Mesophotic Research Articles

Characterizing Geomorphology of Mesophotic Coral Reef Ecosystems in the Southwestern Gulf of Mexico: Implications for Conservation and Management

Coral reefs are the most biodiverse ecosystems on earth and are presently experiencing severe declines globally. Shallow coral reef ecosystems (<30 m) have been studied extensively while mesophotic coral ecosystems (MCE) are poorly studied. As a result, MCE are rarely included in marine reserve design and management, despite their ecological importance and connectivity to shallow reefs. In this study, we assessed the fine-scale topographic complexity, a proxy for structural complexity, for a group of coastal coral reefs in a marine park in the southwestern Gulf of Mexico, in depths between 2 and 49 m. We conducted hydrographic surveys using a semi-portable multibeam echosounder system to produce 3D bathymetry digital terrain models (DTM) with a 2.5 m spatial resolution for three submerged bank reefs and two emerging reefs. From these models, descriptive terrain parameters were calculated for each reef, including slope, aspect, curvature, rugosity and ruggedness. Results show that all reefs are predominantly northeast-southwest oriented, with well-defined leeward and windward sides. For the three submerged bank reefs, structural complexity increased with depth. Estimated mean ruggedness and rugosity were highest at 20–40 m depth range on windward side slopes. Emerging reefs showed high structural complexity, particularly at the 25–40 m depth range. We identified a spur and groove zone with maximum ruggedness (0.26) and rugosity (3.17) values, and four channels with steep slopes (68°) and dispersed mounds. We found that at mesophotic depths (>30 m), southern reefs basements from two distinct reefs merge to form a continuous complex. This has important management implications since presently, only 28.7% of this reef complex (mostly shallow areas) are within the existing limits of the marine park’s core zone. Considering the newly recognized importance of MCE, we propose expanding and reshaping the core zone to include the entire reef complex which mostly encompasses MCE with high structural complexity. Our study illustrates the value of semi-portable MBES for marine planning in developing countries and remote poorly studied areas.

Zooplankton community associated with mesophotic coral reefs in the Colombian Caribbean Sea

The morphological, phenological and ecological characteristics of shallow coral reefs have been widely studied, but only recently have mesophotic corals begun to be studied. Very little is known about these ecosystems and their associated communities, such as the zooplankton community, which constitutes a food source for reefs and for the organisms that inhabit them. In the present study, changes in zooplankton composition were characterised during two hydroclimatic periods in a mesophotic environment of the submarine mountain Bajo Frijol, in Corales de Profundidad National Natural Park, Colombia. Zooplankton was collected using a portable pumping device, which contained three collectors of differing mesh sizes; this allowed the capture of zooplankton ranging in size from 45 to 300 μm. Temperature, conductivity, oxygen and pH were obtained and complemented with MODIS AQUA satellite information. In this ecosystem, the zooplankton organisms with the greatest richness and abundance were small-sized copepods, radiolarians and tintinnids. The oceanography of the area results in greater zooplankton abundance, richness, and diversity in June when the coastal current allows greater chlorophyll-a concentrations in the area and smaller organisms are favoured by greater retention within the corals of the mesophotic system. The biodiversity reported for Bajo Frijol is the first detailed approach to the community of mesophotic coral reefs in the Colombian Caribbean Sea, contributing to new records to the Global Biodiversity Information Facility.

Environmental and biological drivers of white plague disease on shallow and mesophotic coral reefs

