Download Deep-Sea Eukaryote Ecology of the Semi

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
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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-
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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.
In conclusion, the three basin around Japan present
excellent opportunities for comparative studies of species diversity, biomass and production in relation to their
ambient environment.
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