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1. Global community breaks at 60 m on mesophotic coral reefs

   https://doi.org/10.1111/geb.12940  

   Lesser, Michael P. ; Slattery, Marc ; Laverick, Jack H. ; Macartney, Keir J. ; Bridge, Tom C. ; Pandolfi, ed., John ( June 2019 , Global Ecology and Biogeography)

   Mesophotic 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.

   Global MCEs.

   1973–2017.

   Macrophytes, Porifera, Scleractinia, Hydrozoa, Octocorallia, Antipatharia and teleost fishes.

   We 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.

   We 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.

   We 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.

    

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2. Energy Sources of the Depth-Generalist Mixotrophic Coral Stylophora pistillata

   https://doi.org/10.3389/fmars.2020.566663  

   Martinez, Stephane ; Kolodny, Yuval ; Shemesh, Eli ; Scucchia, Federica ; Nevo, Reinat ; Levin-Zaidman, Smadar ; Paltiel, Yossi ; Keren, Nir ; Tchernov, Dan ; Mass, Tali ( November 2020 , Frontiers in Marine Science)

   null (Ed.)

   Energy sources of corals, ultimately sunlight and plankton availability, change dramatically from shallow to mesophotic (30–150 m) reefs. Depth-generalist corals, those that occupy both of these two distinct ecosystems, are adapted to cope with such extremely diverse conditions. In this study, we investigated the trophic strategy of the depth-generalist hermatypic coral Stylophora pistillata and the ability of mesophotic colonies to adapt to shallow reefs. We compared symbiont genera composition, photosynthetic traits and the holobiont trophic position and carbon sources, calculated from amino acids compound-specific stable isotope analysis (AA-CSIA), of shallow, mesophotic and translocated corals. This species harbors different Symbiodiniaceae genera at the two depths: Cladocopium goreaui (dominant in mesophotic colonies) and Symbiodinium microadriaticum (dominant in shallow colonies) with a limited change after transplantation. This allowed us to determine which traits stem from hosting different symbiont species compositions across the depth gradient. Calculation of holobiont trophic position based on amino acid δ 15 N revealed that heterotrophy represents the same portion of the total energy budget in both depths, in contrast to the dogma that predation is higher in corals growing in low light conditions. Photosynthesis is the major carbon source to corals growing at both depths, but the photosynthetic rate is higher in the shallow reef corals, implicating both higher energy consumption and higher predation rate in the shallow habitat. In the corals transplanted from deep to shallow reef, we observed extensive photo-acclimation by the Symbiodiniaceae cells, including substantial cellular morphological modifications, increased cellular chlorophyll a , lower antennae to photosystems ratios and carbon signature similar to the local shallow colonies. In contrast, non-photochemical quenching remains low and does not increase to cope with the high light regime of the shallow reef. Furthermore, host acclimation is much slower in these deep-to-shallow transplanted corals as evident from the lower trophic position and tissue density compared to the shallow-water corals, even after long-term transplantation (18 months). Our results suggest that while mesophotic reefs could serve as a potential refuge for shallow corals, the transition is complex, as even after a year and a half the acclimation is only partial. 

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3. Production of phytodetritus by a coral reef sponge increases from shallow to mesophotic depths

   https://doi.org/10.1002/lno.12342  

   Lesser, Michael P. ; Macartney, Keir J. ; Slattery, Marc ( April 2023 , Limnology and Oceanography)

   On Caribbean coral reefs, sponges are important members of the benthic community and have an important role in consuming particulate organic matter (POM) and dissolved organic matter (DOM), with the subsequent production of detritus that is then shunted into a process now referred to as the “sponge‐loop.” An emergent species of sponge commonly found on Caribbean coral reefs,Agelas tubulata, increases in size and growth rate from shallow (< 30 m) to mesophotic depths (30–150 m) on Grand Cayman Island.A.tubulatadepends largely on heterotrophy across shallow to mesophotic depths and has been shown to utilize detritus on shallow reefs. However, detritus production byA.tubulataon shallow and mesophotic coral reefs has not been previously reported. Here we show, using flow cytometry, that sponge detritus includes a previously unquantified component, phytodetritus. Sponge phytodetritus production was shown experimentally to be greater in sponges from mesophotic depths compared to sponges from shallow coral reefs. Additionally, the size range of this phytodetritus corresponds to the size range of autotrophic picoplankton, primarily prochlorophytes, known to be an important food source for filter‐feeding sponges. Given the known lability of phytodetritus, compared to other more recalcitrant components of the detrital pool, its role in the food web of mesophotic communities combined with the increased availability of live POM, may be an underappreciated component of mesophotic community carbon and nitrogen flow.

