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Journal of Oceanography, Vol. 58, pp. 333 to 341, 2002 Review Deep-Sea Eukaryote Ecology of the Semi-Isolated Basins off Japan PAUL A. TYLER * School of Ocean and Earth Science, University of Southampton, SOC, Southampton SO14 3ZH, U.K. (Received 29 April 2001; in revised form 3 August 2001; accepted 21 August 2001) The Japanese archipelago is surrounded by the Pacific to the east, the Okhotsk Sea to the north, the Sea of Japan to the west and the Okinawa Trough to the south. The last three seas form semi-isolated deep basins, all with potentially tectonic origin but a different primary energy source as well as hydrographic and faunistic history. The Okhotsk Sea is connected to the Pacific through the deep straits between the Kurile Islands. As a result much of the fauna has links with that fauna found at similar depths in the Pacific. By contrast, the Sea of Japan was isolated from the main Pacific during the last ice age and became anoxic. Even today the link is only through narrow shallow straits. As a result the fauna is impoverished and is believed to be composed of cold-adapted eurybathic species rather than true deep-sea species. The deepwater fauna of both these seas derive their energy from sinking surface primary production. The Okinawa Trough has a much younger tectonic history than the Okhotsk Sea or the Sea of Japan. In the Okinawa Trough the most noticeable fauna is associated with hydrothermal activity and chemosynthesis forms the base of the food chain for the bathyal community. The variable nature of these three basins offers excellent opportunities for comparative studies of species diversity, biomass and production in relation to their ambient environment. Keywords: ⋅ Sea of Japan, ⋅ Sea of Okhotsk, ⋅ Okinawa Trough, ⋅ deep sea, ⋅ ecology. and faunistic properties (Fig. 1). To the north of Japan lies the Okhotsk Sea, a basin incised into the continental slope but with water exchange to the main Pacific Ocean through the Kurile Islands. To the west of Japan lies the isolated basin that forms the Sea of Japan (also called the East Sea), connected to the main ocean only through shallow straits. To the south lies the East China Sea the deepest part of which is a back-arc basin referred to as the Okinawa Trough. The purpose of this paper is to review the oceanography of these basins and compare their known deep-sea faunas. 1. Introduction The archipelago of Japan lies on the eastern edge of the Eurasian Plate. To the east of Japan the Pacific Plate subducts below the Eurasian Plate and to the south is found the Philippine Plate that subducts below the Eurasian Plate but overrides the Pacific Plate. These subduction regions have given the eastern side of Japan a turbulent tectonic history and the Japan Trench forms one of the deepest bodies of water in the world. This trench and its associated fauna including hadal species, as well as hydrothermal vent and cold seep species, have been the subject of much study. Hydrographically, to the east of Japan lies the confluence of two major western boundary currents, the warm Kuroshio and the cold Oyashio. In contrast to the eastern seaboard of Japan, there is to the north, west and south of Japan three deep-sea basins with very different morphological, hydrographical 2. Morphology and Origin of the Basins To the north of Japan lies the Okhotsk Sea (Fig. 2). The northern and western parts of the Okhotsk Sea are shallow and slope down to the Deryugin Basin with a maximum depth of 1700 m. In the southeast corner of the Okhotsk Sea is the Yuzno or Kurile Basin with a maximum depth of 3657 m (Zenkevitch, 1963; Freeland et al., 1998). The slope into this basin is in the order of 8 to 10° (Gnibidenko, 1985). Gnibidenko (1985) suggests that the * E-mail address: [email protected] Copyright © The Oceanographic Society of Japan. 