Hadal Research Articles

Biogeographic distribution, ecotype partitioning and controlling factors of Chloroflexi in the sediments of six hadal trenches of the Pacific Ocean

The hadal trenches are “hot spots” for mineralization of organic matter in the deep ocean. Chloroflexi are one of the most dominant and active taxa in trench sediments, serving as important drivers of carbon cycles in hadal trenches. However, current understanding on hadal Chloroflexi is largely restricted to individual trench. This study systematically analyzed the diversity, biogeographic distribution, ecotype partitioning as well as environmental drivers of Chloroflexi in the sediments of hadal trenches, by reanalyzing 16S rRNA gene libraries of 372 samples from 6 trenches around the Pacific Ocean. The results showed that Chloroflexi averagely account for 10.10 % and up to 59.95 % of total microbial communities in the trench sediments. Positive correlations between relative abundance of Chloroflexi and depths down the vertical sediment profiles were observed in all of the sediment cores analyzed, suggesting the increasing significance of Chloroflexi in deeper sediment layers. Overall, trench sediment Chloroflexi were mainly composed of the classes Dehalococcidia, Anaerolineae and JG30-KF-CM66, and four orders i.e. SAR202, Anaerolineales, norank JG30-KF-CM66 and S085, were identified as core taxa that were dominant and prevalent in the hadal trench sediments. A total of 22 subclusters were identified within these core orders, and distinct patterns of ecotype partitioning related with depths down the vertical sediment profiles were observed, suggesting the great diversification of metabolic potentials and environment preference of different Chloroflexi lineages. The spatial distribution of hadal Chloroflexi were found to be significantly related with multiple environmental factors, while depths down the vertical sediment profiles explained the highest proportion of variations. These results provide valuable information for further exploring the roles of Chloroflexi in biogeochemical cycle of the hadal zone, and lay the foundation for understanding the adaptive mechanisms and evolutionary characteristics of microorganisms in hadal trenches.

The phylogeny and metabolic potentials of an n-alkane-degrading Venatorbacter bacterium isolated from deep-sea sediment of the Mariana Trench.

Recently, several reports showed that n-alkanes were abundant in the hadal zone, suggesting that n-alkanes could be an important source of nutrients for microorganisms in hadal ecosystems. To date, most of the published studies on the microbial capacity to degrade hydrocarbons were conducted only at atmospheric temperature and pressure (0.1 MPa), and little is known about whether and which microbes could utilize n-alkanes at in situ environmental conditions in the hadal zone, including low temperature and high hydrostatic pressure (especially >30 MPa). In this study, a piezotolerant bacterium, strain C2-1, was isolated from a Mariana Trench sediment at depth of 5,800 m. Strain C2-1 was able to grow at in situ temperature (4°C) and pressure (58 MPa) with n-alkanes as the sole carbon source. Phylogenetically, strain C2-1 and related strains (TMPB967, ST750PaO-4, IMCC1826, and TTBP476) should be classified into the genus Venatorbacter. Metagenomic analysis using ~5,000 publicly available datasets showed that Venatorbacter has a wide environmental distribution in seawater (38), marine sediments (3), hydrothermal vent plumes (2), Antarctic ice (1), groundwater (13), and marine sponge ecosystems (1). Most Venatorbacter species are non-obligate n-alkane degraders that could utilize, at a minimal, C16-C18 n-alkanes, as well as other different types of carbon substrates, including carbohydrates, amino acids, peptides, and phospholipids. The type II secretion system, extracellular proteases, phospholipase, and endonuclease of Venatorbacter species were robustly expressed in the metatranscriptomes of deep-sea hydrothermal vents, suggesting their important contribution to secondary productivity by degrading extracellular macromolecules. The identification of denitrifying genes suggested a genus-specific ecological potential that allowed Venatorbacter species to be active in anoxic environments, e.g., the oxygen-minimal zone (OMZ) and the deeply buried marine sediments. Our results show that Venatorbacter species are responsible for the degradation of hydrocarbon and extracellular macromolecules, suggesting that they may play an important role in the biogeochemistry process in the Trench ecosystems.

Large effective population size masks population genetic structure in Hirondellea amphipods within the deepest marine ecosystem, the Mariana Trench.

