The First Genome Survey of the Snail Provanna glabra Inhabiting Deep-Sea Hydrothermal Vents.

The snail P. glabra is an endemic species in deep-sea chemosynthetic ecosystems of the Northwest Pacific Ocean. To obtain more genetic information on this species and provide the basis for subsequent whole-genome map construction, a genome survey was performed on this snail from the hydrothermal vent of Okinawa Trough. The genomic size of P. glabra was estimated to be 1.44 Gb, with a heterozygosity of 1.91% and a repeated sequence content of 69.80%. Based on the sequencing data, a draft genome of 1.32 Gb was assembled. Transposal elements (TEs) accounted for 40.17% of the entire genome, with DNA transposons taking the highest proportion. It was found that most TEs were inserted in the genome recently. In the simple sequence repeats, the dinucleotide motif was the most enriched microsatellite type, accounting for 53% of microsatellites. A complete mitochondrial genome of P. glabra with a total length of 16,268 bp was assembled from the sequencing data. After comparison with the published mitochondrial genome of Provanna sp. from a methane seep, 331 potential single nucleotide polymorphism (SNP) sites were identified in protein-coding genes (PCGs). Except for the cox1 gene, nad2, nad4, nad5, and cob genes are expected to be candidate markers for population genetic and phylogenetic studies of P. glabra and other deep-sea snails. Compared with shallow-water species, three mitochondrial genes of deep-sea gastropods exhibited a higher evolutionary rate, indicating strong selection operating on mitochondria of deep-sea species. This study provides insights into the genome characteristics of P. glabra and supplies genomic resources for further studies on the adaptive evolution of the snail in extreme deep-sea chemosynthetic environments.

Adaptation and evolution of the sea anemone Alvinactis sp. to deep-sea hydrothermal vents: A comparison using transcriptomes.

Sea anemones are diverse and ecologically successful members of Anthozoa. They are often found in intertidal and shallow waters, although a few of them inhabit harsher living conditions, such as deep-sea hydrothermal vents. Here, we sequenced the transcriptome of the vent sea anemone Alvinactis sp., which was collected from Edmond vent along the central Indian Ocean ridge at a depth of 3275 m, to explore the molecular mechanisms related to adaptation to vents. Compared with another deep-sea anemone (Paraphelliactis xishaensis) and five shallow water sea anemones, a total of 117 positively selected genes and 46 significantly expanded gene families were found in Alvinactis sp. specifically that may be related to its vent-specific aspect of adaptation. In addition, 127 positively selected genes and 23 significantly expanded gene families that were found in both Alvinactis sp. and P. xishaensis. Among these, vent-specific adaptations of Alvinactis sp. may involve genetic alterations in peroxisome, ubiquitin-mediated protein degradation, oxidative phosphorylation, and endocytosis, and its deep-sea adaptation may involve changes in genetic information processing. Differentially expressed genes between Alvinactis sp. and the deep-sea anemone P. xishaensis were enriched in a variety of pathways related to adaptation, such as energy metabolism, genetic information processing, endocytosis, and peroxisomes. Overall, we provided the first transcriptome of sea anemones that inhabit vents, which enriches our knowledge of deep-sea hydrothermal vent adaptation and the diversity of sea anemones.

Discovery of active off-axis hydrothermal vents at 9° 54'N East Pacific Rise.

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50'N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50'N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.

microRNAs facilitate comprehensive responses of Bathymodiolinae mussel against symbiotic and nonsymbiotic bacteria stimulation.

Bathymodiolinae mussels are dominant species in cold seeps and hydrothermal vents and could harbor endosymbionts in gill bacteriocytes. However, mechanisms underlying the symbiosis have remained largely undisclosed for years. In the present study, the global expression pattern of immune-related genes and miRNAs were surveyed in Gigantidas platifrons during bacterial challenges using enriched symbiotic methane oxidation bacteria MOBs or nonsymbiotic Vibrio. As a result, multiple pattern recognition receptors were found differentially expressed at 12 h and 24 h post bacteria challenges and distinctly clustered between stimulations. Dozens of immune effectors along with signal transducers were also modulated simultaneously during MOB or Vibrio challenge. A total of 459 miRNAs were identified in the gill while some were differentially expressed post MOB or nonsymbiotic bacteria challenge. A variety of immune-related genes were annotated as target genes of aforesaid differentially expressed miRNAs. As a result, biological processes including the immune recognition, lysosome activity and bacteria engulfment were suggested to be dynamically modulated by miRNAs in either symbiotic or nonsymbiotic bacteria challenge. It was suggested that G. platifrons mussels could maintain a robust immune response against invading pathogens while establishing symbiosis with chemosynthetic bacteria with the orchestra of immune-related genes and miRNAs.

Fauna of the Kemp Caldera and its upper bathyal hydrothermal vents (South Sandwich Arc, Antarctica).

