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663
Journal of Oceanography, Vol. 57, pp. 663 to 677, 2001
Keywords:⋅ Hydrothermal vent,⋅ Iheya-Ridge,⋅ Okinawa Trough,⋅
biological commu-nities,
⋅ Calyptogena,⋅ vestimentiferans.
* Corresponding author. E-mail: [email protected]
Copyright © The Oceanographic Society of Japan.
Submersible Observations of the Hydrothermal VentCommunities on
the Iheya Ridge, Mid Okinawa Trough,Japan
SUGURU OHTA1* and DONGSUNG KIM2
1Ocean Research Institute, University of Tokyo, Minamidai
1-15-1, Nakano-ku, Tokyo 164-8639, Japan2Biological 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 Shinkai2000, conducted on June 10 and 11, 1989 and on
May 16 and 17, 1990, several hydro-thermal vents and prosperous
vent associated biological communities were found onthe 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 char-acterized by a relatively thick
blanket of sediments, pleated and/or laminated lavaflows, with
occasional lobate pillows and white and yellow stains. Although no
re-markable shimmering water and thermal anomalies were detected
during the obser-vations, the extraordinarily dense community must
be related to hydrothermal ac-tivities. The community was dominated
by the giant white clam, Calyptogena okutanii,in biomass, and by
Neolepas-type primitive scalpellids and slender
vestimentiferantube-worms in number. The second site, the “Pyramid
Site”, situated only 200 m north-ward from the Calyptogena Site has
typical clear smokers emitting hot water over200°C, and is
characterized by a hard substratum of volcanic rocks and
hydrother-mal slabs. No noteworthy succession was perceived at the
Calyptogena Site over ayear. Many of the vent members occurred at
both sites. However, Calyptogena okutanii,which were confined to
the sediment bottom, Neolepas spp. and larger
vestimentiferantube-worms were found to thrive only at the
Calyptogena Site, being only minor ele-ments in the Pyramid Site.
The global distribution of several groups of organisms isdiscussed
preliminarily in zoogeographical terms based on comparison with
othersubmersible missions and surveys done by surface vessels.
also Okutani et al., 2000; Fujikura et al., 2000). Since1985,
similar communities were found to extend even inthe abyssal depths
on the landward trench slopes and fur-ther to the hadal zone in the
trench axis deeper than 6,000m of the Nankai Trough, the Japan and
Kurile Trenches(Le Pichon et al., 1987; Ohta and Laubier, 1987;
Fujikuraet al., 2000).
In Japan, ecological studies of deep-sea biologicalcommunities
associated with hydrothermalism along theocean-spreading rift
systems started only after 1987, re-lated to a foreign project
surveying the rift systems in theMariana Back-arc Basin (Okutani
and Ohta, 1988; Hessleret al., 1988a; Stein et al., 1988) and the
North Fiji Basin(Auzende et al., 1988).
In 1988 hydrothermal vents and associated biologi-cal
communities were located on the Iheya Ridge and
1. IntroductionIn recent decades many deep-sea
chemosynthesis-
based communities have been reported in severaltectonically and
geomorphologically active settings. Thesea around Japan is
especially noteworthy in this respect,where the Pacific and
Philippine Plates subduct beneaththe Eurasian Plate. In 1984,
prolific deep-sea biologicalcommunities dominated by two giant
clams, Calyptogena(Archivesica) soyoae and C. (A.) okutanii, were
found inthe bathyal zone southeast of Hatsushima Island, SagamiBay,
by a Japanese submersible Shinkai 2000 (Ohta etal., 1987; Sakai et
al., 1987; Hashimoto et al., 1989; see
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664 S. Ohta and D. Kim
Izena Cauldron in the Okinawa Trough (Kato et al., 1989;Kimura
et al., 1989; Tanaka et al., 1989), and we cannow access both
hydrothermal communities on a diver-gent front and cold-seep
communities on the convergentfronts of the earth’s tectonics around
Japan.
After that time, Japanese scientific parties went onto study a
succession of hydrothermal vent ecosystems,such as Minami-Ensei
Knoll (Hashimoto et al., 1995),Kagoshima Bay (Hashimoto et al.,
1993) and volcanicarc on the Izu-Bonin (Takeda et al., 2000), and
in addi-tion the Western South Pacific the Manus Basin, Centraland
South Mariana Basin, and near the triple junction ofthe central
Indian Ocean. However, there are few reportson the general
description of the site characteristics, faunaland ecological
observations of these findings, with theexception of cruise reports
and special items or limitedtaxa. Although recent mainstream
studies are inclined tofollow the genetic lineage of these fauna on
the globalscale, there is also an urgent requirement to prepare
de-tailed descriptions of ecological features, not only thegeneral
characteristics but also the specific features ofeach site to
integrate all of these into a global-scale com-prehension of the
deep-sea chemosynthetic communitiesand the mechanisms by which
species and ecological di-versities are maintained.
The Iheya hydrothermal field in the Mid-OkinawaTrough is one of
the first examples to be described, com-paring its characteristics
with those of the Minami-EnseiKnoll (Hashimoto et al., 1995) and
Izena hydrothermalfield, both situated in the Mid-Okinawa Trough
and sepa-
rated by only several tens or hundreds of kilometers, to-gether
with the intra-field differentiation of the ecologi-cal
characteristics.
Dives Nos. 409 and 410 of the Japanese submersibleShinkai 2000
in 1989, conducted by the ecological groupof the Ocean Research
Institute, University of Tokyo, wereplanned to locate and describe
several types of hydro-thermal vents and vent-associated organisms
at two siteson the Iheya Ridge and to sample basic constituents
ofthe communities. During the dive a new vent communitywas found,
dominated by a giant clams, Calyptogena(Archivesica) okutanii
(undescribed at that time anderected as a new species by Kojima and
Ohta (1997a)),and this was named the “Calyptogena Site”. In
theCalyptogena Site, Calyptogena formed dense beds amongthick
sediment cover with the characteristic faunule of“hydrothermal”
vent communities. No strong indicationof shimmers and bubbling was
observed.
