Metabarcoding offers a powerful approach for assessing benthic biodiversity in remote and understudied deep-sea environments. However, the methodological performance of different DNA extraction strategies remains insufficiently evaluated, especially for benthic meiofaunal communities. In this study, we compared two extraction strategies—community DNA (ComDNA) from isolated benthic metazoans and sedimentary environmental DNA (SedDNA) from whole sediment—to assess benthic metazoan diversity in the Clarion-Clipperton Fracture Zone (CCZ), an area targeted for future polymetallic nodule mining. Using the 18S V1–V2 rRNA marker, we analyzed alpha and beta diversity, taxonomic composition, and the number of metazoan operational taxonomic units (OTUs) recovered by each strategy. ComDNA extractions yielded substantially higher benthic metazoan OTU richness (2,145 OTUs) than SedDNA (392 OTUs), with only 1.2% of OTUs shared between them. Community composition also differed significantly, driven by strategy-specific detection biases. To evaluate the effectiveness of SedDNA for biodiversity monitoring, we modeled the sediment volume required to recover OTU richness comparable to ComDNA samples. Depending on sequencing depth and statistical approach, we estimated that the processing of 27–82 mL of sediment is necessary to match ComDNA-derived richness. Our findings underscore the superior taxonomic resolution of ComDNA extractions but also highlight the potential of optimized SedDNA protocols for scalable biodiversity monitoring. We recommend sediment homogenization, increased sample volume, and a higher sequencing depth of at least 100,000 reads per sample for improving SedDNA-based assessments, particularly in heterogeneous deep-sea environments. These results provide critical methodological guidance for the development of standardized, efficient monitoring strategies in the context of deep-sea mining impact assessments.

In order to understand anthropogenic influences on meiofauna communities, molecular approaches based on meaningful reference libraries are, among other methods, an essential tool. As part of the pilot mission “MGF Ostsee” of the German Marine Research Alliance (DAM), the first curated DNA barcode libraries for harpacticoid copepods from the Baltic Sea were developed, focusing on the Fehmarn Belt and Oderbank. Using an integrative taxonomy approach, mitochondrial COI and nuclear 18S (V1–V2) gene fragments from 818 individuals were sequenced, 629 of which were morphologically identified to species level. The resulting dataset comprises 141 COI and 136 18S sequences, representing 42 species across 30 genera and 15 families, including 25 species sequenced for the first time. Comparison with public databases revealed significant sequence divergence (19–25

Background Monstrilloid copepods have endoparasitic naupliar and postnaupliar stages infecting different groups of marine invertebrates. As adults, they have been recorded as free-swimming, non-feeding planktonic organisms in a wide variety of near-surface coastal and transitional aquatic habitats. The phylogenetic relations of the Monstrilloida, obscured by the lack of antennae and mouthparts, have long been a matter of discussion among copepodologists. Methods Epibenthic samples collected at 2,537 m depth from the Irminger Basin, North Atlantic, yielded an adult male monstrilloid copepod that is unassignable to the Monstrillidae, the only known family of the order Monstrilloida. Herein, we erect a new family of the Monstrilloida to accommodate this individual. We provide a comprehensive morphological description of this specimen, along with a phylogenetic analysis that incorporates three genetic markers (mtCOI, 28S rRNA, and 18S rRNA), which supports the designation of the new family. Results The most striking character in the new family is the remarkably long, slender antennules that are directed posteriorly, thus diverging from the typical monstrillid pattern with rigid, anteriorly directed antennules. Together with the large furca, these characters are considered traits for their adult planktonic life. Another striking character is the presence of a pair of indistinctly segmented, likely vestigial, appendages flanking the oral protuberance, instead of the absence of oral appendages typical of monstrillids. Also, the presence of biramous male fifth legs has never been observed in other monstrilloids, in which male fifth legs are absent or reduced. The distinctive morphological characters exhibited by this new family, along with its deep-sea benthopelagic occurrence, could provide new elements for re-evaluating the phylogenetic position of the Monstrilloida, as well as insights into their biology and ecology.

