Journal of Oceanography, Vol. 57, pp. 663 to 677, 2001
Submersible Observations of the Hydrothermal Vent
Communities on the Iheya Ridge, Mid Okinawa Trough,
Japan
SUGURU O HTA1* and DONGSUNG K IM2
1
Ocean Research Institute, University of Tokyo,
Minamidai 1-15-1, Nakano-ku, Tokyo 164-8639, Japan
2
Biological Oceanography Division, Korea Ocean Research & Development Institute,
Ansan P.O. Box 29, Seoul, Korea
(Received 10 July 2000; in revised form 24 June 2001; accepted 2 July 2001)
During the Dives Nos. 409, 410, 480 and 481 of the Japanese submersible Shinkai
2000, conducted on June 10 and 11, 1989 and on May 16 and 17, 1990, several hydrothermal vents and prosperous vent associated biological communities were found on
the northern slope of the Iheya Ridge in the Mid-Okinawa Trough (27 °32.5′ N,
126 °58.5′ E: depth 1,400 m). The first site we found, the “Calyptogena Site”, was characterized by a relatively thick blanket of sediments, pleated and/or laminated lava
flows, with occasional lobate pillows and white and yellow stains. Although no remarkable shimmering water and thermal anomalies were detected during the observations, the extraordinarily dense community must be related to hydrothermal activities. The community was dominated by the giant white clam, Calyptogena okutanii,
in biomass, and by Neolepas-type primitive scalpellids and slender vestimentiferan
tube-worms in number. The second site, the “Pyramid Site”, situated only 200 m northward from the Calyptogena Site has typical clear smokers emitting hot water over
200 °C, and is characterized by a hard substratum of volcanic rocks and hydrothermal slabs. No noteworthy succession was perceived at the Calyptogena Site over a
year. Many of the vent members occurred at both sites. However, Calyptogena okutanii,
which were confined to the sediment bottom, Neolepas spp. and larger vestimentiferan
tube-worms were found to thrive only at the Calyptogena Site, being only minor elements in the Pyramid Site. The global distribution of several groups of organisms is
discussed preliminarily in zoogeographical terms based on comparison with other
submersible missions and surveys done by surface vessels.
1. Introduction
In recent decades many deep-sea chemosynthesisbased communities have been reported in several
tectonically and geomorphologically active settings. The
sea around Japan is especially noteworthy in this respect,
where the Pacific and Philippine Plates subduct beneath
the Eurasian Plate. In 1984, prolific deep-sea biological
communities dominated by two giant clams, Calyptogena
(Archivesica) soyoae and C. (A.) okutanii, were found in
the bathyal zone southeast of Hatsushima Island, Sagami
Bay, by a Japanese submersible Shinkai 2000 (Ohta et
al., 1987; Sakai et al., 1987; Hashimoto et al., 1989; see
Keywords:
⋅ Hydrothermal vent,
⋅ Iheya-Ridge,
⋅ Okinawa Trough,
⋅ biological communities,
⋅ Calyptogena,
⋅ vestimentiferans.
also Okutani et al., 2000; Fujikura et al., 2000). Since
1985, similar communities were found to extend even in
the abyssal depths on the landward trench slopes and further to the hadal zone in the trench axis deeper than 6,000
m of the Nankai Trough, the Japan and Kurile Trenches
(Le Pichon et al., 1987; Ohta and Laubier, 1987; Fujikura
et al., 2000).
In Japan, ecological studies of deep-sea biological
communities associated with hydrothermalism along the
ocean-spreading rift systems started only after 1987, related to a foreign project surveying the rift systems in the
Mariana Back-arc Basin (Okutani and Ohta, 1988; Hessler
et al., 1988a; Stein et al., 1988) and the North Fiji Basin
(Auzende et al., 1988).
In 1988 hydrothermal vents and associated biological communities were located on the Iheya Ridge and
* Corresponding author. E-mail: [email protected]
Copyright © The Oceanographic Society of Japan.
663
Izena Cauldron in the Okinawa Trough (Kato et al., 1989;
Kimura et al., 1989; Tanaka et al., 1989), and we can
now access both hydrothermal communities on a divergent front and cold-seep communities on the convergent
fronts of the earth’s tectonics around Japan.
After that time, Japanese scientific parties went on
to study a succession of hydrothermal vent ecosystems,
such as Minami-Ensei Knoll (Hashimoto et al., 1995),
Kagoshima Bay (Hashimoto et al., 1993) and volcanic
arc on the Izu-Bonin (Takeda et al., 2000), and in addition the Western South Pacific the Manus Basin, Central
and South Mariana Basin, and near the triple junction of
the central Indian Ocean. However, there are few reports
on the general description of the site characteristics, faunal
and ecological observations of these findings, with the
exception of cruise reports and special items or limited
taxa. Although recent mainstream studies are inclined to
follow the genetic lineage of these fauna on the global
scale, there is also an urgent requirement to prepare detailed descriptions of ecological features, not only the
general characteristics but also the specific features of
each site to integrate all of these into a global-scale comprehension of the deep-sea chemosynthetic communities
and the mechanisms by which species and ecological diversities are maintained.
The Iheya hydrothermal field in the Mid-Okinawa
Trough is one of the first examples to be described, comparing its characteristics with those of the Minami-Ensei
Knoll (Hashimoto et al., 1995) and Izena hydrothermal
field, both situated in the Mid-Okinawa Trough and sepa-
rated by only several tens or hundreds of kilometers, together with the intra-field differentiation of the ecological characteristics.
