In order to assess the gastrointestinal transport function in bathyal bivalves harbouring chemosymbiotic bacteria, the Bathymodiolus japonicus, Bathymodiolus platifrons and Bathymodiolus septemdierum, the uptake of a liquid-phase MRI tracer, gadolinium-diethylenetriamine pentaacetic acid (GdDTPA), was observed for 24 h. The excretion of GdDTPA was then observed for more than 8 days at 5°C. GdDTPA concentration was estimated from the MRI T1 relaxation rate (R1=1/T1). GdDTPA was not taken up into the haemolymph from the seawater, largely reflecting the fluid transport in the gastrointestinal canal and partly the endocytosis/exocytocis of acinar cells. Compared with Mytilus galloprovincialis at 20°C, the uptake rate to the stomach (kis) varied from 60 to 150%, and that of the digestive gland (kidg) was around 70%. The excretion rate from the stomach (kes) was the same as for the Mytilus at 20°C, and almost 3 times that of the Mytilus at 5°C. Therefore, the kis, kes, kidg in the Bathymodiolus species at 5°C were similar to that for the Mytilus at 20°C. The excretion rate from the digestive gland (kedg) was 30-70% of that for the Mytilus at 20°C, and similar to the Mytilus at 5°C. However, the lower kedg value, which indicated a longer retention time, suggested a lower activity of the digestive gland in the Bathymodiolus species, but the gastrointestinal transport was still functioning more than the Mytilus at 5°C. These results suggested that the Bathymodiolus species has two nutrition routes that enable the mussel to adapt and survive in a wide range of environments.
Although the abyssal and hadal zones are defined by faunal composition, the boundary depths vary widely among studies, likely reflecting regional and taxonomic differences. Therefore, it is essential to understand these boundaries within specific benthic groups. We focused on Cumacea, an order within Peracarida and a major component of deep-sea benthic communities. Cumaceans have low dispersal capabilities and are expected to be sensitive to geomorphological barriers. However, community structure analyses based on morphological and DNA data remain limited. Using specimens collected via high-density sampling (39 stations) in the Japan and Kuril–Kamchatka Trenches (3,500–8,000 m), we characterized the abyssal–hadal cumacean species composition and examined the effects of depth and geomorphology on its variation. Morphological and molecular analyses identified 60 morphospecies, 6 of which showed cryptic diversity. Community composition varied significantly with depth, whereas its differences between trenches and slopes were not significant. Our analysis revealed three depth-related clusters of sampling stations corresponding to the upper abyssal (<5,000 m), lower abyssal (5,000–6,500 m), and hadal (>6,500 m) zones, with the lower-abyssal cluster further subdivided into two subclusters (5,000–6,000 m and 6,100–6,500 m). Comparative analyses across additional taxa and regions are essential for determining the generality of these depth-related community boundaries during the abyssal–hadal transition.
The bottom Ekman layer plays a critical role in the exchange of momentum, energy, and materials between the seabed and the overlying deep ocean, yet its detailed features remain poorly understood compared to the upper Ekman layer. Using 222 bottom-reaching velocity profiles obtained in the South China Sea (SCS), we investigated the vertical velocity structure in the bottom layer, and identified three deflection types of the bottom Ekman layer (clockwise, counterclockwise, insignificant) with average deflection angles of 53 ± 34°, 44 ± 26°, and 7 ± 3°, respectively, and thicknesses ranging from 10 to 50 m. The dissipation and diffusivity in the SCS bottom Ekman layer are estimated by assuming a constant value of viscosity coefficient, which show the maximal turbulent dissipation and eddy diffusivity in the Luzon Strait are as high as 10−7 W kg−1 and 10−1 m2 s−1, respectively. The water transport toward the SCS through the bottom Ekman layer via the Luzon Trough is also examined, which reaches 0.03 Sv and contributes 3.6% of the total overflow below the depth of 1920 m. Combined with the suspended sediment concentration of 30 mg L−1 inferred from the backscatter measurements, we estimated a substantial sediment flux of 3.9 Mt y−1 through the bottom Ekman layer from the Luzon Trough to the Manila Trench. This study highlights the significance of bottom Ekman layer in closing energy budget and transporting suspended sediment.
