Red Sea Mesophotic Coral Ecosystems

Mesophotic coral ecosystems (MCEs) are light-dependent coral-associated communities found at 30-150 m depth. Corals inhabiting these deeper reefs are often acclimatized to a limited and blue-shifted light environment, enabling them to maintain the relationship with their photosynthetic algal symbionts (family Symbiodiniaceae) despite the seemingly suboptimal light conditions. Among others, fluorescent proteins produced by the coral host may play a role in the modulation of the quality and spectral distribution of irradiance within the coral tissue through wavelength transformation. Here we examined the bio-optical properties and photosynthetic performances of different fluorescence morphs of two mesophotic coral species Goniopora minor and Alveopora ocellata, in order to test the photosynthesis enhancement hypothesis proposed for coral fluorescence. The green morph of G. minor and the low fluorescence morph of A. ocellata exhibit, in their natural habitats, higher abundance. The morphs also presented different spectral reflectance and light attenuation within the tissue. Nevertheless, chlorophyll a fluorescence-based, and O 2 evolution measurements, revealed only minor differences between the photosynthetic abilities of three fluorescence morphs of the coral G. minor and two fluorescence morphs of A. ocellata. The fluorescence morphs did not differ in their algal densities or chlorophyll concentrations and all corals harbored Symbiodiniaceae from the genus Cladocopium. Thus, despite the change in the internal light quantity and quality that corals and their symbionts experience, we found no evidence for the facilitation or enhancement of photosynthesis by wavelength transformation.

The mesophotic coral Alveopora allingi from the northern Gulf of Eilat/Aqaba, Red Sea, is affected by year-round partial coral-bleaching events. During these events, the migration of Symbiodiniaceae takes place from the coral-host mesoglea to the developed oocytes in bleached parts of colonies of A. allingi but not in the non-bleached parts. Additionally, these oocytes are abnormal, missing part of the structural material of the peripheral areas, and are also significantly larger in the bleached areas of the colonies. Hence, we suggest a parasitic behavior of the symbionts or a commensalism relationship which enhance symbionts' needs during bleaching periods and may boost the gametogenesis development in these corals. We propose that evolutionarily, this behavior may greatly contribute to the symbiont community survival throughout the bleaching period, and it can also be beneficial for the host's persistence and adaptation to bleaching through the acquisition of a specific symbiont community following the bleaching event.

Mesophotic coral ecosystems (MCEs) are characterized by the presence of photosynthetically active organisms such as corals and algae, and associated communities at depths ranging from 30 to 150 m in tropical and subtropical regions. Due to the increased awareness of the potential importance of these reefs as an integral part of coral reef ecosystems (i.e., deep reef refuge, specialized biodiversity, transition zone between shallow and deep-sea environments, and recreational and intrinsic values), interest from the scientific community has grown around the world over the last two decades. Several nations have already made management declarations and started to extend marine protected areas and fishery management to MCEs. The estimated area of Australian MCEs is likely equivalent to that of shallow reef ecosystems down to 30 m; however, Australian MCEs attract limited research effort compared to other major coral reef regions around the world. In this perspective, we briefly explore the reasons for this scarcity of research on mesophotic ecosystems of the Great Barrier Reef (GBR) of Australia (e.g., strict diving regulations, new researchers' involvement, and logistics and cost). At present, research efforts on the mesophotic ecosystems of the GBR are in decline and if this trajectory is maintained, the global disparity in knowledge between MCEs near Australia and those from the other main coral reef regions worldwide will sharpen deeply. We call for action from the research community, grant agencies, and decision-makers toward a wider understanding of these important ecosystems in Australia.