Outbreaks of coral white plague (WP) disease have caused significant regional declines of reef‐building Caribbean corals. Due to a greater availability of epidemiological data, studies have primarily focused on shallow coral reefs (< 30 m). In the U.S. Virgin Islands, however, WP disease prevalence appears to be higher on upper mesophotic (30–40 m) coral reefs when compared with shallow reefs and may be inhibiting coral recovery after environmental disturbances. The aim of this study was to investigate the relationship of environmental drivers with spatio‐temporal patterns of WP prevalence across shallow and mesophotic coral reefs in the U.S. Virgin Islands. We recorded WP prevalence at 13 reef sites (five shallow, three mid‐depth and five upper‐mesophotic reefs) across the south shelf of St. Thomas approximately monthly between 2012 and 2015 using a drop camera method. The influence of environmental factors on disease prevalence was investigated using Bayesian inference with generalized linear mixed‐effect models. We found that WP tended to increase during the beginning of the rainy season (June), and when levels of water turbidity and temperature were higher, and levels of oxygen and salinity lower. The disease prevalence was higher on mesophotic than on shallow or mid‐depth reefs, probably due to higher availability of corals (host), and a possible temperature threshold for WP occurrence that allows long‐term persistence (year‐round) of the disease on upper‐mesophotic reefs. This is the first study to implement the drop camera method to survey a coral disease over several reef sites and depths. This method can be applied on surveys of other rapid tissue loss diseases, such as the newly emergent stony‐coral‐tissue‐loss‐disease (SCTLD).

Gorgonians Are Foundation Species on Sponge-Dominated Mesophotic Coral Reefs in the Caribbean

Foundation species (FS) regulate ecological processes within communities often facilitating biodiversity and habitat complexity. Typically FS are dominant structure-forming taxa; but less dominant taxa having disproportionate ecological impacts to the community can also be FS. Mesophotic coral ecosystems (MCEs) are deep coral reef (∼30–150 m) communities, often dominated by emergent sponges in the Caribbean Basin. Despite the potential competitive advantage of sponges on MCEs, gorgonians are also common constituents of these reefs. Data from the Bahamas demonstrate increased biodiversity and densities of sponges on mesophotic reefs with gorgonians relative to reefs without these species. Drawing upon fifteen years of field surveys at five sites in the Caribbean Basin we assessed in situ interactions between gorgonians and sponges to quantify outcomes consistent with competition (i.e., tissue necrosis and overgrowth). Gorgonians were effective competitors against a variety of sponges, and two allelochemicals produced by Ellisella elongata were mechanistically important in interactions with Agelas clathrodes. We also examined invertebrate recruitment patterns near gorgonians to assess their role in facilitating MCE biodiversity. Our results indicate that live gorgonians, Antillogorgia bipinnata and E. elongata, facilitate biodiverse recruitment into MCEs, indicating that this process is governed by more than passive hydrodynamics. Collectively, these data indicate that these gorgonians exhibit both positive and negative ecological interactions (i.e., facilitation and competition, respectively) with sponges, and other taxa. Thus, these gorgonians are FS of MCE communities within the Caribbean Basin that display several traits contributing to the ecological structure of these understudied communities.

High diversity, abundance and distinct fish assemblages on submerged coral reef pinnacles compared to shallow emergent reefs

Coral reefs exhibit consistent patterns in biodiversity across multiple spatial scales, from local to global clines in species richness, abundance and community structure. Knowledge of fundamental processes driving these patterns is largely derived from studies of shallow, emergent and nearshore reefs. Although research efforts are expanding to deeper mesophotic coral reef ecology, distinct and isolated reef morphologies like submerged pinnacles or seamounts have received scant attention. Despite being potentially important for connectivity and as refugia, the extent to which established patterns and processes in coral reef ecology apply to these systems is unknown. Here we examine the fish and benthic communities associated with coral reefs found on submerged pinnacles in Kimbe Bay, Papua New Guinea. Community structure and diversity metrics are compared with emergent reefs at the same depth in both near and offshore settings. We then explicitly test whether benthic complexity variables known to influence reef fish communities exhibit similar patterns at each reef type. Pinnacles were characterised by 3.70 times the mean fish abundance and 1.98 times the species richness recorded at the same depths on emergent reefs. Fish community structure showed distinct separation across reef morphologies, with pinnacles most similar to offshore reefs. Benthic habitat complexity did not vary across reef types while fish assemblages were weakly related to benthic habitat variables, with reef morphology the most consistent predictor of fish community metrics. The pinnacles in our study support high coral reef fish biodiversity despite their small habitat area and relative isolation by depth and offshore setting. Our results suggest that habitat-specific environmental conditions are generated by the distinct geomorphology of pinnacles. As coastal reefs become more increasingly disturbed, understanding ecological patterns on deep patch reef habitats like pinnacles will be useful to provide a more holistic understanding of coral reef seascapes and their resilience.