The examination of genetic structure in the deep-ocean hadal zone has focused on divergence between tectonic trenches to understand how environment and geography may drive species divergence and promote endemism. There has been little attempt to examine localized genetic structure within trenches, partly because of logistical challenges associated with sampling at an appropriate scale, and the large effective population sizes of species that can be sampled adequately may mask underlying genetic structure. Here we examine genetic structure in the superabundant amphipod Hirondellea gigas in the Mariana Trench at depths of 8126-10,545 m. RAD sequencing was used to identify 3182 loci containing 43,408 single nucleotide polymorphisms (SNPs) across individuals after stringent pruning of loci to prevent paralogous multicopy genomic regions being erroneously merged. Principal components analysis of SNP genotypes resolved no genetic structure between sampling locations, consistent with a signature of panmixia. However, discriminant analysis of principal components identified divergence between all sites driven by 301 outlier SNPs in 169 loci and significantly associated with latitude and depth. Functional annotation of loci identified differences between singleton loci used in analysis and paralogous loci pruned from the data set and also between outlier and nonoutlier loci, all consistent with hypotheses explaining the role of transposable elements driving genome dynamics. This study challenges the traditional perspective that highly abundant amphipods within a trench form a single panmictic population. We discuss the findings in relation to eco-evolutionary and ontogenetic processes operating in the deep sea, and highlight key challenges associated with population genetic analysis in nonmodel systems with inherent large effective population sizes and genomes.

Influence of Extremely High Pressure and Oxygen on Hydrocarbon-Enriched Microbial Communities in Sediments from the Challenger Deep, Mariana Trench

Recent studies reported that highly abundant alkane content exists in the ~11,000 m sediment of the Mariana Trench, and a few key alkane-degrading bacteria were identified in the Mariana Trench. At present, most of the studies on microbes for degrading hydrocarbons were performed mainly at atmospheric pressure (0.1 MPa) and room temperature; little is known about which microbes could be enriched with the addition of n-alkanes under in-situ environmental pressure and temperature conditions in the hadal zone. In this study, we conducted microbial enrichments of sediment from the Mariana Trench with short-chain (SCAs, C7-C17) or long-chain (LCAs, C18-C36) n-alkanes and incubated them at 0.1 MPa/100 MPa and 4 °C under aerobic or anaerobic conditions for 150 days. Microbial diversity analysis showed that a higher microbial diversity was observed at 100 MPa than at 0.1 MPa, irrespective of whether SCAs or LCAs were added. Non-metric multidimensional scaling (nMDS) and hierarchical cluster analysis revealed that different microbial clusters were formed according to hydrostatic pressure and oxygen. Significantly different microbial communities were formed according to pressure or oxygen (p < 0.05). For example, Gammaproteobacteria (Thalassolituus) were the most abundant anaerobic n-alkanes-enriched microbes at 0.1 MPa, whereas the microbial communities shifted to dominance by Gammaproteobacteria (Idiomarina, Halomonas, and Methylophaga) and Bacteroidetes (Arenibacter) at 100 MPa. Compared to the anaerobic treatments, Actinobacteria (Microbacterium) and Alphaproteobacteria (Sulfitobacter and Phenylobacterium) were the most abundant groups with the addition of hydrocarbon under aerobic conditions at 100 MPa. Our results revealed that unique n-alkane-enriched microorganisms were present in the deepest sediment of the Mariana Trench, which may imply that extremely high hydrostatic pressure (100 MPa) and oxygen dramatically affected the processes of microbial-mediated alkane utilization.

Three new deep-sea species of Thyasiridae (Mollusca: Bivalvia) from the northwestern Pacific Ocean

The Thyasiridae is one of the species-richest families in the abyssal and hadal zones of the northwestern Pacific Ocean. Many thyasirid species dominate benthic communities in terms of abundance and play an important role in the functioning of deep-sea ecosystems. Most of the thyasirid species in the region are new to science and have not been described. Based on the material collected from 1954 to 2016 by seven deep-sea expeditions, three new species of Thyasiridae (Parathyasira coani sp. nov., P. pauli sp. nov., and Thyasira kharkovensis sp. nov.) are described from the abyssal and hadal zones (3210–7540 m depth) of the Sea of Okhotsk, the Bering Sea, as well as the Kuril-Kamchatka and Japan trenches. The new species are remarkable among their congeners due to the combination of the following characters: an obliquely-rhomboidal shell with a weak and shallow posterior sulcus and a large prodissoconch with sculpture of lamellated folds. Comparisons with related species are provided.