The “Pyramid Site” (200 m to the north of theCalyptogena Site)
is a typical hydrothermal vent fieldcovered by dacitic volcanic
rocks and slabs of hydrother-mal precipitates. It is interesting to
compare the twonearby communities, separated by only a few
hundredmeters, but having different substrata and, as a
result,different community compositions.
A hydrothermal vent is destined to occlude itselfthrough its own
precipitation activity, and this fate is es-timated to occur on the
order of several decades (Laubierand Desbruyères, 1985). It is
therefore expected that thelongevity 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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Hydrothermal Vent Communities on the Iheya Ridge 665
parable to that of the vent itself. Dives Nos. 480 and 481of
Shinkai 2000 on May 16 and 17, 1990 were planned tovisit the same
spots as Dives Nos. 409 and 410 to com-pare and follow the
succession of the vent fields and thebiological communities over a
roughly one-year period.
2. Location of the Sites and EquipmentThe location and contour
map of the site is shown in
Figs. 1, 2 and 3. The sites are situated on a flat bench ofthe
northern slope of the Iheya Ridge at a depth of around1,400 m in
the Mid-Okinawa Trough (27°32.5 ′ N;126°58.5′ E). To the north of
this dense biological com-munity, there is a topographic high of 50
m in relativealti tude composed of volcanic rocks altered
byhydrothermalism (Fig. 3; see Tanaka et al., 1989).
During Dive No. 409 (June 10, 1990) the southernskirt of the
topographic high was surveyed from west toeast along the 1,400 m
contour. Later this area was ascer-tained to be 150 m to the south
of “fissure emanation”found by Tanaka in 1988, and 200 m to the
south of large-scale vent field of the Pyramid Site. Dive No. 410
(June11, 1989) first visited the Pyramid Site and ended at
theCalyptogena Site.
Both of the Dives No. 480 (May 16, 1990) and No.481 (May 17,
1990) were dedicated to revisiting theCalyptogena Site and the
Pyramid Site on the Iheya Ridgeto obtain additional samples and to
observe the succes-sion, if present, of the biological communities
andhydrothermalism itself.
The following equipment was installed on the sub-mersible:
Dive No. 409: stereo still cameras set perpendicularto the
bottom for the description of the communities,baited traps, hand
net, water-tight, heat-insulated samplebox made of PVC, two coring
tubes.
Dive No. 410: marker buoy and still camera, PVCsample box.
Dive No. 480: stereo still cameras set parallel witheach other
and perpendicular to the bottom for the map-ping of the
communities, two small baited traps and adistensible baited trap,
two coring tubes, scraper, Pt-re-sistance temperature probe.
Dive No. 481: small baited traps, hand net, two cor-ing tubes,
temperature probe, opening-closing planktonnet (modified RMT type)
installed on the payload rack ofthe submersible, another
opening-closing epibenthicplankton net attached to the port-side
sample basket(Kikuchi et al., 1990).
3. Survey Results, Observation and Notes
3.1 Calyptogena SiteThe investigation of the vent community was
initi-
ated along the 1,400 m contour line (1,390–1,410 m) fromwest to
east (Fig. 3). Below 1,410 m deep, the biotaseemed to be
depauperate on steep slopes covered bycoarse debris flow, and the
upslope shallower than 1,390m were rocky and/or covered by talus.
Between the depthsof 1,400 and 1,410 m, the bottom was rather flat
and cov-ered by pale, fine-grained sediment.
Synallactidholothurians of body length between 30 and 40 cm
wereencountered frequently. They were skimming the very
Fig. 2. Detailed bathymetrical map of the Iheya Ridge,
Mid-Okinawa Trough.
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666 S. Ohta and D. Kim
surface of the sediment and fed selectively on organicdetritus.
This suggests that the film of free-living chem-osynthetic bacteria
cover the sediment surface. Hydro-thermal elements may contribute
to the composition ofthe sediment, considering the very turbid
water and densesuspension of the ambient water. The density of
deep-seaeel, Synaphobranchus affinis and/or Ilyophis
brunneus,Aldrovandia affinis and chimaerids was apparently
higherthan those expected from the ordinary deep-sea floor
atcomparable depths. These fishes are not included amongthe usual
members of hydrothermal vent communities.However, the density of
these fishes suggests the prox-imity of prey and a low density of
toxic atmosphere.
After a trip lasting one and a half hour along the 1,400m
contour, we came across a place where laminated darkrocks, oblong
pillow lavas and pleated sheets of volcanicswere outcropping. The
texture of the rocks was not of thetypical MORB (mid-oceanic ridge
basalt) lavas, but ap-parently of eruption rocks, considering the
glassy surfaceand characteristic features. Rounded prickly white
glasssponge Pheronema ijimai aggregates on the rock surface,other
species were Venus’s flower basket, Euplectella sp.,large sea
anemones, and sabellid polychaete worms (di-ameter of tubes ca. 5
mm). They dwell within a 100 mradius around the Calyptogena Site,
as mentioned later.Sponges are filter feeders, and the abundance of
suspen-sion feeders suggests ample quantities of suspended mat-ter.
On the sediment we witnessed dead shells ofCalyptogena sp.