Hadal trenches are extreme and dynamic environments, yet their meiofauna diversity and biogeographic connectivity remains poorly resolved. This study compared abyssal and hadal meiofauna communities from two trench systems in the Northern Pacific, the Aleutian Trench and the Kuril-Kamchatka Trench, using CO1 metabarcoding as the primary genetic marker, with the 18S V1V2 marker as an exploratory comparison. With CO1 five higher meiofauna taxa were retained, being dominated by Crustacea and Nematoda. In comparison, V1V2 additionally detected rare taxa, such as Tardigrada, Gastrotricha and Loricifera. Across both trench systems, abyssal samples exhibited significantly higher observed and expected OTU richness, diversity and evenness than hadal samples. Ordination analyses revealed a clear separation between abyssal and hadal communities, with CO1 capturing a more distinct depth-related gradient but still present in V1V2. The majority of CO1 OTUs (∼84%) were restricted to a single region-habitat group, showing a strong spatial turnover in community composition. Yet a subset of Copepoda OTUs occurred across both trench systems and habitats. Indicator analysis identified OTUs associated with habitats and regions, predominantly linked to copepod lineages from the CCZ, but also including Zosimeidae from the Aleutian Trench. Haplotype networks suggested a mixture of shared and trench restricted variants. Overall, meiofaunal communities were primarily structured by depth (abyssal vs. hadal), with additional regional-specific signals. CO1 proved generally more robust in detected habitat related patterns compared to V1V2 and also showed a higher success regarding legacy samples.

As part of the DAM pilot mission “MGF Ostsee”, a subsector in a marine protected area (MPA) and a neighbouring reference area (REF) in the Oderbank (eastern Baltic Sea, Germany) were sampled for meiofauna in 2021. In both areas, mobile bottom trawling fisheries (MGF) was permitted at the time of the investigations. These shall therefore serve as a baseline for future BACI studies when MGF will be excluded from the MPA. Five sites were sampled in the MPA, and three sites were sampled in the REF. These samples served for morphology-based similarity and diversity analyses to determine whether the communities of MPA and REF differ from each other. Comparisons were carried out both at the level of major meiobenthic taxa and at the species level in the Copepoda Harpacticoida. The taxa inventory revealed that the second-most common major taxon was not Copepoda but Tardigrada, followed by Annelida. Furthermore, the planktonic Calanoida were the dominant copepod taxon instead of Harpacticoida. At major taxon level, no significant differences could be proven between a meiobenthic MPA and a REF community in terms of composition, abundance and diversity, despite minor differences. However, for harpacticoid species, significant differences were detected regarding species composition and abundance values but not regarding diversity.

Submarine deep-sea topography − comprising trenches, ridges, and abyssal plains − functions as a semi-permeable barrier to dispersal. Brooding isopods represent an ideal model for investigating how such features shape species divergence. Here, we applied an integrative framework − combining detailed morphological analyses, a mitochondrial marker (COI), and proteomic fingerprinting using MALDI-TOF MS − to delimit species and elucidate phylogeographic patterns in the deep-sea isopod family Mesosignidae Schulz, 1969 across the Aleutian Trench (AT) and the Kuril-Kamchatka Trench (KKT). Initial attempts to amplify COI with universal primers consistently failed; in response, we designed and validated Kurilosignum-specific primers, achieving ∼ 81 % sequencing success and markedly expanding our genetic dataset. Proteomic profiles corroborated the molecular species clusters, validating this first application of proteomic fingerprinting in Mesosignidae. Molecular analyses revealed four cryptic species within material morphologically identified as Kurilosignum sp., three confined to the AT and one to the KKT. Patterns of haplotype sharing of one Kurilosignum lineage across abyssal sites north and south of the AT indicate recent gene flow, while abyssal and hadal populations suggest limited vertical connectivity with a divergence of 4.5 %. The AT seems to act as a semi-permeable barrier: Kurilosignum species traverse abyssal plains, whereas Japanosignum remains restricted to the southern slope of the AT; however, the small sample size requires additional collections for confirmation. In addition, we formally describe Kurilosignum cardui Kelch & Spies sp. nov. − the second species to be described from the genus − from abyssal depths of the AT. The discovery highlights hidden diversity within a small, low-abundance taxon and demonstrates the power of an integrative framework for resolving taxonomy and biogeography in elusive deep-sea lineages.

