How do we best synergize climate mitigation actions to co-benefit biodiversity?

A multitude of actions to protect, sustainably manage and restore natural and modified ecosystems can have co-benefits for both climate mitigation and biodiversity conservation. Reducing greenhouse emissions to limit warming to less than 1.5 or 2°C above preindustrial levels, as outlined in the Paris Agreement, can yield strong co-benefits for land, freshwater and marine biodiversity and reduce amplifying climate feedbacks from ecosystem changes. Not all climate mitigation strategies are equally effective at producing biodiversity co-benefits, some in fact are counterproductive. Moreover, social implications are often overlooked within the climate-biodiversity nexus. Protecting biodiverse and carbon-rich natural environments, ecological restoration of potentially biodiverse and carbon-rich habitats, the deliberate creation of novel habitats, taking into consideration a locally adapted and meaningful (i.e. full consequences considered) mix of these measures, can result in the most robust win-win solutions. These can be further enhanced by avoidance of narrow goals, taking long-term views and minimizing further losses of intact ecosystems. In this review paper, we first discuss various climate mitigation actions that evidence demonstrates can negatively impact biodiversity, resulting in unseen and unintended negative consequences. We then examine climate mitigation actions that co-deliver biodiversity and societal benefits. We give examples of these win-win solutions, categorized as 'protect, restore, manage and create', in different regions of the world that could be expanded, upscaled and used for further innovation.

Actions to halt biodiversity loss generally benefit the climate.

The two most urgent and interlinked environmental challenges humanity faces are climate change and biodiversity loss. We are entering a pivotal decade for both the international biodiversity and climate change agendas with the sharpening of ambitious strategies and targets by the Convention on Biological Diversity and the United Nations Framework Convention on Climate Change. Within their respective Conventions, the biodiversity and climate interlinked challenges have largely been addressed separately. There is evidence that conservation actions that halt, slow or reverse biodiversity loss can simultaneously slow anthropogenic mediated climate change significantly. This review highlights conservation actions which have the largest potential for mitigation of climate change. We note that conservation actions have mainly synergistic benefits and few antagonistic trade-offs with climate change mitigation. Specifically, we identify direct co-benefits in 14 out of the 21 action targets of the draft post-2020 global biodiversity framework of the Convention on Biological Diversity, notwithstanding the many indirect links that can also support both biodiversity conservation and climate change mitigation. These relationships are context and scale-dependent; therefore, we showcase examples of local biodiversity conservation actions that can be incentivized, guided and prioritized by global objectives and targets. The close interlinkages between biodiversity, climate change mitigation, other nature's contributions to people and good quality of life are seldom as integrated as they should be in management and policy. This review aims to re-emphasize the vital relationships between biodiversity conservation actions and climate change mitigation in a timely manner, in support to major Conferences of Parties that are about to negotiate strategic frameworks and international goals for the decades to come.

Phylogenetic and functional evidence suggests that deep-ocean ecosystems are highly sensitive to environmental change and direct human disturbance.

An understanding of the balance of interspecific competition and the physical environment in structuring organismal communities is crucial because those communities structured primarily by their physical environment typically exhibit greater sensitivity to environmental change than those structured predominantly by competitive interactions. Here, using detailed phylogenetic and functional information, we investigate this question in macrofaunal assemblages from Northwest Atlantic Ocean continental slopes, a high seas region projected to experience substantial environmental change through the current century. We demonstrate assemblages to be both phylogenetically and functionally under-dispersed, and thus conclude that the physical environment, not competition, may dominate in structuring deep-ocean communities. Further, we find temperature and bottom trawling intensity to be among the environmental factors significantly related to assemblage diversity. These results hint that deep-ocean communities are highly sensitive to their physical environment and vulnerable to environmental perturbation, including by direct disturbance through fishing, and indirectly through the changes brought about by climate change.

Comparative population genomics reveals key barriers to dispersal in Southern Ocean penguins.

The mechanisms that determine patterns of species dispersal are important factors in the production and maintenance of biodiversity. Understanding these mechanisms helps to forecast the responses of species to environmental change. Here, we used a comparative framework and genomewide data obtained through RAD-Seq to compare the patterns of connectivity among breeding colonies for five penguin species with shared ancestry, overlapping distributions and differing ecological niches, allowing an examination of the intrinsic and extrinsic barriers governing dispersal patterns. Our findings show that at-sea range and oceanography underlie patterns of dispersal in these penguins. The pelagic niche of emperor (Aptenodytes forsteri), king (A. patagonicus), Adélie (Pygoscelis adeliae) and chinstrap (P. antarctica) penguins facilitates gene flow over thousands of kilometres. In contrast, the coastal niche of gentoo penguins (P. papua) limits dispersal, resulting in population divergences. Oceanographic fronts also act as dispersal barriers to some extent. We recommend that forecasts of extinction risk incorporate dispersal and that management units are defined by at-sea range and oceanography in species lacking genetic data.