333 Fig. 2. Sea of Okhotsk: bathymetry and main geographic features discussed in the text. The main connection between the Deep water of the Okhotsk Sea and the main Pacific Ocean is through deep straits between the Kurile Islands. Fig. 1. The Okhotsk Sea, Sea of Japan and the Okinawa Trough and their relationship to the Japanese archipelago. Yuzno Basin is a classic back-arc basin that is now covered with a thick layer of sediment. The margins of the basin are distinguished by deep faults. The floor of the basin is very level at depths of 3200 to 3300 m with only occasional seamounts rising to ~1500 m depth. On the western side of the basin the Hokkaido-Sakhalin slope is dissected by numerous submarine canyons that cross deep-sea terraces. The northern slope is generally smooth and four spurs have been recognised. The Kurile slope to the southeast of the basin is very steep (up to 25°) and is formed from en echelon short ridges. The basement of the basin is formed from Lower Mesozoic and more ancient rocks. The overlying sediment is divided into a wellstratified upper unit, consisting of alternating turbidites and pelagic oozes, with thin layers of ash laid down in the late Miocene and Pliocene, and a “transparent” lower unit believed to consist of pelagic clays and argillites (Gnibidenko, 1985). The most recent sediments follow the pattern of the upper stratified layer with a decrease in 334 P. A. Tyler grain size towards the centre of the basin where only aleurites (silt-sized fractions) are found. Heat flow in the deep basin is about twice the average for the sea. The deep basin is connected to the Pacific through passages between the Kurile Islands and through these exchanges of water with the Pacific occurs. Zenkevitch (1963) describes the deep bottom surficial sediments of the Okhotsk Sea as forming two sedimentary provinces. In the central basin coarse boulder/shingle and gravel deposits are found in the deepest parts (>1200 m) surrounded by sands at shallower depths. In the Kurile Basin the dominant sediment are clays and diatomaceous oozes. Because of the flow between the Kurile Islands, and local volcanic activity, sediments in the deeper parts of these straits are often coarse. To the west of Japan lies the deep basins of the Sea of Japan connected to the Okhotsk Sea to the north through the shallow Soya Strait (53 m depth) and Tartarskiy Strait (15 m depth) and the East China Sea to the south through the Tsushima Strait (130 m) and the Pacific proper through the Tsugaru Strait (130 m depth) (Kobayashi, 1985; Terazaki, 1999). The Sea of Japan has shallow areas sur- Fig. 3. Sea of Japan: Bathymetry and main geographic features discussed in the text. All the straits mentioned on the figure are shallow (<130 m deep). Fig. 4. Okinawa Trough: Bathymetry and main hydrothermal sites: 1. Minami-Ensei Knoll; 2. Ihelya Deep; 3. Izena cauldron. rounding three wide, flat-bottomed basins, the Japan Basin, the Yamamoto Basin and the Tsushima Basin, with a maximum depth of 4036 m (Fig. 3). The Sea of Japan is underlain by a slab of oceanic lithosphere that has been subducted from the Pacific Ocean along the Japan Trench (Kobayashi, 1985). At present, the deep basins show no evidence of crustal extension and may represent past back-arc basins that have ceased to open. There is evidence that the shallower Toyama Trough, immediately west of Honshu, is of tectonic origin and thus of a young age (Kobayashi, 1985). The floors of all three basins are very flat with only occasional seamounts. Within the basins the sediments show a strongly layered structure and have the two-layer structure, of highly stratified sediment and “transparent” sediments found in seismic profiles in the Okhotsk Sea. The upper layers contain diatomaceous silty clays deposited since the Pliocene and thick turbidites. In contrast to the Okhotsk Sea and the Okinawa Trough, the Sea of Japan was isolated from the Pacific during Pleistocene glacial periods when the sea level was ~140 m lower than present. This isolation resulted in cessation of oxygen-rich inflow and stagnation of the waters of the basin. Sedimentary cores show very well-defined reducing conditions (Kobayashi and Nomura, 1972). Kobayashi (1985) outlines a number of models that ac- count for the origin and age of formation of the Sea of Japan. Resolution