Benthos Research Vol.57,No.1:21-30(2002) BENTHOS RESEARCH

The Japanese Association of Benthology

Estimated Population Densities of Megafauna in Two Chemosynthesis

based Communities:a Cold Seep in Sagami Bay and a Hydrothermal

Vent in the Okinawa Trough

Katsunori Fujikura,Jun Hashimoto and Takashi Okutani

Japan Marine Science and Technology Center,2-15Natsushima-cho,Yokosuka, Kanagawa237-0061,Japan

Abstract:This study investigates the population density characteristics of the megafauna at two previously described chemosynthesis-based communities associated with cold seepage at the Off Hatsushima Island site (OHI)in SagamiBay and hyiarothermalismat the Minami-EnseiKnoll(MEK)in the OkinawaTrough. Quantitativesampling was conducted using a submersible,ROVand Deep Tow Camera array near each com munity.Atotal of26and27megafaunal species were recorded at OHIand MEK,respectively.The dominant organismwith respect to populationdensity was the provannidsnail Provanna glabra at OHI.Thisspecies alsooccurred at MEK,butit hada lowerdensity than at OHI.Instead,Cantraineajamsteci was the dominant gastropodspecies at MEK.Thecommunity at MEKwas dominated with respect to populationdensity by the symbiont-containingmussel Bathymodiolus japonicus,but this specieswas not so commonat OHI.Themean populationdensities of almostall megafaunalspecies,except for a few conspicuouslypredominant species at OHIand MEK,aresimilar to thosefor the dominantspecies in otherdeep-sea photosynthesis-based com munities.Bothsites were characterizedby large populationsof only a few dominantspecies(Provanna glabra,Calyptogenaokutanii,and C.soyoae at OHIand Bathymodiolusjaponicus at MEK).This pattern maybe due to the extremeenvironments that occurnear chemosynthesis-basedcommunities.

Key words:cold-seep community,hydrothermal-vent community,megafauna,Minami-Ensei Knoll,Off Hatsushima Island site,population density

chemosynthesis-based communities,quantitative analy INTRODUCTION ses have estimated the population density or biomass of the dominant megafauna,particularly symbiont The advent of submersible research in the deep sea has containing species.These analyses have been done for revealed the existence of deep-sea chemosynthesis-based the Louisiana Slope(MacDonald et al.1990),the Peru communities(i.e.,hydrothermal-vent and cold-seep vian active margin(Olu et al.1996a),the southern Bar communities)in various locations around the world,in bados prism(Olu et al.1996b,1997)and the Juan de cluding along spreading axes,rift systems,plate conver Fuca Ridge(Grahan&Juniper1996).Recently,the gence zones and hydrocarbon seep areas(see reviews by population density and biomass of non-symbiont Sibuet&Olu1998;Tunnicliffe et al.1998;Van Dover containing species,such as polychaetes,gastropods and 2000).To understand the community structure of crustaceans have been estimated at the Juan de Fuca (Sarrazin&Juniper1999)and the Mid-Atlantic Ridges

Received March24,2002:Accepted May14,2002 (Gebruk et al.2000). In Japanese waters,there are many cold-seep

21 Fujikuraet al.

communities,includingthose in the Japan Trench,the MEK,but since the publicationof that study,our knowl Nankai Trough,Suruga Bay and Sagami Bay,and many edge of the taxonomy and ecology of these communities hydrothermal-ventcommunities,including those in the has progressed.The purpose of the present study is to Okinawa Trough and the Ogasawara(Bonin)Islands present new estimatesof population densitiesat OHI and area(see reviews by Fujikura1997;Kojima2002). MEK based on this new knowledge. Afterthe discoveryof a cold-seepcommunity at the Off Hatsushima Islandsite(OHI)in Sagami Bay(Okutani &Egawa1985)and a hydrothermal-ventcommunity at MATERIALS AND METHODS the Minami-Ensei Knoll(MEK)field in the Okinawa Trough(Hashimoto et al.1995a),the taxonomy of Survey fields and diving surveys megafaunal species(Miura1988;Miura&Hashimoto The locationsand contourmaps of the survey areasare 991a,1991b,1993;Okutani&Fujikura1992;Okutani 1 shown in Fig.1.The ocean floorof Sagami Bay is di et al.1992,1993;Hashimoto&Okutani1994;Kojima& vided intotwo oceanicplates by a plateboundary.The Ohta1997;Machida&Hashimoto1999;Kojima et al. westernpart is on the PhilippineSea Plate,and the east 2000;Kikuchi&Hashimoto2000)and the geochemistry ern part is on the North American Plate.The cold-seep of habitats(Sakaiet al.1987;Masuzawa et al.1992; community at OHI is situatedon the Philippine Sea Hashimoto et al1995b;Tsunogai et al.1996)at these Plate.Thesource of fluidseepage at OHI isthought to be two siteshave been well studied.However there have a mixture of seawater,pore water and freshwater been few quantitativeanalyses of chemosynthesis-based (Tsunogaiet al.1996).The hydrothermalvent commu megafauna in Japan.Fujikuraet al.(1995)reported pre nityat MEK issituated on a back-arcbasin in the north liminaryestimates of the populationdensities at OHI and ern partof the Okinawa Trough under extensionalstress caused by tectonicactivity(Kimura et al.1988).Iso 127。301E128。10'E topic ratiosof dissolvedgasses(CO2and H2S)in 28。401N、130。E150。E hydrothermalfluids imply a magmatic origin(Chiba et 〃'、aAM`R'c・ ・ ノ' M」鶯幽 禦 80、 ・ al.1993). The cold-seep community at OHI was concentrated