Dives Nos. 409 and 410 of the Japanese submersible
Shinkai 2000 in 1989, conducted by the ecological group
of the Ocean Research Institute, University of Tokyo, were
planned to locate and describe several types of hydrothermal vents and vent-associated organisms at two sites
on the Iheya Ridge and to sample basic constituents of
the communities. During the dive a new vent community
was found, dominated by a giant clams, Calyptogena
(Archivesica) okutanii (undescribed at that time and
erected as a new species by Kojima and Ohta (1997a)),
and this was named the “Calyptogena Site”. In the
Calyptogena Site, Calyptogena formed dense beds among
thick sediment cover with the characteristic faunule of
“hydrothermal” vent communities. No strong indication
of shimmers and bubbling was observed.
The “Pyramid Site” (200 m to the north of the
Calyptogena Site) is a typical hydrothermal vent field
covered by dacitic volcanic rocks and slabs of hydrothermal precipitates. It is interesting to compare the two
nearby communities, separated by only a few hundred
meters, but having different substrata and, as a result,
different community compositions.
A hydrothermal vent is destined to occlude itself
through its own precipitation activity, and this fate is estimated to occur on the order of several decades (Laubier
and Desbruyères, 1985). It is therefore expected that the
longevity of the hydrothermal vent communities is com-
Fig. 1. Bottom configuration of the Okinawa Trough. Bottom contour intervals are 200 m.
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S. Ohta and D. Kim
parable to that of the vent itself. Dives Nos. 480 and 481
of Shinkai 2000 on May 16 and 17, 1990 were planned to
visit the same spots as Dives Nos. 409 and 410 to compare and follow the succession of the vent fields and the
biological communities over a roughly one-year period.
2. Location of the Sites and Equipment
The location and contour map of the site is shown in
Figs. 1, 2 and 3. The sites are situated on a flat bench of
the northern slope of the Iheya Ridge at a depth of around
1,400 m in the Mid-Okinawa Trough (27°32.5′ N;
126°58.5′ E). To the north of this dense biological community, there is a topographic high of 50 m in relative
altitude composed of volcanic rocks altered by
hydrothermalism (Fig. 3; see Tanaka et al., 1989).
During Dive No. 409 (June 10, 1990) the southern
skirt of the topographic high was surveyed from west to
east along the 1,400 m contour. Later this area was ascertained to be 150 m to the south of “fissure emanation”
found by Tanaka in 1988, and 200 m to the south of largescale vent field of the Pyramid Site. Dive No. 410 (June
11, 1989) first visited the Pyramid Site and ended at the
Calyptogena Site.
Both of the Dives No. 480 (May 16, 1990) and No.
481 (May 17, 1990) were dedicated to revisiting the
Calyptogena Site and the Pyramid Site on the Iheya Ridge
to obtain additional samples and to observe the succession, if present, of the biological communities and
hydrothermalism itself.
The following equipment was installed on the submersible:
Dive No. 409: stereo still cameras set perpendicular
to the bottom for the description of the communities,
baited traps, hand net, water-tight, heat-insulated sample
box made of PVC, two coring tubes.
Dive No. 410: marker buoy and still camera, PVC
sample box.
Dive No. 480: stereo still cameras set parallel with
each other and perpendicular to the bottom for the mapping of the communities, two small baited traps and a
distensible baited trap, two coring tubes, scraper, Pt-resistance temperature probe.
Dive No. 481: small baited traps, hand net, two coring tubes, temperature probe, opening-closing plankton
net (modified RMT type) installed on the payload rack of
the submersible, another opening-closing epibenthic
plankton net attached to the port-side sample basket
(Kikuchi et al., 1990).
3. Survey Results, Observation and Notes
3.1 Calyptogena Site
The investigation of the vent community was initiated along the 1,400 m contour line (1,390–1,410 m) from
west to east (Fig. 3). Below 1,410 m deep, the biota
seemed to be depauperate on steep slopes covered by
coarse debris flow, and the upslope shallower than 1,390
m were rocky and/or covered by talus. Between the depths
of 1,400 and 1,410 m, the bottom was rather flat and covered by pale, fine-grained sediment. Synallactid
holothurians of body length between 30 and 40 cm were
encountered frequently. They were skimming the very
Fig. 2. Detailed bathymetrical map of the Iheya Ridge, Mid-Okinawa Trough.
Hydrothermal Vent Communities on the Iheya Ridge
665
Fig. 3. Track lines of Dives Nos. 409, 410, 480 and 481 on the
multi-narrow beam echo sounder (“Seabeam”) map of the
relevant site. Ps: Pyramid Site, Cs: Calyptogena Site. Solid
circle on the Dive No. 410 track line indicates the start point.
surface of the sediment and fed selectively on organic
detritus. This suggests that the film of free-living chemosynthetic bacteria cover the sediment surface. Hydrothermal elements may contribute to the composition of
the sediment, considering the very turbid water and dense
suspension of the ambient water. The density of deep-sea
eel, Synaphobranchus affinis and/or Ilyophis brunneus,
Aldrovandia affinis and chimaerids was apparently higher
than those expected from the ordinary deep-sea floor at
comparable depths. These fishes are not included among
the usual members of hydrothermal vent communities.
However, the density of these fishes suggests the proximity of prey and a low density of toxic atmosphere.
After a trip lasting one and a half hour along the 1,400
m contour, we came across a place where laminated dark
rocks, oblong pillow lavas and pleated sheets of volcanics
were outcropping. The texture of the rocks was not of the
typical MORB (mid-oceanic ridge basalt) lavas, but apparently of eruption rocks, considering the glassy surface
and characteristic features. Rounded prickly white glass
sponge Pheronema ijimai aggregates on the rock surface,
other species were Venus’s flower basket, Euplectella sp.,
large sea anemones, and sabellid polychaete worms (diameter of tubes ca. 5 mm). They dwell within a 100 m
radius around the Calyptogena Site, as mentioned later.