The Biology of Seamounts: 25 Years on.

Seamounts are one of the major biomes of the global ocean. The last 25 years of research has seen considerable advances in the understanding of these ecosystems. The interactions between seamounts and steady and variable flows have now been characterised providing a better mechanistic understanding of processes influencing biology. Processes leading to upwelling, including Taylor column formation and tidal rectification, have now been defined as well as those leading to draw down of organic matter from the ocean surface to seamount summit and flanks. There is also an improved understanding of the interactions between seamounts, zooplankton and micronekton communities especially with respect to increased predation pressure in the vicinity of seamounts. Evidence has accumulated of the role of seamounts as hot spots for ocean predators including large pelagic fish, sharks, pinnipeds, cetaceans and seabirds. The complexity of benthic communities associated with seamounts is high and drivers of biodiversity are now being resolved. Claims of high endemism resulting from isolation of seamounts as islands of habitat and speciation have not been supported. However, for species characterised by low dispersal capability, such as some groups of benthic sessile or low-mobility invertebrates, low connectivity between seamount populations has been found with evidence of endemism at a local level. Threats to seamounts have increased in the last 25 years and include overfishing, destructive fishing, marine litter, direct and indirect impacts of climate change and potentially marine mining in the near future. Issues around these threats and their management are discussed.

The challenges of detecting subtle population structure and its importance for the conservation of emperor penguins.

Understanding the boundaries of breeding populations is of great importance for conservation efforts and estimates of extinction risk for threatened species. However, determining these boundaries can be difficult when population structure is subtle. Emperor penguins are highly reliant on sea ice, and some populations may be in jeopardy as climate change alters sea-ice extent and quality. An understanding of emperor penguin population structure is therefore urgently needed. Two previous studies have differed in their conclusions, particularly whether the Ross Sea, a major stronghold for the species, is isolated or not. We assessed emperor penguin population structure using 4,596 genome-wide single nucleotide polymorphisms (SNPs), characterized in 110 individuals (10-16 per colony) from eight colonies around Antarctica. In contrast to a previous conclusion that emperor penguins are panmictic around the entire continent, we find that emperor penguins comprise at least four metapopulations, and that the Ross Sea is clearly a distinct metapopulation. Using larger sample sizes and a thorough assessment of the limitations of different analytical methods, we have shown that population structure within emperor penguins does exist and argue that its recognition is vital for the effective conservation of the species. We discuss the many difficulties that molecular ecologists and managers face in the detection and interpretation of subtle population structure using large SNP data sets, and argue that subtle structure should be taken into account when determining management strategies for threatened species, until accurate estimates of demographic connectivity among populations can be made.

Dispersal in the sub-Antarctic: king penguins show remarkably little population genetic differentiation across their range.

BACKGROUND: Seabirds are important components of marine ecosystems, both as predators and as indicators of ecological change, being conspicuous and sensitive to changes in prey abundance. To determine whether fluctuations in population sizes are localised or indicative of large-scale ecosystem change, we must first understand population structure and dispersal. King penguins are long-lived seabirds that occupy a niche across the sub-Antarctic zone close to the Polar Front. Colonies have very different histories of exploitation, population recovery, and expansion. RESULTS: We investigated the genetic population structure and patterns of colonisation of king penguins across their current range using a dataset of 5154 unlinked, high-coverage single nucleotide polymorphisms generated via restriction site associated DNA sequencing (RADSeq). Despite breeding at a small number of discrete, geographically separate sites, we find only very slight genetic differentiation among colonies separated by thousands of kilometers of open-ocean, suggesting migration among islands and archipelagos may be common. Our results show that the South Georgia population is slightly differentiated from all other colonies and suggest that the recently founded Falkland Island colony is likely to have been established by migrants from the distant Crozet Islands rather than nearby colonies on South Georgia, possibly as a result of density-dependent processes. CONCLUSIONS: The observed subtle differentiation among king penguin colonies must be considered in future conservation planning and monitoring of the species, and demographic models that attempt to forecast extinction risk in response to large-scale climate change must take into account migration. It is possible that migration could buffer king penguins against some of the impacts of climate change where colonies appear panmictic, although it is unlikely to protect them completely given the widespread physical changes projected for their Southern Ocean foraging grounds. Overall, large-scale population genetic studies of marine predators across the Southern Ocean are revealing more interconnection and migration than previously supposed.