The genome of a hadal sea cucumber reveals novel adaptive strategies to deep-sea environments.

How organisms cope with coldness and high pressure in the hadal zone remains poorly understood. Here, we sequenced and assembled the genome of hadal sea cucumber Paelopatides sp. Yap with high quality and explored its potential mechanisms for deep-sea adaptation. First, the expansion of ACOX1 for rate-limiting enzyme in the DHA synthesis pathway, increased DHA content in the phospholipid bilayer, and positive selection of EPT1 may maintain cell membrane fluidity. Second, three genes for translation initiation factors and two for ribosomal proteins underwent expansion, and three ribosomal protein genes were positively selected, which may ameliorate the protein synthesis inhibition or ribosome dissociation in the hadal zone. Third, expansion and positive selection of genes associated with stalled replication fork recovery and DNA repair suggest improvements in DNA protection. This is the first genome sequence of a hadal invertebrate. Our results provide insights into the genetic adaptations used by invertebrate in deep oceans.

Barriers to gene flow in the deepest ocean ecosystems: Evidence from global population genomics of a cosmopolitan amphipod.

The deepest marine ecosystem, the hadal zone, hosts endemic biodiversity resulting from geographic isolation and environmental selection pressures. However, the pan-ocean distribution of some fauna challenges the concept that the hadal zone is a series of isolated island-like habitats. Whether this remains true at the population genomic level is untested. We investigated phylogeographic patterns of the amphipod, Bathycallisoma schellenbergi, from 12 hadal features across the Pacific, Atlantic, Indian, and Southern oceans and analyzed genome-wide single-nucleotide polymorphism markers and two mitochondrial regions. Despite a cosmopolitan distribution, populations were highly restricted to individual features with only limited gene flow between topographically connected features. This lack of connectivity suggests that populations are on separate evolutionary trajectories, with evidence of potential cryptic speciation at the Atacama Trench. Together, this global study demonstrates that the shallower ocean floor separating hadal features poses strong barriers to dispersal, driving genetic isolation and creating pockets of diversity to conserve.

A new oerstediid discovered from wood falls in the Sea of Kumano, Japan: Description of Rhombonemertes rublinea gen. et sp. nov. (Nemertea: Eumonostilifera)

Nemerteans in the family Oestediidae, 1993 occur worldwide, in various marine ecosystems from tropical to polar regions. Recent faunistic studies have revealed unexpected species diversity in upper bathyal to hadal zones, but nemertean taxonomic discoveries remain scarce there. In this study, a new genus and species, Rhombonemertes rublinea gen. et sp. nov. are described for specimens found inside sunken wood logs trawled from the Sea of Kumano, Japan. The new species is placed in the family Oerstediidae based on internal morphological characters (presence of two-layered musculature of the rhynchocoel wall and absence of a vascular plug), as well as on the phylogenetic position nested in Oestediidae revealed by molecular data. The new oerstediid resembles species in the emplectonematid Ischyronemertes Gibson, 1990 in having a short rhynchocoel, diagonal musculature in the body wall, anteriorly undivided longitudinal musculature of the body wall, precerebral septum, 4 eyes, and free-living lifestyle, but differs from all three species in Ischyronemertes in the lozenge-shaped head and lack of accessory nerves in the lateral nerve cords. Comparison of these morphological characters with currently known genera in Oerstediidae warrants the establishment of Rhombonemertes gen. nov.

First eumonostiliferous nemertean from the Nishi-Shichito Ridge, Genrokunemertes obesa gen. et sp. nov. (Eumonostilifera, Nemertea).

Nemerteans are mostly marine, benthic invertebrates, inhabiting intertidal to hadal zones. Recently, they have been recognized from deep sea with environmental DNA (eDNA) metabarcoding of sediment samples whereas any records from the locations and/or the water depth have not been documented in nemertean taxonomic publications to date. It suggests that there are major gaps between deep-sea nemertean fauna observed with eDNA and taxonomic knowledge. During a research expedition in 2021, we obtained a single specimen of eumonostiliferous nemertean from the southern part of Genroku Seamount Chain, the Nishi-Shichito Ridge, where any nemertean species have never been reported. Subsequent morphological and molecular examination reveal that the species is placed in a new genus and herein described as Genrokunemertes obesa gen. et sp. nov. It resembles shallow-water-dwelling Kurilonemertes and Typhloerstedia, but differs from the former in lacking eyes and the latter in possessing well-developed cephalic glands and lacking accessory nerves of lateral nerve cords. In terms of genetic distances based on partial sequences of the cytochrome c oxidase subunit I gene, G. obesa gen. et sp. nov. is closest to Monostilifera sp. Owase collected from Japan, which is characterized by large four eyes; the COI distance is higher than commonly observed interspecific divergences in eumonostiliferans.