of these models suggest that the Sea of Japan is older than 25 Ma and younger than 46 Ma. Heat flow is in the order of ~90 mW m–2 in all three basins. The Okinawa Trough lies to the southwest of Japan and is the youngest of the basins reviewed here. This Trough is the deepest part of the much larger East China Sea and is defined by the 1000 and 2000 m isobaths and has a maximum depth of 2270 m (Fig. 4). It is underlain by crust intermediate between oceanic and continental. The structure is that of a back-arc basin, with the main graben running approximately SW-NE, parallel to the Ryukyus Islands, with smaller grabens trending east-west (Lee et al., 1980; Kobayashi, 1985). Sediment cover in the Okinawa Trough is very thick (1000 to 3000 m) and is well-stratified overlying a highly deformed layer. Volcanic plugs are also found. Towards the southern end of the Trough at ~2050 m depth are found turbidites transported from shallow water (Hyun, 1995). Other sediments are intermediate between neritic and deep-sea sediments (Zheng et al., 1989) comprising terrigenous and biogenic deposits as well as volcanic minerals including pumice and glass. Sedimentation is higher on the west slope of the Trough, compared to the east slope (Zheng et al., 1989). Deep-Sea Eukaryote Ecology of the Semi-Isolated Basins off Japan 335 Continental rifting and crustal separation started in the Okinawa Trough ~2 Ma (Kimura, 1985). An early extensional phase was in the Miocene and subsequent extensional phases occurred in the Pleistocene between 1.9 and 0.5 Ma and is actively occurring at present. Spreading rates are in the order of 2 cm y–1. The very high heat flow in the Okinawa Trough (15.1 to 437 mW m–2) was interpreted by Kobayashi (1985) as being indicative of the existence of hydrothermal circulation. This has been proved to be an accurate prediction. Although now known to be a back-arc basin, the water depth of the Okinawa Trough is too shallow for it to be considered a true oceanic basin. The development of the Okinawa Trough is controlled by the subduction of the Phillipine Plate and the slow seaward extension of the continent (Huang, 1989). The recognised hydrothermal sites known are the Izena Cauldron, the Iheya Deep and Knolls and the Minami-Ensei Knoll all in the central part of the Okinawa Trough. In shallow water at the northern end of the Trough lies the Kagoshima vent site. Hydrothermal mounds (referred to as Natsushima 84-1 but latterly as the Iheya Knolls) were first recognised on a small knoll in the central axial rift of the Okinawa Trough in 1984 and 1986 (Kimura et al., 1988) where heat flow was in the order of 1000 mW m–2. Water temperatures were in the order of 20 to 50°C and showed a methane content of 200 nl kg–1. Subsequent observations at this site have revealed hydrothermal activity is periodic (Kasahara et al., 1995). The Iheya site has also given rise to hydrothermal carbonate chimneys as a result of CO 2-rich magmatic gas (Izawa et al., 1991). Gamo et al. (1987) report hydrothermal activity from the 1540 m-deep Iheya Deep finding significant anomalies in methane, pH, manganese and iron. Carbon dioxide-rich fluids are also found in the JADE hydrothermal field of the Izena Cauldron (Sakai et al., 1990). Precipitation of these fluids formed hydrate pipes standing on the sediment. At the nearby CLAM site in the Iheya Knolls field the mounds are formed from overlapping multi-layered “eaves” emitting hydrothermal fluids from their edges (Gamo et al., 1991). 