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The origin of methylmercury in pelagic fish remains unclear, with many unanswered questions regarding the production and degradation of this neurotoxin in the water column. We used mercury (Hg) stable isotope ratios of marine particles and biota to elucidate the cycling of methylmercury prior to incorporation into the marine food web. The Hg isotopic composition of particles, zooplankton, and fish reveals preferential methylation of Hg within small (< 53 µm) marine particles in the upper 400 m of the North Pacific Ocean. Mass-dependent Hg isotope ratios (δ 202 Hg) recorded in small particles overlap with previously estimated δ 202 Hg values for methylmercury sources to Pacific and Atlantic Ocean food webs. Particulate compound specific isotope analysis of amino acids (CSIA-AA) yield δ 15 N values that indicate more-significant microbial decomposition in small particles compared to larger particles. CSIA-AA and Hg isotope data also suggest that large particles (> 53 µm) collected in the equatorial ocean are distinct from small particles and resemble fecal pellets. Additional evidence for Hg methylation within small particles is provided by a statistical mixing model of even mass–independent (Δ 200 Hg and Δ 204 Hg) isotope values, which demonstrates that Hg within near-surface marine organisms (0–150 m) originates from a combination of rainfall and marine particles. In contrast, in meso- and upper bathypelagic organisms (200–1,400 m), the majority of Hg originates from marine particles with little input from wet deposition. The occurrence of methylation within marine particles is supported further by a correlation between Δ 200 Hg and Δ 199 Hg values, demonstrating greater overlap in the Hg isotopic composition of marine organisms with marine particles than with total gaseous Hg or wet deposition.

Seamounts, pinnacles, and crests are abrupt seafloor features that modify physical processes and ecological patterns. Fishers often target these local bathymetric highs, which can have high catch-per-unit-effort. Increases in the abundance of target species has been qualitatively noted around these features and promontories, however, a quantitative evaluation of local highs as preferred habitat for fishes is still lacking. Here, we used an extensive database of fish abundances (N = 2381) from mesophotic and upper bathyal depths (40-300 m) gathered over 8 yr around the Main Hawaiian Islands to evaluate the effects of macro-scale habitat categorized by combining bathymetric position index (BPI) and slope. A numerical model simulating physical ocean processes was used to test the hypothesis that local highs, herein called crests, are distinct and preferred habitat for many species due to their modified flow conditions. We show that crests host a unique, diverse fish assemblage with double the abundance relative to other habitats. Therefore, fishes are concentrated at crests rather than being uniformly distributed along the island shelf at preferred depths. These habitats are characterized by enhanced current amplitudes and convergence zones. Direct correlations between fish abundance, convergence, and current amplitude strongly suggest that flow-enhanced food availability is likely the driver of habitat-related changes in fish assemblage. These results have important implications for conservation and management. We identify a simple and informative method to classify habitat, quantitatively link bathymetry-induced flow alterations to fish abundances, and provide information for refining definitions of essential fish habitat for species that are important commercial targets worldwide.

A comprehensive understanding of the deep-sea environment and mining’s likely impacts is necessary to assess whether and under what conditions deep-seabed mining operations comply with the International Seabed Authority’s obligations to prevent ‘serious harm’ and ensure the ‘effective protection of the marine environment from harmful effects’ in accordance with the United Nations Convention on the Law of the Sea. A synthesis of the peer-reviewed literature and consultations with deep-seabed mining stakeholders revealed that, despite an increase in deep-sea research, there are few categories of publicly available scientific knowledge comprehensive enough to enable evidence-based decision-making regarding environmental management, including whether to proceed with mining in regions where exploration contracts have been granted by the International Seabed Authority. Further information on deep-sea environmental baselines and mining impacts is critical for this emerging industry. Closing the scientific gaps related to deep-seabed mining is a monumental task that is essential to fulfilling the overarching obligation to prevent serious harm and ensure effective protection, and will require clear direction, substantial resources, and robust coordination and collaboration. Based on the information gathered, we propose a potential high-level road map of activities that could stimulate a much-needed discussion on the steps that should be taken to close key scientific gaps before any exploitation is considered. These steps include the definition of environmental goals and objectives, the establishment of an international research agenda to generate new deep-sea environmental, biological, and ecological information, and the synthesis of data that already exist.

The fate of organic matter (OM) in the deep ocean remains enigmatic, with little understood regarding the flux and its utilization by deep food webs. We used compound‐specific nitrogen stable isotope ratios of source amino acids measured in particle size classes and deep zooplankton (700–1500 m) to determine the contribution of small (0.7–53 μm) vs. large particles (> 53 μm) to their diet at four sites in the North Pacific. Our results show that small particles constitute between 9% and 98% of zooplankton diets, being the contribution higher at sites with lower flux regimes. The contribution of small particles to the diet of deep zooplankton was also higher when biomass of vertical migrators, and therefore actively transported OM, was lower. Climate‐driven changes in primary production and export are expected to shift particle fluxes to smaller size classes, and thus their importance in midwater food webs may become more widespread.