    

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4. To what extent do mesophotic coral ecosystems and shallow reefs share species of conservation interest? A systematic review

   Laverick, J.H. Piango ( January 2018 , Environmental evidence)

   Abstract Background: Globally, shallow-water coral reef biodiversity is at risk from a variety of threats, some of which may attenuate with depth. Mesophotic coral ecosystems (MCEs), occurring from 30 to 40 m and deeper in tropical locations, have been subject to a surge of research this century. Though a number of valuable narrative reviews exist, a systematic quantitative synthesis of published MCE studies is lacking. We conducted a systematic review to collate mesophotic research, including studies from the twentieth century to the present. We highlight current biases in research effort, regarding locations and subject matter, and suggest where more attention may be particularly valuable. Following a notable number of studies considering the potential for mesophotic reefs to act as refuges, it is important to know how comprehensive these sources of recruits and organisms capable of moving to shallow water reefs may be. Methods: We search seven sources of bibliographic data with two search strings, as well as personal libraries. Articles were included if they contained species presence data from both shallower and deeper than 30 m depth on tropical coral reefs. Studies were critically appraised based on the number of species identified and balanced sampling effort with depth. Maximum and minimum depths per species were extracted from each study, along with study region and taxon. We quantified the degree of community overlap between shallow tropical reefs (< 30 m) and reefs surveyed at the same locations below 30 m. Proportions of shallow species, across all studied taxa, observed deeper than 30 m were used to generate log odds ratios and passed to a mixed-effects model. Study location and taxon were included as effect modifiers. Funnel plots, regression tests, fail safe numbers, and analysis of a high validity subgroup contributed to sensitivity analyses and tests of bias. Results: Across all studies synthesised we found two-thirds of shallow species were present on mesophotic reefs. Further analysis by taxon and broad locations show that this pattern is influenced geographically and taxonomically. Community overlap was estimated as low as 26% and as high as 97% for some cases. Conclusions: There is clear support for the hypothesis that protecting mesophotic reefs will also help to conserve shallow water species. At the same time, it is important to note that this study does not address mesophotic-specialist communities, or the ecological forces which would permit refuge dynamics. As we limit our analysis to species only present above 30 m it is also possible diversity found exclusively deeper than 30 m warrants protection in its own right. Further research into relatively ignored taxa and geographic regions will help improve the design of protected areas in future. Keywords: Depth, Community structure, Biodiversity, Coral reefs, Twilight zone, Refuge 

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5. Depth-dependent trophic strategies of Caribbean sponges on mesophotic coral reefs

   https://doi.org/10.3354/meps14059  

   Macartney, KJ ; Pankey, MS ; Clayshulte Abraham, A ; Slattery, M ; Lesser, MP ( July 2022 , Marine Ecology Progress Series)

   Mesophotic coral reef ecosystems (MCEs) are characterized by gradients in irradiance, temperature and trophic resources. As depth increases on Caribbean mesophotic reefs, particulate organic matter increases while dissolved organic matter decreases, and the increase in particulate organic matter is directly related to the increase in sponge abundances and growth rates on MCEs. To further understand the trophic ecology of sponges, changes in microbiome composition and function, stable isotopic composition and proximate biochemical composition of 4 Caribbean reef sponges ( Amphimedon compressa , Agelas tubulata , Plakortis angulospiculatus and Xestospongia muta) were quantified along a shallow to mesophotic depth gradient on Grand Cayman Island. Increases in δ 15 N for all sponges were observed as depth increased, indicating an increasing reliance on heterotrophic food resources. Species-specific changes in symbiotic microbial community composition were also observed as depth increased, and the predicted functional genes associated with nitrogen and carbon cycling showed species-specific changes between depths. Regardless of species-specific changes in microbiome community structure or function, or whether sponges were classified as high microbial or low microbial abundance, sponges increased their consumption of particulate organic matter with increasing depth into the lower mesophotic zone. 

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