Evolutionary dynamics of a common sub-Antarctic octocoral family.

Sequence data were obtained for five different loci, both mitochondrial (cox1, mtMutS, 16S) and nuclear (18S, 28S rDNA), from 64 species representing 25 genera of the common deep-sea octocoral family Primnoidae. We tested the hypothesis that Primnoidae have an Antarctic origin, as this is where they currently have high species richness, using Maximum likelihood and Bayesian inference methods of phylogenetic analysis. Using a time-calibrated molecular phylogeny we also investigated the time of species radiation in sub-Antarctic Primnoidae. Our relatively wide taxon sampling and phylogenetic analysis supported Primnoidae as a monophyletic family. The base of the well-supported phylogeny was Pacific in origin, indicating Primnoidae sub-Antarctic diversity is a secondary species radiation. There is also evidence for a subsequent range extension of sub-Antarctic lineages into deep-water areas of the Indian and Pacific Oceans. Conservative and speculative fossil-calibration analyses resulted in two differing estimations of sub-Antarctic species divergence times. Conservative analysis suggested a sub-Antarctic species radiation occurred ∼52MYA (95% HPD: 36-73MYA), potentially before the opening of the Drake Passage and Antarctic Circumpolar Current (ACC) formation (41-37MYA). Speculative analysis pushed this radiation back into the late Jurassic, 157MYA (95% HPD: 118-204MYA). Genus-level groupings were broadly supported in this analysis with some notable polyphyletic exceptions: Callogorgia, Fanellia, Primnoella, Plumarella, Thouarella. Molecular and morphological evidence supports the placement of Tauroprimnoa austasensis within Dasystenella and Fannyella kuekenthali within Metafannyella.

Too much of a good thing: sea ice extent may have forced emperor penguins into refugia during the last glacial maximum.

The relationship between population structure and demographic history is critical to understanding microevolution and for predicting the resilience of species to environmental change. Using mitochondrial DNA from extant colonies and radiocarbon-dated subfossils, we present the first microevolutionary analysis of emperor penguins (Aptenodytes forsteri) and show their population trends throughout the last glacial maximum (LGM, 19.5-16 kya) and during the subsequent period of warming and sea ice retreat. We found evidence for three mitochondrial clades within emperor penguins, suggesting that they were isolated within three glacial refugia during the LGM. One of these clades has remained largely isolated within the Ross Sea, while the two other clades have intermixed around the coast of Antarctica from Adélie Land to the Weddell Sea. The differentiation of the Ross Sea population has been preserved despite rapid population growth and opportunities for migration. Low effective population sizes during the LGM, followed by a rapid expansion around the beginning of the Holocene, suggest that an optimum set of sea ice conditions exist for emperor penguins, corresponding to available foraging area.

The heart of a dragon: 3D anatomical reconstruction of the 'scaly-foot gastropod' (Mollusca: Gastropoda: Neomphalina) reveals its extraordinary circulatory system.

INTRODUCTION: The 'scaly-foot gastropod' (Chrysomallon squamiferum Chen et al., 2015) from deep-sea hydrothermal vent ecosystems of the Indian Ocean is an active mobile gastropod occurring in locally high densities, and it is distinctive for the dermal scales covering the exterior surface of its foot. These iron-sulfide coated sclerites, and its nutritional dependence on endosymbiotic bacteria, are both noted as adaptations to the extreme environment in the flow of hydrogen sulfide. We present evidence for other adaptations of the 'scaly-foot gastropod' to life in an extreme environment, investigated through dissection and 3D tomographic reconstruction of the internal anatomy. RESULTS: Our anatomical investigations of juvenile and adult specimens reveal a large unganglionated nervous system, a simple and reduced digestive system, and that the animal is a simultaneous hermaphrodite. We show that Chrysomallon squamiferum relies on endosymbiotic bacteria throughout post-larval life. Of particular interest is the circulatory system: Chrysomallon has a very large ctenidium supported by extensive blood sinuses filled with haemocoel. The ctenidium provides oxygen for the host but the circulatory system is enlarged beyond the scope of other similar vent gastropods. At the posterior of the ctenidium is a remarkably large and well-developed heart. Based on the volume of the auricle and ventricle, the heart complex represents approximately 4 % of the body volume. This proportionally giant heart primarily sucks blood through the ctenidium and supplies the highly vascularised oesophageal gland. Thus we infer the elaborate cardiovascular system most likely evolved to oxygenate the endosymbionts in an oxygen poor environment and/or to supply hydrogen sulfide to the endosymbionts. CONCLUSIONS: This study exemplifies how understanding the autecology of an organism can be enhanced by detailed investigation of internal anatomy. This gastropod is a large and active species that is abundant in its hydrothermal vent field ecosystem. Yet all of its remarkable features-protective dermal sclerites, circulatory system, high fecundity-can be viewed as adaptations beneficial to its endosymbiont microbes. We interpret these results to show that, as a result of specialisation to resolve energetic needs in an extreme chemosynthetic environment, this dramatic dragon-like species has become a carrying vessel for its bacteria.