The Characteristics of Lipid Biomarkers from the Abyssal and Hadal Sediments of the Yap Trench and the Influence of V-Shape Topography

Lipid biomarkers from deep-sea sediments have been observed in several studies, but little is known about their occurrence in trench system sediments. Here, we determined the concentrations of lipid biomarkers (fatty acids and neutral lipids) in sediments from the north Yap Trench. Our results showed that short-chain (C12–20) saturated fatty acids (SFAs) contributed more than (C12:1–23:1) monounsaturated fatty acids (MUFAs), and (C16:3–24:4) polyunsaturated fatty acids (PUFAs). Most fatty acids (FAs) suggest that bacteria and algae were the main contributors to marine organic matter. In contrast, terrestrial organic matter (OM) was a minor contributor to long carbon chain fatty acids greater than C20. On the other side, the observed neutral lipids such as alkanes (C14–C27), alkanols (C12–C20), alkenes (C17:1–C26:1), phytol and sterol (C27–29) indicate that phytoplankton and bacteria were the main contributors of organic materials in the sediments, and the carbon chain of neutral lipids C20–29 offering the sources of terrestrial organic matter. The extremely depleted δ13C values of fatty acids give the sources of organic carbon in the sediments from bacteria, algae, and methane-related microbes. This study is important for understanding the biogeochemical activities in deep-sea environments, particularly in the abyss and hadal zones. It will be helpful to understand the sources, transfer, and deposition of organic matter in marine trenches.

Independent radiation of snailfishes into the hadal zone confirmed by Paraliparis selti sp. nov. (Perciformes: Liparidae) from the Atacama Trench, SE Pacific

Snailfishes are among the most rapidly radiating families of marine fishes, resulting in a global distribution from the coastal intertidal to deep subduction trenches. The true diversity and distribution of deep-water snailfishes, particularly at hadal depths (>6000 m) and in the Southern Hemisphere, remain uncertain due to the rarity of samples. Here, we present the snailfish diversity at near-hadal and hadal depths in the Atacama Trench, which runs along the southwest coast of South America. Using free-fall baited cameras and traps, we documented at least three species of hadal snailfishes between 5920 and 7608 m based on distinct morphologies. One snailfish specimen was recovered from 6714 m, which we describe herein as Paraliparis selti sp. nov., based on a combined morphological and molecular taxonomic approach (16S, COI, and Cyt-b). Paraliparis selti sp. nov. is morphologically distinct from described snailfishes due to a combination of high number of; vertebrae (65) particularly the abdominal vertebrae (12), dorsal fin rays (60), anal fin rays (52), and caudal fin rays (8); comparatively low number of pectoral fin rays (18) which forms a deep notch with two widely spaced non-rudimentary rays. Micro-CT was used to minimise dissection of the specimen and to provide a digital holotype. Paraliparis selti sp. nov. highlights the importance of the Liparidae at hadal depths and provides evidence for at least two independent radiations of snailfishes into the hadal zone.

Morphological and molecular evolution of hadal amphipod's eggs provides insights into embryogenesis under high hydrostatic pressure.

Hadal zones are unique habitats characterized by high hydrostatic pressure (HHP) and scarce food supplies. The ability of eggs of species dwelling in hadal zones to develop into normal embryo under high hydrostatic pressure is an important evolutionary and developmental trait. However, the mechanisms underlying the development of eggs of hadal-dwelling species remain unknown due to the difficulty of sampling ovigerous females. Here, morphological and transcriptome analyses of eggs of the “supergiant” amphipod Alicella gigantea collected from the New Britain Trench were conducted. The morphology of A. gigantea eggs, including size, was assessed and the ultrastructure of the eggshell was investigated by scanning electron microscopy. Transcriptome sequencing and molecular adaptive evolution analysis of A. gigantea eggs showed that, as compared with shallow-water Gammarus species, genes exhibiting accelerated evolution and the positively selected genes were mostly related to pathways associated with “mitosis” and “chitin-based embryonic cuticle biosynthetic process”, suggesting that “normal mitosis maintenance” and “cuticle development and protection” are the two main adaptation strategies for survival of eggs in hadal environments. In addition, the concentration of trimethylamine oxide (TMAO), an important osmotic regulator, was significantly higher in the eggs of hadal amphipods as compared to those of shallow-water species, which might promote the eggs’ adaptation abilities. Morphological identification, evolutionary analysis, and the trimethylamine oxide concentration of A. gigantea eggs will facilitate a comprehensive overview of the piezophilic adaptation of embryos in hadal environments and provide a strategy to analyze embryogenesis under high hydrostatic pressure.