We report the first observations of bioluminescence in two giant deep-sea pycnogonid species from the family Colossendeidae, Colossendeis tasmanica Staples, 2007 and Colossendeis minor Schimkewitsch, 1893, both collected from the Australian abyss. Notably, C. tasmanica was abundant enough to allow for detailed study. This species emits a bright deep-blue light (λmax = 449 nm) from its legs, displaying a distal-to-proximal luminescence gradient. Luminescence is triggered by H2O2 and KCl, while adrenaline does not induce light emission. The lack of response to the hot and cold assay suggests that the luminescence system of C. tasmanica is not luciferin-luciferase-based, but instead resembles a photoprotein-based system. Catalase and Fenton assays also elicited light responses, with Fenton's reagent response increasing with calcium concentration. These results suggest that calcium ions, catalase, and H2O2 may be involved in the luminescence mechanism. Histological analysis indicates that light is produced in gut-associated cells within the legs, implying that trophic acquisition of a factor may be necessary. The ecological function of C. tasmanica bioluminescence, whether for anti-predation, predation, or reproduction, remains speculative and warrants further investigation on this abyssal species.
Despite their importance in food webs and predominance among species bycaught in Southern Ocean toothfish fisheries, the trophic ecology of grenadiers of the genus Macrourus, remains poorly understood. Here, we provide the first comprehensive assessment of trophic position, niche segregation and dietary composition of four Macrourus species: M. caml, M. carinatus, M. holotrachys, and M. whitsoni. M. holotrachys had the highest trophic position (highest δ15N values) and the highest mean δ13C values, indicative of a benthic diet; M. caml and M. carinatus had intermediate and to some extent overlapping δ15N and δ13C values, and the widest isotopic niches, and; M. whitsoni, had the lowest δ15N and δ13C values, indicative of a lower trophic position and more pelagic diet. These differences in stable isotope values were reflected in their feeding strategies and diets; although all four species are generalist feeders, with crustaceans and fish as their main prey, M. holotrachys exhibited a more specialized benthic feeding strategy (hence a higher vulnerability to bycatch), whereas M. caml and M. carinatus had broader benthopelagic diets. The niche segregation is likely shaped by depth distribution and habitat preferences, which reduces competition for prey and enables their coexistence. This study is the most comprehensive assessment to date of the trophic ecology of Macrourus species in the Southern Ocean, increasing our understanding of their ecological role in the deep-sea food web and providing valuable insights for their management.
Mesopelagic fish populations are characterised by high species richness and abundance, playing a key role in the active carbon flux in the open ocean. The Cape Verde Frontal Zone (CVFZ), located at the southern boundary of the Canary Current Eastern Boundary Upwelling System, features highly dynamic water mass interactions due to the interplay between the Cape Verde Front and the Mauritanian coastal upwelling. Despite the known ecological importance of mesopelagic fishes in complex oceanographic scenarios such as upwelling zones, frontal systems, and oxygen minimum zones, there are few studies relating them to frontal areas such as the CVZF. Herein, we describe the vertical distribution, composition, and structure of the mesopelagic fish community in the CVFZ. We analysed the diurnal and nocturnal vertical abundance and biomass distribution, the common, dominant, and rare species, and the length-frequency distribution from surface to 1000 m depth during summer 2017. Water temperature, salinity, dissolved oxygen, and chlorophyll-a were measured concurrently. Forty mesopelagic fish species were identified, with Cyclothone spp. dominating numerically and in biomass, especially within oxygen-depleted layers below 350 m. Abundance and biomass were asymmetrical, reflecting contrasting ecological strategies. Diel vertical migrations were evident, with myctophids ascending to surface layers at night and Cyclothone spp. remaining consistently abundant at depth. Seven assemblages were identified, structured by depth, diel period, and environmental gradients. Epipelagic groups were low-diversity and myctophid-dominated, while mesopelagic assemblages showed numerical dominance of small Cyclothone spp. and biomass driven by rare, large-bodied taxa, with clear shifts in dominant species and length-frequency distributions. Strong temperature and dissolved oxygen gradients, oxygen minimum zones, and the depth chlorophyll-a maximum were identified as key drivers of species turnover and vertical distribution. These results highlight the influence of mesoscale hydrographic structures, frontal dynamics, and water mass interactions on mesopelagic fish communities, providing baseline data for this climate-sensitive region and emphasising the need to account for temporal and vertical variability in community assessments.
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.