Significance Mercury is a globally distributed neurotoxic pollutant that can be biomagnified in marine fish to levels that are harmful for consumption by humans and other animals. The degree to which mercury has infiltrated the oceans yields important information on the biogeochemistry of mercury and its expected effects on fisheries during changing mercury emissions scenarios. Mercury isotope measurement of biota from deep-sea trenches was used to demonstrate that surface-ocean-derived mercury has infiltrated the deepest locations in the oceans. It was found that when fish living in the surface ocean die and their carcasses sink (along with marine particles), they transfer large amounts of mercury to the trench foodwebs leading to high concentrations of mercury in trench biota., Mercury isotopic compositions of amphipods and snailfish from deep-sea trenches reveal information on the sources and transformations of mercury in the deep oceans. Evidence for methyl-mercury subjected to photochemical degradation in the photic zone is provided by odd-mass independent isotope values (Δ199Hg) in amphipods from the Kermadec Trench, which average 1.57‰ (±0.14, n = 12, SD), and amphipods from the Mariana Trench, which average 1.49‰ (±0.28, n = 13). These values are close to the average value of 1.48‰ (±0.34, n = 10) for methyl-mercury in fish that feed at ∼500-m depth in the central Pacific Ocean. Evidence for variable contributions of mercury from rainfall is provided by even-mass independent isotope values (Δ200Hg) in amphipods that average 0.03‰ (±0.02, n = 12) for the Kermadec and 0.07‰ (±0.01, n = 13) for the Mariana Trench compared to the rainfall average of 0.13 (±0.05, n = 8) in the central Pacific. Mass-dependent isotope values (δ202Hg) are elevated in amphipods from the Kermadec Trench (0.91 ±0.22‰, n = 12) compared to the Mariana Trench (0.26 ±0.23‰, n = 13), suggesting a higher level of microbial demethylation of the methyl-mercury pool before incorporation into the base of the foodweb. Our study suggests that mercury in the marine foodweb at ∼500 m, which is predominantly anthropogenic, is transported to deep-sea trenches primarily in carrion, and then incorporated into hadal (6,000-11,000-m) food webs. Anthropogenic Hg added to the surface ocean is, therefore, expected to be rapidly transported to the deepest reaches of the oceans.

Hadal snailfishes are the deepest-living fishes in the ocean, inhabiting trenches from depths of ∼6,000 to 8,000 m. While the microbial communities in trench environments have begun to be characterized, the microbes associated with hadal megafauna remain relatively unknown. Here, we describe the gut microbiomes of two hadal snailfishes, Pseudoliparis swirei (Mariana Trench) and Notoliparis kermadecensis (Kermadec Trench) using 16S rRNA gene amplicon sequencing. We contextualize these microbiomes with comparisons to the abyssal macrourid Coryphaenoides yaquinae and the continental shelf-dwelling snailfish Careproctus melanurus. The microbial communities of the hadal snailfishes were distinct from their shallower counterparts and were dominated by the same sequences related to the Mycoplasmataceae and Desulfovibrionaceae. These shared taxa indicate that symbiont lineages may have remained similar to the ancestral symbiont since their geographic separation or that they are dispersed between geographically distant trenches and subsequently colonize specific hosts. The abyssal and hadal fishes contained sequences related to known, cultured piezophiles, microbes that grow optimally under high hydrostatic pressure, including Psychromonas, Moritella, and Shewanella. These taxa are adept at colonizing nutrient-rich environments present in the deep ocean, such as on particles and in the guts of hosts, and we hypothesize they could make a dietary contribution to deep-sea fishes by degrading chitin and producing fatty acids. We characterize the gut microbiota within some of the deepest fishes to provide new insight into the diversity and distribution of host-associated microbial taxa and the potential of these animals, and the microbes they harbor, for understanding adaptation to deep-sea habitats.ImportanceHadal trenches, characterized by high hydrostatic pressures and low temperatures, are one of the most extreme environments on our planet. By examining the microbiome of abyssal and hadal fishes, we provide insight into both the physiology of the deepest-living vertebrates and the microbes which colonize them. Our findings show that there are similar microbial populations in fishes geographically separated by thousands of miles, reflecting strong selection for specific microbial lineages. Only a handful of psychropiezophilic taxa, which do not reflect the diversity of microbial life at great depth, have been successfully isolated in the laboratory. Our examination of deep-sea fish microbiomes shows that typical high-pressure culturing methodologies, which have largely remained unchanged since the pioneering work of Claude ZoBell in the 1950s, may simulate the chemical environment found in animal guts and helps explain why the same deep-sea genera are consistently isolated.