High Carbon Mineralization Rates in Subseafloor Hadal Sediments—Result of Frequent Mass Wasting

AbstractIn the past 20 years, the exploration of deep ocean trenches has led to spectacular new insights. Even in the deepest canyons, an unusual variety of life and unexpectedly high benthic oxygen consumption rates have been detected while microbial processes below the surface of the hadal seafloor remains largely unknown. The information that exist comes from geophysical measurements, especially related to seismic research, and specific component analyses to estimate the carbon export. In contrast, no information is available on metabolic activities in deeper buried sediments of hadal environment. Here we present the first pore water profiles from 15 up to 11 m long sediment cores recovered during three expeditions to two hadal zones, the Japan Trench and the Atacama Trench. Despite low levels of organic debris, our data reveal that rates of microbial carbon turnover along the trench axes can be similar to those encountered in much shallower and more productive oceanic regions. The extreme sedimentation dynamics, characterized by frequent mass wasting of slope sediments into the trenches, result in effective burial of reactive, microbially available, organic material. Our results document the fueling of the deep hadal biosphere with bioavailable material and thus provide important understanding on the function of deep‐sea trenches and the hadal carbon cycle.

Virioplankton assemblages from challenger deep, the deepest place in the oceans

SummaryHadal ocean biosphere, that is, the deepest part of the world’s oceans, harbors a unique microbial community, suggesting a potential uncovered co-occurring virioplankton assemblage. Herein, we reveal the unique virioplankton assemblages of the Challenger Deep, comprising 95,813 non-redundant viral contigs from the surface to the hadal zone. Almost all of the dominant viral contigs in the hadal zone were unclassified, potentially related to Alteromonadales and Oceanospirillales. 2,586 viral auxiliary metabolic genes from 132 different KEGG orthologous groups were mainly related to the carbon, nitrogen, sulfur, and arsenic metabolism. Lysogenic viral production and integrase genes were augmented in the hadal zone, suggesting the prevalence of viral lysogenic life strategy. Abundant rve genes in the hadal zone, which function as transposase in the caudoviruses, further suggest the prevalence of viral-mediated horizontal gene transfer. This study provides fundamental insights into the virioplankton assemblages of the hadal zone, reinforcing the necessity of incorporating virioplankton into the hadal biogeochemical cycles.

Vertical distribution of size-fractionated bacterial communities in the water column of the Atacama Trench

Variation of vertical distribution of bacterial communities through the whole water column of the trench regions has received more and more attention. However, it is still unclear whether there are unique microbial communities in trenches. The study investigated the bacterial composition and diversity in three size-fractions (0.3-0.7μm, 0.7–2.7μm, and >3μm) from the surface to the hadal zone (7500 m) at one site in the Atacama Trench. The results showed that the alpha diversity changed with depth and was the highest at 6000 m and lowest at 1000 m. For different size fractions, the bacterial diversity was higher in 0.7-2.7μm and >2.7μm fractions than 0.3-0.7μm in the surface water, but the diversity in 0.3-0.7μm fraction was the highest in the hadalpelagic, likely reflecting the depth changes of quantity and bioavailability of organic matter. Firmicutes were the most abundant bacteria (36.3 ± 15.6%) in the water depth, while Alphaproteobacteria dominated in the samples at 7500 m for the 0.3–0.7μm fraction (36.3%), and 7000–7500 m for the 0.7–2.7μm fraction (30.6 ± 1.2%). PCoA analysis indicated that the hadalpelagic bacterial communities were more like those in the overlying deep water, but were distinct from the epipelagic in the study area. Though no significant differences among size fractions of bacterial community were observed in the pelagic Atacama Trench, lifestyle preferences existed that some relied on particulate matter, and some preferred to live freely. This suggested the important influence of surface primary production and export production on the bacterial communities in the deep ocean.