DNA metabarcoding of zooplankton biodiversity is used increasingly for monitoring global ocean ecosystems, requiring comparable data from different research laboratories and ocean regions. The MetaZooGene Intercalibration Experiment (MZG-ICE) was designed to examine1 and analyse patterns of variation of DNA sequence data resulting from multi-gene metabarcoding of 10 zooplankton samples carried out by 10 research groups affiliated with the Scientific Committee for Ocean Research (SCOR). Aliquots of DNA extracted from the 10 zooplankton samples were distributed to MZG-ICE groups for metabarcoding of four gene regions: V1-V2, V4 and V9 of nuclear 18S rRNA and mitochondrial COI. Molecular protocols and procedures were recommended; substitutions were allowed as necessary. Resulting data were uploaded to a common repository for centralised statistics and bioinformatics. Based on proportional sequence numbers for abundant phyla, overall patterns of variation were consistent across many-but not all-MZG-ICE groups. V9 showed highest similarity, followed (in order) by V4, V1-V2, and COI. Outlier data were hypothesised to result from the use of different PCR protocols and sequencing platforms, and possible contamination. MZG-ICE results indicated that DNA metabarcoding data from different laboratories and research groups can provide reliable, accurate and valid descriptions of biodiversity of zooplankton throughout the ocean. Recommendations included: pre-screening QA/QC of raw data, detailed records for laboratory protocols, reagents, and instrumentation, and centralised bioinformatics and multivariate statistics. In the absence of universal agreement on standardised protocols or best practices, intercalibration is the best way forward toward validation of DNA metabarcoding of zooplankton diversity for global ocean monitoring.

The northern North Pacific forms a heterogeneous deep-sea environment with dynamic currents, distinct water masses, and complex bathymetry, including the Kuril–Kamchatka (KKT) and Aleutian Trenches (AT). As a climate-sensitive region, warming and sea-ice loss alter surface productivity and organic matter flux, potentially affecting deep-sea benthic communities. We quantified metazoan meiofaunal densities across abyssal and hadal zones of the KKT and AT, integrating samples from the 2022 AleutBio (SO293) expedition with legacy datasets. Gradient-Forest modeling captured broad productivity patterns across higher taxonomic levels, while generalized linear mixed effects models (GLMMs) quantified taxon-specific responses to environmental factors. Meiofaunal densities varied substantially, peaking on the abyssal KKT plain and reaching minima in the hadal AT. Gradient-Forest analysis identified surface chlorophyll a, bottom-water temperature, and monthly primary production as primary drivers of regional density patterns. However, taxon-specific GLMMs revealed that once spatial effects and overdispersion were controlled, bottom-water chemistry and hydrography exerted the most consistent control on local abundances. These results demonstrate a two-tier ecological control: surface-derived food availability governs meiofaunal densities at regional scales, whereas bottom-water chemistry, hydrographic regimes, and grain size fine-tune local distributions. This study provides critical baseline data for understudied deep-sea regions and informs predictions of meiofaunal responses to climate-driven changes.

Marine nematodes, the most abundant meiofauna in benthic ecosystems, drive critical nutrient cycling and carbon sequestration, yet accurate biomass estimation remains challenging due to morphological variability and reliance on simplistic geometric models like Andr & aacute;ssy's formula, which introduced systematic biases across taxa. This study developed RAH NemaCalc, an open-source Python-based tool with Tkinter GUI and OpenCV integration that semi-automates morphometric analysis by segmenting nematodes into 5-20 conical frustums based on length-diameter ratios, calculating lateral surface area with morphology-specific correction factors (k(total) and frustum geometry factor f), and converting to biomass using an empirically determined density of 1.08 g/cm(3) derived from sucrose gradient centrifugation of 50 genera. The tool processed high-resolution images of 187 specimens from diverse habitats- 50 from Lakshadweep Islands, 87 from Clarion Clipperton Zone abyssal sediments, and 50 from the Nemys database yielding processing times of 1.8-2.1 s/image with manual contour verification. Key results showed the frustum method produced significantly different biomass estimates from Andr & aacute;ssy (Wilcoxon p < 0.0001 across datasets), with median volume reductions of 20-28% in coastal/abyssal samples but up to 260% higher biomass in large tapered genera like Linhomoeus and Halichoanolaimus; Bland-Altman analysis confirmed morphology- and size-dependent bias, while 10-20 segments optimized accuracy for tapered forms. RAH NemaCalc thus established a precise, high-throughput standard for nematode biomass quantification, enabling reliable monitoring of anthropogenic impacts like deep-sea mining on ecosystem functions and informing conservation baselines.