Detrimental conditions affecting Xestospongia muta across shallow and mesophotic coral reefs off the southwest coast of Puerto Rico.

Sponges are fundamental components of coral reef communities and, unfortunately, like other major benthic members, they too have been impacted by epizootic and panzootic events. We report on the prevalence of disease-like conditions affecting populations of the giant barrel sponge Xestospongia muta across shallow and mesophotic coral reefs off La Parguera Natural Reserve (LPNR) and Mona Island Marine Reserve (MIMR) in Puerto Rico. Four different conditions affecting X. muta were observed during our surveys, of which 3 have been previously reported: cyclic spotted bleaching (CSB; apparently non-lethal), Xestospongia-tissue wasting disease (X-TWD; apparently lethal), and sponge orange band disease (SOB; sparsely associated with X-TWD infected individuals). Additionally, we describe a fourth condition, Xestospongia-tissue hardening condition (X-THC), a previously unreported disease recently observed along the insular shelf margin off LPNR and MIMR. Within LPNR, a total of 764 specimens of X. muta were inspected and measured. Of these, 590 sponges (72.2%) had CSB, 25 (3.27%) had signs of X-TWD, 7 (0.92%) had SOB, and the remaining 142 (18.6%) were apparently healthy. Three colonies inhabiting upper mesophotic depths on the LPNR insular shelf showed signs of CSB and X-TWD. At MIMR, video-transect surveys revealed a total of 514 colonies, of which 40 (7.78%) had signs of CSB and/or XTWD, 14 (2.72%) were affected by X-THC, while the remaining 460 (89.5%) showed no external signs of disease and appeared healthy. The presence of 4 concomitant disease-like conditions in barrel sponges of Puerto Rico is alarming, and indicative of the deteriorating status of Caribbean coral reefs.

Incident light on mesophotic corals is constrained by reef topography and colony morphology

Mesophotic coral reefs, generally defined as deep reefs between 30 and 150 m, are found worldwide and are largely structured by changes in the underwater light field. Additionally, it is increasingly understood that reef-to-reef variability in topography, combined with quantitative and qualitative changes in the underwater light field with increasing depth, significantly influence the observed changes in coral distribution and abundance. Here, we take a modeling approach to examine the effects of the inherent optical properties of the water column on the irradiance that corals are exposed to along a shallow to mesophotic depth gradient. In particular, the roles of reef topography including horizontal, sloping and vertical substrates are quantified, as well as the differences between mounding, plating and branching colony morphologies. Downwelling irradiance and reef topography interact such that for a water mass of similar optical properties, the irradiance reaching the benthos varies significantly with topography (i.e. substrate angle). Coral morphology, however, is also a factor; model results show that isolated hemispherical colonies consistently ‘see’ greater incident irradiances across depths, and throughout the day, compared to plating and branching morphologies. These modeled geometric-based differences in the incident irradiances on different coral morphologies are not, however, consistent with actual depth-dependent distributions of these coral morphotypes, where plating morphologies dominate as you go deeper. Other factors, such as the cost of calcification, arguably contribute to these differences, but irradiance-driven patterns are a strong proximate cause for the observed differences in mesophotic communities on sloping versus vertical reef substrates.