Near the Calyptogena Site, huge tangles ofvestimentiferan
tube-worms were found (probably con-sisted of more than 1,000
individuals). Among the slen-der and coiled vestimentiferan
tube-worms, we observed
bresiliid shrimp with milky opaque body color, white eyes,long
rostrum and slender walking legs and hippolytidshrimp with orange
body color, black eyes, short rostrumand stout walking legs (Photo
2). Just outside of thevestimentiferan tangles, rather thick
vestimentiferan tube-worms belonging to the genus Lamellibrachia
were seenprojecting their greenish pink obturaculum and
scarletpetal-like gills. Medium-sized mussels and
Neolepas-typecirripeds covered the rock surface. Among the rock
crev-ices, hagfish Eptatretus okinoseanus were aggregating.On the
skirt of the rocks, deep-sea galatheids Shinkaiacrosnieri Baba and
Williams (1998; Photo 4) and Munidasp. and dead shells of
Calyptogena okutanii were scat-tered.
The density gradient of these vent organisms guidedus gradually
to the “heavenly garden” of clams, mussels,cirripeds and shrimps
named Calyptogena Site (Photo 1).The site was characterized by
relatively thick blanket ofsediments, breccia and pyroclastics,
pleated and/or lami-nated lava flow, with occasional lobate pillows
(the tex-ture of the rocks suggests a rather acidic nature like
dacite)and white and yellow stains. Although no
remarkableshimmering water and thermal anomalies were
detectedduring the observation and sampling of the dense patchesof
organisms, the extraordinarily thick community con-vinced us that
it was fueled by hydrothermal chemicals.
Fig. 3. Track lines of Dives Nos. 409, 410, 480 and 481 on
themulti-narrow beam echo sounder (“Seabeam”) map of therelevant
site. Ps: Pyramid Site, Cs: Calyptogena Site. Solidcircle on the
Dive No. 410 track line indicates the start point.
Fig. 4. Distribution of representative vent organisms in
theCalyptogena Site on the Iheya Ridge, Mid-Okinawa Trough.
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Hydrothermal Vent Communities on the Iheya Ridge 667
Photo 1. The giant clam Calyptogena (Archivesica) okutanii
Kojima and Ohta (1997a) colonizing on the sediment and a stonecrab
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
largevestimentiferan tube-worm. Calyptogena Site.
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668 S. Ohta and D. Kim
Photo 3. Voracious shrimps Lebbeus washingtonianus (Rathbun) and
Alvinocaris longirostris Kikuchi and Ohta (1995) aggregateon the
hydrothermal slabs. Pyramid Site.
Photo 4. Galatheid Shinkaia crosnieri Baba and Williams (1998).
They bear Beggiatoa-type filamentous bacteria on abdominalsurface.
Pyramid Site.
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Hydrothermal Vent Communities on the Iheya Ridge 669
The general configuration of the site was a narrowbench on a
southward dipping gentle slope, where darkgrayish brown volcanic
table rocks are outcropping amongblanket of fine to coarse
sediments. The span of the thriv-ing community was about 5 by 20 m
(Fig. 4).
The community was dominated by Calyptogenaokutanii in biomass,
and in number by two primitivescalpellids (akin to Neolepas spp.
undescribed; Photo 2)and slender vestimentiferan tube-worms. It
must be notedthat the C. okutanii were confined to the sediment
bot-tom, and other dominant members were occupying theoutcropping
surface of the volcanic rocks (Photo 1 andFig. 4). Two kinds of
shrimps (bresiliid and hippolytid:Photo 3), two species of mussels,
deep-sea galatheidMunidopsis sp., stone crab Paralomis sp. (cf. P.
verrilli)(Photo 1) were the representative constituents of the
thickcommunity. A larger vestimentiferan tube-wormLamellibrachia
sp. (Photo 2), hyaline sponges Pheronemaijimai and Euplectella sp.
and the fishes such asAldrovandia affinis, chimaeras and
Synaphobranchusaffinis and/or Ilyophis brunneus) surrounded the
field ofdense patches of organisms.
Three dense beds (2 by 4 m, 1 by 1.5 m and 1 by 1.5m) of
Calyptogena occupied the central portion of thecommunity (Fig. 4).
Almost all specimens were livingones, uniform in length (median 14
cm), and half buriedin the sediment in a vertical or oblique
position showingthe openings of their short, reddish incomplete
siphonsamong the gaping shells (Photo 1). They sometimes ex-pired
water jets. The thickness of the sediment was atmost 10 to 15 cm
among the clam beds, and less than 5cm thick outside the beds,
judging from the probing ofsediment temperature sensor.
The Calyptogena of this site was revealed to be com-mon to the
C. (Archivesica) okutanii originally describedon the specimen
collected in Sagami Bay, and differs fromC. (A.) solidissima
reported from the hydrothermal ventfields of the Minami-Ensei Knoll
(Kojima and Ohta,1997a; see also Okutani et al., 1992).
Hemoglobin-richblood, unusually thick gills, atrophied alimentary
canal,and the mode of occurrence are typical characteristics
ofCalyptogena-group inhabiting the cold seepages ofSagami Bay
(Hashimoto et al., 1987), the Japan and KurileTrenches (Ohta and
Laubier, 1987). Typical coccoidsymbionts in the bacteriocytes of
the gill tissue (Kim,1992), ample crystals of native sulfur (based
on opticalmicroscope observations of the preserved
specimens)suggest sulfur-oxidizing chemosynthetic bacteria.
The density was the closest-pack state, and deadspecimens were
scarce. They dominated in biomass, andthe standing crop was
estimated to be more than 10kg m–2, which is 3 to 4 orders of
magnitude higher thanthe average biomass at a comparable depth of
the ordi-nary deep-sea floor (Ohta, 1983).
On the outcropping rocks of the clam beds, twoundescribed
scalpellids both having an ancient body-planwere crowded (Photos 1
and 2). Slender and coiledvestimentiferan tube-worms (probably
belonging to thegenus Alaysia) were entangled. They were muffled
by“snow” of chemical precipitate and/or bacterial mats andsponges.
