Katsunori Fujikura

A New Species of Osedax (Annelida: Siboglinidae) Associated with Whale Carcasses off Kyushu, Japan

Abstract A new whale-bone-eating polychaete species of the genus Osedax was found on sperm whale carcasses submerged off Cape Nomamisaki, Kyushu, Japan, at a depth of approximately 200 m. The new species, Osedax japonicus, is the fourth known species of the genus Osedax and the first species from the western Pacific. Female O. japonicus specimens (1) form dense clusters on whale carcasses; (2) have a body composed of crown, trunk, and root structure; (3) lack a digestive tract; and (4) have bacterium-like particles in the tissue of the root structure. Osedax japonicus shares all these characteristics with O. rubiplumus and O. frankpressi, and items (1) to (3) with O. mucofloris, Osedax japonicus is easily distinguished from the other three known species by oviduct morphology, body length, and palp coloration in females. No males of O. japonicus have yet been found.

Marine Biodiversity in Japanese Waters

To understand marine biodiversity in Japanese waters, we have compiled information on the marine biota in Japanese waters, including the number of described species (species richness), the history of marine biology research in Japan, the state of knowledge, the number of endemic species, the number of identified but undescribed species, the number of known introduced species, and the number of taxonomic experts and identification guides, with consideration of the general ocean environmental background, such as the physical and geological settings. A total of 33,629 species have been reported to occur in Japanese waters. The state of knowledge was extremely variable, with taxa containing many inconspicuous, smaller species tending to be less well known. The total number of identified but undescribed species was at least 121,913. The total number of described species combined with the number of identified but undescribed species reached 155,542. This is the best estimate of the total number of species in Japanese waters and indicates that more than 70% of Japans marine biodiversity remains un-described. The number of species reported as introduced into Japanese waters was 39. This is the first attempt to estimate species richness for all marine species in Japanese waters. Although its marine biota can be considered relatively well known, at least within the Asian-Pacific region, considering the vast number of different marine environments such as coral reefs, ocean trenches, ice-bound waters, methane seeps, and hydrothermal vents, much work remains to be done. We expect global change to have a tremendous impact on marine biodiversity and ecosystems. Japan is in a particularly suitable geographic situation and has a lot of facilities for conducting marine science research. Japan has an important responsibility to contribute to our understanding of life in the oceans.

Hadal disturbance in the Japan Trench induced by the 2011 Tohoku–Oki Earthquake

In situ video observations and sediment core samplings were performed at two hadal sites in the Japan Trench on July, 2011, four months after the Tohoku–Oki earthquake. Video recordings documented dense nepheloid layers extending ~30–50 m above the sea bed. At the trench axis, benthic macrofauna was absent and dead organisms along with turbid downslope current were observed. The top 31 cm of sediment in the trench axis revealed three recent depositions events characterized by elevated 137Cs levels and alternating sediment densities. At 4.9 km seaward from the trench axis, little deposition was observed but the surface sediment contained 134Cs from the Fukushima Dai–ichi nuclear disaster. We argue that diatom blooms observed by remote sensing facilitated rapid deposition of 134Cs to hadal environment and the aftershocks induced successive sediment disturbances and maintained dense nepheloid layers in the trench even four months after the mainshock.

A serpentinite-hosted ecosystem in the Southern Mariana Forearc

Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H2- and CH4-rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the “Shinkai Seep Field (SSF).” The SSF appears to be a serpentinite-hosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.

Gastropods from Recent Hot Vents and Cold Seeps: Systematics, Diversity and Life Strategies

Since the first discovery of hydrothermal vents at the Galapagos Spreading Center in 1977, gastropods have gained considerable attention as a major constituent of the chemosynthesis-based biological communities, especially the colonies of large species like Alviniconcha, Ifremeria and Lepetodrilus, or morphologically strange ones like the scaly-foot snail. Various types of symbiotic relations to bacteria have broadened the interest in them. During more than 30 years numerous vent and seep biotopes have been found mainly in temperate seas, but recently hydrothermal vents off Jan Mayen in the North Atlantic at 80°N and cold seeps off Norway between 67°N and 70°N (Haakon Mosby Mud Volcano) with chemosynthesis-based fauna (see Waren and Bouchet 2001; Desbruyeres et al. 2006: 516–517 for map). Biology of these vent/seep organisms is still in an early state of exploration; much of what is known is summarized in the monographs by Van Dover (2000) and Desbruyeres et al. (2006). Vent/seep taxa have also been a major target in research on higher phylogeny and systematics of gastropods. Several new families were established in the 1980s, based on seemingly “great differences” in morphology.

Microdistribution pattern and biogeography of the hydrothermal vent communities of the Minami-Ensei Knoll in the mid-Okinawa trough, Western Pacific