詩 ＼ 諜:穿 〆_ near1170m depth,and the total community field area o《 ◎ θ'・ 、' was2146.5m2.The seafloor was covered mainly by mud

七1..o宅 。)毛 丁一フ 『 轟ll読3びN and small clasts.Orange deposits covered approximately

'PHILLPP囮NESεAPLATε 亀 !ノ ワ,!'、 m2.Naganuma et al.(1999)suggested 100these orange

28・ 。0,NBu「asianPlate,/・ deposits have high bacterial activity,and Tsunogai et al .

(1996)showed that temperatures are more than10℃

higher below the orange seabed than in ambient water . 35。100N Black layers in the sediment indicative of a reducing e

vironment were attributed to anaerobic bacterial activity.n

At MEK,the observed field was546m2in area and 循 吃 ranged from600-740m in depth .The MEK was a typi cal hydrothermal-vent field covered by volcanic rocks

and slabs of hydrothermal precipitates.Some chimneys 禦壕 were distributed over the seafloor.Water temperature .,。 σ from the vent chimneys was over270℃.Coarse volcanic 34。501N sand and debris surrounded the chimneys.One previous 139。201E139。40「E report on the faunal composition and micro-scale distri ― Plate Boundary Line butions at MEK by Hashimoto et al.(1995a)supple

ments this analysis. Fig.1.(a)Map of Japan showing locations of Sagami Bay Biological surveys were conducted from1989to and the Okinawa Trough.(b)Detailed map showing the loca tion of the hydrothermal-vent community at the Minami-Ensei 1992at OHI(34°59.3-35°00.8′N,139°12 .6-14.1 Knoll(MEK)in the Okinawa Trough.(c)Detailed map show E)using the crewed submersible Shinkai2000,the ′ ROV ing the location of the cold-seep community at the Off Dolphin-3K,and a Deep Tow Camera array(see Hatsushima Island site(OHI)in Sagami Bay .Depths are in www.jamstec.go.jp),and from1988to1992at MEK meters. (28°23.2-24.3′N,127°38-38.8′E)using only the

22 Population densities of chemosynthetic communities

Shinkai2000(Table1).When chemosynthesis-based of species observed in every frame.The number of pho communities were observed during the surveys,we took tographs that contained species in the frame was1,431at 35mm photographs of the communities using the follow OHI and364at MEK.Mean population density(D)was ing cameras:the QI DSC2050A camera of the Shinkai roughly estimated by the following formula: 2000,the PHOTOSEA PS1000A camera of the Dolphin 3K and the BENTHOS MODEL372A camera of the n D(inds./m2)=ƒ°xi/1.5n Deep Tow array.Vehicle positioning was recorded by i=1 the Super Short Base Line(SSBL)method using the where n and xi are the photograph number and the num GPS or LORAN-C. ber of individuals in the frame,respectively.The maxi mum population density of species was calculated from the photograph with the highest number of specimens.In Table1.Summary of survey data from the chemosynthesis the case of small,parasitic or deeply buried animals, based communities at the Off Hatsushima Island- site(OHI) and the Minami-Ensei Knoll(MEK). population densities could not be estimated because these animals could not be counted from photographs.

