A comprehensive investigation of mesophotic coral ecosystems in the Hawaiian Archipelago

Invertebrate sounds from photic to mesophotic coral reefs reveal vertical stratification and diel diversity

Although mesophotic coral ecosystems account for approximately 80% of coral reefs, they remain largely unexplored due to their challenging accessibility. The acoustic richness within reefs has led scientists to consider passive acoustic monitoring as a reliable method for studying both altiphotic and mesophotic coral reefs. We investigated the relationship between benthic invertebrate sounds (1.5-22.5 kHz), depth, and benthic cover composition, key ecological factors that determine differences between altiphotic and mesophotic reefs. Diel patterns of snaps and peak frequencies were also explored at different depths to assess variations in biorhythms. Acoustic recorders were deployed at 20 m, 60 m, and 120 m depths across six islands in French Polynesia. The results indicated that depth is the primary driver of differences in broadband transient sound (BTS) soundscapes, with sound intensity decreasing as depth increases. At 20-60 m, sounds were louder at night. At 120 m depth, benthic activity rhythms exhibited low or highly variable levels of diel variation, likely a consequence of reduced solar irradiation. On three islands, a peculiar peak in the number of BTS was observed every day between 7 and 9 PM at 120 m, suggesting the presence of cyclic activities of a specific species. Our results support the existence of different invertebrate communities or distinct behaviors, particularly in deep mesophotic reefs. Overall, this study adds to the growing evidence supporting the use of passive acoustic monitoring to describe and understand ecological patterns in mesophotic reefs.

Upper mesophotic reef fish assemblages at Bahía de Banderas, Mexico

There is no information on the species associated with the mesophotic reefs of Banderas Bay, located in the central Mexican Pacific. This study analysed the reef fish assemblage from three depths (50, 60 and 70 m) in three sampling sites of the southern submarine canyon of the Bay: Los Arcos, Bajo de Emirio and Majahuitas. Several analyses were performed to test the hypothesis that there are important differences in fish abundance and species composition between sites and depths. Twenty-two species of bony fishes grouped in 14 families were recorded. PERMANOVA results showed that there were no significant differences in fish diversity parameters between sites, indicating a certain uniformity in their distribution. However, nine species were exclusive to one site and depth (five singleton species with only one individual recorded and four unique species recorded only once). On the other hand, there were significant differences between depths, mainly between 50 and 70 m. Diversity decreases with depth and species composition changes. SIMPER, Shade Plot and NMDS analysis show the most representative species at each depth, with at least half of the species (11) recorded only at 50 m and four species at the deeper levels (60 - 70 m). The observed assemblage includes several of the most caught species in the shallow water artisanal fishery, which is the most traditional and common type of fishery in the Bay. In addition, the Pomacanthuszonipectus (Cortés angelfish) is of particular interest, as it has a special protection status in the official Mexican standard (NOM-059-SEMARNAT, 2010) due to its use as an ornamental species in aquaria. We hypothesised that the mesophotic zone may serve as a refuge for these fishes, so we propose that the information obtained is an important basis for new research aimed at the sustainable management of fisheries in the area.

Mesophotic corals in Hawai'i maintain autotrophy to survive low-light conditions

In mesophotic coral ecosystems, reef-building corals and their photosynthetic symbionts can survive with less than 1% of surface irradiance. How depth-specialist corals rely upon autotrophically and heterotrophically derived energy sources across the mesophotic zone remains unclear. We analysed the stable carbon (δ13C) and nitrogen (δ15N) isotope values of a Leptoseris community from the 'Au'au Channel, Maui, Hawai'i (65-125 m) including four coral host species living symbiotically with three algal haplotypes. We characterized the isotope values of hosts and symbionts across species and depth to compare trophic strategies. Symbiont δ13C was consistently 0.5‰ higher than host δ13C at all depths. Mean colony host and symbiont δ15N differed by up to 3.7‰ at shallow depths and converged at deeper depths. These results suggest that both heterotrophy and autotrophy remained integral to colony survival across depth. The increasing similarity between host and symbiont δ15N at deeper depths suggests that nitrogen is more efficiently shared between mesophotic coral hosts and their algal symbionts to sustain autotrophy. Isotopic trends across depth did not generally vary by host species or algal haplotype, suggesting that photosynthesis remains essential to Leptoseris survival and growth despite low light availability in the mesophotic zone.