Antecedent topography such as relic reef terraces as well as biogenic carbonate relief-forming deposits ~30–150 m deep, referred to as mesophotic reefs, provide structural support for diverse mesophotic coral ecosystems (MCEs) that may serve as coral refuges for select light-dependent species. Although terraces at mesophotic depths are found globally, an understanding of their spatial distribution, formation, and relationship with living community composition and lithology is generally lacking. Herein, 2 × 2 m resolution bathymetry from the Gulf of Aqaba (GoA) was examined to define geomorphology features spanning mesophotic depths and compare geomorphology relationships to overlying benthic and lithologic cover. Analysis led to the production of a new map categorizing 12 geomorphology features, including upper mesophotic terraces harboring thriving MCEs. Additionally, a large collection of still imagery (1726 pictures) was obtained at 94 sites and used to define eight unique habitats at mesophotic depths and lithological and biological distribution patterns over vertical and horizontal scales. Study area benthic and lithologic cover was found to be significantly different between geomorphology features and related to GoA geomorphology as well as to seafloor depth and slope, and light attenuation. While these relationships indicated modern cover could not provide a model for producing most underlying geomorphology in the study area, results provided data needed to enhance understanding of geomorphology feature formation history and reef accretion at mesophotic depths. Study results also detailed benthic cover and geomorphology features critical for better identifying and mapping unknown MCE habitats, and for recognizing mesophotic reef spatial relationships and biodiversity patterns in the GoA. These results are especially important considering most northern GoA reefs act as potential refuges, but local anthropogenic development continually stresses shallow GoA reefs and most other shallow coral reefs around the globe continue to degrade.

Coral bleaching, as one of the major threats to the well-being of coral reefs worldwide, has been extensively studied. However, corals from mesophotic coral ecosystems (MCEs), found at 30 to 150 m depth and considered as a potential refuge, have not yet been well studied experimentally under thermal stress. As mesopho-tic corals are also highly fluorescent, and fluorescence under heat stress is known to undergo change, we examined the involvement of fluorescence during heat-induced bleaching, by incorporating both high-and low-fluores-cence morphs in our experiments. We collected colonies of the mesophotic coral Alveopora ocellata and subjected them to elevated temperatures in both winter and summer in controlled laboratory experiments. We followed their physiological response and their bleaching at a high sampling resolution (every 48 h following the temperature ramping period). We found that A. ocellata bleached after a short-term (14 days) thermal stress of ?4°C above ambient sea temperature during the summer, but did not bleach during the winter despite the elevated temperature (?5°C; max temperature of 32°C and 28°C, respectively). After experiencing temperatures higher than 29.5°C, the peak summer temperature, the corals gradually lost their algal symbionts during the summer experiment, while exhibiting an increase in symbiont density during the winter experiment. A similar response was also observed in chlorophyll a concentration, host fluorescence intensity, and maximal quantum yield of PSII (F v /F m). Throughout the experiments (in both seasons and treatments), the high-fluorescence corals presented lower zooxanthellae densities , higher cellular chlorophyll a concentration, and up to sixfold higher fluorescence. The differences found between the two morphs suggest that fluorescence may be favorable under thermal stress, strengthening the possibility of using coral fluorescence as a noninvasive monitoring tool for early detection of bleaching. This demonstration of a bleaching process in a mesophotic coral indicates the vulnerability of MCEs to the increase, in recent decades, in the frequency and intensity of temperature anomalies.

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Population size structure provides information on demographic characteristics, such as growth and decline, enabling post-hoc assessment of spatial differences in susceptibility to disturbance. Nevertheless, very few studies have quantified size data of scleractinian corals along a shallow-mesophotic gradient, partly because of previously inaccessible depths. Here, we report the coral size-frequency distributions at the morphology level (six growth forms) and at the species level for ten representative locally abundant species along a broad depth gradient (5-100 m) in the Gulf of Eilat/Aqaba (GoE/A). A total of 18,865 colonies belonging to 14 families and 45 genera were recorded and measured over four reef sites. Colonies were found to be 11.2% more abundant at mesophotic (40-100 m; 55.6%) depths compared with shallow (5-30 m; 44.4%). The coral taxa exhibited heterogeneity in their size-structure, with marked differences among depths, morphological growth forms, and species. Branching and corymbose corals were more prevalent in shallow waters, while encrusting and laminar forms comprised the majority of mesophotic corals. Nevertheless, massive morphology was the most abundant growth form across all sites and depths (39%), followed by laminar (26%) and encrusting (20%). Corymbose corals (primarily Acroporidae) revealed constrained size at all depths; with the lack of small-size groups indicating populations at risk of decline. Depth-generalist species belonging to massive and laminar morphologies generally exhibited a larger colony size at the mesophotic depths, but were typified by a higher number of small colonies. Furthermore, we refute the widely and long-accepted assertion that Stylophora pistillata is the most abundant coral in the northern GoE/A, and assert that Leptoseris glabra is the one. Here, we provide a baseline for future monitoring of coral population structures, insights to recent ecological dynamics, retrospective assessment of coral community recovery following disturbances and grounds for conservation assessments and management actions.