Highlights • All known observations for Area of Particular Environmental Interest 6 presented. • Assess morphology, sediments, nodules, oceanography, biogeochemistry and ecology. • APEI-6 partially representative of nearby exploration areas yet clear differences. • Present scientific synthesis and management implications for Clarion Clipperton Zone. To protect the range of habitats, species, and ecosystem functions in the Clarion Clipperton Zone (CCZ), a region of interest for deep-sea polymetallic nodule mining in the Pacific, nine Areas of Particular Environmental Interest (APEIs) have been designated by the International Seabed Authority (ISA). The APEIs are remote, rarely visited and poorly understood. Here we present and synthesise all available observations made at APEI-6, the most north eastern APEI in the network, and assess its representativity of mining contract areas in the eastern CCZ. The two studied regions of APEI-6 have a variable morphology, typical of the CCZ, with hills, plains and occasional seamounts. The seafloor is predominantly covered by fine-grained sediments, and includes small but abundant polymetallic nodules, as well as exposed bedrock. The oceanographic parameters investigated appear broadly similar across the region although some differences in deep-water mass separation were evident between APEI-6 and some contract areas. Sediment biogeochemistry is broadly similar across the area in the parameters investigated, except for oxygen penetration depth, which reached >2 m at the study sites within APEI-6, deeper than that found at UK1 and GSR contract areas. The ecology of study sites in APEI-6 differs from that reported from UK1 and TOML-D contract areas, with differences in community composition of microbes, macrofauna, xenophyophores and metazoan megafauna. Some species were shared between areas although connectivity appears limited. We show that, from the available information, APEI-6 is partially representative of the exploration areas to the south yet is distinctly different in several key characteristics. As a result, additional APEIs may be warranted and caution may need to be taken in relying on the APEI network alone for conservation, with other management activities required to help mitigate the impacts of mining in the CCZ.

Industrial interest in deep-sea mineral extraction began decades ago, and today it is at an all-time high, accelerated by global demand for metals. Several seafloor ecosystem disturbance experiments began in the 1970s, including the Disturbance and Recolonization experiment (DISCOL) conducted in the Peru Basin in 1989. A large seafloor disturbance was created by repeatedly ploughing the seafloor over an area of ∼10.8 km2. Though a number of studies in abyssal mining regions have evaluated megafaunal biodiversity and ecosystem responses, few have included quantitative and detailed data on fishes or scavengers despite their ecological importance as top predators. We used towed camera transects (1989–1996, 2015) and baited camera data (1989–1992) to evaluate the fish community at the DISCOL site. The abyssal fish community included 16 taxa and was dominated by Ipnops meadi. Fish density was lower in ploughed habitat at 6 months and 3 years following disturbance but thereafter increased over time. Twenty-six years after disturbance there were no differences in overall total fish densities between reference and experimental areas, but the dominant fish, I. meadi, still exhibited much lower densities in ploughed habitat, likely avoiding these areas and suggesting that the fish community remains affected after decades. At the scale of industrial mining, these results could translate to population-level effects. The scavenging community was dominated by eelpouts (Pachycara spp.), hermit crabs (Probeebei mirabilis) and shrimp. The large contribution of hermit crabs appears to be unique amongst abyssal scavenger studies worldwide. The abyssal fish community at DISCOL was similar to that in the more northerly Clarion–Clipperton Zone (CCZ), though some species have only been observed at DISCOL thus far. Also, further species-level identifications are required to refine this assessment. Additional studies across the polymetallic nodule provinces of the Pacific are required to further evaluate the environmental drivers of fish density, diversity and species biogeographies. This information will be important for the development of appropriate management plans aimed at minimizing human impact from deep-sea mining.

Author(s): Romero-Romero, Sonia; Choy, C Anela; Hannides, Cecelia CS; Popp, Brian N; Drazen, Jeffrey C | Abstract: Many species of micronekton perform diel vertical migrations (DVMs), which ultimately contributes to carbon export to the deep sea. However, not all micronekton species perform DVM, and the nonmigrators, which are often understudied, have different energetic requirements that might be reflected in their trophic ecology. We analyze bulk tissue and whole animal stable nitrogen isotopic compositions (δ 15N values) of micronekton species collected seasonally between 0 and 1250 m depth to explore differences in the trophic ecology of vertically migrating and nonmigrating micronekton in the central North Pacific. Nonmigrating species exhibit depth-related increases in δ 15N values mirroring their main prey, zooplankton. Higher variance in δ 15N values of bathypelagic species points to the increasing reliance of deeper dwelling micronekton on microbially reworked, very small suspended particles. Migrators have higher δ 15N values than nonmigrators inhabiting the epipelagic zone, suggesting the consumption of material during the day at depth, not only at night when they migrate closer to the surface. Migrating species also appear to eat larger prey and exhibit a higher range of variation in δ 15N values seasonally than nonmigrators, likely because of their higher energy needs. The dependence on material at depth enriched in 15N relative to surface particles is higher in migratory fish that ascend only to the lower epipelagic zone. Our results confirm that stark differences in the food habits and dietary sources of micronekton species are driven by vertical migrations.