The discovery of new deep-sea hydrothermal vent communities in the southern ocean and implications for biogeography.

Since the first discovery of deep-sea hydrothermal vents along the Galápagos Rift in 1977, numerous vent sites and endemic faunal assemblages have been found along mid-ocean ridges and back-arc basins at low to mid latitudes. These discoveries have suggested the existence of separate biogeographic provinces in the Atlantic and the North West Pacific, the existence of a province including the South West Pacific and Indian Ocean, and a separation of the North East Pacific, North East Pacific Rise, and South East Pacific Rise. The Southern Ocean is known to be a region of high deep-sea species diversity and centre of origin for the global deep-sea fauna. It has also been proposed as a gateway connecting hydrothermal vents in different oceans but is little explored because of extreme conditions. Since 2009 we have explored two segments of the East Scotia Ridge (ESR) in the Southern Ocean using a remotely operated vehicle. In each segment we located deep-sea hydrothermal vents hosting high-temperature black smokers up to 382.8°C and diffuse venting. The chemosynthetic ecosystems hosted by these vents are dominated by a new yeti crab (Kiwa n. sp.), stalked barnacles, limpets, peltospiroid gastropods, anemones, and a predatory sea star. Taxa abundant in vent ecosystems in other oceans, including polychaete worms (Siboglinidae), bathymodiolid mussels, and alvinocaridid shrimps, are absent from the ESR vents. These groups, except the Siboglinidae, possess planktotrophic larvae, rare in Antarctic marine invertebrates, suggesting that the environmental conditions of the Southern Ocean may act as a dispersal filter for vent taxa. Evidence from the distinctive fauna, the unique community structure, and multivariate analyses suggest that the Antarctic vent ecosystems represent a new vent biogeographic province. However, multivariate analyses of species present at the ESR and at other deep-sea hydrothermal vents globally indicate that vent biogeography is more complex than previously recognised.

Body Size Versus Depth: Regional and Taxonomical Variation in Deep-Sea Meio- and Macrofaunal Organisms.

Body size (weight per individual) is an important concept in ecology. It has been studied in the deep sea where a decrease in size with increasing depth has often been found. This has been explained as an adaptation to food limitation where size reduction results in a lowered metabolic rate and a decreased energetic requirement. However, observations vary, with some studies showing an increase in size with depth, and some finding no depth correlation at all. Here, we collected data from peer-reviewed studies on macro- and meiofaunal abundance and biomass, creating two datasets allowing statistical comparison of factors expected to influence body size in meio- and macrofaunal organisms. Our analyses examined the influence of region, taxonomic group and sampling method on the body size of meiofauna and macrofauna in the deep sea with increasing depth, and the resulting models are presented. At the global scale, meio- and macrofaunal communities show a decrease in body size with increasing depth as expected with the food limitation hypothesis. However, at the regional scale there were differences in trends of body size with depth, either showing a decrease (e.g. southwest Pacific Ocean; meio- and macrofauna) or increase (e.g. Gulf of Mexico; meiofauna only) compared to a global mean. Taxonomic groups also showed differences in body size trends compared to total community average (e.g. Crustacea and Bivalvia). Care must be taken when conducting these studies, as our analyses indicated that sampling method exerts a significant influence on research results. It is possible that differences in physiology, lifestyle and life history characteristics result in different responses to an increase in depth and/or decrease in food availability. This will have implications in the future as food supply to the deep sea changes as a result of climate change (e.g. increased ocean stratification at low to mid latitudes and reduced sea ice duration at high latitudes).