These two groups of organisms dominated innumber on the outcropping
rocks.
The giant clams and the tube-worms were sampledwith the
manipulator. Ophiothrichid ophiuroids were en-countered among the
slender tube-worms. Possibly theyare feeding on the detritus
trapped among the bush, ratherthan taking shelter among them.
We could differentiate two species of shrimpsthrough the
porthole of the submersible. During DivesNos. 409 and 410 they
escaped easily from the hand netoperated by manipulator, except
when the clashed clamswere used as a lure. During Dives Nos. 480
and 481, theywere efficiently collected by the baited traps. The
bresiliidshrimp was described as a new species
Alvinocarislongirostris Kikuchi and Ohta (1995) and is very akin
toand congeneric with Alvinocaris lusca Williams andChace (1982)
described on the hydrothermal vents of theEast Pacific Rise about
7,000 km away. The hippolytidshrimp was identified as Lebbeus
washingtonianus. Thisspecies also occurs in the Minami Ensei Knoll
vent fields(Hashimoto et al., 1995); the southern extension of
geo-graphical distribution to the Okinawa Trough and the
oc-currence of the hippolytid shrimp bound tohydrothermalism are
noteworthy (Kikuchi and Ohta,1995).
Amphipods were the first to be attracted to the smalltraps
baited with clam meat. Alvinocaris and Lebbeusfollowed the
amphipods, but they were rather cautious atthe entrance to the
traps. They sometimes stretched theirlegs to grasp the bait at the
entrance, and took out themoiety of the food. However, before long,
the traps werefull of the two shrimps. The density of the shrimps
in thefields was estimated to be 200 individuals per squaremeter,
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 camesomewhat later, walking on their
stout legs. Although theyare not large predators, they are
voracious and swift inmotion, and probably play the role of most
influentialconsumers.
Although the mussels Bathymodiolus spp. are sub-ordinate to the
Calyptogena, they fasten themselves withbyssus to the rock
substratum and also to slender andcoiled vestimentiferan
tube-worms. Two species of mus-sels occurred in this field and are
described anew asBathymodiolus platifrons Hashimoto and Okutani
(1994)and B. aduloides Hashimoto and Okutani (1994). Theshells are
medium-sized (3–10 cm) compared to the con-
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670 S. Ohta and D. Kim
generic species of the East Pacific Rise, the Mariana Back-arc
Basin and the North Fiji Basin. As a member of thefamily Mytilidae
of the order Filibranchia, the gill fila-ment is rather thin and
essentially free. However, eachfilament is unusually wide, and most
of the “gill tissue”is transformed into the bacteriocytes layer
bearing chem-osynthetic bacteria, and this is reminiscent of
theCalyptogena group (Kim, 1992). The gill, however, stillfunctions
as a respiratory and filter feeding organ, andthe alimentary canal
is intact. The mussels can collectflocks of free-living
chemosynthetic bacteria to feed on(Lilley et al., 1983). The high
infection rate of exoparasiticpolychaetes aiming to snatch off the
particles on the gillalso supports this (Miura and Ohta, 1991).
Therefore themussels are mixotrophs depending nutritionally on
theirown alimentary canal and chemosynthetic endosymbionts(Kim,
1992; Kim and Ohta, 2000).
Large fish, Synaphobranchus and/or Ilyophis and astone crab
Paralomis sp. roamed around the site as preda-tors. Two eelpouts
occurred among the dense communi-ties. They seem to be endemic to
this hydrothermal ventfield. They usually lay with their bodies
motionless, andnever showed feeding behavior. They paid no
attentionto the baited traps containing swordfish Cololabis
saira,whereas synaphobranchids and amphipods are attractedquickly
to the baited trap.
A large, yellow-colored stone crab Paralomis sp. cf.P. verrilli
(Photo 1) was captured by the manipulator ofthe submersible.
Carapace width attained 15 cm. In con-trast to the case of
Paralomis jamsteci reported from thehydrothermal vent field of the
Minami-Ensei Knoll(Takeda and Hashimoto, 1990). They are not bound
tohydrothermal vents, but are attracted and accumulate justas in
the case of Paralomis multispina in Sagami Bay(Hashimoto et al.,
1987). However, they must be quiteresistant to the chemical
atmosphere of the vent fields.Although they can expel the shrimps
and pick up and feedon the clams, mussels, cirripedes and
polychaetes amongthe sediment with their stout chelae, their
sluggish mo-tion may not cope with the quick response of the
twoshrimps and amphipods.
A few individuals of deep-sea galatheid with flat ros-trum
Shinkaia crosnieri (Photo 4) were also recognizedin the field, but
they remained almost motionless, and wecould not imagine how and
what items they feed on.
White patches of about 10 by 30 cm were scatteredhere and there
in the field. We tried in vain to collect thewhite stuff with
coring tubes with core catchers. Theywere easily blown off by the
bow wave, because they arenot sticky as in the case of filamentous
bacterial mats.They are probably composed mainly of chemical
precipi-tates such as carbonates, sulfates and/or silicates.