Aim Coral reefs shift between distinct communities with depth throughout the world. Yet, despite over half a century of research on coral reef depth gradients, researchers have not addressed the driving force of these patterns. We present a theoretical, process‐based model of light’s influence on the shallow to mesophotic reef transition as a single quantitative framework. We also share an interactive web application. Moving beyond depth as an ecological proxy will enhance research conducted on deeper coral reefs. Location Global; subtropical and tropical coral reefs, oligotrophic and turbid coastal waters. Time period Present day (2020). Major taxa Scleractinia. Methods We constructed ordinary differential equations representing the preferred light environments of shallow and mesophotic Scleractinia. We projected these as depth bands using light attenuation coefficients from around the world, and performed a sensitivity analysis. Results We found light relationships alone are sufficient to capture major ecological features across coral reef depth gradients. Our model supports the depth limits currently used in coral reef ecology, predicting a global range for the shallow‐upper mesophotic boundary at 36.1 m ± 5.6 and the upper‐lower mesophotic boundary at 61.9 m ± 9.6. However, our model allows researchers to move past these fixed depth limits, and quantitatively predict the depths of reef zones in locations around the world. Main conclusions The use of depth as a proxy for changes in coral reef communities offers no guidance for environmental variation between sites. We have shown it is possible to use light to predict the depth boundaries of reef zones as a continuous variable, and to accommodate this variability. Predicting the depths of reef zones in unusual light environments suggests that shallow‐water turbid reefs should be considered as mesophotic coral ecosystems. Nonetheless, the current depth‐based heuristics are relatively accurate at a global level.

Most studies to date on the various life-history aspects of scleractinian corals (e.g. reproduction, connectivity, and physiology) have focused on their innate habitats. However, comprehensive data on the ability of both shallow and mesophotic corals to contend in the coming decades with the different environmental conditions they may encounter due to new habitats or environmental changes (e.g. eutrophication), are scarce. Long-term cross-transplantation experiments assessing the potential responses and acclimatization ability of corals are thus needed in order to expand our knowledge. Here we examined the survivorship and changes in the photobiological acclimatization of corals following their cross-transplantation between two different depths (5–10 m and 45 m) and two sites characterized by different abiotic conditions (i.e. light, nutrient, and sedimentation regime). This year-long in-situ experiment was performed on five depth-generalist coral species. Depth of origin and the species’ particular morphology were found to be the strongest predictors of survivorship. Physiological responses occurred mainly among those corals that had been translocated from deep-to-shallow water, and were expressed in a significant reduction in chlorophyll-a concentration and algal density, as well as changes in photosynthetic parameters (e.g. minimal/maximal saturating points, Ek and Em, and rETRmax). Our findings contribute to the existing knowledge on the ability of species-specific coral responses to contend with dramatic changes in their environment. The findings presented here contribute to assessment of the physiological and ecological consequences for corals of the long-term environmental changes that result from extreme environmental events.

The recognition of the microbiota complexity and their role in the evolution of their host is leading to the popularization of the holobiont concept. However, the coral holobiont (host and its microbiota) is still enigmatic and unclear. Here, we explore the complex relations between different holobiont members of a mesophotic coral Euphyllia paradivisa. We subjected two lines of the coral—with photosymbionts, and without photosymbionts (apo-symbiotic)—to increasing temperatures and to antibiotics. The different symbiotic states were characterized using transcriptomics, microbiology and physiology techniques. The bacterial community’s composition is dominated by bacteroidetes, alphaproteobacteria, and gammaproteobacteria, but is dependent upon the symbiont state, colony, temperature treatment, and antibiotic exposure. Overall, the most important parameter determining the response was whether the coral was a symbiont/apo-symbiotic, while the colony and bacterial composition were secondary factors. Enrichment Gene Ontology analysis of coral host’s differentially expressed genes demonstrated the cellular differences between symbiotic and apo-symbiotic samples. Our results demonstrate the significance of each component of the holobiont consortium and imply a coherent link between them, which dramatically impacts the molecular and cellular processes of the coral host, which possibly affect its fitness, particularly under environmental stress.