Hadal trenches remain among the least explored marine environments, with much of their faunal diversity still unknown. It is still debated whether trenches act as dispersal barrier for benthic organisms potentially leading to distinct faunal communities within and around them. This study aimed to investigate the meiofauna diversity in and around the Aleutian trench using a multi-gene metabarcoding approach. Two genetic markers, CO1 and 18S, were used to assess the potential uniqueness of hadal, abyssal and bathyal habitats. To improve the accuracy of taxonomic assignments, a curated reference library of harpacticoid species was created. All sampling sites revealed unique meiofaunal communities; however, both markers consistently detected a clear separation between hadal and abyssal assemblages. While certain taxa were underrepresented in the CO1 dataset compared to 18S, likely due to primer bias, the overall patterns in community structure remained consistent. Besides depth, Chl-a (for CO1) and TOC (for 18S) emerged as key environmental factors influencing meiofauna community composition. Notably, eight COI OTUs were detected across sites spanning the trench, five of which were also detected in the Clarion-Clipperton Zone, suggesting that the trench does not necessarily act as a strict barrier to dispersal. One of these OTUs was assigned to genus Dorsiceratus − a taxon rarely encountered in previous deep-sea surveys − demonstrating the potential of metabarcoding, especially when combined with curated reference libraries, to reveal hidden biodiversity.

Abstract. The Clarion-Clipperton Zone (CCZ) situated in the central east Pacific holds major portions of manganese nodule deposits and is therefore subject to intense exploration for future deep-sea mining. However, mining rises multiple concerns. Among others about its direct or indirect impact on abyssal environment. The more, proper evaluation of deep-sea protection plans to be applied there is still hindered by insufficient knowledge of the abyssal fauna diversity and their assemblages. Amphipoda are speciose and abundant in all marine habitats and were proven to be important food source for higher trophic levels. These crustaceans are brooders with no planktonic larval stage, so migration of species depend only on swimming capacities of adult animals. In the CCZ macrofauna-sized and relatively mobile, epibenthic Amphipoda, not collected with commonly used box corers or multicorers, remained unstudied. The present work aims to fill in this gap in knowledge by: 1) characterizing the diversity of amphipods across the CCZ and one Area of Particular Environmental Interest (APEI), 2) studying the amphipod species distribution and assemblages as well as 3) inferring the connectivity between eastern and western parts of the CCZ and other deep-sea regions. The material was collected with epibenthic sledge from three contractor zones (UK, Singapore, Germany) and from APEI-06. Specimens were sorted into families and preferably morphospecies and subjected to cytochrome c oxidase subunit I gene (COI) barcoding. Within total of 708 individuals 23 known and one undescribed families were identified. The barcoding allowed to obtain 581 sequences that were ascribed to 207 Molecular Operational Taxonomic Units (MOTUs). The families that dominated barcoded material in terms of abundance and species richness were: Pardaliscidae (123 ind., 44 MOTUs), Eusiridae (122 ind., 32 MOTUs) and Synopiidae (74 ind., 31 MOTUs). Only 17 known species were identified; another 48 are considered as new to science. Almost 70% of MOTUs were singletons or doubletons and 186 MOTUs were unique for the present study. The remaining 21 taxa appeared to be broadly distributed. Among the MOTUs recorded only in the CCZ majority was distributed within 400 km distance. Just six species reached or exceeded 1000 km distance of occurrence. Additionally only one species was shared between contractor areas and the studied APEI that was supposed to be preserved area representative for the mining zones. The cluster analysis of the fauna collected during three expeditions to the German contractor areas revealed almost no similarity between the West and East part of the CCZ. Additionally, within the eastern part of the studied region geographic proximity appeared to have no influence on station clustering. The study confirmed low abundance and high species richness of deep-sea amphipod fauna of the CCZ and their highly variable assemblages. Moreover, it has shown weak connectivity between eastern and western parts of the region as well as between the contractor zones, APEIs and other deep-sea regions of the World. All the above suggest high vulnerability of the assemblages studied and, with the present state of knowledge, weak representativeness of Areas of Particular Environmental Interest for the mining zones. In order to better understand the deep-sea biodiversity and develop Regional Environmental Management Plans for the whole CCZ area two main recommendations are to be made: 1) to conduct more intensive sampling program of APEIs parallel to monitoring studies of contractor zones with the goal to better assess their usefulness as reservoirs of biodiversity for the mining areas, 2) to include the epibenthic sledge among standard gears used for sampling in order to avoid missing important part of deep-sea fauna in the monitoring studies.