The family Hamacanthidae (Demospongiae: Merliida) is reported from the abyssal plains for the first time, represented by two new species of Hamacantha (Vomerula) collected from the polymetallic nodule fields of the Clarion-Clipperton Zone (CCZ), Pacific Ocean. Hamacantha (V.) abyssalis sp. nov. is characterized by a single category of styles and diancistras, whilst lacking other microscleres such as sigmas, toxas, or raphides. Hamacantha (V.) apodiancistra sp. nov., likely the most abundant hamacanthid in the abyssal plains, is remarkable for the complete absence of diancistras-an apomorphy previously considered diagnostic of Hamacantha. Molecular phylogenetic analyses however confirmed its placement within Hamacantha, suggesting a secondary loss of this synapomorphy. The discovery of these species nearly doubles the known bathymetric range of Hamacantha to the abyssal zone and possibly highlights evolutionary trends toward reductions in body size, skeletal complexity, and spicule diversity in deep-sea demosponges.
Knowledge on the diversity of Loxosomatidae, a group of solitary entoprocts, remains quite limited for the abyssal and hadal zones. Only three species have been reported from depths below 3500 m, all from the northwestern Pacific. Here, we describe the loxosomatid Loxosomella namacola sp. nov. from Molpadiodemas holothuroids collected from two localities at depths of 6093-6102 m and 7285-7288 m in the Kuril-Kamchatka Trench, northwestern Pacific. Holothuroidea is a novel host group for Loxosomatidae, and our specimens from 7285-7288 m represent the deepest record to date for Entoprocta. Loxosomella namacola sp. nov. has a widened root-like stalk end that is embedded in the host body. This structure suggests a close relationship between L. namacola sp. nov. and Loxosomella marcusorum, which has a star-shaped plate embedded in the body of its echiuran host. We determined partial sequences for the 18S rRNA (18S), 28S rRNA, and cytochrome c oxidase subunit I (COI) genes from L. namacola sp. nov. to aid in future DNA barcoding and for inference of its phylogenetic position in Loxosomatidae (18S only). We observed no nucleotide differences among COI sequences from the same locality, but a 1.53% p-distance between sequences from the two localities. In an 18S-based phylogenetic tree (1508 positions), L. namacola sp. nov. was the sister taxon to its abyssal congener Loxosomella profundorum found on an unidentified anthozoan species in Corallimorpharia.
Organic carbon burial in continental margin sediments represents a key component of the global carbon cycle, and cold-seep environments may have been potential burial hotspots. This study examines the complex ironsulfur interactions in ferruginous sediments of the Haima cold-seep area, within the Qiongdongnan Basin, South China Sea, revealing previously overlooked mechanisms that may reduce carbon burial efficiency. Geochemical and magnetic analyses indicate that the iron reduction and cryptic sulfur cycle initiated by sulfur disproportionation actively occur near the sulfate-methane transition zone (SMTZ). These processes promote the reductive dissolution of Fe-(oxyhydr)oxides, leading to the OC burial rate that is 65% +/- 17% (n =14) lower than the global average for comparable SMTZ depths. Our findings hence propose the necessity of accounting for ironsulfur interactions when evaluating marine carbon burial efficiency. Ignoring these processes may lead to a potentially misleading overestimation of carbon sequestration in analogous ferruginous environments.
Bottom trawl fishing (BTF) is one of the most destructive fishing practices, generating long-term impacts on benthic habitats and removing habitat-structuring species such as millenary deep-sea corals essential for demersal assemblages. Using the ROV SuBastian aboard the R/V Falkor (too), we assessed the effects of BTF on deep benthic communities of the Juan Fern & aacute;ndez Archipelago (JFA) seamounts, Chile, more than 25 years after a BTF moratorium. We also surveyed "Solito", a seamount in the Desventuradas ecoregion that, to our knowledge, has never been trawled. ROV video data were compared with historical trawling locations from the early 2000s. Evidence of past trawling was found across all JFA seamounts, whereas none was detected at Solito. Non-trawled areas displayed significantly higher cover of live habitat-forming corals and greater abundances of cnidarians and arthropods, while trawled areas were dominated by soft sediments and by high densities of the urchin Dermechinus horridus. We hypothesize that BTF disturbed substrates favor the proliferation of D. horridus. Habitat complexity was also significantly higher in non-trawled areas, indicating that BTF causes long-lasting simplification of benthic structure. Our results demonstrate persistent ecological degradation in the JFA >25 years after the moratorium, including reduced habitat complexity and altered species composition. While the JFA seamounts currently fall within Marine Protected Areas, Solito remains unprotected. We recommend establishing long-term biophysical monitoring to evaluate habitat recovery and species succession, and implementing spatial management measures for Solito to safeguard this pristine and diverse seamount from present and future anthropogenic impacts.