The Clarion-Clipperton Zone (CCZ) is a 4 million km2 area in the eastern Central Pacific Ocean exhibiting large variability in environmental parameters, particularly oxygen and primary production, that is being targeted for deep-sea polymetallic nodule mining. This remote region’s pelagic biology is very poorly sampled, including for micronekton and zooplankton that provide essential ecosystem services such as carbon flux and support for commercial fisheries. We built a baseline of deep scattering layer (DSL) depths and vertical migration behaviors, proxies for mesopelagic micronekton and zooplankton communities, using shipboard acoustic Doppler current profiler datasets. Acoustic data (38 kHz, 75 kHz) were compiled from research cruises passing near or through the CCZ (2004–2019), and environmental data (mean midwater oxygen partial pressure, surface chlorophyll-a, and sea surface height anomaly) were assembled from the World Ocean Atlas and satellite oceanographic datasets. Our results suggest that midwater oxygen, associated with the Eastern Tropical Pacific Oxygen Minimum Zone (OMZ), is the strongest predictor of daytime DSL depths and the proportions of midwater populations that undergo vertical migration in this region. We used these relationships to predict micronekton and zooplankton behaviors across the CCZ, including licensed mining exploration areas and no-mining reserves. While the OMZ encompasses most licensed exploration areas, the current network of reserves lies outside of the core OMZ and ultimately may not represent or protect the pelagic OMZ fauna at highest risk from mining impacts. This research will further assist in developing resource exploitation regulations by the International Seabed Authority, and will provide mesopelagic baseline information for monitoring changes that may occur in the CCZ once industrial-scale mining begins.

Trophic ecology of detrital-based food webs is still poorly understood. Abyssal plains depend entirely on detritus and are among the most understudied ecosystems, with deposit feeders dominating megafaunal communities. We used compound-specific stable isotope ratios of amino acids (CSIA-AA) to estimate the trophic position of three abundant species of deposit feeders collected from the abyssal plain of the Northeast Pacific (Station M; ~ 4000 m depth), and compared it to the trophic position of their gut contents and the surrounding sediments. Our results suggest that detritus forms the base of the food web and gut contents of deposit feeders have a trophic position consistent with primary consumers and are largely composed of a living biomass of heterotrophic prokaryotes. Subsequently, deposit feeders are a trophic level above their gut contents making them secondary consumers of detritus on the abyssal plain. Based on δ13C values of essential amino acids, we found that gut contents of deposit feeders are distinct from the surrounding surface detritus and form a unique food source, which was assimilated by the deposit feeders primarily in periods of low food supply. Overall, our results show that the guts of deposit feeders constitute hotspots of organic matter on the abyssal plain that occupy one trophic level above detritus, increasing the food-chain length in this detritus-based ecosystem.

The abyssal seafloor makes up three quarters of the ocean floor, and it is generally characterized as a food-limited habitat with low numbers of megafauna, particularly fishes. Baited camera observations from three abyssal seamount summits in the equatorial Pacific challenge this idea. On each of two deployments at the southernmost seamount, over 100 synaphobranchid eels (Ilyophis arx) were recorded feeding on standard bait (1 kg mackerel). This is the highest number of fishes per kg of bait ever recorded below 1000 m, including observations from large organic falls such as cetacean and shark carcasses. It is also the highest number that has ever been recorded at carrion of any kind or size at abyssal depths. We suggest an abyssal ‘seamount effect’ may be responsible, highlighting the potential importance of seamounts in structuring abyssal communities.

Environmental Protection Requires Accurate Application of Scientific Evidence.&nbsp

The deep sea (>500 m ocean depth) is the largest global habitat, characterized by cool temperatures, low ambient light, and food-poor conditions relative to shallower waters. Deep-sea teleosts generally grow more slowly than those inhabiting shallow water. However, this is a generalization, and even amongst deep-sea teleosts, there is a broad continuum of growth rates. The importance of potential drivers of growth rate variability amongst deep-sea species, such as temperature, food availability, oxygen concentration, metabolic rate, and phylogeny, have yet to be fully evaluated. We present a meta-analysis in which age and size data were collected for 53 species of teleosts whose collective depth ranges span from surface waters to 4000 m. We calculated growth metrics using both calendar and thermal age, and compared them with environmental, ecological, and phylogenetic variables. Temperature alone explained up to 30% of variation in the von Bertalanffy growth coefficient (K, yr-1), and 21% of the variation in the average annual increase in mass (AIM, %), a metric of growth prior to maturity. After correcting for temperature effects, depth was still a significant driver of growth, explaining up to 20 and 10% of the remaining variation in K and AIM, respectively. Oxygen concentration also explained ~11% of remaining variation in AIM following temperature correction. Relatively minor amounts of variation may be explained by food availability, phylogeny, and the locomotory mode of the teleosts. We also found strong correlation between growth and metabolic rate, which may be an underlying driver also related to temperature, depth, and other factors, or the 2 parameters may simply covary as a result of being linked by evolutionary pressures. Evaluating the influence of ecological and/or environmental drivers of growth is a vital step in understanding both the evolution of life history parameters across the depth continuum as well as their implications for species’ resilience to increasing anthropogenic stressors.

