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 (<40 m) and MCEs (40–100 m), exhibiting markedly different relationships with light. The depth ranges and morphology of 47 coral genera were better explained by light than depth, mainly, due to photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) (1% at 76 and 36 m, respectively). Branching coral species were found mainly at shallower depths, that is, down to 36 m. Among the abundant upper‐mesophotic specialist corals, Leptoseris glabra, Euphyllia paradivisa, and Alveopora spp. were found strictly between 40 and 80 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 MCE 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 worldwide on the basis of light quality data.

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 longaccepted 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.

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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.

Light quality is a crucial physical factor driving coral distribution 32 along depth gradients. Currently, a 30 m depth limit, based on SCUBA regulations, 33 separates shallow and deep mesophotic coral ecosystems (MCEs). This definition, 34 however, fails to explicitly accommodate environmental variation. Here, we posit a 35 novel definition for a regional or reef-to-reef outlook of MCEs based on the light vs. 36 coral community-structure relationship. A combination of physical and ecological 37 methods enabled us to clarify the ambiguity in relation to that issue. To characterize 38 coral community structure with respect to the light environment, we conducted wide-39 scale spatial studies at five sites along shallow and MCEs of the Gulf of Eilat/Aqaba 40 (0-100 m depth). Surveys were conducted by Tech-diving and drop-cameras, in 41 addition to one year of light spectral measurements. We quantify two distinct coral 42 assemblages: shallow (<40 m), and MCEs (40-100 m), exhibiting markedly different 43 relationships with light. The depth ranges and morphology of 47 coral genera, was 44 better explained by light than depth, mainly, due to the Photosynthetically Active 45 Radiation (PAR) and Ultra Violet Radiation (1% at 76 m and 36 m, respectively). 46Branching coral species were found mainly at shallower depths i.e., down to 36 m. 47Among the abundant upper mesophotic specialist-corals, Leptoseris glabra, Euphyllia 48 paradivisa and Alveopora spp., were found strictly between 36-76 m depth. The only 49 lower mesophotic-specialist, Leptoseris fragilis, was found deeper than 80 m. We 50 suggest that shallow coral genera are light-limited below a level of 1.25% surface 51 PAR and that the optimal PAR for mesophotic communities is at 7.5%. This study 52 contributes to moving MCEs ecology from a descriptive-phase into identifying key 53 ecological and physiological processes structuring MCE coral communities. 54Moreover, it may serve as a model enabling the description of a coral zonation world-55 wide on the basis of light quality data. 56 3