Supplementary materials: Materials and Methods Additional results and figures

Sex change has been widely studied in animals and plants. However, the conditions favoring sex change, its mode and timing remain poorly known. Here, for the first time in stony corals, we report on a protandrous (youngest individuals are males) repetitive sex change exhibited by the fungiid coral Herpolitha limax across large spatial scales (the coral reefs of Japan, Jordan and Israel) and temporal scales (2004–2017). In contrast to most corals, this species is a daytime spawner (08:00–10:00 AM) that spawned at the same time/same date across all the study sites. The sporadically scattered populations of H. limax among the coral reefs of Eilat (Israel) and Aqaba (Jordan) exhibited significantly slower growth, earlier sex change, and lower percentages of reproduction and sex change in comparison to the densely aggregated populations in Okinawa (Japan). At all sites, sex ratio varied among years, but was almost always biased towards maleness. Growth rate decreased with size. We conclude that comparable to dioecious plants that display labile sexuality in response to energetic and/or environmental constraints, the repetitive sex change displayed by H. limax increases its overall fitness reinforcing the important role of reproductive plasticity in the Phylum Cnidaria in determining their evolutionary success.

The reproductive patterns of coral communities can vary temporally and geographically, raising interesting questions regarding natural selection and ecology. Species of the cnidarian genus Millepora have received the common name “fire corals” due to the painful sting inflicted on humans by the release of venom from their stinging cells. During the course of a multiyear study of reproductive behavior and reproduction timing of corals and other inhabitants of the coral reef in the Gulf of Eilat/Aqaba, northern Red Sea, we observed numerous reproductive events of the three Millepora species found in the Red Sea and provided novel information regarding the periodicity and timing of medusae release events.

With shallow coral reefs suffering from an ongoing rapid decline in many regions of the world, the interest in studies on mesophotic coral ecosystems (30–150 m) is growing rapidly. While most photoacclimation responses in corals were documented within the upper 30 m of reefs, in the present study we transplanted fragments of a strictly mesophotic species from the Red Sea, Euphyllia paradivisa, from 50 m to 5 m for a period of 3 years. Following the retrieval of the corals, their physiological and photosynthetic properties of the corals were tested. The transplanted corals presented evidence of photosynthetic acclimation to the shallow habitat, lower sensitivity to photoinhibition, and a high survival percentage, while also demonstrating a reduced ability to utilize low light compared to their mesophotic counterparts. This long-term successful transplantation from a mesophotic depth to a shallow habitat has provided us with insights regarding the ability of mesophotic corals and their symbionts to survive and withstand shallow environments, dominated by a completely different light regime. The extensive characterization of the photobiology of E. paradivisa, and its photoacclimation response to a high-light environment also demonstrates the plasticity of corals and point out to mechanisms different than those reported previously in shallower corals.

Mesophotic coral ecosystems (MCEs) and temperate mesophotic ecosystems (TMEs) occur at depths of roughly 30–150 m depth and are characterized by the presence of photosynthetic organisms despite reduced light availability. Exploration of these ecosystems dates back several decades, but our knowledge remained extremely limited until about a decade ago, when a renewed interest resulted in the establishment of a rapidly growing research community. Here, we present the ‘mesophotic.org’ database, a comprehensive and curated repository of scientific literature on mesophotic ecosystems. Through both manually curated and automatically extracted metadata, the repository facilitates rapid retrieval of available information about particular topics (e.g. taxa or geographic regions), exploration of spatial/temporal trends in research and identification of knowledge gaps. The repository can be queried to comprehensively obtain available data to address large-scale questions and guide future research directions. Overall, the ‘mesophotic.org’ repository provides an independent and open-source platform for the ever-growing research community working on MCEs and TMEs to collate and expedite our understanding of the occurrence, composition and functioning of these ecosystems. Database URL: http://mesophotic.org/