© 2020 The Authors The abyssal seafloor of the Clarion Clipperton Zone (CCZ) in the central Pacific has the largest known deposits of polymetallic nodules and associated benthic faunal communities with high biodiversity. The environmental factors that structure these communities, both at regional and local scales, are not well understood. In this study, seabed image surveys were used to assess distribution patterns in invertebrate and fish megafauna (>1 cm) at multiple scales in relation to key environmental factors: food supply to the seabed varying at the regional scale (hundreds of km), seabed geomorphological variations varying at the broad local scale (tens of km), and seabed nodule cover varying at the fine local scale (tens of meters). We found significant differences in megafaunal density and community composition between all study areas. Variations in faunal density did not appear to match with regional productivity gradients, although faunal density generally decreased with increasing water depth (from E to W). In contrast, geomorphology and particularly nodule cover appeared to exert strong control on local faunal abundance and community composition, but not in species richness. Local variations in faunal density and beta-diversity, particularly those driven by nodule presence (within study areas), were of comparable magnitude to those observed at a regional level (between study areas). However, regional comparisons of megabenthic assemblages showed clear shifts in dominance between taxonomic groups (perceivable even at Phylum levels) across the mid-eastern CCZ seabed, suggesting a higher regional heterogeneity than was previously thought.

Hadal trenches house distinct ecosystems but we know little about their sources of nutrition or trophic structures. We evaluated megafaunal food web structure and nutritional sources in the Kermadec and Mariana trenches using carbon and nitrogen stable isotope analysis (δ15N and δ13C values) of bulk tissues and proteinaceous individual amino acids (AAs). In the Kermadec Trench, bulk δ15N values ranged from 5.8‰ in trench sediment to 17.5‰ in tissues of the supergiant amphipod, Allicela gigantea. δ15N values of detritivores were much higher than those of sediments (by 7.5‰ more). The δ13C values ranged from −21.4‰ in sediments to −17.3‰ in the brittle star, Ophiolimna sp., and did not co-vary with δ15N values. In the Mariana Trench, only bait-attending fauna and surface sediments were available for analysis. Mariana Trench fishes, amphipods, and sediments had slightly lower δ15N values than those from the Kermadec Trench, possibly because the Mariana Trench lies under more oligotrophic surface waters. We found evidence for multiple food inputs to the system in each trench, namely substantially higher δ15N values in detritivores relative to sediment and high variability in δ13C values. Trophic levels determined from isotopic analysis of individual AAs in the Kermadec Trench ranged from three for detritivores to five for fishes. Source AA δ15N values were variable (range of ~7.0‰ in average δ15N source AA values), with much of this variation occurring in small amphipods. For the other fauna sampled, there was a significant increase in δ15N source AA values with increasing collection depth. This increase could reflect larger amounts of highly microbially reworked organic matter with increasing depth or sporadic input from turbidity flows. Although further sampling across a broader faunal diversity will be required to understand these food webs, our results provide new insights into hadal trophic interactions and suggest that trench food webs are very dynamic.

Deep-sea fisheries occur at depths between 200 and 1800 m, using bottom trawls, long lines, and occasionally pots and gillnets. These fisheries were of minor interest and value until the mid-1980s when large stocks of fish were discovered, mostly on high-seas seamounts. However, because of the life-history characteristics of deep-dwelling fish, most seamount fish stocks were soon overfished, and few have recovered. Total deep-sea fish catch since 1950 represents about 3 per cent of the global catch, yet the environmental harm caused to deep-sea bottom communities by bottom trawling is extensive and long lasting, far exceeding the value of the fishery. In response, the United Nations has passed several resolutions since 2004 requiring the establishment of regional fisheries management organisations (RFMOs) who would be responsible for setting catch limits for the target species and requiring actions that would limit the damage to the habitat by fishing gear. To date, the latter of these two requirements, at least, has not been successfully met.