Abstract From 1988 to 1992, a series of deep-sea surveys was conducted to characterize hydrothermal vent fields on the Minami-Ensei Knoll, approximately 140 km west of Amami-Ohshima Island, southwest Japan, with a multi-narrow beam mapping system (Sea Beam), deep tow observing systems and the submersible Shinkai 2000. The vent fields were centered around the depressions on the western slope of the knoll. The hydrothermal vents emitted superheated water over 269°C through chimneys. Diffuse fluid discharged from fissures in rocks. Numerous patches of grayish white hydrothermal stains were observed on the bottom of coarse sand. Vent-associated biological communities consisted of sponges, vestimentiferans, alvinellid and polynoid polychaetes, cerithiid and trochid gastropods, lepetrodrillid limpets, vesicomyid clams, mytilid bivalves, bresiliid and hippolytid shrimp, zoarcid and cynoglossid fish, and lithodid and galatheid crabs. The hydrothermal vent communities of the Minami-Ensei Knoll showed many similarities to those of the Kaikata Seamount, the Mariana Back-Arc Spreading Center, the North Fiji Basin and the Lau Basin, as well as the cold seep communities of Sagami Bay. There may be considerable interchange among the Minami-Ensei knoll communities and other chemosynthetic communities in the Western Pacific despite the 1000 km distance separating these communities and the existence of Ryukyu Trench and Ryukyu Arc. These discoveries, as well as other more recent findings around Japan, contribute significantly to our understanding of the biogeography of the hydrothermal vent and cold seep communities in the Western Pacific.

Phylogeny of Hydrothermal-Vent-Endemic Gastropods Alviniconcha spp. from the Western Pacific Revealed by Mitochondrial DNA Sequences

Mitochondrial genes for cytochrome oxidase I (COI) from hydrothermal-vent–endemic gastropods of the genus Alviniconcha were sequenced to determine the phylogenetic relationships among specimens from three areas in the western Pacific. Individuals of Alviniconcha hessleri were collected at two vent fields (depths 1470 m and 3600 m) in the Mariana Trough. Specimens collected in the North Fiji Basin could be divided into two genetically distinct groups, both of which also differed from A. hessleri from the Mariana Trough. None of the specimens of the genus Alviniconcha collected in the Manus Basin differed genetically from the dominant group from the North Fiji Basin. We suggest that the specimens of the genus Alviniconcha analyzed in the present study can be tentatively classified into A. hessleri and two undescribed species.

Life habit of vesicomyid clam,Calyptogena soyoae, and hydrogen sulfide concentration in interstitial waters in Sagami Bay, Japan

In and around the beds of vesicomyid clam (Calytogena soyoae) located off Hatsushima Island in Sagami Bay, central Japan, hydrogen sulfide concentration in bottom water and interstitial water was measured every 10 cm from just above seafloor to 40 cm deep usingin situ separative dialysis bags. While hydrogen sulfide over 0.01 mmol/kg was not measured from the seawater just above the dense clam beds, the concentration of hydrogen sulfide increased rapidly below 10 cm deep. The results indicate that the habit of the clam is correlated with high concentration of hydrogen sulfide contained in pore waters of sediments between depths of 10 and 20 cm from the bottom surface. Concentrations of hydrogen sulfide ranging from approximately 0.05 mmol/kg to 0.6 mmol/kg might be suitable requirement for the habitat ofC. soyoae.

Phylogenetic relationships among species of Calyptogena (Bivalvia: Vesicomyidae) collected around Japan revealed by nucleotide sequences of mitochondrial genes

Phylogenetic relationships among the seven species of deep-sea giant clams Calyptogena (Bivalvia: Vesicomyidae) collected around Japan were examined using parts of nucleotide sequences of mitochondrial genes for cytochrome oxidase I (COI) and cytochrome oxidase III (COIII) and the encoded amino acid sequences. The seven species were C. soyoae (Sagami Bay), C. fausta (Suruga Bay), C. kaikoi (Nankai Trough), C. nautilei (Nankai Trough), C. phaseoliformis (Japan Trench), C. solidissima (Minami-Ensei Knoll, Okinawa Trough) and Calyptogena sp. (Iheya Ridge, Okinawa Trough). A clear phylogenetic split was observed between one group of three species (C. kaikoi, C. phaseoliformis and C. fausta) and the remaining species. This clustering corresponds to the two previously described subgenera within the genus Calyptogena (Calyptogean and Ectenagena) with the exception of the placement of C. nautilei, which had been placed in the subgenus Ectenagena. Genetic distances between two haplotypes of C. soyoae were 0.043 for the COI region and 0.055 for the COIII region, and three amino acid substitutions were detected with the COIII region. Calyptogena sp. from the Iheya Ridge could be distinguished from one of the two haplotypes (type A) of C. soyoae by only a single nucleotide substitution, a result that suggests that Calyptogena sp. of the Iheya Ridge diverged from C. soyoae after the two haplotypes had diverged, and it is now isolated from C. soyoae in Sagami Bay.

EN ECHELON PATTERNS OF CALYPTOGENA COLONIES IN THE JAPAN TRENCH

The distribution of Calyptogena phaseoliformis colonies in right-stepping en echelon patterns was observed by the Japanese submersible Shinkai 6500 at the foot of the landward escarpment of the northern Japan Trench at around 6437–6274 m depth. The north-south–trending Sanriku Escarpment has a thrust origin and is subparallel to the trench axis along which the Pacific plate is being subducted beneath the North America or Okhotsk plate at about 300° at a rate of about 7.8 to 8.3 cm/yr. The trends of colonies are concentrated at 250°, 300°, and 330°: each trend matches either an antithetic riedel shear, extension fracture, or synthetic riedel shear, respectively, within a left-lateral shear regime caused by the oblique subduction. Methane- and hydrogen sulfide–bearing fluid advection from depth occurs essentially along the thrust fault, but finally seeps along the fractures at the sea floor. This supplies energy to the food chain through bacteria utilizing hydrogen sulfide, then eventually sustains the Calyptogena colonies. Because the clams select the best places to survive, the geometric arrangement of the clam colonies provides a kinematic indicator of relative plate motions.