Faunal assemblages at hydrothermal vents associated with island-arc volcanism are less well known than those at vents on mid-ocean ridges and back-arc spreading centres. This study characterizes chemosynthetic biotopes at active hydrothermal vents discovered at the Kemp Caldera in the South Sandwich Arc. The caldera hosts sulfur and anhydrite vent chimneys in 1375-1487 m depth, which emit sulfide-rich fluids with temperatures up to 212°C, and the microbial community of water samples in the buoyant plume rising from the vents was dominated by sulfur-oxidizing Gammaproteobacteria. A total of 12 macro- and megafaunal taxa depending on hydrothermal activity were collected in these biotopes, of which seven species were known from the East Scotia Ridge (ESR) vents and three species from vents outside the Southern Ocean. Faunal assemblages were dominated by large vesicomyid clams, actinostolid anemones, Sericosura sea spiders and lepetodrilid and cocculinid limpets, but several taxa abundant at nearby ESR hydrothermal vents were rare such as the stalked barnacle Neolepas scotiaensis. Multivariate analysis of fauna at Kemp Caldera and vents in neighbouring areas indicated that the Kemp Caldera is most similar to vent fields in the previously established Southern Ocean vent biogeographic province, showing that the species composition at island-arc hydrothermal vents can be distinct from nearby seafloor-spreading systems. δ 13C and δ 15N isotope values of megafaunal species analysed from the Kemp Caldera were similar to those of the same or related species at other vent fields, but none of the fauna sampled at Kemp Caldera had δ 13C values, indicating nutritional dependence on Epsilonproteobacteria, unlike fauna at other island-arc hydrothermal vents.

Compositional and Functional Shifts in the Epibiotic Bacterial Community of Shinkaia crosnieri Baba & Williams (a Squat Lobster from Hydrothermal Vents) during Methane-Fed Rearing.

The hydrothermal vent squat lobster Shinkaia crosnieri Baba & Williams harbors an epibiotic bacterial community, which is numerically and functionally dominated by methanotrophs affiliated with Methylococcaceae and thioautotrophs affiliated with Sulfurovum and Thiotrichaceae. In the present study, shifts in the phylogenetic composition and metabolic function of the epibiont community were investigated using S. crosnieri individuals, which were reared for one year in a tank fed with methane as the energy and carbon source. The results obtained indicated that indigenous predominant thioautotrophic populations, such as Sulfurovum and Thiotrichaceae members, became absent, possibly due to the lack of an energy source, and epibiotic communities were dominated by indigenous Methylococcaceae and betaproteobacterial methylotrophic members that adapted to the conditions present during rearing for 12 months with a supply of methane. Furthermore, the overall phylogenetic composition of the epibiotic community markedly changed from a composition dominated by chemolithotrophs to one enriched with cross-feeding heterotrophs in addition to methanotrophs and methylotrophs. Thus, the composition and function of the S. crosnieri epibiotic bacterial community were strongly affected by the balance between the energy and carbon sources supplied for chemosynthetic production as well as that between the production and consumption of organic compounds.

A new deep water chiton (Mollusca: Polyplacophora) from hydrothermal vent ecosystems in the Okinawa Trough, Japan.

Recent expeditions exploring deep-sea hydrothermal vent ecosystems in the Okinawa Trough, East China Sea resulted in the collection of a hitherto undescribed species of polyplacophoran mollusc found living at three different vent fields at depths of 950-1178 m. This new chiton is a member of the small lepidopleuran family Protochitonidae and is morphologically similar to Hanleyella japonica Saito, 1997. The two species differ in small morphological differences of the valve shape and elevation, are divergent in the standard molecular barcoding mitochondrial gene cytochrome oxidase I (COI), and furthermore the known distribution range of H. japonica is considerably more northern and also shallower. The new species is described herein as Hanleyella henrici n. sp. Additional in situ observations taken in the course of collecting material for this study indicates that chitons are more abundant in the vicinity of hydrothermal vents than was previously appreciated, and perhaps more speciose.

Unanticipated discovery of two rare gastropod molluscs from recently located hydrothermally influenced areas in the Okinawa Trough.