Despite rapidly growing interest in deep-sea mineral exploitation, environmental research and management have focused on impacts to seafloor environments, paying little attention to pelagic ecosystems. Nonetheless, research indicates that seafloor mining will generate sediment plumes and noise at the seabed and in the water column that may have extensive ecological effects in deep midwaters (1), which can extend from an approximate depth of 200 meters to 5 kilometers. Deep midwater ecosystems represent more than 90% of the biosphere (2), contain fish biomass 100 times greater than the global annual fish catch (3), connect shallow and deep-sea ecosystems, and play key roles in carbon export (4), nutrient regeneration, and provisioning of harvestable fish stocks (5). These ecosystem services, as well as biodiversity, could be negatively affected by mining. Here we argue that deep-sea mining poses significant risks to midwater ecosystems and suggest how these risks could be evaluated more comprehensively to enable environmental resource managers and society at large to decide whether and how deep-sea mining should proceed. Midwater animal biodiversity: Squid, fish, shrimp, copepods, medusa, filter-feeding jellies, and marine worms are among the midwater creatures that could be affected by deep sea mining. Photos by E. Goetze, K. Peijnenburg, D. Perrine, Hawaii Seafood Council (B. Takenaka, J. Kaneko), S. Haddock, J. Drazen, B. Robison, DEEPEND (Dante Fenolio), and MBARI. Interest in deep-sea mining for sulfide deposits near hydrothermal vents, polymetallic nodules on the abyssal seafloor, and cobalt-rich crusts on seamounts (6) has grown substantially in the last decade. Equipment and system development are already occurring. The International Seabed Authority (ISA), the international organization created under the United Nations Convention on the Law of the Sea (UNCLOS) to manage deep-sea mining beyond national jurisdiction, is developing mineral exploitation regulations, the Mining Code. Currently, 30 ISA exploration contracts cover over 1.5 million … [↵][1]1To whom correspondence should be addressed. Email: jdrazen{at}hawaii.edu. [1]: #xref-corresp-1-1

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Seamounts are ubiquitous global features often characterized by biological hotspots of diversity, biomass, and abundance, though the mechanisms responsible are poorly understood. One controversial explanation suggests seamount-induced chlorophyll enhancements (SICE) subsidize seamount ecosystems. Using a decade of satellite chlorophyll data, we report substantial long-term chlorophyll enhancements around 17% of Pacific seamounts and 45% of shallow (

Hadal trenches are isolated habitats that cover the greatest ocean depths (6,500–11,000 m) and are believed to host high levels of endemism across multiple taxa. A group of apparent hadal endemics is within the snailfishes (Liparidae), found in at least five geographically separated trenches. Little is known about their biology, let alone the reasons for their success at hadal depths around the world. This study investigated the life history of hadal liparids using sagittal otoliths of two species from the Kermadec (Notoliparis kermadecensis) and Mariana (Pseudoliparis swirei) trenches in comparison to successful abyssal macrourids found at the abyssal-hadal transition zone. Otoliths for each species revealed alternating opaque and translucent growth zones that could be quantified in medial sections. Assuming these annuli represent annual growth, ages were estimated for the two hadal liparid species to be from five to 16 years old. These estimates were compared to the shallower-living snailfish Careproctus melanurus, which were older than described in previous studies, expanding the potential maximum age for the liparid family to near 25 years. Age estimates for abyssal macrourids ranged from eight to 29 years for Coryphaenoides armatus and six to 16 years for C. yaquinae. In addition, 18O/16O ratios (δ18O) were measured across the otolith using secondary ion mass spectrometry (SIMS) to investigate the thermal history of the three liparids, and two macrourids. Changes in δ18O values were observed across the otoliths of C. melanurus, C. armatus, and both hadal liparids, the latter of which may represent a change of >5 °C in habitat temperature through ontogeny. The results would indicate there is a pelagic larval stage for the hadal liparids that rises to a depth above 1000 m, followed by a return to the hadal environment as these liparids grow. This result was unexpected for the hadal liparids given their isolated environment and large eggs, and the biological implications and plausibility of interpretations of these data are discussed. This study presents a first look at the life history of some of the deepest-living fishes through otolith analyses.

Pelagic animals that form deep sea scattering layers (DSLs) represent an important link in the food web between zooplankton and top predators. While estimating the composition, density and location of the DSL is important to understand mesopelagic ecosystem dynamics and to predict top predators’ distribution, DSL composition and density are often estimated from trawls which may be biased in terms of extrusion, avoidance, and gear-associated biases. Instead, location and biomass of DSLs can be estimated from active acoustic techniques, though estimates are often in aggregate without regard to size or taxon specific information. For the first time in the open ocean, we used a DIDSON sonar to characterize the fauna in DSLs. Estimates of the numerical density and length of animals at different depths and locations along the Kona coast of the Island of Hawaii were determined. Data were collected below and inside the DSLs with the sonar mounted on a profiler. A total of 7068 animals were counted and sized. We estimated numerical densities ranging from 1 to 7 animals/m3 and individuals as long as 3 m were detected. These numerical densities were orders of magnitude higher than those estimated from trawls and average sizes of animals were much larger as well. A mixed model was used to characterize numerical density and length of animals as a function of deep sea layer sampled, location, time of day, and day of the year. Numerical density and length of animals varied by month, with numerical density also a function of depth. The DIDSON proved to be a good tool for open-ocean/deep-sea estimation of the numerical density and size of marine animals, especially larger ones. Further work is needed to understand how this methodology relates to estimates of volume backscatters obtained with standard echosounding techniques, density measures obtained with other sampling methodologies, and to precisely evaluate sampling biases.