Abstract Light quality is a crucial physical factor driving coral distribution along depth gradients. Currently, a 30 m depth limit, based on SCUBA regulations, separates shallow and deep mesophotic coral ecosystems (MCEs). This definition, however, fails to explicitly accommodate environmental variation. Here, we posit a novel definition for a regional or reef‐to‐reef outlook of MCEs based on the light vs. coral community–structure relationship. A combination of physical and ecological methods enabled us to clarify the ambiguity in relation to the mesophotic definition. To characterize coral community structure with respect to the light environment, we conducted wide‐scale spatial studies at five sites along shallow and MCEs of the Gulf of Eilat/Aqaba (0–100 m depth). Surveys were conducted by technical‐diving and drop‐cameras, in addition to one year of light spectral measurements. We quantify two distinct coral assemblages: shallow (

Light quality is a crucial physical factor driving coral distribution along depth gradients. Currently, a 30 m depth limit, based on SCUBA regulations, separates shallow and deep mesophotic coral ecosystems (MCEs). This definition, however, fails to explicitly accommodate environmental variation. Here, we posit a novel definition for a regional or reef-to-reef outlook of MCEs based on the light vs. coral community-structure relationship. A combination of physical and ecological methods enabled us to clarify the ambiguity in relation to that issue. To characterize coral community structure with respect to the light environment, we conducted wide-scale spatial studies at five sites along shallow and MCEs of the Gulf of Eilat/Aqaba (0-100 m depth). Surveys were conducted by Tech-diving and drop-cameras, in addition to one year of light spectral measurements. We quantify two distinct coral assemblages: shallow (<40 m), and MCEs (40-100 m), exhibiting markedly different relationships with light. The depth ranges and morphology of 47 coral genera, was better explained by light than depth, mainly, due to the Photosynthetically Active Radiation (PAR) and Ultra Violet Radiation (1% at 76 m and 36 m, respectively). Branching coral species were found mainly at shallower depths i.e., down to 36 m. Among the abundant upper mesophotic specialist-corals, Leptoseris glabra, Euphyllia paradivisa and Alveopora spp., were found strictly between 36-76 m depth. The only lower mesophotic-specialist, Leptoseris fragilis, was found deeper than 80 m. We suggest that shallow coral genera are light-limited below a level of 1.25% surface PAR and that the optimal PAR for mesophotic communities is at 7.5%. This study contributes to moving MCEs ecology from a descriptive-phase into identifying key ecological and physiological processes structuring MCE coral communities. Moreover, it may serve as a model enabling the description of a coral zonation world-wide on the basis of light quality data.

Mesophotic coral ecosystems (MCEs) and temperate mesophotic ecosystems (TMEs) have received increasing research attention during the last decade as many new and improved methods and technologies have become more accessible to explore deeper parts of the ocean. However, large voids in knowledge remain in our scientific understanding, limiting our ability to make scientifically based decisions for conservation and management of these ecosystems. Here, we present a list of key research and conservation questions to enhance progress in the field. Questions were generated following an initial open call to MCE and TME experts, representing a range of career levels, interests, organizations (including academia, governmental, and nongovernmental), and geographic locations. Questions were refined and grouped into eight broad themes: (1) Distribution, (2) Environmental and Physical Processes, (3) Biodiversity and Community Structure, (4) Ecological Processes, (5) Connectivity, (6) Physiology, (7) Threats, and (8) Management and Policy. Questions were ranked within themes, and a workshop was used to discuss, refine, and finalize a list of 25 key questions. The 25 questions are presented as a guide for MCE and TME researchers, managers, and funders for future work and collaborations.