Characterization of fish assemblages in mesophotic reefs of Cuba

Although mesophotic coral reef ecosystems (MCEs) are widespread and diverse, they remain largely unexplored. The Caribbean MCEs are among the most studied in the world, but detailed information regarding Cuban MCEs is limited. In this study we present the first analyses of the ichthyofauna of the Cuban MCEs based on data from a research cruise in June 2017. Forty-two sites were sampled around the Archipelago of Cuba using a Remotely Operated Vehicle at depths between 30 and 180 m. Overall, 151 species were observed, including 13 threatened species. Differences in ichthyofauna were observed between marine protected areas (MPAs) and unprotected areas, with greater abundances and species richness inside national parks. Invasive lionfish had greater abundances on the north coast, while the south coast had greater abundances and richness of threatened and commercially valuable species. A detailed analysis (Generalized Lineal Model) showed effects of depth, protection level, reef type (seamount or fringing reef), and region on abundance (as fish observed per minute), but no interaction between them. A transition was observed in the fish assemblages between the 30–60 m and 60–100 m depth ranges. Species abundance and richness decreased with increasing depth. The most represented families of commercial importance (Serranidae, Lutjanidae, and Carangidae) were well represented in the upper, middle, and lower areas of the MCEs, but dominated in deeper areas. The percentage of herbivores in the fish assemblage decreased with increasing depth, contrary to what was observed with piscivores and invertebrate feeders/piscivores. Twenty-seven functional groups were identified, with cryptic micro-invertivores and cryptic planktivores being the most abundant, while roving macro-invertivores and roving piscivores were the most functionally redundant. This research constitutes a valuable contribution to the study and management of ichthyofauna in Cuba and highlights the effectiveness of MPAs in the conservation of fish biodiversity.

Mesophotic coral reef community structure: the constraints of imagery collected by unmanned vehicles

There is a critical need to quantify and monitor mesophotic coral reef community structure and function at multiple spatial and temporal scales. Because accessing these habitats is costly in terms of infrastructure and effort, often for a modest return in data, many investigators collect digital imagery using transect techniques from unmanned platforms. Specifically, remotely operated vehicles and autonomous underwater vehicles are used because they operate at deeper depths for extensive periods of time, can carry an array of oceanographic and imaging instruments, and can collect and archive extensive amounts of video and still imagery. However, substrate angle, camera angle, and vehicle position above the benthos creates varying degrees of error in the imagery due to parallax and geometric distortion. Photogrammetry conducted on 2D photographs from uncorrected 3D imagery can over- or under-estimate the percent cover, biomass estimates, and abundance of the benthic groups of interest. Here we illustrate these errors and emphasize the requirement for post-processing of imagery to ensure that these data can be used for valid quantitative ecological descriptions of mesophotic benthic communities in the future.

Widest geographic distribution of a shallow and mesophotic antipatharian coral (Anthozoa: Hexacorallia): Antipathes grandis VERRILL, 1928 \u2013 confirmed by morphometric and molecular analyses

We provide the first record of the shallow and mesophotic (< 150 m depth) antipatharian coral Antipathes grandis VERRILL, 1928 from the Indian Ocean. First described from Hawaii, A. grandis was recently found on a mesophotic coral reef in SW Madagascar. Its identity was confirmed by means of morphological and molecular analyses (cytochrome c oxidase subunit I). These findings not only expand our knowledge of the potential geographic range of antipatharian corals, but also challenge the belief that only deep-sea (> 200 m depth) benthic taxa have such wide geographic distributions.

A novel fish sampling system for ROVs

Mesophotic coral reefs (~30–150 m) and deeper reefs (>150 m) represent some of the least explored and poorly documented habitats in our oceans. These deep ecosystems are both high in biodiversity and ecologically distinct from shallower reefs. They are also equally threatened by natural and anthropogenic factors. Thanks to advancements in remote undersea technologies, our access to and understanding of these poorly studied environments continues to expand. With current data now suggesting more stratified populations of reef fishes and a continuous yield of new species discovered in the mesophotic zone, now under global pressures of climate change and overfishing, the need to explore these ecosystems has never been greater. Most deep-sea (>200 m) biodiversity studies rely on dredging and trawling technologies that are not selective or targeted. Here we present on the design and field demonstration of a compact, low-cost, targeted reef fish sampling system intended for use on inspection-class ROVs in the mesophotic zone. During a research expedition to the Western Province of the Solomon Islands, the system successfully collected multiple novel fish species at depths ranging from 90 to 187 m. The results obtained during this expedition highlight the utility of ROV-based collecting systems, add to the growing knowledge of mesophotic reef communities, and inspire new techniques for sampling mobile faunas at depth.