Sponges are filter feeders, and the abundance of suspension feeders suggests ample quantities of suspended matter. On the sediment we witnessed dead shells of
Calyptogena sp.
Near the Calyptogena Site, huge tangles of
vestimentiferan tube-worms were found (probably consisted of more than 1,000 individuals). Among the slender and coiled vestimentiferan tube-worms, we observed
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S. Ohta and D. Kim
Fig. 4. Distribution of representative vent organisms in the
Calyptogena Site on the Iheya Ridge, Mid-Okinawa Trough.
bresiliid shrimp with milky opaque body color, white eyes,
long rostrum and slender walking legs and hippolytid
shrimp with orange body color, black eyes, short rostrum
and stout walking legs (Photo 2). Just outside of the
vestimentiferan tangles, rather thick vestimentiferan tubeworms belonging to the genus Lamellibrachia were seen
projecting their greenish pink obturaculum and scarlet
petal-like gills. Medium-sized mussels and Neolepas-type
cirripeds covered the rock surface. Among the rock crevices, hagfish Eptatretus okinoseanus were aggregating.
On the skirt of the rocks, deep-sea galatheids Shinkaia
crosnieri Baba and Williams (1998; Photo 4) and Munida
sp. and dead shells of Calyptogena okutanii were scattered.
The density gradient of these vent organisms guided
us gradually to the “heavenly garden” of clams, mussels,
cirripeds and shrimps named Calyptogena Site (Photo 1).
The site was characterized by relatively thick blanket of
sediments, breccia and pyroclastics, pleated and/or laminated lava flow, with occasional lobate pillows (the texture of the rocks suggests a rather acidic nature like dacite)
and white and yellow stains. Although no remarkable
shimmering water and thermal anomalies were detected
during the observation and sampling of the dense patches
of organisms, the extraordinarily thick community convinced us that it was fueled by hydrothermal chemicals.
Photo 1. The giant clam Calyptogena (Archivesica) okutanii Kojima and Ohta (1997a) colonizing on the sediment and a stone
crab Paralomis sp. (cf. P. verrilli (Benedict)). Scalpellids probably akin to genus Neolepas cover the outcropping rocks.
Calyptogena Site.
Photo 2. Two forms of primitive scalpellids probably akin to the genus Neolepas. They are often attached on a tube of the large
vestimentiferan tube-worm. Calyptogena Site.
Hydrothermal Vent Communities on the Iheya Ridge
667
Photo 3. Voracious shrimps Lebbeus washingtonianus (Rathbun) and Alvinocaris longirostris Kikuchi and Ohta (1995) aggregate
on the hydrothermal slabs. Pyramid Site.
Photo 4. Galatheid Shinkaia crosnieri Baba and Williams (1998). They bear Beggiatoa-type filamentous bacteria on abdominal
surface. Pyramid Site.
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S. Ohta and D. Kim
The general configuration of the site was a narrow
bench on a southward dipping gentle slope, where dark
grayish brown volcanic table rocks are outcropping among
blanket of fine to coarse sediments. The span of the thriving community was about 5 by 20 m (Fig. 4).
The community was dominated by Calyptogena
okutanii in biomass, and in number by two primitive
scalpellids (akin to Neolepas spp. undescribed; Photo 2)
and slender vestimentiferan tube-worms. It must be noted
that the C. okutanii were confined to the sediment bottom, and other dominant members were occupying the
outcropping surface of the volcanic rocks (Photo 1 and
Fig. 4). Two kinds of shrimps (bresiliid and hippolytid:
Photo 3), two species of mussels, deep-sea galatheid
Munidopsis sp., stone crab Paralomis sp. (cf. P. verrilli)
(Photo 1) were the representative constituents of the thick
community. A larger vestimentiferan tube-worm
Lamellibrachia sp. (Photo 2), hyaline sponges Pheronema
ijimai and Euplectella sp. and the fishes such as
Aldrovandia affinis, chimaeras and Synaphobranchus
affinis and/or Ilyophis brunneus) surrounded the field of
dense patches of organisms.
Three dense beds (2 by 4 m, 1 by 1.5 m and 1 by 1.5
m) of Calyptogena occupied the central portion of the
community (Fig. 4). Almost all specimens were living
ones, uniform in length (median 14 cm), and half buried
in the sediment in a vertical or oblique position showing
the openings of their short, reddish incomplete siphons
among the gaping shells (Photo 1). They sometimes expired water jets. The thickness of the sediment was at
most 10 to 15 cm among the clam beds, and less than 5
cm thick outside the beds, judging from the probing of
sediment temperature sensor.
The Calyptogena of this site was revealed to be common to the C. (Archivesica) okutanii originally described
on the specimen collected in Sagami Bay, and differs from
C. (A.) solidissima reported from the hydrothermal vent
fields of the Minami-Ensei Knoll (Kojima and Ohta,
1997a; see also Okutani et al., 1992). Hemoglobin-rich
blood, unusually thick gills, atrophied alimentary canal,
and the mode of occurrence are typical characteristics of
Calyptogena-group inhabiting the cold seepages of
Sagami Bay (Hashimoto et al., 1987), the Japan and Kurile
Trenches (Ohta and Laubier, 1987). Typical coccoid
symbionts in the bacteriocytes of the gill tissue (Kim,
1992), ample crystals of native sulfur (based on optical
microscope observations of the preserved specimens)
suggest sulfur-oxidizing chemosynthetic bacteria.
The density was the closest-pack state, and dead
specimens were scarce. They dominated in biomass, and
the standing crop was estimated to be more than 10
kg m–2, which is 3 to 4 orders of magnitude higher than
the average biomass at a comparable depth of the ordinary deep-sea floor (Ohta, 1983).