Niche processes mainly dictate the successful establishment and coexistence of species along gradients of community drivers such as energy availability and environmental stress. Deep-sea chemosynthetic habitats such as hydrothermal vents and wood falls show concomitant gradients of energy availability and stress because productivity is fueled by potentially toxic, chemically reduced compounds. However, the specific processes and constraints driving the assembly of these eco-evolutionarily related communities remain poorly understood. Here, we infer community assembly processes from species, functional, and isotopic diversity patterns of vent and wood-fall assemblages using a colonization experiment along a hydrothermal gradient, from vent periphery to diffuse-flow habitats. We hypothesized that despite the high environmental stress, the high productivity of vent diffuse-flow and wood-fall habitats increases niche space due to niche partitioning and/or competitive exclusion processes, allowing more species to coexist in high densities. As predicted, at moderate levels of stress at vent diffuse-flow habitats, productivity increases niche space and supports denser and more species-rich assemblages with many ecological strategies, suggesting niche partitioning and/or competitive exclusion of functionally similar species. However, as stress increases at habitats under higher venting influence, species richness, abundance, and functional diversity decrease, suggesting environmental filtering and/or competitive dominance of better adapted species. Similarly, the vent periphery supports fewer species, individuals, trophic levels, and reduced niche space, suggesting that the scarcer food resources act as an environmental filter. Unexpectedly, food resources at wood substrata at the vent periphery, harboring typical wood-fall communities, support very speciose and dense assemblages of reduced functional diversity. This suggests that compared to vent diffuse-flow habitats, the reduced habitat availability, or longevity, of wood falls reduces niche space and promotes environmental filtering and/or competitive dominance processes. Thus, different processes and constraints appear to drive the assembly of communities along hydrothermal vent gradients and between the eco-evolutionarily related vent and wood-fall communities.

Despite its remoteness, the deep sea is not spared from the impact of human activities. The emergence of industrial-scale deep-sea mining of polymetallic nodules on the abyssal plains (4-5 km depth) of the Clarion-Clipperton fracture zone in the equatorial Pacific is potentially the most threatening undertaking at present time. A primary aspect that must be comprehensively assessed in order to safeguard these abyssal benthic communities through the designation of marine protected areas and the spatial organization of prospective mining fields is connectivity, i.e., the continuity of species distributions in space and time. In this review we provide the current state of knowledge on connectivity through an examination of published literature focused on several animal groups from all benthic faunal size classes (megafauna, macrofauna, meiofauna) as well as Foraminifera and microbes. We highlight the main challenges associated with assessing connectivity in the deep sea and outline the key parameters required to achieve the idealised connectivity study for fauna and microbes.