3. Hydrography A series of cyclonic gyres form the surface circulation in the Okhotsk Sea. During the winter months there is considerable surface cooling as a result of the cold Siberian winds that results in surface freezing over much of the sea, which leads to increased water density and large scale convective vertical mixing down to 400 m (Kitani, 1972). In the spring, surface salinity is greatly reduced by ice melt and river runoff from the terrestrial spring melt. In summer the surface circulation is decoupled from the deep circulation by the formation of a strong thermocline. The limitation on convective mix- 336 P. A. Tyler ing in the Okhotsk Sea is the inflow of dense Pacific seawater, through the deep passages between the Kurile Islands, into the deepest parts of the sea. The deep water masses in the Okhotsk Sea consist of a transient layer between 150 and 750 m with a temperature of 0 to 2.0°C and salinity of 33.2 to 33.8 and deep and bottom waters from 750 m to the bottom (temperature 1.8 to 2.5°C and salinity 34 to 34.5) (Nishimura, 1983; Saidova, 1997; Freeland et al., 1998). The cold intermediate water, overlying the transient water, spreads out across the Okhotsk Sea and flows southeastwards through the straits of the Kurile Islands into the Pacific proper. Deep Pacific water enters the Okhotsk Sea through inter alia the Krzenshtern Strait, and sinks into the Kurile Basin. The deep-water circulation is cyclonic. This deep water may mix vigorously upwards with the dicothermal water to give the “transient water” (Yasuoka, 1967). The distribution of the deep benthic fauna of the Okhotsk Sea is influenced by the influx of deep Pacific water. Oxygen concentration decreases with depth in the Okhotsk Sea with values of 50% saturation in the shallowest parts of the Kurile Basin whilst in the deepest parts oxygen concentration is only 0.7 ml L –1 at 1500 m (9.2% saturation) (Zenkevitch, 1963). Saidova (1997) reports values of 2.0 to 2.3 ml L –1 (80 to 100 µmol kg–1 (Freeland et al., 1998)) below 1500 m in the Kurile Basin. This is probably a function of the deep water of the Kurile Basin being of Pacific origin. Four water masses are recognised in the Sea of Japan: the Tsushima Current Water; Central Water; Intermediate Water and Deep/Bottom Waters (modified from Nishimura, 1969). Surface waters are affected by prevailing surface currents, the Tsushima Current bringing warm water in from the south whilst the Liman Current brings in cold water from the north. The only direct connection with the Pacific is through the Tsugaru Strait and is only in the order of 2 sverdrups (Nof, 2000). The variation in surface temperature affects surface primary production and thus the supply of particulates to the deep seabed (Nishimura, 1983). Winter convection forms deep cold water masses in the northwest of the Sea of Japan that sink to the deep-sea bed and spread throughout the deep basins (Nishimura, 1969). This water forms a distinctive Bottom Water confined morphologically to the deep Sea of Japan (Uda, 1938; Gamo et al., 1986; Kim, 1997). The mixing of this bottom water into the intermediate and upper layers is by slow eddy turbulence. Mixing of the bottom water with intermediate water gives Sea of Japan Deep Water. The deep and bottom water occupy the Sea of Japan from 300 m to the bed at >3000 m. Recently, Kim (1997) has redefined these water masses. In the southern part of the Sea of Japan winter cooling gives rise to convectional mixing that sinks to between ~100 and ~300 m giving rise to Japan Sea Inter- mediate Water (Miyazaki, 1952). The deep waters of the Sea of Japan remain well aerated (Zenkevitch, 1963) although this has not always been in the case in the past (Terazaki, 1999). Bottom temperatures (θ ) range between 0.03 and 0.12°C (Kobayashi, 1985) and the salinity is 34.08 to 34.14, both being lower than the Okhotsk Sea and adjacent Pacific (Zenkevitch, 1963; Kobayashi, 1985; Terezaki, 1999). A significant feature of the bottom water of the Japan Sea is their high oxygen concentration resulting from the sinking of cold surface waters in winter. As a result the calcium carbonate compensation depth is as shallow as 2000 m (Kobayashi, 1985). The hydrography of the Okinawa Trough is a subset of the circulation of the East China Sea (Nishimura, 1983). Surface flow over the Okinawa Trough is formed by the Kuroshio, which branches to flow into the Japan Sea as the Tsushima Current (~5% of flow) or continues between the northern Ryukyu Islands and Kyushu and up the east side of Japan as the main Kuroshio (Nishimura, 1983). Flow within the Okinawa Trough itself is poorly known except for localised observations round hydrothermal structures (Mitsuzawa, 1990). At the Iheya site flow periodicity in currents was ~11 d with a maximum velocity of 20 cm s–1. Hydrothermal water diffused horizontally on a scale of 150 to 600 m and vertically up to 200 m (Mitsuzawa, 1990). 