Comparative Analysis of the Mitochondrial Genome of Galatheanthemum sp. MT-2020 (Actiniaria Galatheanthemidae) From a Depth of 9,462 m at the Mariana Trench

The hadal zone, which represents the deepest marine habitat on Earth (6,000–11,000 m), is a harsh environment mainly characterized by extremely high hydrostatic pressure, and this habitat is believed to have a high degree of endemism. The deep-sea anemone family Galatheanthemidae comprises two valid species exclusively from the hadal; however, no other information about this family is currently available. In the present study, a sea anemone was collected from a depth of 9,462 m at the Mariana Trench and was defined as Galatheanthemum sp. MT-2020 (Actiniaria Galatheanthemidae). The mitochondrial genome of Galatheanthemum sp. MT-2020 was circular, was 16,633 bp in length, and contained two ribosomal RNA genes, 13 protein-coding genes and two transfer RNA genes. The order of the genes of Galatheanthemum sp. MT-2020 was identical to that of the majority of the species of the order Actiniaria. The value of the AT-skew was the lowest in the whole mitochondrial genome, with a positive GC skew value for the atp8 gene, while other species, except Antholoba achates, had the negative values of the GC skew. Galatheanthemum sp. MT-2020 was clustered with another abyssal species, Paraphelliactis xishaensis, in the phylogenetic tree, and these species diverged in the early Jurassic approximately 200 Mya from the shallow-sea species. The usage ratio of valine, which is one of the five amino acids with the strongest barophilic properties, in the mitochondrial genomes of the two abyssal species was significantly higher than that in other species with habitats above the depth of 3,000 m. The ω (dN/dS) ratio of the genomes was 2.45-fold higher than that of the shallow-sea species, indicating a slower evolutionary rate. Overall, the present study is the first to provide a complete mitogenome of sea anemones from the hadal and reveal some characteristics that may be associated with adaptation to an extreme environment.

Exponential growth of hadal science: perspectives and future directions identified using topic modelling

Abstract The hadal zone is a cluster of deep-ocean habitats that plunge to depths of 6000–11000 m below sea level. Research of the deepest marine zone has occurred on a disjunct timeline and slower than shallower zones. Over the past 20 years, research efforts have surged with greater sampling capabilities and an expansion of expeditions. We aimed to assess the state of hadal science by quantitively assessing the publishing landscape. We applied a topic modelling approach and fit a Latent Dirichlet Allocation model for 12 topics to 520 abstracts from peer-reviewed papers, reviews, and conference proceedings available on the Web of Science's Core Collection between 1991 and 2021. The model outputs were analysed with ecological modelling approaches to identify the main lines of research, track trends over time, and identify strengths and gaps. We found that hadal science is occurring across all five broad disciplines of oceanography and engineering. Hadal research has exponentially grown in the past 30 years, a trend that shows no signs of slowing. The expansion is most rapidly occurring to understand the biogeochemistry of trenches, the functions of microbial communities, and the unique biodiversity inhabiting these ecosystems, and then the application of ‘omics techniques to understand hadal life. The topic trends over time are largely driven by available technology to access and sample the deepest depths and not necessarily the pursuit of specific scientific questions, i.e. the hadal research topics are bounded by the capabilities of available exploratory vehicles. We propose three recommendations for future hadal research: (1) conduct multifeature studies that include all hadal geomorphologies across their depth range, (2) establish a programme for seasonal or long-term sampling, and (3) strengthen cross-disciplinary research. This continued acceleration in hadal research is pertinent for this last marine frontier given its vulnerability to multiple anthropogenic pressures and cascading threats from global change.

Distribution of hadal genera depends on the lower limits of their bathymetrical ranges

A list of genera represented in the hadal zone (>6000 m), 22 trenches, two trench faults, and two basins was compiled for Tanaidacea, Isopoda, Bivalvia and five classes of Echinodermata. Among these 161 genera, hadal endemics comprised only 4%, which is lower than the previous estimate. Geographic and bathymetric distributions were examined in four groups of genera based on their lower bathymetric distribution limits: i) 6001–7000 m, ii) 7001–8000 m, iii) 8001–9000 m and iv) greater than 9000 m. These groups were compared by the percentage of genera with a narrow and wide geographical distribution in the hadal zone and in the world ocean from intertidal to hadal depths. Most of the hadal genera were characterized by a wide geographic range at shallower depths and a narrow geographic range in the hadal zone. Almost half of the 161 genera considered in this study had lower limits at 6001–7000 m. The percentage of narrowly distributed in the hadal genera found only in one trench decreases from 72% to 4% from the first to fourth group, whereas the proportion of widely distributed genera known from four or more trenches increases from 3% to 70%. Lower limit of the vertical distribution is positively correlated with the number of trenches and other hadal areas where the genera occurred. The wide distribution of the deepest-dwelling genera at hadal depths suggests that the most successful colonization of the hadal zone results from the high tolerance of some genera to multiple factors, including hydrostatic pressure, food distribution, topography, and potential ecological interactions. The lower hadal zone seems to be inhabited mainly by ubiquists.