Accelerating climate change driven by greenhouse gas emissions is profoundly impacting marine biodiversity. Some species are shifting their geographic ranges toward more favourable environments, while others lose suitable habitat and face extinction. We examined the effects of climate change on benthic peracarid species (Amphipoda, Cumacea, and Isopoda) in the Pacific Arctic, Pacific Subarctic, and North Pacific temperate ecosystems—regions characterized by environmental sensitivity and unique topography. Using Maximum Entropy (MaxEnt) modelling, we identified key environmental drivers of species occurrence and projected horizontal and vertical shifts until 2050 and 2100 under RCP 2.6 and 8.5 scenarios. We further analysed how ecological traits such as diet, mobility, and life habit correlate with distributional responses. While most species shifted northward, some exhibited unexpected directional changes, likely due to regional differences in climate velocity, environmental change, and species-specific responses. Temperature emerged as a primary determinant for many species, with dissolved oxygen and salinity also playing critical roles. Our findings indicate that traits—particularly diet and bathymetric distribution—affect potential habitat expansions or contractions, categorizing species as potential “winners” or “losers.” Despite challenges including deep-sea data limitations and complex trait interactions, our study provides vital insights into future peracarid distributions. These results underscore the need for refined predictive models and targeted conservation strategies to enhance ecosystem resilience and sustainable resource management in a warming ocean.

The aim of this study was to investigate and compare the composition of meiofaunal organisms in and around the Aleutian Trench, with a detailed focus on Harpacticoida. Overall, Nematoda were the most abundant Taxon (88.5 – 86.6%) followed by Copepoda (4.8 – 3.8%). The highest concentrations of these two taxa were observed in the northern rim of the trench and the Bering Sea, while Loricifera demonstrated higher abundances in the southern rim. Furthermore, a greater number of harpacticoid families were identified in samples from the northern rim and the Bering Sea compared to the other sampling sites. A total of 16 different harpacticoid families were identified, with Ectinosomatidae being the most abundant. Samples from the Aleutian trench exhibited low diversity and the lowest number of encountered families. The distribution and composition of meiofaunal organisms were found to be influenced by water depth, silt and clay content, and TOC. For the harpacticoid families, the most influential factors were water depth and Chl-a. The families Ectinosomatidae and Pseudotachidiidae contribute the most to Bray-Curtis dissimilarities. Out of seven selected harpacticoid genera, no genera showed a significant high abundance in the Aleutian trench, while Pseudotachidius showed a higher abundance in the Bering Sea compared to the northern rim of the trench and within the trench, and Zosime showed a higher abundance in the Bering Sea and in the northern rim compared to the southern rim of the trench.

Tetragoniceps bermudensis sp. nov. (Copepoda, Harpacticoida, Tetragonicipitidae) is described based on an ovigerous female collected from Roadside Cave, a tidally influenced anchialine cavern in Bermuda. The new taxon represents the 16th recorded species of Tetragoniceps Brady, 1880. It can be distinguished from congeneric species by the length:width ratio of the caudal rami (approximately 10 times longer than wide), its cephalothorax with a smooth dorsal surface, and the diagnostic setal formula of its pereiopods 1–5. Tetragoniceps bermudensis sp. nov. is the first record of Tetragoniceps from Bermuda and the first known anchialine species in the genus globally. Based on our description of the new species, we provide a revised key to the species of Tetragoniceps. In addition, we include an updated table of salient morphological characters for the females of the genus, providing grounds for a preliminary analysis of the phylogenetic relationships of its constituent species.

Extreme morphological disparity within Mollusca has long confounded efforts to reconstruct a stable backbone phylogeny for the phylum. Familiar molluscan groups—gastropods, bivalves, and cephalopods—each represent a diverse radiation with myriad morphological, ecological, and behavioral adaptations. The phylum further encompasses many more unfamiliar experiments in animal body-plan evolution. In this work, we reconstructed the phylogeny for living Mollusca on the basis of metazoan BUSCO (Benchmarking Universal Single-Copy Orthologs) genes extracted from 77 (13 new) genomes, including multiple members of all eight classes with two high-quality genome assemblies for monoplacophorans. Our analyses confirm a phylogeny proposed from morphology and show widespread genomic variation. The flexibility of the molluscan genome likely explains both historic challenges with their genomes and their evolutionary success.

