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Habitat differentiation is the essential force for shaping animal and plant biogeography. Distribution patterns of microeukaryotes, which are an assemblage of highly diverse and phylogenetically distant groups, across multiple marine habitats remain largely unknown. We hypothesize that microeukaryote communities vary across different types of marine habitats at the regional scale similar as macroorganisms, and they are shaped by multiple factors. Herein, we evaluated the microeukaryotic diversity and connectivity in sediments from intertidal flat through continental shelf to deep-sea floor, including habitats of deep-sea plain, seamount, and hydrothermal vent, using high-throughput DNA sequencing. Microeukaryote communities were clustered into their own sub-groups according to the types of the habitats, where the communities of abundant taxa tended to gather with nearby habitats, while the rare taxa were mainly shaped by the types of the habitats. The SourceTracker analysis showed that the most probable sources for each community composition, particularly for those from seamounts and hydrothermal vent, were identified unknown. The continental shelf and deep-sea plain showed a relatively larger fraction of sources from each other, indicating a stronger source-sink dynamic than that among other habitats. The habitat differentiation contributed more to community variations than the spatial variables and water depth did. Co-occurrence network analysis indicated more complex bio-interactions in the neritic zone, where Cercozoa might play fundamental roles in the stability of the community than those in the deep sea, where ciliates were more connected with other taxa. Altogether, our findings highlight that the microeukaryotic communities are clearly shaped by the habitat differentiation, and this study also identifies the most probable source of dispersal for each community of different habitats. • The microeukaryotic communities were clearly shaped by the habitat differentiation. • The most probable source of dispersal for each community of different habitats was identified. • The continental shelf and deep-sea plain showed a relatively stronger source-sink dynamic. • More complex bio-interactions were observed in the neritic zone than those in the deep sea. [ABSTRACT FROM AUTHOR]

Cetacean falls provide a large amount of organic matter to the deep ocean, an otherwise oligotrophic environment. These events are now believed to maintain endemic species and offer intermediate habitats for species in other deep-sea ecosystems. Only about 70 modern whale falls have been reported in the world's oceans to date. We discovered the first known dolphin fall in the South China Sea in March 2020, on the middle slope of the Zhongnan seamount. We set up a camera at the site to observe animals visiting the dolphin carcass for 30 consecutive days from March 29 to April 28, 2020 and made four additional HOV dives to the dolphin carcass. In total, we observed 14 visible megafauna species visiting this dolphin fall, including six fish species, five crustacean species, two echinoderm species, and one mollusk species, of which most were scavengers or predators. This was the first record of many of these taxa in the deep South China Sea. Furthermore, two bone-eating worms were found in a piece of the dolphin bone. The worms are identified as new species in the genus Osedax based on DNA analysis. The findings of this work suggest that cetacean falls occur frequently in the South China Sea and help to fill a knowledge gap regarding the biodiversity of cetacean falls in the West Pacific. • A 1.8-m dolphin fall was found in the South China Sea for the first time. • We observed 14 visible megafauna species visiting this dolphin fall. • Two bone-eating worms were found in a piece of the dolphin bone. [ABSTRACT FROM AUTHOR]

Deep-water seamount ecosystems are sensitive to human activities and have a slow recovery rate after being disturbed. Bottom trawling and potential deep-sea mineral extraction could severely damage benthic communities on seamounts and seriously impact deep-sea ecosystems. Inadequate knowledge of the distribution of megabenthos on seamounts or their community structure hinders deep-sea conservation and management. In this study, based on a multidisciplinary dataset generated from recordings taken by human-occupied vehicle (HOV) and remotely operated vehicles (ROVs) along transects and environmental variables, a range of megabenthic morphotaxa were observed on two adjacent deep-water seamounts and predicted using species distribution models (SDMs). Accordingly, based on the predicted distribution of each morphotaxon, five distinct communities were identified through cluster analysis. The results of SDMs showed that environmental variables varyingly impacted the distribution of different morphotaxa, among which the average velocity and eastness direction of near-bottom currents, bathymetric position index (BPI), and backscatter intensity exerted the most significant influence on megabenthic distribution patterns. The distribution of five distinct communities showed a similarity of community composition on the two deep-water seamounts, suggesting a potential connectivity between the two seamounts. The distribution of communities revealed the spatial characteristics of vulnerability of deep-water seamounts at the community level, which could provide a direct basis for marine spatial planning of deep-sea ecosystems. • A range of megabenthic morphotaxa on deep-water seamounts was predicted. • Currents, bathymetric position index and backscatter were key abiotic factors. • Five megabenthic communities on deep-water seamounts was identified. • Similarity in community composition of adjacent deep-water seamounts was found. [ABSTRACT FROM AUTHOR]