Limited knowledge of the molecular evolution of deep-sea fish proteomes so far suggests that a few widespread residue substitutions in cytosolic proteins binding hydrophilic ligands contribute to resistance to the effects of high hydrostatic pressure (HP). Structure-function studies with additional protein systems, including membrane bound proteins, are essential to provide a more general picture of adaptation in these extremophiles. We explored molecular features of HP adaptation in proteins binding hydrophobic ligands, either in lipid bilayers (cytochrome P450 1A - CYP1A) or in the cytosol (the aryl hydrocarbon receptor - AHR), and their partners P450 oxidoreductase (POR) and AHR nuclear translocator (ARNT), respectively. Cloning studies identified the full-length coding sequence of AHR, CYP1A and POR, and a partial sequence of ARNT from Coryphaenoides armatus, an abyssal gadiform fish thriving down to 5000m depth. Inferred protein sequences were aligned with many non-deep-sea homologs to identify unique amino acid substitutions of possible relevance in HP adaptation. Positionally unique substitutions of various physicochemical properties were found in all four proteins, usually at sites of strong-to-absolute residue conservation. Some were in domains deemed important for protein-protein interaction or ligand binding. In addition, some involved removal or addition of beta-branched residues; local modifications of beta-branched residue patterns could be important to HP adaptation. In silico predictions further suggested that some unique substitutions might substantially modulate the flexibility of the polypeptide segment in which they are found. Repetitive motifs unique to the abyssal fish AHR were predicted to be rich in glycosylation sites, suggesting that post-translational changes could be involved in adaptation as well. Recombinant CYP1A and AHR showed functional properties (spectral characteristics, catalytic activity and ligand binding) that demonstrate proper folding at 1atm, indicating that they could be used as deep-sea fish protein models to further evaluate protein function under pressure. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone".

Interest in the mining of polymetallic nodules, sulfide deposits, and ferromanganese crusts in the deep sea is growing. While the impacts of deep-sea mining on benthic systems are somewhat understood, there is little information available on pelagic impacts via, for example, the discharge plume. The impacts of these suspended sediments on pelagic communities can have consequences for the fishing industry if their target species are directly or indirectly impacted. Here, we quantify how tuna fisheries on the high seas spatially intersect with mining areas for which exploration contracts have been issued. We explore the spatial intersection of potential mining impacts by investigating different areal extensions around the mining areas as proxies for the spread of potential sediment plumes and movements of tunas. We investigate which countries are fishing in these areas and what percentage of their catches from Regional Fisheries Management Organization (RFMO) areas may intersect with mining activities. We show that for countries that catch the most in mining areas, their catches from mining areas constitute small to moderate parts of their RFMO catches. There are some exceptions amongst these nations and other, often small island nations and/or developing countries, which can derive a high percentage of their RFMO catch from within mining areas, especially if mining plumes spread to a larger extent. RFMOs and other fishery stakeholders should be aware of these potential spatial intersections with their industry and seek to contribute to the development of mining regulations and policies as another stakeholder in the blue economy.

Despite high rates of photosynthetic oxygen production during the day, the warm waters of coral reefs are susceptible to hypoxia at night due to elevated respiration rates at higher temperatures that also reduce the solubility of oxygen. Hypoxia may be a challenge for coral-reef fish that hide in the reef to avoid predators at night. Triggerfishes (Balistidae) are found in a variety of reef habitats, but they also are known to find refuge in reef crevices and holes at night, which may expose them to hypoxic conditions. The critical oxygen tension (P crit) was determined as the point below which oxygen uptake could not be maintained to support standard metabolic rate (SMR) for five species of triggerfish. The triggerfishes exhibited similar levels of hypoxia tolerance as other coral-reef and coastal marine fishes that encounter low oxygen levels in their environment. Two species, Rhinecanthus rectangulus and R. aculeatus, had the lowest P crit (~ 3.0 kPa O2), comparable to the most hypoxia-tolerant obligate coral-dwelling gobies, while Odonus niger and Sufflamen bursa were moderately tolerant to hypoxia (P crit ~ 4.5 kPa), and Xanthichthys auromarginatus was intermediate (P crit ~ 3.7 kPa). These differences in P crit were not due to differences in oxygen demand, as all the species had a similar SMR once mass differences were taken into account. The results suggest that triggerfish species are adapted for different levels of hypoxia exposure during nocturnal sheltering within the reef.