Biogeochemical variability and trophic status of reef water column following a coral bleaching event

Although resource availability is important for coral reef nutrient cycling and trophic ecology, it is poorly understood how particulate resources may change following an oceanic heat stress event. Here, carbon and nitrogen concentrations and stable isotope values (δ13C and δ15N) of particulate organic matter (POM; 10 and 30 m) were compared from before and after the 2016 mass coral bleaching event in the Maldives, Indian Ocean. To characterize the trophic status of the post-bleaching water column, dissolved inorganic nutrients across shallow to mesophotic coral reefs (10–50 m) and plankton δ13C and δ15N were measured. Eight months after the mass bleaching, an oligotrophic water column with decreased particulate organic nitrogen and variable yet increased mean δ13C of POM was observed. Elemental and isotopic data of particulate resources from pre- and post-bleaching revealed three oceanographic regimes related to ocean mixing. This study highlights the importance of characterizing water column biogeochemistry to understand nutrient cycling in coral reef ecosystems especially post-disturbance.

Fine-scale structure among mesophotic populations of the great star coral Montastraea cavernosa revealed by SNP genotyping.

Mesophotic reefs (30‐150 m) have been proposed as potential refugia that facilitate the recovery of degraded shallow reefs following acute disturbances such as coral bleaching and disease. However, because of the technical difficulty of collecting samples, the connectivity of adjacent mesophotic reefs is relatively unknown compared with shallower counterparts. We used genotyping by sequencing to assess fine‐scale genetic structure of Montastraea cavernosa at two sites at Pulley Ridge, a mesophotic coral reef ecosystem in the Gulf of Mexico, and downstream sites along the Florida Reef Tract. We found differentiation between reefs at Pulley Ridge (~68 m) and corals at downstream upper mesophotic depths in the Dry Tortugas (28–36 m) and shallow reefs in the northern Florida Keys (Key Biscayne, ~5 m). The spatial endpoints of our study were distinct, with the Dry Tortugas as a genetic intermediate. Most striking were differences in population structure among northern and southern sites at Pulley Ridge that were separated by just 12km. Unique patterns of clonality and outlier loci allele frequency support these sites as different populations and suggest that the long‐distance horizontal connectivity typical of shallow‐water corals may not be typical for mesophotic systems in Florida and the Gulf of Mexico. We hypothesize that this may be due to the spawning of buoyant gametes, which commits propagules to the surface, resulting in greater dispersal and lower connectivity than typically found between nearby shallow sites. Differences in population structure over small spatial scales suggest that demographic constraints and/or environmental disturbances may be more variable in space and time on mesophotic reefs compared with their shallow‐water counterparts.

Massive bioconstructions built by Neopycnodonte cochlear (Mollusca, Bivalvia) in a mesophotic environment in the central Mediterranean Sea

The present paper provides a multidisciplinary fine-scale description of a Mediterranean mesophotic new habitat dominated by the bivalve Neopycnodonte cochlear (Poli, 1795), building large and thick pinnacles on vertical cliffs at two study areas along the southern Italian coast. The pinnacles, constituted by a multilayered aggregation of living and dead specimens of N. cochlear, were interconnected with each other to form a framework of high structural complexity, never observed before for this species. The bioconstruction, considerably extended, resulted very complex and diversified in the associated community of structuring organisms. This latter included 165 taxa attributable to different ecological groups occurring in different microhabitats of the bioconstruction. Among the secondary structuring taxa there were scleractinians, serpulids and bryozoans, all contributing to the deposition of calcium carbonate, and poriferans, helping to bind shells together or eroding carbonate by boring species. In comparison with coralligenous sensu stricto and the recently described Mediterranean mesophotic coral reef, the Neopycnodonte bioconstruction showed peculiar features, since it lacked the major contribution of encrusting coralline algae and scleractinians as reef builders, respectively.