4. Characteristics of the Bathyal and Abyssal Faunas There is a patchy distribution of biomass in the deep Okhotsk Sea and both biomass and species composition are dominated by the low oxygen concentration. Zenkevitch (1963) identified two zones of deep-sea benthic fauna in the Kurile Basin. The main part of the Kurile Basin contains immobile filter feeders including the pennatulids Pavonaria and Umbellula, crinoids, Culeolus, sabellid worms and the pogonophoran Lamellisabella zachsi. In this zone the biomass is at its lowest at ~30.5 g m–2. The low benthic biomass may be related to the particularly low primary production in the surface waters of the central part of the Okhotsk Sea. Much of this fauna has close links with similar depths in the Pacific. The deepest part of the basin is dominated by a zone of bottom feeders including the polychaete families Maldanidae and Capitellidae, the holothurian family Molpadidae and the echinoid Brisaster and the asteroid Ctenodiscus. Mean biomass in this zone is ~102 g m–2. Zenkevitch (1963) noted the high incidence of gigantism amongst Okhotsk Sea deep fauna with the barnacle Balanus evermanni, the holothurian Psychropotes raripes and the polychaete Potamilla symbiotica all displaying gigantism. There are suggestions (Nishimura, 1983) that the deep-water fauna of the Okhotsk Sea is primitive in character being dominated by hyalosponges, polychelid and homarid decapod crustaceans, porcellanasteriid seastars and elasipod holothurians. Data for other deep-water taxa are patchy although deep-water cruises give rise to the descriptions of new deep-water species such as isopods (Kussakin and Malyutina, 1989), shrimp (Komai and Amaoka, 1989) and ascidians (Sanamyan, 1992). Data on deep-water crabs suggest a zonation with depth in the bathyal zone (Nizyaev, 1992). To improve our resolution in understanding the deep water environment of the Okhotsk Sea Saidova (1997) has described the benthic foraminiferal communities of the Okhotsk Sea. In the Kurile deeps the foram communities are dominated by Globobulimina auriculata, Trochammina abyssorum living in the deepest parts on sediments with an organic content reaching 2%. By contrast, Miliolinella recenta, Bolivina pseudodecussata and Elphidium batialis living at depth but on sediments of <1.5% organic carbon. The fish biomass at mesopelagic depths in the Okhotsk Sea is poor (Lapko and Radchenko, 2000). The mesopelagic layer between 500 and 1500 m contains 61 species of fish, many corresponding to the intermediate waters of the sub-Arctic Pacific. Over the same depth range sixteen species of squid are found (Lapko, 1995). Recently, Tuponogov (1997) has described the seasonal migration of the grenadier Coryphaenoides pectoralis at bathypelagic depths in the Okhotsk Sea. Individuals migrate to the northern Kurile Islands to reproduce before migrating south. These migrations are over several hundred kilometers. Zenkevitch (1963) and Nishimura (1966, 1968, 1969, 1983) have suggested the fauna of the deep Sea of Japan is composed of cold-adapted eurybathic species with affinities to Arctic species rather than a true deep-sea fauna. The biomass and species diversity of bottom living fauna of the Sea of Japan decreases markedly with depth. Zenkevitch (1963) records only 25 species between 2000 and 3000 m and five macrofaunal species from below 3000 m (Terazaki, 1999). Down to 2000 m the fauna is dominated by the cnidarians Primnoa resdaeformis pacifica, Caryophyllia clavus and Lafoeina maxima, the echinoderms, Thaumatometra tenuis, Ctenodiscus crispatus and Luidiaster tuberculatus, the polychaetes Nephthys longisetosa. Harmothoe impar and Jasmineria pacifica, as well as decapods and molluscs, particularly the Buccinidae. Nishimura (1966) calls this community the “taraba community III” suggesting it extends from about 300 m to ~1500 m below which the community