Global status, impacts, and management of rocky temperate mesophotic ecosystems

The ecology and function of rocky temperate mesophotic ecosystems (TMEs) remain poorly understood globally despite their widespread distribution. They typically occur at 20-150 m (the limit of photosynthesis), and on rocky substratum they support rich benthic communities and mobile fauna. We determined the distribution of rocky TMEs, their conservation status, and their most characteristic biological groups. Rocky TMEs were dominated by algae, turf-invertebrate matrices (<50 m only), sponges, bryozoans, and cnidarians. The community composition of TMEs differed significantly from shallow (0-15 m) subtidal reefs. Data were geographically biased and variable, available only from the North and South Atlantic, Mediterranean, and Temperate Australasia. Degree of protection of rocky TMEs varied considerably across the world. The biggest threats to rocky TMEs were identified changes in temperature, sedimentation rates, nutrient concentrations, and certain fishing types. We propose a conservation framework to inform future rocky TME management and conservation, highlighting the need to recognize the importance of these biologically diverse and functionally important ecosystems.

Mesophotic coral bleaching associated with changes in thermocline depth

As global temperatures continue to rise, shallow coral reef bleaching has become more intense and widespread. Mesophotic coral ecosystems reside in deeper (30-150 m), cooler water and were thought to offer a refuge to shallow-water reefs. Studies now show that mesophotic coral ecosystems instead have limited connectivity with shallow corals but host diverse endemic communities. Given their extensive distribution and high biodiversity, understanding their susceptibility to warming oceans is imperative. In this multidisciplinary study of an atoll in the Chagos Archipelago in the central Indian Ocean, we show evidence of coral bleaching at 90 m, despite the absence of shallow-water bleaching. We also show that the bleaching was associated with sustained thermocline deepening driven by the Indian Ocean Dipole, which might be further enhanced by internal waves whose influence varied at a sub-atoll scale. Our results demonstrate the potential vulnerability of mesophotic coral ecosystems to thermal stress and highlight the need for oceanographic knowledge to predict bleaching susceptibility and heterogeneity.

Plumbing the depths with environmental DNA (eDNA): Metabarcoding reveals biodiversity zonation at 45-60 m on mesophotic coral reefs

Mesophotic coral ecosystems (MCEs) are tropical reefs found at depths of ~30-150 m, below the region most heavily impacted by heat stress and other disturbances. Hence, MCEs may serve as potential refugia for threatened shallow reefs, but they also harbour depth-endemic fauna distinct from shallow reefs. Previous studies have characterized biodiversity patterns along depth gradients, but focussed primarily on conspicuous taxa (fishes, corals, etc.). Environmental DNA (eDNA) metabarcoding offers a more holistic approach to assess biodiversity patterns across the tree of life. Here, we use three metabarcoding assays targeting fishes (16S rRNA), eukaryotes (18S rDNA) and metazoans (COI) to assess biodiversity change from the surface to ~90 m depth across 15-m intervals at three sites within the Hawaiian Archipelago. We observed significant community differences between most depth zones, with distinct zonation centred at 45-60 m for eukaryotes and metazoans, but not for fishes. This finding may be attributable to the higher mobility of reef fishes, although methodological limitations are likely a contributing factor. The possibility for MCEs to serve as refugia is not excluded for fishes, but invertebrate communities >45 m are distinct, indicating limited connectivity for the majority of reef fauna. This study provides a new approach for surveying biodiversity on MCEs, revealing patterns in a much broader context than the limited-taxon studies that comprise the bulk of our present knowledge.

Corals at the edge of environmental limits: A new conceptual framework to re-define marginal and extreme coral communities

The worldwide decline of coral reefs has renewed interest in coral communities at the edge of environmental limits because they have the potential to serve as resilience hotspots and climate change refugia, and can provide insights into how coral reefs might function in future ocean conditions. These coral communities are often referred to as marginal or extreme but few definitions exist and usage of these terms has therefore been inconsistent. This creates significant challenges for categorising these often poorly studied communities and synthesising data across locations. Furthermore, this impedes our understanding of how coral communities can persist at the edge of their environmental limits and the lessons they provide for future coral reef survival. Here, we propose that marginal and extreme coral communities are related but distinct and provide a novel conceptual framework to redefine them. Specifically, we define coral reef extremeness solely based on environmental conditions (i.e., large deviations from optimal conditions in terms of mean and/or variance) and marginality solely based on ecological criteria (i.e., altered community composition and/or ecosystem functioning). This joint but independent assessment of environmental and ecological criteria is critical to avoid common pitfalls where coral communities existing outside the presumed optimal conditions for coral reef development are automatically considered inferior to coral reefs in more traditional settings. We further evaluate the differential potential of marginal and extreme coral communities to serve as natural laboratories, resilience hotspots and climate change refugia, and discuss strategies for their conservation and management as well as priorities for future research. Our new classification framework provides an important tool to improve our understanding of how corals can persist at the edge of their environmental limits and how we can leverage this knowledge to optimise strategies for coral reef conservation, restoration and management in a rapidly changing ocean.