Abstract According to the field experiment implemented by the cruise with the observation by Submersible Jiaolong in No. 37 and 38 Cruise of China Ocean Mineral Resources R&D Association, detailed investigations about northern Yap Trench, including the water properties, bottom currents, turbulent mixing were presented. The deep watermass properties indicate that the deep water originates from East Mariana Basin (EMB) and East Caroline Basin (ECB), as part of the west propagating Lower Circumpolar Deep Water (LCPW). The volume transport results further prove it. The variability of salinity, potential temperature presents semidiurnal variations. The isothermal level variations indicate the existence of semidiurnal internal tide. The phase of salinity lags about 12 h than that of potential temperature. The salinity serves as the compensation on the isopycnal. The bottom currents mainly consist of the low frequency motions, near inertial motions, semidiurnal and diurnal motions. The strong semidiurnal motions derived by Lanczos band-pass filter and reconstructed current indicate the existence of internal tide in the western side of Yap Trench. Enhanced turbulent mixing is observed at the central part and slope of the northern Yap Trench, and weakens the stratification. The strong mixing seems to be caused by internal tides dissipation. Though mesoscale eddies are active around the Yap Trench, the bottom currents are not directly associated with mesoscale eddies. Highlights • The watermass properties suggest the deep water originates from LCPW. • The bottom current is influenced by low frequency motions, near inertial motions, semidiurnal motions and diurnal motions. • Enhanced turbulent mixing is observed at the central part and slope of northern Yap Trench. [ABSTRACT FROM AUTHOR]

The Southwest Indian Ridge (SWIR) is an important section of the global mid-ocean ridge system providing a corridor for dispersion of vent fauna between the Pacific and Atlantic Oceans. However, very few studies focusing on the biodiversity and biogeography of this region have been performed. To better understand the biological baseline and biogeography of vent fields on SWIR, we have described the faunal assemblages at three hydrothermal fields on SWIR, the biogeographical relationships among the three fields and fields on neighboring ridges as well as community dynamics at Longqi on a short time-scale (3 years). Communities at two fields, Duanqiao and Tiancheng, were reported for the first time, and the Longqi community was characterized more comprehensively. Collected specimens, videos and images were examined and taxa identification was conducted by both morphological and molecular approaches (cytochrome oxidase I (COI) or 16 S ribosomal RNA gene). In total, 39 species were recorded, 18 of which were reported from SWIR for the first time. According to the multivariate analysis of occurrence of fauna from 16 vent fields on four ridge systems, the SWIR vent sites were generally more similar to Central Indian Ridge. Within Indian Ocean, Longqi and Duanqiao formed a subcluster, which appeared to be intermediate between the Central Indian Ridge, Mid-Atlantic Ridge and East Scotia Ridge. Distinct faunal zonation was described at Tiancheng and Longqi. Comparison of vent fauna discovered in 2011 and 2014/2015 revealed an interannual constancy in faunal zonation on most vents at Longqi, except DFF1 chimney, which displayed a successional change from an early stage featuring high-temperature venting and low species diversity, to a more mature stage characterized by diffuse fluid and more colonized taxa. Thus, the present study is the first interannual variation study on the ultra-slow spreading ridges, and will expand our understanding of biodiversity and biogeography of vent fields on SWIR. [ABSTRACT FROM AUTHOR]

Increasing threats to seamount ecosystems potentially from fishing and seabed mining are creating an urgent demand for research on megafauna to inform conservation and management. We surveyed the community structure of benthic megafauna on the Weijia Seamount, which is covered by cobalt-rich ferromanganese crusts, in the northwestern Pacific Ocean. A multidisciplinary dataset was generated from video recordings taken by a human occupied vehicle (HOV) and remotely operated vehicle (ROV) along transects and bathymetry-derived environmental variables. The spatial variation of community structure and its environmental and spatial factors impacting the community were then explored. A total of 2779 individuals of the megabenthos (from depths of 1533–2763 m, covering 24.4 km in distance and with a maximum of 109 individuals per 100 m by linear observation), was counted and classified into 76 morphospecies, which was dominated by crinoids, ophiuroids, hexactinellids and octocorals. Moderately abundant and diverse megafauna, high beta diversity and distinct assemblages were present in the Weijia Seamount, suggesting a highly patchy pattern within a single seamount. Six clusters of megafauna were revealed, with significant differences in species composition and biodiversity. The results of statistical analyses showed that the megafaunal community was impacted by at least eight environmental variables (latitude, depth, slope, curvature, rugosity, bathymetric position index, topography type, and substrate type), and excluding aspect. Among these variables, the topography and substrate types played key roles in the occurrence of small-scale aggregations of hexactinellids; depth-related variables were key factors, but no gradual depth-related change in terms of abundance, richness or species turnover was observed. This case study on the megabenthic community of the Weijia Seamount can serve as an environmental baseline, providing a reference status for seamount conservation and management. • A highly patchy pattern of megafauna was illustrated on a single seamount. • Local sponge aggregations were impacted by elevated topography and hard substrates. • Communities dominated by ophiuroids and crinoids were observed frequently. • A multidisciplinary dataset was used to serve as an environmental baseline. [ABSTRACT FROM AUTHOR]