On the outcropping rocks of the clam beds, two
undescribed scalpellids both having an ancient body-plan
were crowded (Photos 1 and 2). Slender and coiled
vestimentiferan tube-worms (probably belonging to the
genus Alaysia) were entangled. They were muffled by
“snow” of chemical precipitate and/or bacterial mats and
sponges. These two groups of organisms dominated in
number on the outcropping rocks.
The giant clams and the tube-worms were sampled
with the manipulator. Ophiothrichid ophiuroids were encountered among the slender tube-worms. Possibly they
are feeding on the detritus trapped among the bush, rather
than taking shelter among them.
We could differentiate two species of shrimps
through the porthole of the submersible. During Dives
Nos. 409 and 410 they escaped easily from the hand net
operated by manipulator, except when the clashed clams
were used as a lure. During Dives Nos. 480 and 481, they
were efficiently collected by the baited traps. The bresiliid
shrimp was described as a new species Alvinocaris
longirostris Kikuchi and Ohta (1995) and is very akin to
and congeneric with Alvinocaris lusca Williams and
Chace (1982) described on the hydrothermal vents of the
East Pacific Rise about 7,000 km away. The hippolytid
shrimp was identified as Lebbeus washingtonianus. This
species also occurs in the Minami Ensei Knoll vent fields
(Hashimoto et al., 1995); the southern extension of geographical distribution to the Okinawa Trough and the occurrence of the hippolytid shrimp bound to
hydrothermalism are noteworthy (Kikuchi and Ohta,
1995).
Amphipods were the first to be attracted to the small
traps baited with clam meat. Alvinocaris and Lebbeus
followed the amphipods, but they were rather cautious at
the entrance to the traps. They sometimes stretched their
legs to grasp the bait at the entrance, and took out the
moiety of the food. However, before long, the traps were
full of the two shrimps. The density of the shrimps in the
fields was estimated to be 200 individuals per square
meter, and Alvinocaris accounted for 80 to 90% of them.
There was a slight difference in the approach to the bait.
Alvinocaris leapt at the clam meat, whereas Lebbeus came
somewhat later, walking on their stout legs. Although they
are not large predators, they are voracious and swift in
motion, and probably play the role of most influential
consumers.
Although the mussels Bathymodiolus spp. are subordinate to the Calyptogena, they fasten themselves with
byssus to the rock substratum and also to slender and
coiled vestimentiferan tube-worms. Two species of mussels occurred in this field and are described anew as
Bathymodiolus platifrons Hashimoto and Okutani (1994)
and B. aduloides Hashimoto and Okutani (1994). The
shells are medium-sized (3–10 cm) compared to the con-
Hydrothermal Vent Communities on the Iheya Ridge
669
generic species of the East Pacific Rise, the Mariana Backarc Basin and the North Fiji Basin. As a member of the
family Mytilidae of the order Filibranchia, the gill filament is rather thin and essentially free. However, each
filament is unusually wide, and most of the “gill tissue”
is transformed into the bacteriocytes layer bearing chemosynthetic bacteria, and this is reminiscent of the
Calyptogena group (Kim, 1992). The gill, however, still
functions as a respiratory and filter feeding organ, and
the alimentary canal is intact. The mussels can collect
flocks of free-living chemosynthetic bacteria to feed on
(Lilley et al., 1983). The high infection rate of exoparasitic
polychaetes aiming to snatch off the particles on the gill
also supports this (Miura and Ohta, 1991). Therefore the
mussels are mixotrophs depending nutritionally on their
own alimentary canal and chemosynthetic endosymbionts
(Kim, 1992; Kim and Ohta, 2000).
Large fish, Synaphobranchus and/or Ilyophis and a
stone crab Paralomis sp. roamed around the site as predators. Two eelpouts occurred among the dense communities. They seem to be endemic to this hydrothermal vent
field. They usually lay with their bodies motionless, and
never showed feeding behavior. They paid no attention
to the baited traps containing swordfish Cololabis saira,
whereas synaphobranchids and amphipods are attracted
quickly to the baited trap.
A large, yellow-colored stone crab Paralomis sp. cf.
P. verrilli (Photo 1) was captured by the manipulator of
the submersible. Carapace width attained 15 cm. In contrast to the case of Paralomis jamsteci reported from the
hydrothermal vent field of the Minami-Ensei Knoll
(Takeda and Hashimoto, 1990). They are not bound to
hydrothermal vents, but are attracted and accumulate just
as in the case of Paralomis multispina in Sagami Bay
(Hashimoto et al., 1987). However, they must be quite
resistant to the chemical atmosphere of the vent fields.
Although they can expel the shrimps and pick up and feed
on the clams, mussels, cirripedes and polychaetes among
the sediment with their stout chelae, their sluggish motion may not cope with the quick response of the two
shrimps and amphipods.
A few individuals of deep-sea galatheid with flat rostrum Shinkaia crosnieri (Photo 4) were also recognized
in the field, but they remained almost motionless, and we
could not imagine how and what items they feed on.
White patches of about 10 by 30 cm were scattered
here and there in the field. We tried in vain to collect the
white stuff with coring tubes with core catchers. They
were easily blown off by the bow wave, because they are
not sticky as in the case of filamentous bacterial mats.
They are probably composed mainly of chemical precipitates such as carbonates, sulfates and/or silicates.