Assembly rules of coral reef fish communities along the depth gradient

Coral reefs are home to some of the most studied ecological assemblages on the planet. However, differences in large-scale assembly rules have never been studied using empirical quantitative data stratified along the depth gradient of reefs. Consequently, little is known about the small- and regional-scale effects of depth on coral reef assemblages. Using a large dataset of underwater surveys, we observed that the influence of classic biogeographic drivers on the species richness of coral reef fishes changes significantly with depth, shaping distinct assemblages governed by different rules in mesophotic coral ecosystems. We show that a general pattern of decreased taxonomic and functional richness of reef fish assemblages with depth results from convergent filtering of species composition and trophic strategies on deeper reefs across ocean basins and that at smaller scales deep-reef communities are less influenced by regional factors than shallower reefs.

Mesophotic depths hide high coral cover communities in French Polynesia

The rapid decline of shallow coral reefs has increased the interest in the long-understudied mesophotic coral ecosystems (MCEs). However, MCEs are usually characterised by rather low to moderate scleractinian coral cover, with only a few descriptions of high coral cover at depth. Here, we explored eight islands across French Polynesia over a wide depth range (6 to 120 m) to identify coral cover hotspots at mesophotic depths and the co-occurrent biotic groups and abiotic factors that influence such high scleractinian cover. Using Bayesian modelling, we found that 20 out of 64 of studied deep sites exhibited a coral cover higher than expected in the mesophotic range (e.g. as high as 81.8 % at 40 m, 74.5 % at 60 m, 53 % at 90 m and 42 % at 120 m vs the average expected values based on the model of 31.2 % at 40 m, 22.8 % at 60 m, 14.6 % at 90 m and 9.8 % at 120 m). Omitting the collinear factors light-irradiance and depth, these 'hotspots' of coral cover corresponded to mesophotic sites and depths characterised by hard substrate, a steep to moderate slope, and the dominance of laminar corals. Our work unveils the presence of unexpectedly and unique high coral cover communities at mesophotic depths in French Polynesia, highlighting the importance of expanding the research on deeper depths for the potential relevance in the conservation management of tropical coral reefs.

Thermal stress responses of the antipatharian Stichopathes sp. from the mesophotic reef of Mo'orea, French Polynesia

Antipatharians, also called black corals, are present in almost all oceans of the world, until extreme depths. In several regions, they aggregate in higher densities to form black coral beds that support diverse animal communities and create biodiversity hotspots. These recently discovered ecosystems are currently threatened by fishing activities and illegal harvesting for commercial purposes. Despite this, studies dedicated to the physiology of antipatharians are scarce and their responses to global change stressors have remained hardly explored since recently. Here, we present the first study on the physiological responses of a mesophotic antipatharian Stichopathes sp. (70-90 m) to thermal stress through a 16-d laboratory exposure (from 26 to 30.5 °C). Oxygen consumption measurements allowed identifying the physiological tipping point of Stichopathes sp. (T_opt = 28.3 °C; 2.7 °C above mean ambient condition). Our results follow theoretical predictions as performances start to decrease beyond T_opt, with lowered oxygen consumption rates, impairment of the healing capacities, increased probability of tissue necrosis and stress responses activated as a function of temperature (i.e. increase in mucocyte density and total antioxidant capacity). Altogether, our work indicates that Stichopathes sp. lives at suboptimal performances during the coldest months of the year, but also that it is likely to have low acclimatization capacity and a narrow thermal breadth.