A Mesophotic Hotel: The Octocoral Bebryce cf grandicalyx as a Host

Mesophotic coral reefs are ecosystems found below 30 m and down to 150 m depth. Even though interest in these ecosystems has increased in recent years, several aspects still need further attention, including symbiotic associations. During survey of the upper mesophotic reefs in Faafu Atoll, Republic of Maldives, we observed a stable, and likely mutualistic, association between the octocoral Bebryce cf grandicalyx, a sponge belonging to the family Suberitidae, and the hydrozoan Zanclea timida. Additionally, several other organisms were found dwelling on the octocoral and sponge surface, including amphipods, barnacles, entoprocts, flatworms, brittle stars, and foraminiferans. These photographs illustrate the article “A mesophotic hotel: the octocoral Bebryce cf grandicalyx as a host” by D. Maggioni, S. Montano, O. Voigt, D. Seveso and P. Galli published in Ecology. https://doi.org/10.1002/ecy.2950

Global community breaks at 60 m on mesophotic coral reefs

Global Ecology and BiogeographyVolume 29, Issue 4 p. 766-766 CORRIGENDUMFree Access Global community breaks at 60 m on mesophotic coral reefs This article corrects the following: Global community breaks at 60 m on mesophotic coral reefs Michael P. Lesser, Marc Slattery, Jack H. Laverick, Keir J. Macartney, Tom C. Bridge, John Pandolfi, Volume 28Issue 10Global Ecology and Biogeography pages: 1403-1416 First Published online: June 24, 2019 First published: 02 March 2020 https://doi.org/10.1111/geb.13066AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL In Lesser et al. (2019) page 1411, scaling on the y-axis of panel B in Figure 5 was incorrect. The “significant effect of depth” and “figure legend” remains the same, while the “magnitude of the absolute numbers” is different. The correct version is shown below: Figure 5Open in figure viewerPowerPoint Linear regression of particulate organic matter (POM) versus depth on Grand Cayman. Points are means (±SE), with the regression equation, overall model fit R2-value and p value. (a) Particulate organic carbon (POC). (b) Particulate organic nitrogen (PON) The online version has been corrected. The author apologizes for this error and any confusion this may cause. REFERENCE Lesser, M. P., Slattery, M., Laverick, J. H., Macartney, K. J., & Bridge, T. C. (2019). Global community breaks at 60 m on mesophotic coral reefs. Global Ecology and Biogeography, 28(10), 1403– 1416. https://doi.org/10.1111/geb.12940 Volume29, Issue4April 2020Pages 766-766 FiguresReferencesRelatedInformation

Light-Harvesting in Mesophotic Corals is Powered by a Spatially Efficient Photosymbiotic System between Coral Host and Microalgae

The coral-algal photosymbiosis fuels global coral-reef primary productivity, extending from sea level to as deep as 150 m (i.e., mesophotic). Currently, it is largely unknown how such mesophotic reefs thrive despite extremely limited light conditions. Here, we show that corals exhibit a plastic response to mesophotic conditions that involves a spatially optimized regulation of the bio-optical properties by coral host and symbiont. In contrast to shallow corals, mesophotic corals absorbed up to three-fold more light, resulting in excellent photosynthetic response under light conditions of only ~3% of the incident surface irradiance. The enhanced light harvesting capacity of mesophotic corals is regulated by average refractive index fluctuations in the coral skeleton that give rise to optical scattering and facilitate light transport and absorption by densely pigmented host tissue. The results of this study provide fundamental insight into the energy efficiency and light-harvesting mechanisms underlying the productivity of mesophotic coral reef ecosystems, yet also raise concerns regarding their ability to withstand prolonged environmental disturbances.

Zooxanthellate, Sclerite-Free, and Pseudopinnuled Octocoral Hadaka nudidomus gen. nov. et sp. nov. (Anthozoa, Octocorallia) from Mesophotic Reefs of the Southern Ryukyus Islands