Surrounding the Calyptogena Site, large
vestimentiferan tube-worms, roughly 1 cm in the diam-
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S. Ohta and D. Kim
eter of distal opening, 30 to 70 cm in total length, were
scattered on the basement rock covered thinly by coarse
sediments. The anterior 1/3 of the tube was held perpendicular to the sea floor, whereas the remaining posterior
portions were attached among the rock crevices and/or
beneath the boulders, hence the obturacula occupies between 10 and 20 cm above the bottom. They reminded us
of asparagus in the field. This species does not form a
gregarious bush as was observed in the case of the
vestimentiferans of Sagami Bay (Hashimoto et al., 1987;
Ohta, 1990b). The highest density was 20 individuals per
square meter. Pale greenish obturacula projected into the
water column as slender funnels. This corresponds to the
“pistil” of a flower, and the lamellate gill slits form petals of scarlet color (due to the presence of hemoglobin in
the body fluid). The general configuration of the “flower”
and the occurrence of several distinct trumpet-shaped
corrugations on the distal portion of their tubes differentiate them from the possibly congeneric species of
Lamellibrachia found in the cold seepages in Sagami Bay
(Ohta, 1990b). Most of them bear a few individuals of
smooth Neolepas-type cirripeds on the distal portions of
the tubes (Photo 2). They slowly but regularly protrude
and draw in the “flowers” at the openings.
The prickly scalpellid, Neolepas-type sp. A (Photo
2; left) dominated in number. They gregariously cover
most of the rock surface. Another species of smooth
scalpellid, Neolepas sp. B (Photo 2; right), as inclined to
dwell in relative solitude on the rocks or on the tubes of
vestimentiferan tube-worms. Most of the Neolepas sp. B
occupied the distal extremities of the tubes. This at least
suggests that the cirripede finds its own position after the
growth of vestimentiferan tube-worm, and further suggests that they are rather opportunistic in reproduction
strategy. The cirripeds in the hydrothermal vents are
equipped with very long and fine appendages adapted to
collect the fine flocks of free-living bacteria and/or bacterial clots suspended in the ambient seawater (Newman,
1979; Newman and Hessler, 1989; Yamaguchi and
Newman, 1997a, b).
We noticed small ophiuroids walking among the mats
of coiled vestimentiferan tube-worms or on the
outcropping rocks. Also Ophiura-type ophiuroids occurred among the Calyptogena beds. They are not negligible members in number, and the dimensions were almost uniform.
Although Dives Nos. 480 and 481 were conducted
with the theme in mind, i.e., to follow the temporal variation (succession) of the biological communities at the
Calyptogena Site over a year, we could not notice the
slightest difference in the hydrothermalism and the vent
communities. The Calyptogena Site seems to be in a stable climax phase.
Fig. 5. Generalized map showing the central part of the Pyramid Site (after Gamo et al., 1991). Small concentric circles
and squares denote the location of marker buoys and/or scale
deployed during submersible dives.
Many ctenophores were noticed during the dives. We
must take into account the occurrence of “hydrothermal
plankton” in the vent fields.
3.2 Pyramid Site
The second site, the “Pyramid Site”, situated only
200 m northward to the Calyptogena Site (Fig. 2), has
typical clear smokers and is characterized by the hard
substratum of volcanic rocks and hydrothermal slabs.
Dive No. 481 was dedicated to ecological observations and sampling of the Pyramid Site. Following the
network of broad and white altered streaks and the
shimmering of hot water, we approached the western flank
of the vent field (Fig. 5). The Pyramid Site as a whole
consists of hard substrata of huge volcanic rocks covered
by hydrothermal precipitation slabs composed mainly of
calcium carbonate and magnesium carbonate (Gamo et
al., 1991). In the eastern part, a pyramidal edifice stands
several meters high as a conical tower (Fig. 5). It emits
hot water (more than 200°C) mainly through the small
central collapsed cauldron, as well as through the gaps in
the skirt. Slender and coiled vestimentiferan tube-worms,
probably of the genus Alaysia, mussels Bathymodiolus,
shrimps of Alvinocaris and Lebbeus, galatheids and
eelpouts are the main constituents of the vent opening
community. Dense beds of mussels and small sponges
encircled the vent opening community, and large potatoshaped sponges (5–10 cm in diameter) populated the periphery. Limpets of 1.5 cm diameter were sometimes attached to the rocks and bivalves.
The most dominant organisms in number were
shrimps, Alvinocaris longirostris and Lebbeus
washingtonianus (50 individuals per 50 × 50 cm quadrat),
and Bathymodiolus platyfrons and B. aduloides were scat-
tered in threes and fives (20 individuals per 2 × 2 m
quadrat). The most dominant organisms in biomass must
be mats of several species of sponges. The density of freeliving deep-sea galatheid Munidopsis were around the
1/4 of that of the mussels. On the other hand, larger
vestimentiferan tube-worms, Calyptogena and Neolepastype cirripeds were not witnessed in the Pyramid Site.
Shrimps of unusual appearance were sometimes
found. Body length reaches 7 to 8 cm (ca. 1.3 times larger
than and about twice as wide as the ordinary ones), and
they bear “white moss” over 2/3 of the cephalothorax.
They must be full-grown and senile individuals of
Alvinocaris, considering the body’s pale opaque color,
colorless eyes, long rostrum and relatively slender walking legs. The “moss” suggests the tufts of filamentous
bacteria, but its ecological implications remain as open
question due to the failure of sample collection.
Eelpouts lie among the beds, flexed in sinusoidal
fashion. Sometimes they maintain this posture even in
the water column. Again, they never pay attention to
baited traps. Possibly they may be satiated in the dense
communities.
It must be noted that four asteroids Henricia sp. were
seen on the rocks.
The top and middle portion of the pyramid (a hydrothermal chimney) are blurred with shimmering hot water. The surface of the pyramidal body was covered with
the mossy tubes of vent-specific polychaetes, Paralvinella
hessleri Desbruyères and Laubier (1989; see also Miura
and Ohta, 1991).