Surrounding the Calyptogena Site, largevestimentiferan
tube-worms, roughly 1 cm in the diam-
eter of distal opening, 30 to 70 cm in total length,
werescattered on the basement rock covered thinly by
coarsesediments. The anterior 1/3 of the tube was held
perpen-dicular to the sea floor, whereas the remaining
posteriorportions were attached among the rock crevices
and/orbeneath the boulders, hence the obturacula occupies be-tween
10 and 20 cm above the bottom. They reminded usof asparagus in the
field. This species does not form agregarious bush as was observed
in the case of thevestimentiferans of Sagami Bay (Hashimoto et al.,
1987;Ohta, 1990b). The highest density was 20 individuals persquare
meter. Pale greenish obturacula projected into thewater column as
slender funnels. This corresponds to the“pistil” of a flower, and
the lamellate gill slits form pet-als of scarlet color (due to the
presence of hemoglobin inthe body fluid). The general configuration
of the “flower”and the occurrence of several distinct
trumpet-shapedcorrugations on the distal portion of their tubes
differen-tiate them from the possibly congeneric species
ofLamellibrachia found in the cold seepages in Sagami Bay(Ohta,
1990b). Most of them bear a few individuals ofsmooth Neolepas-type
cirripeds on the distal portions ofthe tubes (Photo 2). They slowly
but regularly protrudeand draw in the “flowers” at the
openings.
The prickly scalpellid, Neolepas-type sp. A (Photo2; left)
dominated in number. They gregariously covermost of the rock
surface. Another species of smoothscalpellid, Neolepas sp. B (Photo
2; right), as inclined todwell in relative solitude on the rocks or
on the tubes ofvestimentiferan tube-worms. Most of the Neolepas sp.
Boccupied the distal extremities of the tubes. This at
leastsuggests that the cirripede finds its own position after
thegrowth of vestimentiferan tube-worm, and further sug-gests that
they are rather opportunistic in reproductionstrategy. The
cirripeds in the hydrothermal vents areequipped with very long and
fine appendages adapted tocollect the fine flocks of free-living
bacteria and/or bac-terial clots suspended in the ambient seawater
(Newman,1979; Newman and Hessler, 1989; Yamaguchi andNewman, 1997a,
b).
We noticed small ophiuroids walking among the matsof coiled
vestimentiferan tube-worms or on theoutcropping rocks. Also
Ophiura-type ophiuroids oc-curred among the Calyptogena beds. They
are not negli-gible members in number, and the dimensions were
al-most uniform.
Although Dives Nos. 480 and 481 were conductedwith the theme in
mind, i.e., to follow the temporal vari-ation (succession) of the
biological communities at theCalyptogena Site over a year, we could
not notice theslightest difference in the hydrothermalism and the
ventcommunities. The Calyptogena Site seems to be in a sta-ble
climax phase.
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Hydrothermal Vent Communities on the Iheya Ridge 671
Fig. 5. Generalized map showing the central part of the Pyra-mid
Site (after Gamo et al., 1991). Small concentric circlesand squares
denote the location of marker buoys and/or scaledeployed during
submersible dives.
Many ctenophores were noticed during the dives. Wemust take into
account the occurrence of “hydrothermalplankton” in the vent
fields.
3.2 Pyramid SiteThe second site, the “Pyramid Site”, situated
only
200 m northward to the Calyptogena Site (Fig. 2), hastypical
clear smokers and is characterized by the hardsubstratum of
volcanic rocks and hydrothermal slabs.
Dive No. 481 was dedicated to ecological observa-tions and
sampling of the Pyramid Site. Following thenetwork of broad and
white altered streaks and theshimmering of hot water, we approached
the western flankof the vent field (Fig. 5). The Pyramid Site as a
wholeconsists of hard substrata of huge volcanic rocks coveredby
hydrothermal precipitation slabs composed mainly ofcalcium
carbonate and magnesium carbonate (Gamo etal., 1991). In the
eastern part, a pyramidal edifice standsseveral meters high as a
conical tower (Fig. 5). It emitshot water (more than 200°C) mainly
through the smallcentral collapsed cauldron, as well as through the
gaps inthe skirt. Slender and coiled vestimentiferan
tube-worms,probably of the genus Alaysia, mussels
Bathymodiolus,shrimps of Alvinocaris and Lebbeus, galatheids
andeelpouts are the main constituents of the vent openingcommunity.
Dense beds of mussels and small spongesencircled the vent opening
community, and large potato-shaped sponges (5–10 cm in diameter)
populated the pe-riphery. Limpets of 1.5 cm diameter were sometimes
at-tached to the rocks and bivalves.
The most dominant organisms in number wereshrimps, Alvinocaris
longirostris and Lebbeuswashingtonianus (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 mquadrat).
The most dominant organisms in biomass mustbe mats of several
species of sponges. The density of free-living deep-sea galatheid
Munidopsis were around the1/4 of that of the mussels. On the other
hand, largervestimentiferan tube-worms, Calyptogena and
Neolepas-type cirripeds were not witnessed in the Pyramid Site.
Shrimps of unusual appearance were sometimesfound. Body length
reaches 7 to 8 cm (ca. 1.3 times largerthan and about twice as wide
as the ordinary ones), andthey bear “white moss” over 2/3 of the
cephalothorax.They must be full-grown and senile individuals
ofAlvinocaris, considering the body’s pale opaque color,colorless
eyes, long rostrum and relatively slender walk-ing legs. The “moss”
suggests the tufts of filamentousbacteria, but its ecological
implications remain as openquestion due to the failure of sample
collection.
Eelpouts lie among the beds, flexed in sinusoidalfashion.
Sometimes they maintain this posture even inthe water column.
Again, they never pay attention tobaited traps. Possibly they may
be satiated in the densecommunities.
It must be noted that four asteroids Henricia sp. wereseen on
the rocks.
The top and middle portion of the pyramid (a hydro-thermal
chimney) are blurred with shimmering hot wa-ter. The surface of the
pyramidal body was covered withthe mossy tubes of vent-specific
polychaetes, Paralvinellahessleri Desbruyères and Laubier (1989;
see also Miuraand Ohta, 1991).