A Biophysical and Economic Profile of South Georgia and the South Sandwich Islands as Potential Large-Scale Antarctic Protected Areas.

The current hiatus in the establishment of a network of marine protected areas (MPAs) in the Antarctic means that other routes to conservation are required. The protection of overseas territories in the Antarctic and sub-Antarctic represents one way to advance the initiation of such a network. This review of the physical and biological features of the United Kingdom (U.K.) overseas territories of South Georgia and South Sandwich Islands (SGSSI) is undertaken to estimate the importance of the islands in terms of marine conservation in the Southern Ocean and globally. The economy and management of SGSSI are also analysed, and the question of whether the islands already have sufficient protection to constitute part of an Antarctic network of MPAs is assessed. The SGSSI comprise unique geological and physical features, a diverse marine biota, including a significant proportion of endemic species and globally important breeding populations of marine predators. Regardless of past exploitation of biotic resources, such as seals, whales and finfish, SGSSI would make a significant contribution to biological diversity in an Antarctic network of MPAs. At present, conservation measures do not adequately protect all of the biological features that render the islands so important in terms of conservation at a regional and global level. However, a general lack of data on Antarctic marine ecosystems (particularly needed for SGSSSI) makes it difficult to assess this fully. One barrier to achieving more complete protection is the continuing emphasis on fishing effort in these waters by U.K. government. Other non-U.K. Antarctic overseas territories of conservation importance are also compromised as MPAs because of the exploitation of fisheries resources in their waters. The possible non-use values of SGSSI as well as the importance of ecosystem services that are indirectly used by people are outlined in this review. Technology is improving the potential for management of remote MPAs, particularly in the context of incursion by illegal fishing activities and use of satellite surveillance for enforcement of fisheries and conservation regulations. The conflict between commercial exploitation and conservation of Antarctic marine living resources is explored.

Phylogeny and systematics of deep-sea sea pens (Anthozoa: Octocorallia: Pennatulacea).

Molecular methods have been used for the first time to determine the phylogeny of families, genera and species within the Pennatulacea (sea pens). Variation in ND2 and mtMutS mitochondrial protein-coding genes proved adequate to resolve phylogenetic relationships among pennatulacean families. The gene mtMutS is more variable than ND2 and differentiates all genera, and many pennatulacean species. A molecular phylogeny based on a Bayesian analysis reveals that suborder Sessiliflorae is paraphyletic and Subselliflorae is polyphyletic. Many families of pennatulaceans do not represent monophyletic groups including Umbellulidae, Pteroeididae, and Kophobelemnidae. The high frequency of morphological homoplasy in pennatulaceans has led to many misinterpretations in the systematics of the group. The traditional classification scheme for pennatulaceans requires revision.

Habitat Protection Indexes - new monitoring measures for the conservation of coastal and marine habitats.

A worldwide call to implement habitat protection aims to halt biodiversity loss. We constructed an open-source, standardized, and reproducible workflow that calculates two indexes to monitor the extent of coastal and marine habitats within protected areas and other effective area-based conservation measures. The Local Proportion of Habitats Protected Index (LPHPI) pinpoints the jurisdictions with the greatest opportunity to expand their protected or conserved areas, while the Global Proportion of Habitats Protected Index (GPHPI) showcases which jurisdictions contribute the most area to the protection of these habitats globally. We also evaluated which jurisdictions have the highest opportunity to contribute globally to protecting habitats by meeting a target of 30% coverage. We found that Areas Beyond National Jurisdiction (ABNJ) have the greatest potential to do so. Our workflow can also be easily extended to terrestrial and freshwater habitats. These indexes are helpful to monitor aspects of the Sustainable Development Goal 14 and the emerging post-2020 global biodiversity framework, to understand the current status of international cooperation on coastal and marine habitats conservation.

Discovering marine biodiversity in the 21st century.