The galatheids Shinkaia crosnieri gathered gregariously on the diffuse chimney top, hiding almost all of its
surface, where a rather high temperature gradient and high
concentration of hydrogen sulfide are expected. Apparently they are tolerant to these environments where no
other members except the Paralvinella dare to invade.
They may probably feed on the polychaetes and “culture”
filamentous bacteria on their abdominal surface (Photo
4).
Groups of sabellid polychaetes were observed on the
flanks and overhang of the chimney. The habitat of the
sabellid polychaetes is separated by, say, 1 m from that
of paralvinellids. They are not exposed directly to the
warm water.
4. Discussion
Since the discovery of hydrothermalism and the association of deep-sea communities along the Galapagos
Rift in 1977, many interdisciplinary works have been
performed to explain the unusual prosperity of the biota.
Demonstration of the occurrence of free-living
chemoautotrophs (Lilley et al., 1983; Jannasch and Mottl,
1985; Johnson et al., 1986) and symbiosis with
chemoautotrophic bacteria among several large repre-
Hydrothermal Vent Communities on the Iheya Ridge
671
sentatives of the vent organisms (Felbeck, 1981;
Cavanaugh et al., 1981) explained the prosperity, up to
20–30 kg m –2 standing stock (Hessler and Smithey, 1983;
Somero et al., 1983; Laubier and Desbruyères, 1984; Stein
et al., 1988). The detailed explanation of the nutritional
backgrounds of this kind of ecosystem is ubiquitous in
many references.
Now we have available data on the biological samples of the Okinawa vent fields (the Iheya Ridge, the Izena
Cauldron and the Minami-Ensei Knoll; e.g. Hashimoto
et al., 1995; Okutani et al., 2000; Fujikura et al., 2000;
Okutani and Fujiwara, 2000), the vent fields along the
volcanic front of the Izu-Ogasawara Ridge (Kaikata
Seamount, Kasuga Seamount, Mokuyo and Suiyo
Seamounts; e.g. Takeda et al., 2000), and the vent fields
of the back-arc and/or microplate rift systems (Mariana
Back-arc Basin, Manus Basin, the North Fiji Basin and
the Lau Basin; e.g. Desbruyères et al., 1994) of the Western and Southwestern Pacific areas. A comparison with
those of the Juan de Fuca Ridge, the East Pacific Rise of
the Eastern Pacific (see the review by Tunnicliffe, 1991),
together with the data set of the subduction zones along
the Northwestern Pacific (Sibuet and Olu, 1998; Ohta and
Hashimoto, unpublished data), demands that we start
study the global distribution and generalization of the
deep-sea ecosystems bound to tectonics.
As for the dominant members of the vent fields—
i.e., Calyptogena, Bathymodiolus, vestimentiferan tubeworms, shrimps, cirripedes, and polychaetes—morphological work and molecular phylogeny studies are now
under way by many collaborators. The amino acid
sequencing of the hemoglobin molecules together with
the systematic positioning of Calyptogena and
vestimentiferan tube-worms have already been demonstrated (Suzuki et al., 1988, 1989a, 1989b, 1989c, 1990a,
1990b), and the systematics and phylogeny of
Calyptogena and vestimentiferan tube-worms of the Western and South Pacific are now being fixed. Here we focus very briefly on the problems of zoogeography of the
vent and cold-seep organisms around Japan.
4.1 Notes on biogeography of vent organisms around
Japan
To start with the white giant clams Calyptogena, we
can count at least more than 10 species of extant
Calyptogena sensu lato around Japanese waters, ranging
from several hundred meters to the depth of 6,800 m
(Metivier et al., 1986; see descriptions and review by
Okutani et al., 2000; Fujikura et al., 2000). Among them,
the species from the Minami-Ensei Knoll and Iheya-Izena
fields in the Okinawa Trough are associated with
hydrothermalism, and most of the remainders are recorded
from cold seepage areas, and a few undescribed species
are not related to active tectonics, as will be discussed
672
S. Ohta and D. Kim
later. However, all Japanese species are collected from
sediment floor, contrasting strikingly to the Calyptogena
magnifica found among crevices of basaltic rocks, eventually without sediment cover (Hessler and Smithey, 1983;
Hessler et al., 1985). The latter species is reported to take
up hydrogen sulfide through the foot muscle protruding
downward into the crevices. However, all Japanese
Calyptogena together with that found recently in the
Manus Basin, Solomon Sea, exclusively inhabit the sediment bottom (Ohta, unpublished data) and sometimes they
roam about the field with their pelecypods (Ohta and
Laubier, 1987). The Pyramid Site where hard substrata
dominate, and volcanism and hydrothermalism are more
active than in the Calyptogena Site lacked the
Calyptogena. So far, we can conclude that the giant clam
of the western side of the Pacific needs soft substrata.
Calyptogena are well-known organisms that house
symbiotic bacteria within their gills (hence, they are seeming “producers”), and they are often the dominant member of the vent and seep communities around the Japanese waters and along EPR. On the other hand, in the
hydrothermal fields of the back-arc (and/or microplate)
rift systems of the Western and Central Pacific, the dominant members bearing chemosynthetic symbionts were
several species of large gastropods, such as Alviniconcha
hessleri and Ifremeria nautilei (the Mariana Basin:
Okutani and Ohta, 1988; Ohta, 1988 in Hessler et al.,
1988a; Stein et al., 1988; the Manus Basin: Both et al.,
1986; the North Fiji Basin: Kojima et al., 2001; Lau Basin: Bouchet and Warren, 1991). We must note that the
distance between the hydrothermal vent fields of the
Okinawa Trough and the rift systems of the Western and
South Pacific regions is several thousands of kilometers,
whereas the Okinawa Trough is separated from the EPR
by more than ten thousand kilometers, if we do not include the Calyptogena of the cold-seep areas.