Coral sclerochronology is a powerful tool for understanding environmental and ecological changes on coral reefs. Geochemical, isotopic, and skeletal density banding analyses along the major growth axis of massive coral skeletons from tropical shallow-water reefs have been used successfully to reconstruct decadal- to centennial-scale histories of climate and coral growth with annual to seasonal resolution. However, little is known about how coral sclerochronological approaches could capture environmental and/or physiological changes in mesophotic coral ecosystems (MCEs), which occur at depths ranging from 30 to 150 m. We compared the oxygen and carbon isotopes and growth records of coral from upper MCEs with those from adjacent shallow reefs by examining Porites corals collected at 4 and 40 m from Okinawa, Japan, and from 5 and 50 m water depths from the Gulf of Eilat, Red Sea, Israel. Porites corals in MCEs exhibited low calcification rates, but still recorded distinct seasonal to interannual variability in oxygen and carbon isotope signals consistent with coral isotopic records on shallow reefs. The amplitudes of the seasonality in the isotopic records in corals from MCEs were larger than those expected from seasonal environmental variations and those recorded in shallow reefs, indicating that coral growth rate and/or physiological changes affected skeletal isotopic composition. Our results suggest that sclerochronological records have great potential for reconstructing environmental and ecological characteristics of MCEs. The isotopic records of MCE corals are also more influenced by physiological processes such as symbiotic photosynthesis, calcification rates, and trophic levels than their shallow-water counterparts.KeywordsCoral sclerochronologyOxygen isotopeCarbon isotopeMesophotic coral ecosystems Porites corals

The mesophotic coral ecosystems (MCEs) of Eilat, in the Northern Red Sea, are among the best-studied worldwide, as demonstrated by the high number of publications from the region. Nonetheless, Eilat’s MCEs remain relatively unexplored compared to its shallow reefs. Its MCEs host diverse benthic communities that are potentially linked ecologically to shallow reefs. Here, we summarize the history of MCE research and compare the shallow and mesophotic reefs using long-term biotic and abiotic data. Eilat’s MCEs exhibit lower fluctuations in temperature, light, sedimentation, and a decreased frequency of shore-related disturbances than adjacent shallow reefs, supporting the hypothesis that key environmental parameters become more stable with increasing depth. However, nutrient concentrations are more variable in MCEs than nearby shallow reefs. We provide a novel definition of the upper (30–80 m) and lower (80–160 m) mesophotic zone boundaries in Eilat, based on the degree of light penetration, as well as the relative abundance of major fauna and flora. Scleractinian coral diversity increases with depth, as well as the abundance of specialist taxa. Corals (93 spp.) comprise the major organisms contributing to living benthic cover. A mass coral-bleaching event took place in 2015 that exclusively affected MCEs, and we discuss the event’s potential mechanisms and consequences for shallow vs. mesophotic coral assemblages. Protection and regulations of MCEs are needed to maintain and support these unique ecosystems.

Mesophotic coral ecosystems (MCEs) have historically been considered more stable than shallow reefs and thus suggested to provide refuge to coral reef communities against natural and anthropogenic impacts. Despite this assumption, a growing body of literature has shown that deep reefs are not immune to natural disturbance. Here, based on our in situ observations, we propose that disturbance may actually represent an important mechanism for maintaining biodiversity in MCEs, as is the case for shallow reefs. Our observations suggest that disturbances can provide microhabitat and space necessary for the recruitment and occurrence of different species, increasing overall diversity. Since bioerosion rates are lower at depth, and most well‐developed coral reefs on MCEs are formed by dense aggregations of a single or a few species, intermediate levels of disturbance could represent a critical driver of community structure balancing. Therefore, instead of long‐term stability, intermediate disturbances should be expected on MCEs. However, high frequency and intensity of natural disturbances, or their association with anthropogenic stressors, might have stronger negative impacts on MCEs than on shallower reefs due to slower coral growth and calcification rates.

Euphyllia paradivisa is a strictly mesophotic coral in the reefs of Eilat that displays a striking color polymorphism, attributed to fluorescent proteins (FPs). FPs, which are used as visual markers in biomedical research, have been suggested to serve as photoprotectors or as facilitators of photosynthesis in corals due to their ability to transform light. Solar radiation that penetrates the sea includes, among others, both vital photosynthetic active radiation (PAR) and ultra-violet radiation (UVR). Both types, at high intensities, are known to have negative effects on corals, ranging from cellular damage to changes in community structure. In the present study, fluorescence morphs of E. paradivisa were used to investigate UVR response in a mesophotic organism and to examine the phenomenon of fluorescence polymorphism. E. paradivisa, although able to survive in high-light environments, displayed several physiological and behavioral responses that indicated severe light and UVR stress. We suggest that high PAR and UVR are potential drivers behind the absence of this coral from shallow reefs. Moreover, we found no significant differences between the different fluorescence morphs’ responses and no evidence of either photoprotection or photosynthesis enhancement. We therefore suggest that FPs in mesophotic corals might have a different biological role than that previously hypothesized for shallow corals.