Project funding was provided by the Colonel Willys E. Lord, DVM and Sandina L. Lord Endowed Scholarship (to ZSO), the Carol Ann and Myron K. Hayashida Scholarship (to ZSO), and the NMFS-Sea Grant Population Dynamics Fellowship (to ZSO). The BotCam data were collected, processed, organized, and provided by: V. Moriwake, C. Demarke, B. Alexander, C. Kelley, W. Misa, J. Friedman, D. Tokishi, B. Richards, D. Kobayashi, and A. Rollo. Funding for the BotCam work was provided by the State of Hawaii Division of Aquatic Resources and the Federal Aid in Sport Fish Restoration program (F17R35-study IX), NOAA Pacific Islands Regional Office, Kahoolawe Island Reserve Commission, NOAA Pacific Island Fisheries Science Center, the Joint Institute for Marine and Atmospheric Research at the University of Hawaii at Manoa, the NOAA Fisheries Office of Science and Technology, and the NOAA Fisheries Advanced Science at Technology Working Group. All funding sources had no involvement in the study design, collection, analysis, and interpretation of the data nor in the writing of this manuscript. This manuscript is SOEST contribution #10019 and HIMB contribution #1686.

Metabolic enzyme activities of muscle tissue have been useful and widely-applied indicators of whole animal metabolic capacity, particularly in inaccessible systems such as the deep sea. Previous studies have been conducted at atmospheric pressure, regardless of organism habitat depth. However, maximum reaction rates of some of these enzymes are pressure dependent, complicating the use of metabolic enzyme activities as proxies of metabolic rates. Here, we show pressure-related rate changes in lactate and malate dehydrogenase (LDH, MDH) and pyruvate kinase (PK) in six fish species (2 hadal, 2 abyssal, 2 shallow). LDH maximal reaction rates decreased with pressure for the two shallow species, but, in contrast to previous findings, it increased for the four deep species, suggesting evolutionary changes in LDH reaction volumes. MDH maximal reaction rates increased with pressure in all species (up to 51±10% at 60 MPa), including the tide pool snailfish, Liparis florae (activity increase at 60 MPa 44±9%), suggesting an inherent negative volume change of the reaction. PK was inhibited by pressure in all species tested, including the hadal liparids (up to 34±3% at 60 MPa), suggesting a positive volume change during the reaction. The addition of 400 mM TMAO counteracted this inhibition at both 0.5 and 2.0 mM ADP concentrations for the hadal liparid, Notoliparis kermadecensis. We revisit depth-related trends in metabolic enzyme activities according to these pressure-related rate changes and new data from seven abyssal and hadal species from the Kermadec and Mariana trenches. Results show that, with abyssal and hadal species, pressure-related rate changes are another variable to be considered in the use of enzyme activities as proxies for metabolic rate, in addition to factors such as temperature and body mass. Intraspecific increases in tricarboxylic acid cycle enzymes with depth of capture, independent of body mass, in two hadal snailfishes suggest improved nutritional condition for individuals deeper in the hadal zone, likely related to food availability. These new data inform the discussion of factors controlling metabolism in the deep sea, including the visual interactions hypothesis and extend published trends to the planet's deepest-living fishes.

The Clarion-Clipperton Zone (CCZ) is one of the richest manganese nodule provinces in the world and has recently become a focus area for manganese nodule mining interests. However, this vast area remains poorly studied and highly undersampled. In this study, the abyssal bait-attending fauna is documented for the first time using a series of baited camera deployments in various locations across the CCZ. A bait-attending community intermediate between those typical of the California margin and Hawaii was found in the larger CCZ area, generally dominated by rattail fishes, dendrobranchiate shrimp, and zoarcid and ophidiid fishes. Additionally, the western and eastern ends of the CCZ had different communities, with the western region characterized by decreased dominance of rattails and small shrimps and increased dominance of ophidiids (especially Bassozetus sp. and Barathrites iris ) and large shrimps. This trend may be related to increasing distance from the continental margin. We also test the hypothesis that bait-attending communities change across the CCZ in response to key environmental predictors, especially topography and nodule cover. Our analyses showed that higher nodule cover and elevated topography, as quantified using the benthic positioning index (BPI), increase bait-attending community diversity. Elevated topography generally had higher relative abundances, but taxa also showed differing responses to the BPI metric and bottom temperature, causing significant community compositional change over varying topography and temperatures. Larger individuals of the dominant scavenger in the CCZ, Coryphaenoides spp., were correlated with areas of higher nodule cover and with abyssal hills, suggesting these areas may be preferred habitat. Our results suggest that nodule cover is important to all levels of the benthic ecosystem and that nodule mining could have negative impacts on even the top-level predators and scavengers in the CCZ. Additionally, there is continuous change in diversity, dominance, and relative abundance across the CCZ and across gradients in bathymetric and oceanographic variables. This work increased the understanding of the biogeography of the demersal scavengers and top predators as well as the key environmental drivers of their distributions across the CCZ in order to better predict and manage the impacts of nodule mining.