The richness, composition, and functioning of many ecosystems is reliant upon habitat complexity provided by foundation species. One remarkable example of this is at cold-water coral (CWC) reefs formed by the cosmopolitan azooxanthellate coral Desmophyllum pertusum (syn. Lophelia pertusa). These reefs often harbor assemblages with species richness rivaling or exceeding coral reefs in shallow waters. Reflecting this ecological importance and their inherent vulnerability, CWC reefs have been classified as Vulnerable Marine Ecosystems (VMEs), leading to their inclusion in international conservation frameworks and targeted spatial protections. However, it is not clear what level of coral coverage is required to augment biodiversity enough to be classified as a VME, or to what degree CWC reef-associated biodiversity is reliant upon the cover of living coral, dead coral structure, or other abiotic habitat types. Here, we use video imagery and in situ collections of intact CWC assemblages from D. pertusum reefs of the Blake Plateau (Southeast USA) to test a number of hypotheses relating the richness and composition of reef-associated faunal assemblages to the cover of biogenic (i.e. live & dead-standing coral, coral rubble) and abiotic (e.g. sediment, rock) habitat, depth, and bathymetry-derived terrain variables. Our results reveal dead-standing coral cover, bathymetric position index, and depth as the primary determinants of megafaunal richness and composition from video imagery, while the ratio of living versus dead coral was the strongest influence of macrofaunal richness from physical collections. This study contributes to a growing body of knowledge necessary to disentangle the drivers of CWC reef biodiversity and composition globally, which is imperative given the threats that projected changes in ocean conditions would pose to the structural complexity and integrity of CWC reefs.
Caudofoveata are often an abundant group in deep-sea sediments, although its diversity remainspoorly known in many regions such as the South Atlantic. Within Caudofoveata, Falcidens Salvini-Plawen, 1968 is currently the second most speciose genus, with about 40 described species and distributed worldwide. These species exhibit a great morphological diversity, having species with slender, stoutish or tailed bodies. A new species of Falcidens is described here, based on specimens from sediment samples collected along the Brazilian coast. It is described in detail using scanning electron and birefringence microscopy. In addition, species distribution modelling was also applied to understand its potential range and ecology. Moreover, and a brief review of the genus is provided. Falcidens porrectus sp. nov. has a very long and slender body, a “U”-shaped oral disc, and a radula with symphysis; its large arrow-shaped sclerites differ from those of other elongated species of Falcidens. Externally, it is very similar to F. sagittiferus Salvini-Plawen, 1968 from northern Europe, but the latter has a midventral suture line and a more stoutish body morphology. Species distribution modelling suggests a potentially wide distributionacross tropical and subtropical areasof the western Atlantic, primarily associated with the continental slope. The number of Falcidens species recorded from the Brazilian coast is increased to five highlightingthe considerable morphological and taxonomic diversity of the genus in this regionand contributing to improved knowledge of deep sea Caudofoveata in the Southern Hemisphere.
Over the past few decades, many new taxa of the order Zoantharia (Cnidaria: Anthozoa: Hexacorallia) have been described from the deep sea, particularly from the Pacific and Atlantic oceans. However, research on deep-sea zoantharians from the Indian Ocean remains sparse. Here, we formally describe a new species of parazoanthid associated with the glass sponge Aphrocallistes beatrix from the western Andaman Sea, Vitrumanthus sankalpi n. sp., based on specimens collected from 1050 to 1250 m, and morphological and molecular analyses of the specimens. Vitrumanthus sankalpi n. sp. can be distinguished from its congeners by the number of tentacles, mesoglea thickness, and shape of siphonoglyph, as well as unique molecular sequences of investigated DNA markers. Limited research on the biodiversity in Indian Ocean deep-sea habitats could mean that more species of zoantharians await discovery from this region. Further research on the biodiversity of zoantharians on seamounts and the deep sea will help us better understand the biogeography and evolution of this group of organisms.
Although the deep sea (>200 m) represents most of the ocean's habitat, biodiversity baselines remain poorly resolved due to fragmented records and taxonomic uncertainty among morphologically similar and cryptic taxa. The deep-sea isopod genus Bathynomus exemplifies this problem, where unresolved species identities and sparse molecular references limit interpretation of distributional patterns. To systematically resolve sporadic historical Bathynomus identifications, we assembled and standardized a global occurrence database from public archives such as the Global Biodiversity Information Facility (GBIF) and digitized literature, implemented automated data-integrity checks (e.g., inland coordinates, aquarium records, published misidentifications and unverified descriptions), and linked records to available mitochondrial barcodes within a relational database. To demonstrate molecular analyses, we conducted trap-based surveys in the Bahamas and Japan to obtain 102 specimens and recover high-quality tissue for sequencing and phylogenetic placement. We collected 15,564 georeferenced records from 1994 sampling events, including 689 observations linked to mitochondrial barcodes, and supported the hypothesis that verified species taxa do not span multiple ocean basins. We reconstructed and annotated the first B. giganteus mitochondrial genome (14,968 bp) and evaluated genetic distances to validate specimen identity. Uneven spatial sampling effort and limited mitochondrial markers for congeners continue to challenge Bathynomus research, demonstrating the need for expanded standardized sampling and improved barcode references.