BACKGROUND: The deep-sea hydrothermal vent is one of the most 'extreme' environments in the marine realm. Few species are capable of inhabiting such ecosystems, despite extremely high productivity there supported by microbial chemosynthesis, leading to high biomass and low species richness. Although gastropod molluscs are one of the main constituents of megafaunal communities at vent ecosystems, most species belong to several typical families (e.g., Provannidae, Peltospiridae, Lepetodrilidae) specialised and adapted to life at vents. METHODS: During recent surveys of Okinawa Trough hydrothermal vent systems, two snails atypical of vent ecosystems were unexpectedly found in newly discovered hydrothermally influenced areas. Shell and radular characteristics were used to identify the gastropods morphologically. RESULTS: One species was a vetigastropod, the calliostomatid Tristichotrochus ikukoae (Sakurai, 1994); and the other was a caenogastropod, the muricid Abyssotrophon soyoae (Okutani, 1959). Both gastropods were previously only known from regular non-chemosynthetic deep-sea and very rare-only two definitive published records exist for T. ikukoae and three for A. soyoae. The radula formula of Tristichotrochus ikukoae is accurately reported for the first time and based on that it is returned to genus Otukaia. For both species, barcode sequences of the cytochrome c oxidase I (COI) gene were obtained and deposited for future references. DISCUSSION: These new records represent the second record of calliostomatids from vents (third from chemosynthetic ecosystems) and the third record of muricids from vents (tenth from chemosynthetic ecosystems), and extend the distribution of both species to the southwest. Neither family has been recorded at chemosynthetic ecosystems in the western Pacific. Both were from weakly diffuse flow areas not subject to high temperature venting but were nevertheless associated with typical vent-reliant taxa such as Lamellibrachia tubeworms and Bathymodiolus mussels. These new records show that these species are capable of tolerating environmental stress associated with weak hydrothermally influenced areas, despite not being vent endemic species, adding to the list of known vent/non-vent species intersections. This signifies that such weakly influenced areas may provide key habitats for them, and that such areas may play a role in the evolution of biological adaptations to 'extreme' chemosynthetic ecosystems.

Do larvae from deep-sea hydrothermal vents disperse in surface waters?

Larval dispersal significantly contributes to the geographic distribution, population dynamics, and evolutionary processes of animals endemic to deep-sea hydrothermal vents. Little is known as to the extent that their larvae migrate vertically to shallower waters and experience stronger currents and richer food supplies. Here, we first provide evidence from early life-history traits and population genetics for the surface dispersal of a vent species. Planktotrophic larvae of a red blood limpet, Shinkailepas myojinensis (Gastropoda: Neritimorpha: Phenacolepadidae), were cultured to observe their swimming behavior and to evaluate the effects of temperature on survival and growth. In addition, the population structure was analyzed based on 1.2-kbp mitochondrial DNA sequences from 77 specimens that cover the geographic and bathymetric distributions of the species (northwest Pacific, 442-1,227 m in depth). Hatched larvae constantly swam upward at 16.6-44.2 mm/min depending on temperature. Vertical migration from hydrothermal vents to the surface, calculated to take ~4-43 d, is attainable given their lengthy survival time without feeding. Fed larvae best survived and grew at 25°C (followed by 20°C), which approximates the sea surface temperature in the geographic range of the species. Little or no growth was observed at the temperature of the vent habitat where adult limpets occur (≤15°C). Population genetic analyses showed no differentiation among localities that are <1,350 km apart. The larvae of S. myojinensis most likely migrate to the surface water, where high phytoplankton biomass and strong currents enable their growth and long distance dispersal over many months. Sea surface temperature may represent a critical factor in determining the geographic distribution of many vent endemic species with a planktotrophic early development, and in turn the faunal composition of individual vent sites and regions.

Deepest and hottest hydrothermal activity in the Okinawa Trough: the Yokosuka site at Yaeyama Knoll.

Since the initial discovery of hydrothermal vents in 1977, these 'extreme' chemosynthetic systems have been a focus of interdisciplinary research. The Okinawa Trough (OT), located in the semi-enclosed East China Sea between the Eurasian continent and the Ryukyu arc, hosts more than 20 known vent sites but all within a relatively narrow depth range (600-1880 m). Depth is a significant factor in determining fluid temperature and chemistry, as well as biological composition. However, due to the narrow depth range of known sites, the actual influence of depth here has been poorly resolved. Here, the Yokosuka site (2190 m), the first OT vent exceeding 2000 m depth is reported. A highly active hydrothermal vent site centred around four active vent chimneys reaching 364°C in temperature, it is the hottest in the OT. Notable Cl depletion (130 mM) and both high H2 and CH4 concentrations (approx. 10 mM) probably result from subcritical phase separation and thermal decomposition of sedimentary organic matter. Microbiota and fauna were generally similar to other sites in the OT, although with some different characteristics. In terms of microbiota, the H2-rich vent fluids in Neuschwanstein chimney resulted in the dominance of hydrogenotrophic chemolithoautotrophs such as Thioreductor and Desulfobacterium. For fauna, the dominance of the deep-sea mussel Bathymodiolus aduloides is surprising given other nearby vent sites are usually dominated by B. platifrons and/or B. japonicus, and a sponge field in the periphery dominated by Poecilosclerida is unusual for OT vents. Our insights from the Yokosuka site implies that although the distribution of animal species may be linked to depth, the constraint is perhaps not water pressure and resulting chemical properties of the vent fluid but instead physical properties of the surrounding seawater. The potential significance of these preliminary results and prospect for future research on this unique site are discussed.