Community similarity and species overlap between habitats provide insight into the deep reef refuge hypothesis

The deep reef refuge hypothesis (DRRH) postulates that mesophotic coral ecosystems (MCEs) may provide a refuge for shallow coral reefs (SCRs). Understanding this process is an important conservation tool given increasing threats to coral reefs. To establish a better framework to analyze the DRRH, we analyzed stony coral communities in American Sāmoa across MCEs and SCRs to describe the community similarity and species overlap to test the foundational assumption of the DRRH. We suggest a different approach to determine species as depth specialists or generalists that changes the conceptual role of MCEs and emphasizes their importance in conservation planning regardless of their role as a refuge or not. This further encourages a reconsideration of a broader framework for the DRRH. We found 12 species of corals exclusively on MCEs and 183 exclusively on SCRs with another 63 species overlapping between depth zones. Of these, 19 appear to have the greatest potential to serve as reseeding species. Two additional species are listed under the U.S. Endangered Species Act, Acropora speciosa and Fimbriaphyllia paradivisa categorized as an occasional deep specialist and a deep exclusive species, respectively. Based on the community distinctiveness and minimal species overlap of SCR and MCE communities, we propose a broader framework by evaluating species overlap across coral reef habitats. This provides an opportunity to consider the opposite of the DRRH where SCRs support MCEs.

Amino acid δ13C and δ15N analyses reveal distinct species-specific patterns of trophic plasticity in a marine symbiosis

Compound-specific isotope analyses (CSIA) and multivariate "isotope fingerprinting" track biosynthetic sources and reveal trophic interactions in food webs. However, CSIA have not been widely applied in the study of marine symbioses. Here, we exposed a reef coral (Montipora capitata) in symbiosis with Symbiodiniaceae algae to experimental treatments (autotrophy, mixotrophy, heterotrophy) to test for trophic shifts and amino acid (AA) sources using paired bulk (δ¹³C, δ¹⁵N) and AA-CSIA (δ¹³C_AA, δ¹⁵N_AA). Treatments did not influence carbon or nitrogen trophic proxies, thereby not supporting nutritional plasticity. Instead, hosts and symbionts consistently overlapped in essential- and nonessential-δ¹³C_AA (11 of 13 amino acids) and trophic- and source-δ¹⁵N_AA values (9 of 13 amino acids). Host and symbiont trophic-δ¹⁵N_AA values positively correlated with a plankton end-member, indicative of trophic connections and dietary sources for trophic-AA nitrogen. However, mass balance of AA-trophic positions (TP_Glx-Phe) revealed heterotrophic influences to be highly variable (1-41% heterotrophy). Linear discriminant analysis using M. capitata mean-normalized essential-δ¹³C_AA with previously published values (Pocillopora meandrina) showed similar nutrition isotope fingerprints (Symbiodiniaceae vs. plankton) but revealed species-specific trophic strategies. Montipora capitata and Symbiodiniaceae shared identical AA-fingerprints, whereas P. meandrina was assigned to either symbiont or plankton nutrition. Thus, M. capitata was 100% reliant on symbionts for essential-δ¹³C_AA and demonstrated autotrophic fidelity and contrasts with trophic plasticity reported in P. meandrina. While M. capitata AA may originate from host and/or symbiont biosynthesis, AA carbon is Symbiodiniaceae-derived. Together, AA-CSIA/isotope fingerprinting advances the study of coral trophic plasticity and are powerful tools in the study of marine symbioses.

Unexplored Refugia with High Cover of Scleractinian Leptoseris spp. and Hydrocorals Stylaster flabelliformis at Lower Mesophotic Depths (75–100 m) on Lava Flows at Reunion Island (Southwestern Indian Ocean)

Despite increased attention over the last decade on Mesophotic Coral Ecosystems (MCEs) [...]

Mesophotic Ecosystems: The Link between Shallow and Deep-Sea Habitats

Mesophotic ecosystems (MEs) are characterized by the presence of light-dependent organisms, found at depths ranging from ~30 to 150 m in temperate, subtropical and tropical regions. These communities occasionally create massive reef structures with diverse but characteristic morphologies, which serve as the framework builders of those ecosystems. In many localities, MEs are physically linked with shallow and deep-sea habitats, and while taxa from both environments share this space, a unique and endemic biodiversity is also found. The main MEs studied to date are the mesophotic coral ecosystems (MCEs) and the temperate mesophotic ecosystems (TMEs), which have received increased attention during the last decade. As shallow coral reef ecosystems are among the most threatened habitats on Earth, the potential of MEs to act as refugia and contribute to the resilience of the whole ecosystem has been a subject of scrutiny. New technologies and methods have become more available to study these deeper parts of the reef ecosystems, yielding many new discoveries. However, basic gaps in knowledge remain in our scientific understanding of the global diversity of MEs, limiting our ability to recognize biogeographic patterns and to make educated decisions for the management and conservation of these ecosystems.

Why do mesophotic coral ecosystems have to be protected?