We review the current knowledge of the biodiversity of the ocean as well as the levels of decline and threat for species and habitats. The lack of understanding of the distribution of life in the ocean is identified as a significant barrier to restoring its biodiversity and health. We explore why the science of taxonomy has failed to deliver knowledge of what species are present in the ocean, how they are distributed and how they are responding to global and regional to local anthropogenic pressures. This failure prevents nations from meeting their international commitments to conserve marine biodiversity with the results that investment in taxonomy has declined in many countries. We explore a range of new technologies and approaches for discovery of marine species and their detection and monitoring. These include: imaging methods, molecular approaches, active and passive acoustics, the use of interconnected databases and citizen science. Whilst no one method is suitable for discovering or detecting all groups of organisms many are complementary and have been combined to give a more complete picture of biodiversity in marine ecosystems. We conclude that integrated approaches represent the best way forwards for accelerating species discovery, description and biodiversity assessment. Examples of integrated taxonomic approaches are identified from terrestrial ecosystems. Such integrated taxonomic approaches require the adoption of cybertaxonomy approaches and will be boosted by new autonomous sampling platforms and development of machine-speed exchange of digital information between databases.

Changes in zooplankton communities from epipelagic to lower mesopelagic waters.

Zooplankton form a trophic link between primary producers and higher trophic levels, and exert significant influence on the vertical transport of carbon through the water column ('biological carbon pump'). Using a MultiNet we sampled and studied mesozooplankton communities (i.e. >0.2 mm) from six locations around Bermuda targeting four depth zones: ∼0-200 m, ∼200-400 m, ∼400-600 m (deep-scattering layer), and ∼600-800 m. Copepoda, our focal taxonomic group, consistently dominated samples (∼80% relative abundance). We report declines in zooplankton and copepod abundance with depth, concurrent with decreases in food availability. Taxonomic richness was lowest at depth and below the deep-scattering layer. In contrast, copepod diversity peaked at these depths, suggesting lower competitive displacement in these more food-limited waters. Finally, omnivory and carnivory, were the dominant trophic traits, each one affecting the biological carbon pump in a different way. This highlights the importance of incorporating data on zooplankton food web structure in future modelling of global ocean carbon cycling.

Investigating the environmental drivers of deep-seafloor biodiversity: A case study of peracarid crustacean assemblages in the Northwest Atlantic Ocean.

The deep-sea benthos covers over 90% of seafloor area and hosts a great diversity of species which contribute toward essential ecosystem services. Evidence suggests that deep-seafloor assemblages are structured predominantly by their physical environment, yet knowledge of assemblage/environment relationships is limited. Here, we utilized a very large dataset of Northwest Atlantic Ocean continental slope peracarid crustacean assemblages as a case study to investigate the environmental drivers of deep-seafloor macrofaunal biodiversity. We investigated biodiversity from a phylogenetic, functional, and taxonomic perspective, and found that a wide variety of environmental drivers, including food availability, physical disturbance (bottom trawling), current speed, sediment characteristics, topographic heterogeneity, and temperature (in order of relative importance), significantly influenced peracarid biodiversity. We also found deep-water peracarid assemblages to vary seasonally and interannually. Contrary to prevailing theory on the drivers of deep-seafloor diversity, we found high topographic heterogeneity (at the hundreds to thousands of meter scale) to negatively influence assemblage diversity, while broadscale sediment characteristics (i.e., percent sand content) were found to influence assemblages more than sediment particle-size diversity. However, our results support other paradigms of deep-seafloor biodiversity, including that assemblages may vary inter- and intra-annually, and how assemblages respond to changes in current speed. We found that bottom trawling negatively affects the evenness and diversity of deep-sea soft-sediment peracarid assemblages, but that predicted changes in ocean temperature as a result of climate change may not strongly influence continental slope biodiversity over human timescales, although it may alter deep-sea community biomass. Finally, we emphasize the value of analyzing multiple metrics of biodiversity and call for researchers to consider an expanded definition of biodiversity in future investigations of deep-ocean life.

Low connectivity between shallow, mesophotic and rariphotic zone benthos.

Worldwide coral reefs face catastrophic damage due to a series of anthropogenic stressors. Investigating how coral reefs ecosystems are connected, in particular across depth, will help us understand if deeper reefs harbour distinct communities. Here, we explore changes in benthic community structure across 15-300 m depths using technical divers and submersibles around Bermuda. We report high levels of floral and faunal differentiation across depth, with distinct assemblages occupying each depth surveyed, except 200-300 m, corresponding to the lower rariphotic zone. Community turnover was highest at the boundary depths of mesophotic coral ecosystems (30-150 m) driven largely by taxonomic turnover and to a lesser degree by ordered species loss (nestedness). Our work highlights the biologically unique nature of benthic communities in the mesophotic and rariphotic zones, and their limited connectivity to shallow reefs, thus emphasizing the need to manage and protect deeper reefs as distinct entities.