Benthic communities inhabiting the periphery of active hydrothermal vent fields have not been well characterized, including how they are influenced by adjacent active vents. In this study, we investigated patterns in the distribution of megafauna relative to distance from active venting, substrate type, and seabed terrain attributes using multivariate techniques. Video footage of the seabed and bathymetric data were collected in 2018 and 2019 at an active vent field (similar to 20 km from Kairei) within the Federal Institute for Geosciences and Natural Resources (BGR) contract area on the Central Indian Ridge. The distribution of individual taxa revealed possible habitat preferences that may be related to local water dynamics or food availability. We identified three megafaunal assemblages: a 'vent-associated' assemblage located near active vent sites and dominated by symbiont-hosting shrimp and predatory anemones; a 'sponge-dominated' assemblage linked to bathymetry rather than vent activity; and an 'anemone-dominated' assemblage that also included taxa from the other assemblages. Total faunal abundance decreased with distance from vent activity up to approximately 200 m, suggesting a transition zone of vent influence. Although our findings indicate that active vents play a role in structuring benthic megafaunal communities on the surrounding deep-sea environment, the extent of vent influence and drivers of faunal abundance are still not well constrained. We discuss sampling methods that may build research capacity for studying benthic communities at active vent fields where non-vent faunal abundance can be low. Establishing ecological baselines for hydrothermal vent ecosystems is essential to inform environmental management and sustainable deep-ocean governance.
Glass sponges (Hexactinellida) are key structural components of deep-sea ecosystems, often harboring diverse invertebrates. Yet associations with molluscs, and particularly the vermiform, shell-less Solenogastres (Mollusca, Aplacophora), remain virtually undocumented. The genus Apodomenia is exceptional among solenogasters in completely lacking a foot and pedal groove, a feature that departs from the traditional morphological distinctions separating solenogasters and caudofoveates, the other aplacophoran lineage. Until now, the genus was represented by a single species, A. enigmatica described from the Southern Ocean living in association with the hexactinellid sponge Rossella sp. Here we describe Apodomenia mimica sp. nov., based on the study of two specimens discovered in the hexactinellid Euplectella suberea in the Northwest Atlantic Ocean on the Mississippi-Alabama continental slope (USA). The new species exhibits striking camouflage, mimicking the fibrous skeleton of its host. These findings extend the ecological and geographic range of Apodomenia and indicate that different genera of hexactinellid sponges may harbor specialized but overlooked molluscan associates. Notably, a second solenogaster species was also recovered from the same sponge, suggesting for the first time the co-occurrence of multiple solenogaster lineages within a single host. Taken together, this work highlights glass sponges as reservoirs of overlooked biodiversity.
As a key transport mechanism of the deep-sea sediment "source sink" system, the motion characteristics of turbidity currents are closely related to seabed erosion, and their coupling directly regulates the sediment transport mechanics and sedimentary system evolution in submarine canyons. However, the mechanism of acceleration-deceleration conversion in turbidity currents driven by erosion remains unclear. This study is based on the principle of the density Froude number, and uses a comprehensive method combining flume experiments, numerical simulations, and theoretical analysis to systematically explore the influence and underlying mechanisms of bed erosion on the flow characteristics of turbidity currents. Develop a numerical model that couples bottom erosion and sedimentation processes, and verify its reliability using experimental data. The core innovation lies in identifying and parameterizing the threshold densimetric Froude number range that characterizes the acceleration deceleration transition of turbidity current under erosion conditions, establishing the functional relationship between this threshold and environmental entrainment coefficient with slope and grain size, revealing the natural limiting effect of erosion induced flow resistance on turbidity current velocity, and elucidating the inherent mechanism of non infinite growth of turbidity current velocity. The study provides a new theoretical perspective on the coupling dynamics of turbidity currents and erodible layers by comparing erosive and non-erosive scenarios. Furthermore, based on a comprehensive research methodology, it elucidates and quantifies the laws governing velocity evolution under the influence of density currents and bed erosion-deposition, revealing the limitations of traditional density current dynamic mechanisms that cannot be quantitatively characterized.