The galatheids Shinkaia crosnieri gathered gregari-ously on the
diffuse chimney top, hiding almost all of itssurface, where a
rather high temperature gradient and highconcentration of hydrogen
sulfide are expected. Appar-ently they are tolerant to these
environments where noother members except the Paralvinella dare to
invade.They may probably feed on the polychaetes and
“culture”filamentous bacteria on their abdominal surface
(Photo4).
Groups of sabellid polychaetes were observed on theflanks and
overhang of the chimney. The habitat of thesabellid polychaetes is
separated by, say, 1 m from thatof paralvinellids. They are not
exposed directly to thewarm water.
4. DiscussionSince the discovery of hydrothermalism and the
as-
sociation of deep-sea communities along the GalapagosRift in
1977, many interdisciplinary works have beenperformed to explain
the unusual prosperity of the biota.Demonstration of the occurrence
of free-livingchemoautotrophs (Lilley et al., 1983; Jannasch and
Mottl,1985; Johnson et al . , 1986) and symbiosis
withchemoautotrophic bacteria among several large repre-
-
672 S. Ohta and D. Kim
sentatives of the vent organisms (Felbeck, 1981;Cavanaugh et
al., 1981) explained the prosperity, up to20–30 kg m–2 standing
stock (Hessler and Smithey, 1983;Somero et al., 1983; Laubier and
Desbruyères, 1984; Steinet al., 1988). The detailed explanation of
the nutritionalbackgrounds of this kind of ecosystem is ubiquitous
inmany references.
Now we have available data on the biological sam-ples of the
Okinawa vent fields (the Iheya Ridge, the IzenaCauldron and the
Minami-Ensei Knoll; e.g. Hashimotoet al., 1995; Okutani et al.,
2000; Fujikura et al., 2000;Okutani and Fujiwara, 2000), the vent
fields along thevolcanic front of the Izu-Ogasawara Ridge
(KaikataSeamount, Kasuga Seamount, Mokuyo and SuiyoSeamounts; e.g.
Takeda et al., 2000), and the vent fieldsof the back-arc and/or
microplate rift systems (MarianaBack-arc Basin, Manus Basin, the
North Fiji Basin andthe Lau Basin; e.g. Desbruyères et al., 1994)
of the West-ern and Southwestern Pacific areas. A comparison
withthose of the Juan de Fuca Ridge, the East Pacific Rise ofthe
Eastern Pacific (see the review by Tunnicliffe, 1991),together with
the data set of the subduction zones alongthe Northwestern Pacific
(Sibuet and Olu, 1998; Ohta andHashimoto, unpublished data),
demands that we startstudy the global distribution and
generalization of thedeep-sea ecosystems bound to tectonics.
As for the dominant members of the vent fields—i.e.,
Calyptogena, Bathymodiolus, vestimentiferan tube-worms, shrimps,
cirripedes, and polychaetes—morpho-logical work and molecular
phylogeny studies are nowunder way by many collaborators. The amino
acidsequencing of the hemoglobin molecules together withthe
systematic positioning of Calyptogena andvestimentiferan tube-worms
have already been demon-strated (Suzuki et al., 1988, 1989a, 1989b,
1989c, 1990a,1990b), and the systematics and phylogeny
ofCalyptogena and vestimentiferan tube-worms of the West-ern and
South Pacific are now being fixed. Here we fo-cus very briefly on
the problems of zoogeography of thevent and cold-seep organisms
around Japan.
4.1 Notes on biogeography of vent organisms aroundJapanTo start
with the white giant clams Calyptogena, we
can count at least more than 10 species of extantCalyptogena
sensu lato around Japanese waters, rangingfrom several hundred
meters to the depth of 6,800 m(Metivier et al., 1986; see
descriptions and review byOkutani et al., 2000; Fujikura et al.,
2000). Among them,the species from the Minami-Ensei Knoll and
Iheya-Izenafields in the Okinawa Trough are associated
withhydrothermalism, and most of the remainders are recordedfrom
cold seepage areas, and a few undescribed speciesare not related to
active tectonics, as will be discussed
later. However, all Japanese species are collected fromsediment
floor, contrasting strikingly to the Calyptogenamagnifica found
among crevices of basaltic rocks, even-tually without sediment
cover (Hessler and Smithey, 1983;Hessler et al., 1985). The latter
species is reported to takeup hydrogen sulfide through the foot
muscle protrudingdownward into the crevices. However, all
JapaneseCalyptogena together with that found recently in theManus
Basin, Solomon Sea, exclusively inhabit the sedi-ment bottom (Ohta,
unpublished data) and sometimes theyroam about the field with their
pelecypods (Ohta andLaubier, 1987). The Pyramid Site where hard
substratadominate, and volcanism and hydrothermalism are moreactive
than in the Calyptogena Site lacked theCalyptogena. So far, we can
conclude that the giant clamof the western side of the Pacific
needs soft substrata.
Calyptogena are well-known organisms that housesymbiotic
bacteria within their gills (hence, they are seem-ing “producers”),
and they are often the dominant mem-ber of the vent and seep
communities around the Japa-nese waters and along EPR. On the other
hand, in thehydrothermal fields of the back-arc (and/or
microplate)rift systems of the Western and Central Pacific, the
domi-nant members bearing chemosynthetic symbionts wereseveral
species of large gastropods, such as Alviniconchahessleri 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
Ba-sin: Bouchet and Warren, 1991). We must note that thedistance
between the hydrothermal vent fields of theOkinawa Trough and the
rift systems of the Western andSouth Pacific regions is several
thousands of kilometers,whereas the Okinawa Trough is separated
from the EPRby more than ten thousand kilometers, if we do not
in-clude the Calyptogena of the cold-seep areas.