Mesophotic coral ecosystems (MCEs; ~30-150 m depth) are among the most biologically diverse and least protected ecosystems in the world's oceans. However, discussions regarding the conservation of these unique ecosystems are scarce. To address this issue, we identified the features of MCEs that demonstrate they should be considered as a global conservation priority. Some MCEs are characterized by their well-preserved and unique seascapes; their narrow environmental tolerance and high vulnerability to anthropogenic effects; and their slow recovery and reduced reproductive performance. The unique biodiversity of MCEs includes depth-adapted specialist species and new species, most of which are threatened or important fishery resources. MCEs also provide refuge against human stressors, valuable ecosystem services, and ecological connectivity. MCEs generally meet the criteria to be classified as Ecologically and Biologically Significant Marine Areas under the Convention on Biological Diversity. However, we highlight that many MCEs worldwide are threatened and not yet adequately protected by fishery regulations, marine protected areas, or considered in marine spatial planning. Establishing MCEs as a global conservation priority requires the designation of national, international, transnational, public, and private policies.

Fish biodiversity patterns of a mesophotic-to-subphotic artificial reef complex and comparisons with natural substrates

Artificial reefs act as high-rugosity habitats and are often deployed to enhance fishing; however, the effects of man-made features on fish communities can be unpredictable and are poorly understood in deeper waters. In this study, we used a submersible to describe a deep-water artificial reef complex (93–245 m) off of Ewa Beach, Oahu, Hawaii, USA, and evaluated possible conservation and/or fisheries-related contributions. Sixty-eight species were recorded, with larger features supporting greater diversity of species. Species composition changed strongly with depth and a faunal break was detected from 113–137 m. While the features supported diverse fish communities, they were not similar to those on natural substrates, and were numerically dominated by only two species, Lutjanis kasmira and Chromis verater. Depth-generalist and endemic species were present at levels comparable to natural substrates, but were less abundant and species-rich than at biogenic Leptoseris reefs at similar depths. While the non-native L. kasmira was highly abundant, its presence and abundance were not associated with discernable changes in the fish community, and was not present deeper than 120 m. Finally, five species of commercially- and recreationally-important ‘Deep 7’ fisheries species were also observed, but the artificial reef complex was mostly too shallow to provide meaningful benefits.

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.

Three new species of Chromis (Teleostei, Pomacentridae) from mesophotic coral ecosystems of the Philippines

Three new species of Chromis (Perciformes, Pomacentridae) from the Philippines, collected between 75–150 m depth, are described by a combination of morphological features and their coloration. Chromisguntingsp. n. was found in Batangas and Oriental Mindoro, and differs from its congeners in body depth (2.1–2.2 in SL), and color of adults, light brown, with a silver area on the anterior end and a bilateral black margin along the exterior side of the tail. It is most similar to C.scotochiloptera, with a 5.3% genetic divergence in COI. Chromishangganansp. n. was found around Lubang Island. Body depth (1.9–2.0 in SL) and adult coloration (yellowish with dark black outer margins on dorsal and anal fins) also separate this species from its congeners. It is most similar to C.pembae, with a 2.5% genetic divergence. Chromisbowesisp. n. was found in Batangas, and also differs from its congeners by the combination of body depth (1.5–1.6 in SL), and color of adults (brownish grey in the dorsal side to whitish on the ventral side, with alternating dark and light stripes in the sides of body). It is most similar to C.earina, with a 3.6% genetic divergence in COI.

Deep-Reef Fish Communities of the Great Barrier Reef Shelf-Break: Trophic Structure and Habitat Associations

The ecology of habitats along the Great Barrier Reef (GBR) shelf-break has rarely been investigated. Thus, there is little understanding of how associated fishes interact with deeper environments. We examined relationships between deep-reef fish communities and benthic habitat structure. We sampled 48 sites over a large depth gradient (54–260 m) in the central GBR using Baited Remote Underwater Video Stations and multibeam sonar. Fish community composition differed both among multiple shelf-break reefs and habitats within reefs. Epibenthic cover decreased with depth. Deep epibenthic cover included sponges, corals, and macro-algae, with macro-algae present to 194 m. Structural complexity decreased with depth, with more calcified reef, boulders, and bedrock in shallower depths. Deeper sites were flatter and more homogeneous with softer substratum. Habitats were variable within depth strata and were reflected in different fish assemblages among sites and among locations. Overall, fish trophic groups changed with depth and included generalist and benthic carnivores, piscivores, and planktivores while herbivores were rare below 50 m. While depth influenced where trophic groups occurred, site orientation and habitat morphology determined the composition of trophic groups within depths. Future conservation strategies will need to consider the vulnerability of taxa with narrow distributions and habitat requirements in unique shelf-break environments.