The attenuation of light with increasing depth, along with reduced exposure to wave stress, plays an important role in vertically structuring coral reef communities. Benthic photosynthetic organisms exhibit different depth distributions and abundance patterns which cause changes in community composition of associated reef fauna. This vertical zonation in coral reef community structure suggests special adaptations in response to the changing environmental regime with depth including changes in light intensity, light spectrum, and angular distribution. At the lower depth limits of mesophotic coral ecosystems (MCEs), both light and temperature can become limiting factors with the latter playing an important role at higher latitudes. The available evidence indicates that different species can exhibit distinct and sometimes opposing photophysiologi-cal adaptations with increasing depth. Some zooxanthel-late corals appear to maximize ambient light utilization at the expense of efficiency, while others appear to maximize efficiency. Coral holobiont adaptations to mesophotic depths include changes in colony morphology, algal sym-bionts, pigment physiology, skeletal properties, and metabolic strategy. Given the scarcity of physiological studies at depths >60 m, the current understanding of how obligate zooxanthellate corals and other light-dependent organisms can inhabit such a broad depth distribution is far from complete. This chapter summarizes the ecologically relevant aspects of light and temperature regimes of MCEs, as well as the depth-related photophysiological and adap-tive strategies of coral holobionts.

Due to increasing frequency of disturbances to shallow reefs, it has been suggested that Mesophotic Coral Ecosystems (MCEs, 30–150 m depth) may serve as a refuge for corals and a source of larvae that can facilitate the recovery of shallow degraded reefs. As such, they have received increased attention in the past decade, yet remained understudied regarding recruitment dynamics. Here we describe coral recruitment dynamics on settlement tiles and their adjacent natural habitats (10 m vs. 50 m depths) in Eilat, over a period of 5.5 years. The tiles were deployed along three sites onto 18 racks (3 at each depth and at each site). Recruitment patterns varied both temporally and spatially, ending up to two-fold higher juvenile density and higher recruitment rates at mesophotic sites. Settlement surface preference changed with depth, favoring exposed surfaces in mesophotic waters and cryptic surfaces in shallow waters. Juvenile assemblages differed between depths and were distinct from adjacent natural habitats. Over half of the recruited genera overlapped between depths. We suggest that Eilat MCEs serve as a larval sink, providing settlement grounds for shallow-reef propagules. In view of their significance, we call for the protection of these unique and distinct deep-reef habitats.

Corals and their photosymbionts experience inherent changes in light along depth gradients, leading them to have evolved several well-investigated photoacclimation strategies. As coral calcification is influenced by light (a process described as LEC—‘light-enhanced calcification’), studies have sought to determine the link between photosynthesis and calcification, but many puzzling aspects still persist. Here, we examine the physiology of Euphyllia paradivisa, a coral species found at a wide range of depths but that is strictly mesophotic in the Red Sea; and also examines the coupling between photosynthesis and LEC by investigating the response of the coral under several controlled light regimes during a long-term experiment. E. paradivisa specimens were collected from 40 to 50 m depth and incubated under three light conditions for a period of 1 year: full-spectrum shallow-water light (approx. 3 m, e.g. shallow-light treatment); blue deep-water light (approx. 40 m, e.g. mesophotic-light treatment) or total darkness (e.g. dark treatment). Net photosynthesis remained similar in the shallow-light-treated corals compared to the mesophotic-light-treated corals, under both low and high light. However, calcification increased dramatically with increasing light intensity in the shallow-light-treated corals, suggesting a decoupling between these processes. Photoacclimation to shallow-water conditions was indicated by enhanced respiration, a higher density of zooxanthellae per polyp and lower chlorophyll a content per cell. The dark-treated corals became completely bleached but did not lower their metabolism below that of the mesophotic-light-treated corals. No Symbiodinium clade shift was found following the year-long light treatments. We conclude that E. paradivisa, and its original symbiont clade, can adapt to various light conditions by controlling its metabolic rate and growth energy investment, and consequently induce LEC.