peters out. Below 2000 m the qualitatively and quantitatively poor fauna includes a variety of polychaetes, the brittle star Ophiura leptoctenia, the molluscs Pecten randolfi and Axinus sp. as well as a number of peracarid crustaceans (Zenkevitch, 1963). Owing to the postPleistocene age of the deep Sea of Japan, the fauna has not had time to acquire an endemic character of its own Deep-Sea Eukaryote Ecology of the Semi-Isolated Basins off Japan 337 and is “evolutionarily young”. Zenkevitch (1963) considers the only true endemic forms to be the polychaetes Harmothoe derjugini and Tharyx pacifica, the echinoderm Pedicellaster orientalis and the decapod Chionectis angulatus bathyalis. Nishimura (1966) was more conservative, suggesting that only Harmothoe derjugini is a truly endemic deep-sea polychaete species. All other species found in the deep waters of the Sea of Japan are eurybathic forms found in cold water of the Pacific and Bering Sea. A number of boreal forms have also invaded the Sea of Japan. Vinogradov (cited in Zenkevitch, 1963) identified the low temperature and salinity of the Sea of Japan rather than its recent history for the low diversity. Nishimura refers to the deep fauna as a pseudo-abyssal fauna and supports Vinogradov in suggesting the unique cold temperatures and salinity account for the penetration and success of a cold-adapted secondary deep-sea fauna, and the failures of archaic deep-sea faunas to colonise the deep waters especially after periods of anoxia during the Pleistocene low sea level stands (Terazaki, 1999). Some 20 species of deep-water fish of the Sea of Japan are recognised (Zenkevitch, 1963) and are characterised by cold water species from the families Zoarcidae, Cottidae, Liparidae, Lumpenidae and Pleuronectidae (Nishimura, 1968, 1983). Nishimura (1968) lists 8 deepwater zoarcids, 12 deep-water cottids and 16 deepwater species of Liparidae. The poverty of invertebrate species is reflected in the diversity of the fish fauna. Of particular interest to Nishimura (1968) was the poverty of macrourids in the Sea of Japan compared to the deep water on the Pacific side of Japan. Approximately 50 species of macrourid are known from off southern Japan in deep water whereas in the Sea of Japan there were one or two species, of which there was taxonomic uncertainty. The same feature is seen in the Myctophidae where some 33 species are seen on the Pacific side of Japan and only two within the Sea of Japan (Nishimura, 1968). Our knowledge of the deep-water fauna of the Okinawa Trough is dominated by recent observations of hydrothermal vent faunas. Data on non-vent faunas are very limited. Feng and Huang (1997) describe the distribution of small gastropods in surface sediments on the northwest side of the Okinawa Trough. Most species were related to inner shelf and offshore shallow-water species and may be a relict of the Late Pleistocene low sealevel stands. Feng and Huang suggest that a minority of these deep-sea species are derived via upwelling events formed by the Kuroshio along the northern edges of the Okinawa Trough. Of the three main described hydrothermal sites in the Okinawa Trough, the best studied site is that of the Minami-Ensei Knoll described in detail by Hashimoto et al. (1995). This study examined three sites, observed as 338 P. A. Tyler 100 to 1000 m depressions, associated with the western slope of the Minami-Ensei Knoll in the mid-Okinawa Trough. The Minami-Ensei Knoll (minimum depth 550 m) is situated in the northern part of the central graben and is surrounded by many small knolls and depressions. Such depressions, approximately 100 m deep, are interpreted as small calderas formed by volcanic activity. The distribution of the fauna is determined by the substratum type, whether rock or sedimentary, and by the presence of venting fluids, whether point source or diffuse. As a broad rule rocky substrata are associated with high and low temperature vents and sedimentary environments with low temperature diffuse hydrothermal fluid ~5 to 10°C higher than the ambient seawater. Three depressions were examined by