Biodiversity and Functional Ecology of Mesophotic Coral Reefs

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.

Spatial variation in the morphological traits of Pocillopora verrucosa along a depth gradient in Taiwan

Pocillopora verrucosa is a widely distributed depth-generalist coral that presents plasticity in its skeletal macro- and microstructure in response to environmental gradients. Light and water movement, which covary with depth, are the main environmental drivers of morphological plasticity in this genus; however, assessing environmentally-induced plasticity may be confounded by the extent of interspecific variation in Pocillopora. We examine the morphology of 8 typed P. verrucosa specimens collected along a depth gradient ranging from 7 to 45 meters and comprising 3 sites throughout Ludao, Taiwan. We measured 36 morphological characters, 14 which are novel, in 3 regions on the corallum-the apex, branch and base-in order to quantify their relationship to site and depth. We found significant correlation between depth and 19 morphological characters, notably branch verruca area, branch verruca height, base verruca spacing, base spinule length, and branch corallite area. 60% of microstructural characters and 25% of macrostructural characters showed a correlative relation to depth, suggesting that depth acclimatization is manifested primarily at the microstructural level. Canonical discriminant analysis of all morphometric characters by depth supports clustering into 3 groups: an overlapping 7m and 15m group, a 23-30m group, and a 38-45m group. Canonical discriminant analysis by site supports clustering into low- and high-current sites, differentiated primarily by branch septa width, base septa width, pre-terminal branch width, terminal branch maximum length, and terminal branch minimum length. We conclude that distinctive patterns of morphological variation in mesophotic specimens of P. verrucosa could reflect the effects of abiotic parameters such as light and water flow. Elucidating the mechanisms behind the morphological changes that occur in response to environmental gradients can help clarify the role that physiological plasticity plays in the acclimatization of corals to the unique environmental settings of mesophotic coral ecosystems.

Shifting reef fish assemblages along a depth gradient in Pohnpei, Micronesia

Mesophotic coral ecosystems (MCEs) continue to be understudied, especially in island locations spread across the Indo-Pacific Ocean. Pohnpei is the largest island in the Federated States of Micronesia, with a well-developed barrier reef, and steep slopes that descend to more than 1,000 m. Here we conducted visual surveys along a depth gradient of 0 to 60 m in addition to video surveys that extend to 130 m, with 72 belt transects and 12 roving surveys using closed-circuit rebreathers, to test for changes in reef fish composition from shallow to mesophotic depths. We observed 304 fish species across 47 families with the majority confined to shallow habitat. Taxonomic and trophic positions at 30 m showed similar compositions when compared against all other depths. However, assemblages were comprised of a distinct shallow (<30 m) and deep (>30 m) group, suggesting 30 m as a transition zone between these communities. Shallow specialists had a high probability of being herbivores and deep specialists had a higher probability of being planktivores. Acanthuridae (surgeonfishes), Holocentridae (soldierfishes), and Labridae (wrasses) were associated primarily with shallow habitat, while Pomacentridae (damselfishes) and Serranidae (groupers) were associated with deep habitat. Four species may indicate Central Pacific mesophotic habitat: Chromis circumaurea, Luzonichthys seaver, Odontanthias borbonius, and an undescribed slopefish (Symphysanodon sp.). This study supports the 30 m depth profile as a transition zone between shallow and mesophotic ecosystems (consistent with accepted definitions of MCEs), with evidence of multiple transition zones below 30 m. Disturbances restricted to either region are not likely to immediately impact the other and both ecosystems should be considered separately in management of reefs near human population centers.

Habitat geography around Hawaii's oceanic islands influences tiger shark (Galeocerdo cuvier) spatial behaviour and shark bite risk at ocean recreation sites

We compared tiger shark (Galeocerdo cuvier) spatial behaviour among 4 Hawaiian Islands to evaluate whether local patterns of movement could explain higher numbers of shark bites seen around Maui than other islands. Our sample consisted of 96 electronically-tagged (satellite and acoustic transmitters) tiger sharks, individually tracked for up to 6 years. Most individuals showed fidelity to a specific ‘home’ island, but also swam between islands and sometimes ranged far (up to 1,400 km) offshore. Movements were primarily oriented to insular shelf habitat (0–200 m depth) in coastal waters, and individual sharks utilized core-structured home ranges within this habitat. Core utilization areas of large tiger sharks were closer to high-use ocean recreation sites around Maui, than around Oahu. Tiger sharks routinely visited shallow ocean recreation sites around Maui and were detected on more days overall at ocean recreation sites around Maui (62–80%) than Oahu (<6%). Overall, our results suggest the extensive insular shelf surrounding Maui supports a fairly resident population of tiger sharks and also attracts visiting tiger sharks from elsewhere in Hawaii. Collectively these natural, habitat-driven spatial patterns may in-part explain why Maui has historically had more shark bites than other Hawaiian Islands.