Community No.of observations Community area(m2)Depth(m)

15(Sinkai2000) RESULTS OHI8(Dolphin-3K)2146.5 830-1230 Off Hatsushima Island site(OHI) 14(Deep Tow Camera) Twenty-six megafaunal species occurred at this site MEK15(Shinkai2000)546 600-740 (Table2).The provannid snail Provanna glabra was the most abundant megafaunal species with respect to mean population density.Relatively high densities of two vesicomyid clams,Calyptogena okutanii and C. Analytical methods soyoae,and a conid whelk,Oenopota sagamiana,were Biological samples were collected either directly using also observed(Table2).P.glabra was distributed most manipulators or with scoop samplers attached to the ma widely and crept over hard substrata including the shell nipulators of the submersible.These samples were par surfaces of Calyptogena and Bathymodiolus bivalves, tially sorted and fixed in10%buffered formaldehyde rock outcrops,and sediments(Fig.2a,c).The maximum seawater solution onboard ship.Megafaunal species population density of P.glabra recorded was1073inds./ were identified both from samples and photographs. m2.Live Calyptogena clams formed densely clustered Mean population density was estimated for the beds among thick sediments,and these clusters ranged in dominant species.We assumed that the population den size from a few to over100m2(Fig.2a).Beds of empty sity of each species did not fluctuate during the study pe clam shells were also observed in and around the live riod.Total numbers of individuals were directly counted clam beds.Calyptogena clams occurred sparsely over the from photographs.Calyptogena soyoae and C.okutanii orange areas of the seabed.Locally,the maximum popu could not be reliably identified to species level using lation density of the integrated Calyptogena species(C. only photographs.This was also true for Bathymodiolus soyoae and C.okutanii)recorded was352inds./m2. japonicus,B.platifrons and B.aduloides.When cold A high population density of the conid whelk seep communities were first discovered at OHI, Oenopota sagamiana was observed concentrated over Calyptogena was treated as a single species,C.soyoae the orange areas of the seabed(Fig.2b).No specimens (Okutani&Egawa1985;Ohta et al.1987;Hashimoto et were observed in any other habitat.Another conid whelk, al.1989).However,molecular and detailed morphologi Phymorhynchus buccinoides,was also limited in its dis cal analyses have also identified a sibling species,C. tribution,aggregating on the shell surfaces of okutanii(Kojima&Ohta1997).The mean surface area Bathymodiolus mussels and on outcrops over the orange covered by each photograph was estimated to be ap areas(Fig.2c).Locally,the maximum population densi proximately1.5m2based on the dimensions of known ties of O.sagamiana and P.buccinoides were219inds./ objects such as the manipulators and sample baskets of m2and63inds./m2,respectively.The limpet the submersibles,and the chain of the Deep Tow Camera Bathyacmaea nipponica and the trochid snail Margarites array.The field area of the chemosynthesis-based com hinkai were representative faunal constituentss at this munity at each field was determined using photographs field and crept mainly on the shell surfaces of

23 Fujikura et al.

Table2.Faunal list of species and their population densities at the cold-seep community at the Off Hatsushima Island site(OHI) and at the hydrothermal-vent community at the Minami-Ensei Knoll (MEK) .+:present,but population density not estimated,-:not present.

Species 。HI(ind翻m識(indsノ ㎡)Species。HI(ind翫 蒜mmuni鴇EK(ind8/㎡)

PolychaetaBivalvia 燃灘 鰯 鷹 旛ll撚 鰯;雛i難i}忽}5あ

∫乃'ηんo'・∫080〃～'8η∫'5+-Bo〃zy〃 ～04'01㍑5sp .-0.1 ∫17'〃んα'n・sp・ 一+ム 〃c'ηo〃1αyo5乃'40'+一 翫 撫 ・伽 ・ 伽 燃 ・+-C・ 励 ・c81〃'遡c'α+一 瀦1朧1欝 翫,、=工 蹴 瀦 器 、}127b=

unidentifiedNautihniellidaespecies+-Colylo'08θ ηα30〃61'53伽o--0 .2 1)αro1〃18〃0118、 ∬18r'一+Cirripedia