However, considering that Calyptogena is also thedominant
species in the cold-seep fields along subduc-tion zones, and
further they seem not to be obligatorilybound to a well-defined
chemosynthetic environment(Ohta, unpublished data), the pan-Pacific
distribution canbe understood. Recent finding of the occurrence
ofCalyptogena on the Rodriguez Triple Junction in the Cen-tral
Indian Ocean (Ohta, unpublished data) may shed lighton another
possible propagation route.
Two species of the exoparasite polychaetes to thegills of
mussels and clams in the Iheya Ridge vent fieldswere reported as
new to science; Shinkai longipedata andBranchipolynoe pettiboneae.
They have, so far, been re-ported only from the type locality. On
the other hand,paralvinellid polychaete of the Iheya-Izena fields
wasconcluded to be conspecific to Paralvinella hessleri re-ported
from the Mariana Back-arc Basin, the North FijiBasin and the Lau
Basin in the central South Pacific
-
Hydrothermal Vent Communities on the Iheya Ridge 673
(Miura and Ohta, 1991), and the species is even conge-neric with
those recorded along the entire region of theEast Pacific Rise. The
forms of Paralvinella are reportedfrom every hydrothermal vent
field in the world. Theylay off-spring within their tubes, hence
they must behandicapped in their larval dispersal. This mode of
re-production is still puzzling problem for the ecologists(Laubier
and Desbruyères, 1985).
Vestimentiferan and pogonophoran tube-worms arealso famous
representative members of the vent and coldseep fields, depending
for their nutrition on symbioticchemoautotrophs. The phylum
Vestimentifera was erectedin 1981, and so far 5 families and more
than 10 specieshave 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/orspecies, from
hydrothermal fields and cold seepage, willprobably belong to the
genera Lamellibrachia, Escarpia,Arcovestia and Alaysia. The genus
Lamellibrachia wasoriginally reported from supposedly reduced
environmentof cold seepage (Webb, 1969; Hecker, 1985). As a
gen-eral rule the vestimentiferans, at least the adult stage
ofthem, completely lack alimentary canals. However, thespongy
posterior portion of their bodies, the“trophosome”, without
exception, houses symbiotic bac-teria (Kim, 1992).
After the finding of the most ancient form ofcirripedes,
Neolepas zevinae in the vent fields of the EPR,a wealth of
collections of so-called living fossils of thecirripedes was found,
to which Japanese groups contrib-uted greatly. This lead to the
systematics and phylogenyof the cirripedes drastically renewed
(Newman, 1979;Newman and Hessler, 1989; Yamaguchi and Newman,1997a,
b). Neoverruca brachylepadoformis Newman andHessler (1989) from the
Mariana Back-arc Basin, situ-ates the main stock of the
Verrucomorpha, Eoverrucaohtai from the vent fields of the North
Fiji Basin situatesthe main stock dividing Verrucomorpha
andBalanomorpha.
The Neolepas zevinae are based on very limited num-bers of
specimens, and they were a minor group in thevent fields of the
EPR. However, Neolepas-type sp. A andNeolepas sp. B in the
Calyptogena Site of the Iheya Ridgedominated in number and also in
the standing crop, andthey are comparable to those of clams and
mussels (seePhotos 1 and 2). From a geological point of view,
theOkinawa Trough is considered to be an incipient back-arc rift
system just beginning to spread open. The occur-rence of the most
ancient forms of cirripedes already ar-rive at the rift system is
thus astonishing.
We collected Alvinocaris longirostris and Lebbeuswashingtonianus
in the Okinawa Trough. However, wecould not find typical rimicarid
shrimps at these sites in
the Okinawa Trough. The rimicarid group dominates inthe Mariana
Back-arc Basin, in the North Fiji Basin andthe Lau Basin of the
Western Pacific, and along the Mid-Atlantic and Central Indian
ridge systems. Occurrence ofAlvinocaris and no collection of
rimicarid group suggestthe affinity of the fauna of the Okinawa
Trough to that ofthe EPR.
During the successive dives Nos. 411 and 412 in 1989to the Izena
Hole in the Okinawa Trough (ca. 40 km southto the Iheya Ridge)
during the same leg of the submers-ible, we had the chance to
inspect the biological samplesfrom a black smoker. On the ores
collected from the flankof the black smoker, we found undescribed
cirripeds andlimpets. 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 werenot collected from the Izena Cauldron.
Paralvinella hessleri were also found among the oredeposits,
which were famous for very wideeurythermalism, together with very
wide zoogeographicaldistribution, as mentioned before.
No bythograeid crabs have been collected, so far,from both of
the Iheya and Izena fields. Vestimentiferantube-worms occur in both
sites.
Although the Izena Hole fields have a sediment-cov-ered habitat,
rather limited numbers of Calyptogena wereencountered in the Izena
fields. This may due to; 1) anepisodic “explosion event”, which
must occur in the Izenasmoker area (Kato et al., 1989) will
depopulate the lessmobile species like Calyptogena; 2) a high
concentrationof carbon dioxide in the Izena Site as clathrate or
bub-bles will be awkward for Calyptogena; and/or 3) Japa-nese
species group of Calyptogena require rather thicksediment cover as
a reactor bed, where the reduction ofsulfate from seawater will
occur coupled with the inputof methane beneath the sediment,
because all theCalyptogena living in methane-rich environments
have,without exception, sulfur-related symbionts and high
con-centration of native sulfur within their soft tissue. So
far,the study of stable isotope ratios of sulfur favors the
lasthypothesis (Sakai et al., 1990, 1991; Kim et al., 1990).
Apart from the geographical distribution,bathymetrical
segregation of vent and cold seep associ-ates also deserves to be
studied in near future (Kojimaand Ohta, 1997b; Fujikura et al.,
2000).