Hadal Biodiversity, Habitats and Potential Chemosynthesis in the Java Trench, Eastern Indian Ocean

The Java Trench is the only subduction trench in the Indian Ocean that extends to the hadal zone (&amp;gt; 6,000 m water depth), and except for sevenbenthic trawls acquired around the 1950s, there has been little to no sampling at hadal depths undertaken since. In 2019, we undertook a 5-day expedition comprising a scientific dive using a full ocean depth-rated submersible, the DSV Limiting Factor, seven hadal-lander deployments, and high-resolution bathymetric survey. The submersible performed a video transect from the deepest point of the trench, up a 150 m high near-vertical escarpment located on the forearc, and then across a plateau at a depth of ∼7,050 m to make in situ observations of the habitat heterogeneity and biodiversity inhabiting these hadal depths. We found the Java Trench hadal community to be diverse and represented by 10 phyla, 21 classes, 34 orders and 55 families, with many new records and extensions in either depth or geographic range, including a rare encounter of a hadal ascidian. The submersible transect revealed six habitats spanning the terrain. The deepest trench axis comprised fine-grained sediments dominated by holothurians, whereas evidence of active rock slope failure and associated talus deposits were prevalent in near-vertical and vertical sections of the escarpment. Sediment pockets and sediment pouring down the steep wall in “chutes” were commonly observed. The slope terrain was dominated by two species in the order Actiniaria and an asteroid, as well as 36 instances of orange, yellow, and white bacterial mats, likely exploiting discontinuities in the exposed bedrock, that may indicate a prevalence of chemosynthetic input into this hadal ecosystem. Near the top of the escarpment was an overhang populated by &amp;gt; 100 hexactinellid (glass) sponges. The substrate of the plateau returned to fine-grained sediment, but with a decreased density and diversity of epifauna relative to the trench floor. By providing the first visual insights of the hadal habitats and fauna of the Java Trench, this study highlights how the habitat heterogeneity influences patchy species distributions, and the great benefit of using a hadal-rated exploratory vehicle to comprehensively assess the biodiversity of hadal ecosystems.

The Multi-Ocean Distribution of the Hadal Amphipod, Hirondellea dubiaDahl, 1959 (Crustacea, Amphipoda)

The hadal zone (depths &amp;gt; 6,000 m) comprises isolated features that host high degrees of endemism, with species inhabiting only a single feature or a group of adjacent features. The amphipod, Hirondellea dubia is an abundant scavenger at the hadal depths of the Kermadec, Tonga, and New Hebrides trenches in the Southwest Pacific Ocean, particularly at depths &amp;gt; 9,500 m. However, several records have hinted that H. dubia may not be restricted to the Southwest Pacific nor be exclusively hadal. Here, we present new records of H. dubia recovered by baited landers deployed to five hadal trenches during three expeditions and assess the geographic and vertical distribution of this species. To rule out the possibility of cryptic diversity, morphological taxonomy was paired with two mitochondrial markers (16S and COI) to test for species boundaries among the new and published molecular data using four delineation approaches (ABGD, CD-HIT, GMYC, and bPTP). We found H. dubia to be a single species and not restricted to the Southwest Pacific or hadal depths. This species’ geographic range extends from the Southwest Pacific to the Mariana Trench (Northwest Pacific), the Clarion-Clipperton Zone (Central Pacific), the South Sandwich Trench (Southern), and the Puerto Rico Trench (North Atlantic). This amphipod’s vertical range spans over half of the ocean’s total depth, between 4,700 and 10,817 m. This study presents an extraordinary geographic range extension to a species long considered endemic to a cluster of three Southwest Pacific trenches and shows a pan-oceanic distribution across extremely isolated hadal features.