Hashimoto et al. (1995). The first (670 to 780 m depth) appeared to be inactive but contained dead shells of Calyptogena solidissima, suggesting that this depressions, and maybe others, were subject to intermittent venting. The second depressions was a little larger at 1000 m diameter. Sediment cover was very heterogeneous consisting of breccias, fine and coarse sands and small pieces of pumice. As with the first depression, dead vesicomyid clams were found in heaps but associated with these were a few living C. solidissima. More significant, however, was the 200 m-diameter patch of, as yet, undescribed vestimentiferans on the flat bottom of the depression where low temperature (positive temperature anomaly of 0.1°C at 30 cm in sediment) diffuse venting occurred. Densities were estimated as 10 m –2. Two undescribed species of vestimentiferan were also found on the steep inner slope of the east wall of the depression with a positive temperature anomaly of 2.9°C (ambient water was 7.0°C). Hydrogen sulphide levels were ~2.6 ppm and methane 2200 nl kg –1. Apparently grazing on the outside of the vestimentiferan tubes was the gastropod Cantrainea jamsteci whilst the limpets Puncturella parvinobilis, Bathyacmaea secunda and Lepetodrilus japonicus as well as Provanna glabra were collected on sediment close to the venting fluids. Close to the vestimentiferan clump were found individuals of Calyptogena solidissima and Bathymodiolus aduloides. B. aduloides has also been taken from the Izena Cauldron and the Iheya Ridge in the mid Okinawa Trough (Hashimoto and Okutani, 1994). There appears to be a number of non-vent species associated with this site including the lithodid genus Paralomis and the seastar Ceremaster misakiensis. Ohta and Kim (1992) also reported the presence of the non-vent crab Geryon affinis granulatus associated with the MinamiEnsei Knoll. The most active hydrothermal site at Minami-Ensei Knoll was a depression ~1200 m in diameter with a maximum depth of 720 m. At the base of this depression were a series of black and white smokers forming chimneys 50 to 250 cm high, one having an exit temperature of 269°C. Carbon dioxide levels were high and a substance believed to be CO2 hydrate emerged from the seafloor. Closest to the vent openings were the alvinellid Paralvinella hessleri whilst the bresiliid shrimp Alvinocaris was attached to the outer surface of the vents. Surrounding the vents were dense aggregations of Bathymodiolus japonicus and dead colonies were found nearby. The population appeared to be actively recruiting as suggested by the wide range of size classes. In the mantle cavity of the mussel were found Branchipolynoe pettiboneae and Mytilidiphila okinawaensis (Miura and Hashimoto, 1991, 1993). The surface of the shells were being grazed by the limpets Puncturella parvinobilis, Bathyacmaea secunda and Lepetodrilus japonicus whilst Paralomis jamsteci and a species of Munidopsis were found closely associated with the mussel bed. Living and dead assemblages of Calyptogena solidissima were also found associated with a positive temperature anomaly of 0.3°C. Very small clumps of vestimentiferans were found on the floor and north and south slopes of the depression. As with the second depression Cantrainea jamsteci and Paralomis were common and the gastropod Neptunea insularis was found crawling over bacterial mats. The associated fauna was very similar to that found in the second depression. The other hydrothermal sites in the Okinawa Trough are not as well characterised as the Minami-Ensei Knolls. On the northern slope of the Iheya Ridge (~1400 m depth) two vent communities were identified. One was a sedimentary site dominated by Calyptogena sp., an unidentified vestimentiferan and the primitive barnacle Neolepas sp. (Kim and Ohta, 1991). The second site was a rock substratum with Bathymodiolus sp., Alvinocaris, Lebbus sp. and Paralomis sp. (Ohta, 1990). Sulphur isotope analysis of the vent fauna suggested that Bathymodiolus, Calyptogena and the vestimentiferan tube worms were nutritionally supported by endosymbiotic chemolithoautotrophic