However, considering that Calyptogena is also the
dominant species in the cold-seep fields along subduction zones, and further they seem not to be obligatorily
bound to a well-defined chemosynthetic environment
(Ohta, unpublished data), the pan-Pacific distribution can
be understood. Recent finding of the occurrence of
Calyptogena on the Rodriguez Triple Junction in the Central Indian Ocean (Ohta, unpublished data) may shed light
on another possible propagation route.
Two species of the exoparasite polychaetes to the
gills of mussels and clams in the Iheya Ridge vent fields
were reported as new to science; Shinkai longipedata and
Branchipolynoe pettiboneae. They have, so far, been reported only from the type locality. On the other hand,
paralvinellid polychaete of the Iheya-Izena fields was
concluded to be conspecific to Paralvinella hessleri reported from the Mariana Back-arc Basin, the North Fiji
Basin and the Lau Basin in the central South Pacific
(Miura and Ohta, 1991), and the species is even congeneric with those recorded along the entire region of the
East Pacific Rise. The forms of Paralvinella are reported
from every hydrothermal vent field in the world. They
lay off-spring within their tubes, hence they must be
handicapped in their larval dispersal. This mode of reproduction is still puzzling problem for the ecologists
(Laubier and Desbruyères, 1985).
Vestimentiferan and pogonophoran tube-worms are
also famous representative members of the vent and cold
seep fields, depending for their nutrition on symbiotic
chemoautotrophs. The phylum Vestimentifera was erected
in 1981, and so far 5 families and more than 10 species
have been described from the region along the EPR (Jones,
1985, 1987, 1988) and Japanese waters (Hashimoto et al.,
1993; Miura et al., 1997).
Japanese vestimentiferans of about ten forms and/or
species, from hydrothermal fields and cold seepage, will
probably belong to the genera Lamellibrachia, Escarpia,
Arcovestia and Alaysia. The genus Lamellibrachia was
originally reported from supposedly reduced environment
of cold seepage (Webb, 1969; Hecker, 1985). As a general rule the vestimentiferans, at least the adult stage of
them, completely lack alimentary canals. However, the
spongy posterior portion of their bodies, the
“trophosome”, without exception, houses symbiotic bacteria (Kim, 1992).
After the finding of the most ancient form of
cirripedes, Neolepas zevinae in the vent fields of the EPR,
a wealth of collections of so-called living fossils of the
cirripedes was found, to which Japanese groups contributed greatly. This lead to the systematics and phylogeny
of the cirripedes drastically renewed (Newman, 1979;
Newman and Hessler, 1989; Yamaguchi and Newman,
1997a, b). Neoverruca brachylepadoformis Newman and
Hessler (1989) from the Mariana Back-arc Basin, situates the main stock of the Verrucomorpha, Eoverruca
ohtai from the vent fields of the North Fiji Basin situates
the main stock dividing Verrucomorpha and
Balanomorpha.
The Neolepas zevinae are based on very limited numbers of specimens, and they were a minor group in the
vent fields of the EPR. However, Neolepas-type sp. A and
Neolepas sp. B in the Calyptogena Site of the Iheya Ridge
dominated in number and also in the standing crop, and
they are comparable to those of clams and mussels (see
Photos 1 and 2). From a geological point of view, the
Okinawa Trough is considered to be an incipient backarc rift system just beginning to spread open. The occurrence of the most ancient forms of cirripedes already arrive at the rift system is thus astonishing.
We collected Alvinocaris longirostris and Lebbeus
washingtonianus in the Okinawa Trough. However, we
could not find typical rimicarid shrimps at these sites in
the Okinawa Trough. The rimicarid group dominates in
the Mariana Back-arc Basin, in the North Fiji Basin and
the Lau Basin of the Western Pacific, and along the MidAtlantic and Central Indian ridge systems. Occurrence of
Alvinocaris and no collection of rimicarid group suggest
the affinity of the fauna of the Okinawa Trough to that of
the EPR.
During the successive dives Nos. 411 and 412 in 1989
to the Izena Hole in the Okinawa Trough (ca. 40 km south
to the Iheya Ridge) during the same leg of the submersible, we had the chance to inspect the biological samples
from a black smoker. On the ores collected from the flank
of the black smoker, we found undescribed cirripeds and
limpets. The latter turned out to be another “missing link”
which bridges the Verrucomorpha and Balanomorpha
(T. Yamaguchi, personal communication). Incidentally,
two Neolepas-type forms found in the Iheya fields were
not collected from the Izena Cauldron.
Paralvinella hessleri were also found among the ore
deposits, which were famous for very wide
eurythermalism, together with very wide zoogeographical
distribution, as mentioned before.
No bythograeid crabs have been collected, so far,
from both of the Iheya and Izena fields. Vestimentiferan
tube-worms occur in both sites.
Although the Izena Hole fields have a sediment-covered habitat, rather limited numbers of Calyptogena were
encountered in the Izena fields. This may due to; 1) an
episodic “explosion event”, which must occur in the Izena
smoker area (Kato et al., 1989) will depopulate the less
mobile species like Calyptogena; 2) a high concentration
of carbon dioxide in the Izena Site as clathrate or bubbles will be awkward for Calyptogena; and/or 3) Japanese species group of Calyptogena require rather thick
sediment cover as a reactor bed, where the reduction of
sulfate from seawater will occur coupled with the input
of methane beneath the sediment, because all the
Calyptogena living in methane-rich environments have,
without exception, sulfur-related symbionts and high concentration of native sulfur within their soft tissue. So far,
the study of stable isotope ratios of sulfur favors the last
hypothesis (Sakai et al., 1990, 1991; Kim et al., 1990).