Below the Mesophotic

Mesophotic coral ecosystems, which occur at depths of ~40 to 150 m, have received recent scientific attention as potential refugia for organisms inhabiting deteriorating shallow reefs. These ecosystems merit research in their own right, as they harbor both depth-generalist species and a distinctive reef-fish fauna. Reef ecosystems just below the mesophotic are globally underexplored, and the scant recent literature that mentions them often suggests that mesophotic ecosystems transition directly into those of the deep sea. Through submersible-based surveys in the Caribbean Sea, we amassed the most extensive database to date on reef-fish diversity between ~40 and 309 m at any single tropical location. Our data reveal a unique reef-fish assemblage living between ~130 and 309 m that, while taxonomically distinct from shallower faunas, shares strong evolutionary affinities with them. Lacking an existing name for this reef-faunal zone immediately below the mesophotic but above the deep aphotic, we propose "rariphotic." Together with the "altiphotic," proposed here for the shallowest reef-faunal zone, and the mesophotic, the rariphotic is part of a depth continuum of discrete faunal zones of tropical reef fishes, and perhaps of reef ecosystems in general, all of which warrant further study in light of global declines of shallow reefs.

Conventional and technical diving surveys reveal elevated biomass and differing fish community composition from shallow and upper mesophotic zones of a remote United States coral reef

The world’s coral reefs appear to be in a global decline, yet most previous research on coral reefs has taken place at depths shallower than 30 m. Mesophotic coral ecosystem (depths deeper than ~30 m) studies have revealed extensive, productive habitats and rich communities. Despite recent advances, mesophotic coral ecosystems remain understudied due to challenges with sampling at deeper depths. The few previous studies of mesophotic coral ecosystems have shown variation across locations in depth-specific species composition and assemblage shifts, potentially a response to differences in habitat or light availability/water clarity. This study utilized scuba to examine fish and benthic communities from shallow and upper mesophotic (to 45 m) zones of Flower Garden Banks National Marine Sanctuary (FGBNMS, 28°0ʹN; 93°50ʹW) from 2010–2012. Dominant planktivores were ubiquitous in shallow and upper mesophotic habitats, and comparisons with previous shallow research suggest this community distribution has persisted for over 30 years. Planktivores were abundant in shallow low-relief habitats on the periphery of the coral reef, and some of these sites that contained habitat transitioning from high to low relief supported high biomass of benthic predators. These peripheral sites at FGBNMS may be important for the trophic transfer of oceanic energy to the benthic coral reef. Distinct differences between upper mesophotic and shallow communities were also observed. These included greater overall fish (as well as apex predator) biomass in the upper mesophotic, differences in apex predator community composition between depth zones, and greater percent cover of algae, rubble, sand, and sponges in the upper mesophotic. Greater fish biomass in the upper mesophotic and similar fish community composition between depth zones provide preliminary support that upper mesophotic habitats at FGBNMS have the capacity to serve as refugia for the shallow-water reefs. Diving surveys of the upper mesophotic and shallow-water coral reef have revealed valuable information concerning the reef fish community in the northern Gulf of Mexico, with implications for the conservation of apex predators, oceanic coral reefs, and the future management of FGBNMS.

Snapshot recordings provide a first description of the acoustic signatures of deeper habitats adjacent to coral reefs of Moorea

Acoustic recording has been recognized as a valuable tool for non-intrusive monitoring of the marine environment, complementing traditional visual surveys. Acoustic surveys conducted on coral ecosystems have so far been restricted to barrier reefs and to shallow depths (10–30 m). Since they may provide refuge for coral reef organisms, the monitoring of outer reef slopes and describing of the soundscapes of deeper environment could provide insights into the characteristics of different biotopes of coral ecosystems. In this study, the acoustic features of four different habitats, with different topographies and substrates, located at different depths from 10 to 100 m, were recorded during day-time on the outer reef slope of the north Coast of Moorea Island (French Polynesia). Barrier reefs appeared to be the noisiest habitats whereas the average sound levels at other habitats decreased with their distance from the reef and with increasing depth. However, sound levels were higher than expected by propagation models, supporting that these habitats possess their own sound sources. While reef sounds are known to attract marine larvae, sounds from deeper habitats may then also have a non-negligible attractive potential, coming into play before the reef itself.

