance), contributing to the structuring of lionfish distributions. We suggest, therefore, that cold-water upwelling currents may be fueling the food chain in certain locations, resulting in high abundances of prey fishes and thus lionfish. Understanding the factors that influence lionfish distributions will ultimately increase the efficacy of management strategies, which, as the data presented here suggest, must incorporate mesophotic lionfish populations.

The detrimental effects of invasive lionfishes (Pterois volitans and Pterois miles) on western Atlantic shallow reefs are well documented, including declines in coral cover and native fish populations, with disproportionate predation on critically endangered reef fish in some locations. Yet despite individuals reaching depths >100 m, the role of mesophotic coral ecosystems (MCEs; reefs 30–150 m) in lionfish ecology has not been addressed. With lionfish control programs in most invaded locations limited to 30 m by diving restrictions, understanding the role of MCEs in lionfish distributions remains a critical knowledge gap potentially hindering conservation management. Here we synthesise unpublished and previously published studies of lionfish abundance and body length at paired shallow reef (0–30 m) and MCE sites in 63 locations in seven western Atlantic countries and eight sites in three Indo-Pacific countries where lionfish are native. Lionfish were found at similar abundances across the depth gradient from shallow to adjacent MCEs, with no difference between invaded and native sites. Of the five invaded countries where length data were available three had larger lionfish on mesophotic than shallow reefs, one showed no significant difference, while the fifth represented a recently invaded site. This suggests at least some mesophotic populations may represent extensions of natural ontogenetic migrations. Interestingly, despite their shallow focus, in many cases culling programs did not appear to alter abundance between depths. In general, we identify widespread invasive lionfish populations on MCE that could be responsible for maintaining high densities of lionfish recruits despite local shallow-biased control programs. This study highlights the need for management plans to incorporate lionfish populations below the depth limit of recreational diving in order to address all aspects of the local population and maximise the effectiveness of control efforts.

Mesophotic coral reef ecosystems remain largely unexplored with only limited information available on taxonomic composition, abundance and distribution. Yet, mesophotic reefs may serve as potential refugia for shallow-water species and thus understanding biodiversity, ecology and connectivity of deep reef communities is integral for resource management and conservation. The Caribbean coral, Montastraea cavernosa, is considered a depth generalist and is commonly found at mesophotic depths. We surveyed abundance and size-frequency of M. cavernosa populations at six shallow (10m) and six upper mesophotic (45m) sites in Bermuda and found population structure was depth dependent. The mean surface area of colonies at mesophotic sites was significantly smaller than at shallow sites, suggesting that growth rates and maximum colony surface area are limited on mesophotic reefs. Colony density was significantly higher at mesophotic sites, however, resulting in equal contributions to overall percent cover. Size-frequency distributions between shallow and mesophotic sites were also significantly different with populations at mesophotic reefs skewed towards smaller individuals. Overall, the results of this study provide valuable baseline data on population structure, which indicate that the mesophotic reefs of Bermuda support an established population of M. cavernosa.