Deep-sea hydrothermal vents host chemosynthesis-based ecosystems inhabited chiefly by specially adapted animals that do not live anywhere else, and depth has been shown to be a major driver of species composition at vents around Japan. Though the Ryukyu region in southern Japan is home to many hot vents, only two – Minami-Ensei Knoll and Yoron Hole – have been found shallower than 1000 m. Here, we report the discovery of a new vent field on the Amami Rift northwest off Amami Ōshima at 630 m deep. A total of 29 macrofaunal species were recorded from Amami Rift, including 19 vent specialists. Comparison of species composition across the three shallow Ryukyu vents revealed only three shared species, highlighting that all three display distinct community structure. Amami Rift exhibits distinct zonation patterns and is generally more similar to Minami-Ensei than Yoron Hole, but the presence of key taxa such as the sulphide worm Paralvinella and the mussel ‘Bathymodiolus’ platifrons as well as the absence of the symbiotic squat lobster Shinkaia and the limpet Lepetodrilus exemplify its difference with Minami-Ensei. Furthermore, the non-vent specific predators seen in these two sites were completely different. Overall, the Amami Rift vent field can be considered a shallow vent with a unique set of fauna, warranting future research on the mechanisms shaping disparate macrofaunal diversity between nearby shallow vents such as Amami Rift and Minami-Ensei. The unusual geological setting of Amami Rift at the converging point of Okinawa Trough and Ryukyu Arc may influence fluid chemistry to drive such differences.

Two species of dirivultid copepod (Siphonostomatoida), Stygiopontius senckenbergi Ivanenko & Ferrari, 2013 and Dirivultus kaiko sp. nov., were discovered from hydrothermal vent fields in the Okinawa Trough, the western North Pacific. Since S. senckenbergi was originally described based on two adult males from the New Ireland Fore-Arc system, Papua New Guinea, the discovery here represents the second record. This species was attached on ventral setae covered with filamentous bacteria of the deep-sea squat lobster Shinkaia crosnieri (Decapoda: Munidopsidae). The female of S. senckenbergi is described for the first time as well as the nauplius I and the copepodid IV. Sexual dimorphism is shown on several appendages (i.e. antennule, leg 2 and leg 5). The nauplius I shows typical lethithotrophic characteristics. Dirivultus kaiko sp. nov., which was found on the tentacular crown of the siboglinid tubeworm Lamellibrachia columna, differs from its two congeners by the antennule, maxilliped and leg 5 of both sexes. These are the second and third dirivultid species to be reported from Japan.

A variety of biomarkers were examined from Ocean Drilling Program Core 1202B to reconstruct temperature and phytoplankton community structures in the southern Okinawa Trough since 20 ka. Two molecular temperature proxies ( $${\rm U}_{{37}}^{{{\rm K}\prime}} $$ and TEX86) show 5°C to ~6°C warming during the glacial-interglacial transition. Prior to the Holocene, the $${\rm U}_{{37}}^{{{\rm K}\prime}} $$ -derived temperature was generally 1°C to 4°C higher than TEX86-derived temperature. This difference, however, was reduced to <1°C in the Holocene. Correspondingly, the phytoplankton biomarkers (e.g., C37:2 alkenone, brassicasterol, C30 1,15 diol, and dinosterol) indicate a shift of planktonic community structures, with coccolithophorids becoming more abundant in the Holocene at the expense of diatoms/dinoflagellates. This shift is related to the variability of nutrients, temperature, and salinity in the Okinawa Trough, likely controlled by the sea level and the intensity of the Kuroshio Current. The phytoplankton community change may have had profound implications for atmospheric CO2 fluctuations during glacial-interglacial cycles since diatoms and dinoflagellates have a higher efficiency of the biological pump than coccolithophorids.

Fan worms (Sabellidae: Polychaeta) are known to inhabit deep-sea environments and some species have been reported from hydrothermal vents. However, this is the first record of a Bispira species from this environment. The specimens identified as Bispira wireni were collected at the Izena Hole, Okinawa Trough, Japan. We describe the specimens from this location and provide new insights about the morphological features of this species, previously reported from sediments at 150–300 m in Sagami Bay.

The galatheid crab Shinkaia crosnieri, is the sole member of the subfamily Shinkaiinae. It is abundant and forms dense beds around active hydrothermal vents in the Okinawa Trough. Thousands of filamentous bacteria attached to the plumose setae on the ventral surface of this crab were observed using field-emission scanning electron microscopy and transmission electron microscopy. Nucleic acids were extracted from the filamentous bacteria, and the phylotypes of 16S rRNA genes were identified from 81 clones. These phylotypes were divided into three groups: Epsilonproteobacteria (74%); Gammaproteobacteria (20%); and Bacteroidetes (6%). Gamma- and major phylotypes of Epsilonproteobacteria were also detected using fluorescence in situ hybridization analysis. These Epsilon- and Gammaproteobacteria were closely related to cultured and uncultured bacteria from hydrothermal vent fields including episymbionts of vent-associated invertebrates such as Rimicaris exoculata, Alvinella pompejana, the scaly-foot snail, Kiwa hirsuta etc. The carbon isotopic compositions of the muscle of S. crosnieri and in filamentous bacteria were similar. The muscle of S. crosnieri contained monounsaturated C16 and C18 fatty acids, which are known to be characteristic of sulphur-oxidizing bacteria in H2S-rich marine habitats. Through the video images transmitted by a submersible and a remotely operated vehicle, S. crosnieri was observed to comb out its ventral setae using the third maxilliped and appeared to consume the contents. These evidences suggest the epibiotic association between S. crosnieri and the filamentous bacteria attached to the ventral setae of the crab, but the details of role and function are still unclear at the present study.