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  1. Abstract Aim

    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.

    Location

    Global MCEs.

    Time period

    1973–2017.

    Major taxa studied

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

    Methods

    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.

    Results

    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.

    Main conclusions

    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. Abstract

    Sponges are important ecological and functional components of coral reefs. Recently, a new hypothesis about the functional ecology of sponges in organic matter recycling pathways, the sponge‐loop hypothesis, in which dissolved and particulate organic matter is taken up by sponges and shunted to higher trophic levels as detritus, has been proposed and demonstrated for shallow (< 30 m) cryptic species. However, support for this hypothesis at mesophotic depths (∼ 30–150 m) is lacking. Here, we examined detritus production, a prerequisite of the sponge loop pathway, in a reciprocal transplant experiment, usingHalisarca caeruleafrom water depths of 10 and 50 m. Detritus production was significantly lower in mesophotic sponges compared to shallow samples ofH. caerulea. Additionally, detritus production rates in transplanted sponges moved in the direction of rates observed for resident conspecifics. The microbiome of these sponge populations was also significantly different between shallow and mesophotic depths, and the microbial communities of the transplanted sponges also shifted in the direction of their new depth in 10 d largely driven by changes inOxyphotobacteria,Acidimicrobiia,Nitrososphaeria,Nitrospira,Deltaproteobacteria, andDadabacteriia. This occurred in an environment where the availability of both dissolved and particulate trophic resources changed significantly across the shallow to mesophotic depth gradient where these sponge populations were found. These results suggest that changes in sponge detritus production are primarily driven by differential quality and quantity of trophic resources, as well as their utilization by the sponge host, and its microbiome, along the shallow to mesophotic depth gradient.

     
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  3. Mesophotic coral reefs, currently defined as deep reefs between 30 and 150 m, are linked physically and biologically to their shallow water counterparts, have the potential to be refuges for shallow coral reef taxa such as coral and sponges, and might be a source of larvae that could contribute to the resiliency of shallow water reefs. Mesophotic coral reefs are found worldwide, but most are undescribed and understudied. Here, we review our current knowledge of mesophotic coral reefs and their functional ecology as it relates to their geomorphology, changes in the abiotic environment along depth gradients, trophic ecology, their reproduction, and their connectivity to shallow depths. Understanding the ecology of mesophotic coral reefs, and the connectivity between them and their shallow water counterparts, is now a primary focus for many reef studies as the worldwide degradation of shallow coral reefs, and the ecosystem services they provide, continues unabated. 
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  4. 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. 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 mby 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. 
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