4.2 Succession of the vent fields and vent faunaNo apparent
change was observed in the biota and
emission of venting fluid in both sites of the Iheya Ridgeover
13 months. The trial of the 1-year continuous re-cording of the
fluid venting activity in the Pyramid Siteof the Iheya Ridge using
temperature probes failed dueto heavy chemical corrosion of the
stainless steel hous-
-
674 S. Ohta and D. Kim
ing of the temperature probe. However, the
temperaturemeasurements of each year showed almost the same
value.
There are several reports to describe the successionof the vent
and vent organisms over a few years (Laubierand Desbruyères, 1985;
Hessler et al., 1985, 1988b; Fustecet al., 1987; Campbell et al.,
1988; Johnson et al., 1988).According to these reports,
vestimentiferan tube-wormsand Calyptogena, both of which are
obligatorily in sym-biosis with chemoautolithotrophs, are climax
phase rep-resentatives; mussels, one of the typical mixotrophs,
canbe both a pioneering species and also the declining
phaseinhabitant. At any rate we have a good field to test
thesehypothesis around the Japanese waters in near future.
4.3 Occurrence of echinoderms around vent fieldsAlthough we
assumed that the dense biological com-
munities, especially those in the Calyptogena Site of theIheya
Ridge are hydrothermal ones, we could not per-ceive the
characteristic shimmering of hot water or gasbubbling as observed
in the typical vents of the Izena andIheya fields (Sakai et al.,
1990). Here temperature probesshowed only slight thermal anomalies
of +0.3–0.5°C com-pared to the ambient seawater temperature of
3.1°C. Thesite may be a field of low-temperature, methane rich
ema-nation often reported on a sediment-covered back-arcbasin
(Ishibashi et al., 1990; Gamo et al., 1991).
Formerly, the echinoderms, in general, were thoughtto be so
sensitive to the chemical constituents of theseawater that they
avoid the vent environments full oftoxic substances such as heavy
metals, hydrogen sulfideand aromatic hydrocarbons (Hessler and
Smithey, 1983;Grassle, 1986). In particular, it is often reported
that evena trace amount of heavy metals disturbs the early
devel-opment. However it is noticed that a group of
brisingidasteroids always encircles the hydrothermal vents as
aperipheral member of vent communities in the MarianaBack-arc
Basin, North Fiji Basin and also in the vent fieldsof N13° (on the
East Pacific Rise). Just on the vent open-ings of the diffuse
hydrothermal vent fields of the NorthFiji Basin and also of the
Manus Basin, a group ofchiridotid holothurian (Trochodota sp.
undescribed) werefound to occur as if they are bond to the vent
openingenvironment. Furthermore, large five-armed
asteroidDistorasterias stichantha were observed to pray on theclam
Calyptogena soyoae and/or C. okutanii in their densebeds and
together with small ophiuroids (Ohta, 1990a),and large synallactid
holothurians were roaming aroundthe Calyptogena bed in the cold
seepage of the TenryuCanyon (Ohta and Laubier, 1987).
Fairly large numbers of echinoderms were encoun-tered in the
hydrothermal vent fields (though not verynear the vents) of the
Okinawa Trough. Taking accountof the fact that the hydrothermal
emission in the Okinawa
Trough is thought to be of gas-liquid separated type be-neath
the sea floor (Sakai et al., 1990), and is also alteredchemically
through the thick sediment, as is usual in theisland-arc type rift
systems (Ishibashi et al., 1990), theemitted water is not always
rich in hydrogen sulfide andheavy metals of crustal origin.
Our frequent encounters with deep-sea eels,Aldrovandia affinis
and chimaeras, that are not bound tohydrothermalism, suggest that
the atmosphere of the Iheyafields are not so toxic to these fishes.
Therefore, the Iheyasite is a rather open ecosystem as compared to
other typi-cal vent fields, such as those of Juan de Fuca
Ridge.
Incorporating additional survey results, the Iheyahydrothermal
field is becoming one of the more inten-sively explored vent
fields, and we can depict detailedgeological and ecological
settings over a fairly wide area,and this ecological description
will offer good basis forthe intercomparison of other typical vent
fields as thoseof the Minami Ensei Knoll (Hashimoto et al., 1995)
sepa-rated only 100 km, and that of the North Fiji
Basin(Desbruyères et al., 1994) separated by several
thousandkilometers.
As a whole, the Iheya vent field has typical island-arc
hydrothermalism, compared to those of the mid-oce-anic ridge
hydrothermalism. It embraces both the hardsubstratum composed of
hydrothermal slabs and mounds,and is also influenced by the thick
sedimentation exem-plified by the chemical composition of the vent
fluid(Sakai et al., 1990, 1991) and the bottom features. Thefield
is a complex of heterogeneous ecological habitatswithin a
relatively short span, and concomitantly the mi-cro-distribution of
vent fauna to each habitat was regu-lated by the spatial allocation
of the vent type and also bythe succession stage of the vents. At
the same time, theIheya Field was situated on a rather convex
topography,and the faunal composition suggested a rather
“open”ecosystem, contrasting with that of the
Minami-Enseihydrothermal field which is situated in caldrons and
filledby hydrothermal atmosphere in the topographic
depres-sions.
Anyway, we are looking for compilation of goodecological
description of every vent field including de-tailed and dynamic
descriptions of the life-forms, alloca-tion and adaptation to the
special environmental settings,and the mode of larval dispersal to
synthesize with thecompilation of a knowledge of the morphology and
sys-tematic lineage of constituent vent organisms.
AcknowledgementsWe would like to express to the captain Mr.
Ochi
and crew of the tendership Natsushima and the formercommander
Mr. K. Danno and diving support team ofShinkai 2000.
-
Hydrothermal Vent Communities on the Iheya Ridge 675
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