Fungi associated with mesophotic macroalgae from the 'Au'au Channel, west Maui are differentiated by host and overlap terrestrial communities

Mesophotic coral ecosystems are an almost entirely unexplored and undocumented environment that likely contains vast reservoirs of undescribed biodiversity. Twenty-four macroalgae samples, representing four genera, were collected from a Hawaiian mesophotic reef at water depths between 65 and 86 m in the 'Au'au Channel, Maui, Hawai'i. Algal tissues were surveyed for the presence and diversity of fungi by sequencing the ITS1 gene using Illumina technology. Fungi from these algae were then compared to previous fungal surveys conducted in Hawaiian terrestrial ecosystems. Twenty-seven percent of the OTUs present on the mesophotic coral ecosystem samples were shared between the marine and terrestrial environment. Subsequent analyses indicated that host species of algae significantly differentiate fungal community composition. This work demonstrates yet another understudied habitat with a moderate diversity of fungi that should be considered when estimating global fungal diversity.

A survey of fishes associated with Hawaiian deep-water Halimeda kanaloana (Bryopsidales: Halimedaceae) and Avrainvillea sp. (Bryopsidales: Udoteaceae) meadows

The invasive macroalgal species Avrainvillea sp. and native species Halimeda kanaloana form expansive meadows that extend to depths of 80 m or more in the waters off of O‘ahu and Maui, respectively. Despite their wide depth distribution, comparatively little is known about the biota associated with these macroalgal species. Our primary goals were to provide baseline information on the fish fauna associated with these deep-water macroalgal meadows and to compare the abundance and diversity of fishes between the meadow interior and sandy perimeters. Because both species form structurally complex three-dimensional canopies, we hypothesized that they would support a greater abundance and diversity of fishes when compared to surrounding sandy areas. We surveyed the fish fauna associated with these meadows using visual surveys and collections made with clove-oil anesthetic. Using these techniques, we recorded a total of 49 species from 25 families for H. kanaloana meadows and surrounding sandy areas, and 28 species from 19 families for Avrainvillea sp. habitats. Percent endemism was 28.6% and 10.7%, respectively. Wrasses (Family Labridae) were the most speciose taxon in both habitats (11 and six species, respectively), followed by gobies for H. kanaloana (six species). The wrasse Oxycheilinus bimaculatus and cardinalfish Apogonichthys perdix were the most frequently-occurring species within the H. kanaloana and Avrainvillea canopies, respectively. Obligate herbivores and food-fish species were rare in both habitats. Surprisingly, the density and abundance of small epibenthic fishes were greater in open sand than in the meadow canopy. In addition, species richness was also higher in open sand for Avrainvillea sp. We hypothesize that the dense holdfasts and rhizoids present within the meadow canopy may impede benthic-dwelling or bioturbator species, which accounted for 86% and 57% of individuals collected in sand adjacent to H. kanaloana and Avrainvillea sp. habitats, respectively. Of the 65 unique species recorded in this study, 16 (25%) were detected in clove oil stations alone, illustrating the utility of clove-oil anesthetic in assessing the diversity and abundance of small-bodied epibenthic fishes.

Identifying zooplankton community changes between shallow and upper-mesophotic reefs on the Mesoamerican Barrier Reef, Caribbean

Mesophotic coral ecosystems (MCEs, reefs 30–150 m) are understudied, yet the limited research conducted has been biased towards large sessile taxa, such as scleractinian corals and sponges, or mobile taxa such as fishes. Here we investigate zooplankton communities on shallow reefs and MCEs around Utila on the southern Mesoamerican Barrier Reef using planktonic light traps. Zooplankton samples were sorted into broad taxonomic groups. Our results indicate similar taxonomic zooplankton richness and overall biomass between shallow reefs and MCEs. However, the abundance of larger bodied (>2 mm) zooplanktonic groups, including decapod crab zoea, mysid shrimps and peracarid crustaceans, was higher on MCEs than shallow reefs. Our findings highlight the importance of considering zooplankton when identifying broader reef community shifts across the shallow reef to MCE depth gradient.