Shallow water coral reefs are the most diverse marine ecosystems, but there is an immense gap in knowledge when it comes to understanding the diversity of the vast majority of marine biota in these ecosystems. This is especially true when it comes to understudied small and cryptic coral reef taxa in understudied ecosystems, such as mesophotic coral reef ecosystems (MCEs). MCEs were reported in Japan almost fifty years ago, although only in recent years has there been an increase in research concerning the diversity of these reefs. In this study we describe the first stoloniferous octocoral from MCEs, Hadaka nudidomus gen. nov. et sp. nov., from Iriomote and Okinawa Islands in the southern Ryukyus Islands. The species is zooxanthellate; both specimens host Cladocopium LaJeunesse &amp; H.J.Jeong, 2018 (formerly Symbiodinium ‘Clade C’) and were collected from depths of ~33 to 40 m. Additionally, H. nudidomus gen. nov. et sp. nov. is both sclerite-free and lacks free pinnules, and both of these characteristics are typically diagnostic for octocorals. The discovery and morphology of H. nudidomus gen. nov. et sp. nov. indicate that we still know very little about stoloniferous octocoral diversity in MCEs, their genetic relationships with shallower reef species, and octocoral–symbiont associations. Continued research on these subjects will improve our understanding of octocoral diversity in both shallow and deeper reefs.

Global community breaks at 60 m on mesophotic coral reefs

AbstractAimMesophotic coral ecosystems (MCEs) are unique communities that support a high proportion of depth‐endemic species distinct from shallow‐water coral reefs. However, there is currently little consensus on the boundaries between shallow and mesophotic coral reefs and between upper versus lower MCEs because studies of these communities are often site specific. Here, we examine the ecological evidence for community breaks, defined here as species loss, in fish and benthic taxa between shallow reefs and MCEs globally.LocationGlobal MCEs.Time period1973–2017.Major taxa studiedMacrophytes, Porifera, Scleractinia, Hydrozoa, Octocorallia, Antipatharia and teleost fishes.MethodsWe used random‐effects models and breakpoint analyses on presence/absence data to identify regions of higher than expected species loss along a depth gradient of 1–69 m, based on a meta‐analysis of 26 studies spanning diverse photoautotrophic and heterotrophic taxa. We then investigated the extent to which points of high faunal turnover can be explained by environmental factors, including light, temperature and nutrient availability.ResultsWe found evidence for a community break, indicated by a significant loss of shallow‐water taxa, at ~ 60 m across several taxonomically and functionally diverse benthic groups and geographical regions. The breakpoint in benthic composition is best explained by decreasing light, which is correlated with the optical depths between 10 and 1% of surface irradiance. A concurrent shift in the availability of nutrients, both dissolved and particulate organic matter, and a shift from photoautotroph to heterotroph‐dominated assemblages also occurs at ~ 60 m depth.Main conclusionsWe found evidence for global community breaks across multiple benthic taxa at ~ 60 m depth, indicative of distinct community transitions between shallow and mesophotic coral ecosystems. Changes in the underwater light environment and the availability of trophic resources along the depth gradient are the most parsimonious explanations for the observed patterns.

Genetic connectivity dynamics of the giant barrel sponge, Xestospongia muta, across the Florida Reef Tract and Gulf of Mexico

Resolving the genetic connectivity of coral reef taxa is necessary to understand the community dynamics of these increasingly threatened ecosystems. Herein, we assess the fine scale genetic connectivity of six populations of the Atlantic giant barrel sponge, Xestospongia muta (Schmidt, 1870), using microsatellite markers. This survey included populations from across the Florida Reef Tract and the Gulf of Mexico, including sponges from the Pulley Ridge Habitat Area of Particular Concern, a mesophotic coral reef located approximately 250 km from Florida's (USA) southwestern coast, and the Flower Garden Banks National Marine Sanctuary, the northernmost western Atlantic coral reef ecosystem. Overall, significant population structure was found (F ST = 0.020, 95% CI: 0.011–0.031), with X. muta individuals from the Flower Garden Banks showing the highest levels of genetic differentiation relative to all other surveyed populations (F ST > 0.048). Despite the high levels of population structuring observed, some horizontal and/ or vertical connectivity was found between neighboring reef systems, including evidence of gene flow between mesophotic (Pulley Ridge Habitat of Particular Concern) and photic (Dry Tortugas) reef tracts. Furthermore, largely negligible levels of first-generation migration among discrete genetic populations was observed, suggesting that the persistence of most populations of X. muta is highly dependent on self-recruitment. Thus, while the Pulley Ridge mesophotic reef may provide sponge recruits to shallow photic reefs, its role as a potential refuge and propagule source appears limited to only small geographic scales.