Phylogeographic analyses have advanced our understanding of evolutionary processes in the deep sea, yet patterns of genetic variation and population divergence at abyssal depths remain poorly understood. The bivalve Ledella ultima is one of the most abundant protobranchs in the abyssal Atlantic, making it a valuable model organism for studying phylogeographic patterns and population connectivity. However, evidence for sex-specific heteroplasmic mtDNA challenges the assessment of genetic structure using mitochondrial markers alone. To address this, we used mtDNA (COI, 16S), single-nucleotide polymorphisms (SNPs) from 2b-RAD, and proteomic profiles to examine the population structure of L. ultima across seven Atlantic basins spanning over 10,000 km in latitude. Five mitochondrial lineages with a lack of geographic structure were consistently identified by COI and 16S. Conversely, SNP and proteomic data did not mirror these findings, denoting that heteroplasmic mtDNA inflates intraspecific genetic divergence in this gonochoric species. Despite the SNP data revealing overall low genetic divergence, subtle genetic structure was detected by admixture analyses supporting two source populations: one in the north and central Atlantic, and a second in the south Atlantic, with moderate admixture in the Brazil and Cape basins. Proteomic fingerprinting revealed two basin-separated groups with patterns distinct from the nuclear data, suggesting environmentally driven shifts in protein expression. Our findings underscore the value of integrating nuclear genomic and proteomic tools to decipher population connectivity at abyssal depths, where minimal genetic differentiation necessitates fine-scale analyses.

Hydroacoustic mapping has recently challenged the long-held view of a uniform abyssal seafloor by predicting substantial habitat heterogeneity in this environment. The RUBBLE expedition M205 validated these predictions in the Vema Fracture Zone (VFZ) with visual surveys at six locations using a towed deep-sea camera system. Exposed rock outcrops and varied hard substrates were consistently confirmed in areas of high hard-rock exposure, while moderately predicted areas contained a mix of sediments, cobbles, and pebbles; low-potential sites were almost entirely covered in sediment. Although a detailed quantification is beyond the scope of this report, visual correspondence supports the reliability of hydroacoustic predictions for abyssal habitats. Notably, this study pioneers the application of hydroacoustic-based seafloor characterization at abyssal depths—extending methods formerly focused on bathyal zones to the planet’s largest benthic environment and enabling direct ground-truthing of habitat models below 5,000 m. These findings highlight abyssal habitat heterogeneity, confirm the utility of hydroacoustic tools for broad-scale benthic mapping, and establish a baseline for future research on deep-sea biodiversity and ecological dynamics.

Organic falls, hydrothermal vent fields and seeps rely primarily on chemosynthetic organic production leading to a carbon enriched habitat set against the oligotrophic deep-sea background. These habitats are typically inhabited by novel faunal assemblages with similar functional characteristics and often show harsh environmental gradients over relatively small scales with severe effects on diversity and density compared to the benthic background fauna. While the importance of particulate organic matter (POM) has been pointed out, the presence of POM as local accumulated organic deposits within hydrothermal vent fields has not been studied. Here, we present the first observations of organic deposits within active vents in the Indian Ocean including observations of giant capitellid worms inhabiting these organic mats with a length of up to 30 cm. This potential new species occurred in shrimp carcasses, the exuvia of Rimicaris kairei, on two hydrothermal vent sites from the Indian Ocean. We used imagery to build a high-resolution 3D reconstruction model of one side of a chimney complex, including the 4.8 m2 surface area of the exuvia deposit. The capitellids occurred in dense aggregations and may contribute significantly to the food web of active hydrothermal vent fields, representing a possible overlooked food source for benthic and demersal predators. In addition, we report on observations on further organic deposit types and compare their associated epifauna along the mid-ocean ridges of the Indian Ocean. The exuvia deposits where capitellids were present appeared to be compacted and compressed implicating a potential role in stabilizing seafloor habitat. This is the first record of the family Capitellidae in organic deposits at hydrothermal vent fields, where they may serve as ecosystem engineers.