Apart from the geographical distribution,
bathymetrical segregation of vent and cold seep associates also deserves to be studied in near future (Kojima
and Ohta, 1997b; Fujikura et al., 2000).
4.2 Succession of the vent fields and vent fauna
No apparent change was observed in the biota and
emission of venting fluid in both sites of the Iheya Ridge
over 13 months. The trial of the 1-year continuous recording of the fluid venting activity in the Pyramid Site
of the Iheya Ridge using temperature probes failed due
to heavy chemical corrosion of the stainless steel hous-
Hydrothermal Vent Communities on the Iheya Ridge
673
ing of the temperature probe. However, the temperature
measurements of each year showed almost the same value.
There are several reports to describe the succession
of the vent and vent organisms over a few years (Laubier
and Desbruyères, 1985; Hessler et al., 1985, 1988b; Fustec
et al., 1987; Campbell et al., 1988; Johnson et al., 1988).
According to these reports, vestimentiferan tube-worms
and Calyptogena, both of which are obligatorily in symbiosis with chemoautolithotrophs, are climax phase representatives; mussels, one of the typical mixotrophs, can
be both a pioneering species and also the declining phase
inhabitant. At any rate we have a good field to test these
hypothesis around the Japanese waters in near future.
4.3 Occurrence of echinoderms around vent fields
Although we assumed that the dense biological communities, especially those in the Calyptogena Site of the
Iheya Ridge are hydrothermal ones, we could not perceive the characteristic shimmering of hot water or gas
bubbling as observed in the typical vents of the Izena and
Iheya fields (Sakai et al., 1990). Here temperature probes
showed only slight thermal anomalies of +0.3–0.5°C compared to the ambient seawater temperature of 3.1°C. The
site may be a field of low-temperature, methane rich emanation often reported on a sediment-covered back-arc
basin (Ishibashi et al., 1990; Gamo et al., 1991).
Formerly, the echinoderms, in general, were thought
to be so sensitive to the chemical constituents of the
seawater that they avoid the vent environments full of
toxic substances such as heavy metals, hydrogen sulfide
and aromatic hydrocarbons (Hessler and Smithey, 1983;
Grassle, 1986). In particular, it is often reported that even
a trace amount of heavy metals disturbs the early development. However it is noticed that a group of brisingid
asteroids always encircles the hydrothermal vents as a
peripheral member of vent communities in the Mariana
Back-arc Basin, North Fiji Basin and also in the vent fields
of N13° (on the East Pacific Rise). Just on the vent openings of the diffuse hydrothermal vent fields of the North
Fiji Basin and also of the Manus Basin, a group of
chiridotid holothurian (Trochodota sp. undescribed) were
found to occur as if they are bond to the vent opening
environment. Furthermore, large five-armed asteroid
Distorasterias stichantha were observed to pray on the
clam Calyptogena soyoae and/or C. okutanii in their dense
beds and together with small ophiuroids (Ohta, 1990a),
and large synallactid holothurians were roaming around
the Calyptogena bed in the cold seepage of the Tenryu
Canyon (Ohta and Laubier, 1987).
Fairly large numbers of echinoderms were encountered in the hydrothermal vent fields (though not very
near the vents) of the Okinawa Trough. Taking account
of the fact that the hydrothermal emission in the Okinawa
674
S. Ohta and D. Kim
Trough is thought to be of gas-liquid separated type beneath the sea floor (Sakai et al., 1990), and is also altered
chemically through the thick sediment, as is usual in the
island-arc type rift systems (Ishibashi et al., 1990), the
emitted water is not always rich in hydrogen sulfide and
heavy metals of crustal origin.
Our frequent encounters with deep-sea eels,
Aldrovandia affinis and chimaeras, that are not bound to
hydrothermalism, suggest that the atmosphere of the Iheya
fields are not so toxic to these fishes. Therefore, the Iheya
site is a rather open ecosystem as compared to other typical vent fields, such as those of Juan de Fuca Ridge.
Incorporating additional survey results, the Iheya
hydrothermal field is becoming one of the more intensively explored vent fields, and we can depict detailed
geological and ecological settings over a fairly wide area,
and this ecological description will offer good basis for
the intercomparison of other typical vent fields as those
of the Minami Ensei Knoll (Hashimoto et al., 1995) separated only 100 km, and that of the North Fiji Basin
(Desbruyères et al., 1994) separated by several thousand
kilometers.
As a whole, the Iheya vent field has typical islandarc hydrothermalism, compared to those of the mid-oceanic ridge hydrothermalism. It embraces both the hard
substratum composed of hydrothermal slabs and mounds,
and is also influenced by the thick sedimentation exemplified by the chemical composition of the vent fluid
(Sakai et al., 1990, 1991) and the bottom features. The
field is a complex of heterogeneous ecological habitats
within a relatively short span, and concomitantly the micro-distribution of vent fauna to each habitat was regulated by the spatial allocation of the vent type and also by
the succession stage of the vents. At the same time, the
Iheya Field was situated on a rather convex topography,
and the faunal composition suggested a rather “open”
ecosystem, contrasting with that of the Minami-Ensei
hydrothermal field which is situated in caldrons and filled
by hydrothermal atmosphere in the topographic depressions.
Anyway, we are looking for compilation of good
ecological description of every vent field including detailed and dynamic descriptions of the life-forms, allocation and adaptation to the special environmental settings,
and the mode of larval dispersal to synthesize with the
compilation of a knowledge of the morphology and systematic lineage of constituent vent organisms.
Acknowledgements
We would like to express to the captain Mr. Ochi
and crew of the tendership Natsushima and the former
commander Mr. K. Danno and diving support team of
Shinkai 2000.
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