sulphur-oxidizing bacteria (Kim et al., 1989) whilst galatheids benefitted from this chemosynthetic food web. However, the main source of sulphide was sulphate reduction by bacteria rather than reduced sulphur of volcanic origin. Nearby were the sponges Pheronema sp. and Euplectella sp. At a shallower site (1045 m) Bathymodiolus sp. was also found and associated with this habitat was a distinctive foraminiferal assemblage (Akimoto and Hattori, 2000). Forams were mainly agglutinated with few calcareous species suggesting the low pH of the environment may dissolve calcareous forms. Shirayama (1992) identifies the meiofauna of the mussel beds and finds it to be dominated by nematodes of which Neochromadora was the dominant genus. Other taxa included harpacticoid copepods, polychaetes and micromolluscs. 5. Discussion The three major basins to the north, west and south of Japan offer contrasting scenarios for their deep-water benthic fauna. Tectonically, there is evidence that all three may have some form of back-arc origin but this is only truly apparent in the Okinawa Trough. The fauna of each basin differs from that of the other basins driven by the morphology of the basin and the origin of primary energy. The Okhotsk Sea is semi-isolated by has connections to the main Pacific Ocean through gaps in the Kurile island chain that allows exchange of water between the sea and the Pacific Ocean. Such exchange will also carry reproductive propagules and thus the fauna between the Okhotsk Sea and the Pacific share many species. Data for both surface production and vertical flux are few for the Okhotsk Sea. Mordasova (1997) reports that surface primary production is highest and very seasonal over the shelf regions beginning at the ice edge. Over the deepwater regions the chlorophyll biomass is 0.2 to 0.4 mg m–3 rising to 1.0 mg m–3 inside the Kurile Island in the southeastern corner of the Okhotsk Sea. Outside the Kurile Islands in the open Pacific chlorophyll values increase to 5.0 mg m –3 that increase the phytoplankton biomass in the Okhotsk Sea as a result of hydrographic incursions. Mordosova classifies the Okhotsk Sea as a “eutrophic” as it supports high invertebrate and fish production. No data are available for the flux of surface production to deeper waters but the patchy deep-sea benthic biomass may reflect the patchy effect of incursions of Pacific Water into the Okhotsk Sea. The Sea of Japan has no deep-water connections to the Pacific and as a result the species diversity is impoverished. During the last glacial period the Sea of Japan basin was isolated and became anoxic. In the post-glacial period the circulation pattern has been invigorated by the surface production of cold dense water resulting from winter surface cooling and this has ventilated the deeper waters allowing reinvasion of the deep areas. The low species diversity may reflect the short period of time for colonisation, as is seen in the Mediterranean since the last drying out of that sea. Surface production in the Sea of Japan is low, patchy and seasonal. In the northwest production is high whilst in the southeast it is low (Nishimura, 1969, 1983). There are no data for vertical flux to the deep seabed of the Sea of Japan but the very low biomass suggests flux is very limited. The Okinawa Trough is also a relatively young deepsea area but the fauna, in terms of both species diversity and biomass, is biased by chemosynthetic production. The faunal composition is similar to that of other back-arc basins and is a mixture of the typical hard substratum vent fauna and that of sediment-covered seabed with diffuse venting. The Guaymas Basin would appear to be a natural comparator. Biomass, as in all vent environments, is Deep-Sea Eukaryote Ecology of the Semi-Isolated Basins off Japan 339 very patchy and relies not on surface production but on production and export as a result of hydrothermal venting. Diversity is relatively low, when compared to similar depths in a non-vent environment (Gage and Tyler, 1991) but is typical of vent communities. 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