ObturataハZ{～016ρo、 ∫sp .1+一

ムo'η81"わ1roc乃'osp・0.3-〈 匹60161フ α、∫sp .2-+

Alyぶ'o-likevestimentiferamO .2-Decapoda

unidentifiedspecies1 ,-1.4A1レ 〃70cor'310η8か05'r'∫0.1- unidentifiedspecies2-05Alv'ηocα グ'5わr8レ 〃8130η'5-0 .4 GastropodaA1り'〃ocαr'51ε 配roん0105-2 .8 」P〃〃c加r8〃oρorレ'〃oわ 〃'3-+乙6わ 加 〃5wo5痂 η8'oη'oη 配5-1 .4

∫8rroolo〃'oレ6～ ・∫'"η6～η'{戸gr'co1α+一 、乙εわわ6〃 ∫sp .+- Bo"7yoc〃70{～o'1ψ ρo〃'cα4 ・1-Porocroη80ηsp .-0 .1

8・ 伽 α η・ω ・8c〃 η4α 一 〇.4unidentifiedByth・9・aeidaespecies-+

ム・1・・'・""〃 ・ ノ… 卿c〃 ・-0 .8Pα …1・ 加 ・ ノ・耐6c'-0.3 ル10rgor"己 ∫ ・∫乃"1ん α'4・2-1)αrolo〃1酌sp .一>0 .1

C・ 〃'禰6・ ノ・襯8d-5 .35伽 た・∫αc・ ・3漉r'-0.3

1)rovoη'κ 」810わ κ0309.90 .1Vertebrate

O{～,～oρo∫05080'η'θ,708.2一 乙yco4a∫sp .1+-

1)17)'〃70"7y〃c1り 〃5わ 〃cc〃 ηo'oF{～・㌢0.3一 乙yco46～5sp .2+- BivaiviaunidentifiedZ・arcidaespecies1+-

Acノ ・・燃 ノ・傭 ・'・'+-unidentifiedZ・arcidaespecies2-0 .1

3y〃21フ 加r〃 ∫c£oz'8η ∫01'5->0.1

Calyptogena clams.Lower population densities of two usually buried approximately20cm below the sediment Bathymodiolus species,B.platifrons and B.japonicus, surface.The commensal polychaete worm Shinkai were found attached via their byssi to rock outcrops. sagamiensis was collected from within the mantle cavity Live mussels formed densely clustered beds of a few of C.soyoae and C.okutanii.Another commensal square meters in area.These mussels were confined to polychaete,Natsushima bifurcata,was collected from only three rock outcrops in the field .The maximum inte within A.johnsoni.An unidentified nautiliniellid species grated population density of the two Bathymodiolus spe was found from within the mantle cavity of Conchocele cies,B.platifrons and B.japonicus,reached366inds ./ disjuncta(T.Miura,personal communication).Two m2.Sprawling tangles of tubes of pogonophorans , species of polychaetes,Protomystides hatsushimaensis Lamellibrachia sp.and an Alaysia-like vestimentiferan, and Nicomache ohtai,were occasionally collected from occurred over the muddy sediments,with their posterior sediments in the Calyptogena beds.Three crustacean ends buried in the sediment or under rocks .Sometimes species(an unidentified Neolepas barnacle,Alvinocaris both species were found at the same microhabitat .The longirostris and Lebbeus sp.)occurred on one rock out maximum population densities of Lamellibrachia sp .and crop surface covered with grayish-white deposits.The the Alaysia-like vestimentiferan recorded were20inds ./ population densities of these deep-buried clams, m2and101inds./m2,respectively. polychaete worms and crustaceans could not be esti Some shells of the bivalves Acharax johnsoni , mated. Lucinoma yoshidai and Conchocele disjuncta were pre Buccinid whelks,Buccinum soyomaruae and sent among the Calyptogena beds .Live clams were Neptunea acutispiralis,and an anomuran crab,

24 1%磁 姦 聡b攣 .繕漸記

ジも羅 鑛醗il灘勢 ごlllぐ甥 、、 .じ'『c認礎 雛無 鋼蟻羅 欝畢欝 讐 こ/一諜∵ 避 露'ジ:∵鑑回 ,

'・ ・メ,瓢離 。 ・4・ひ 識 隔 ・ド 霞 細 噸 塁.∴ ン 靴..議 塁きご 憾 驚 、 ♂ヂ 馨 ・ζ=一一ヅ・逐∵ こ ㌧ 義 鞭/ ,

Population densitiesFig.2.Chemosynthesis-based of chemosynthetic communities communities at the Off Hatsushima Island site(OHI)and the Minami-Ensei Knoll(MEK) .The mean surface area in each photograph was approxi mately1.5m2.All photos taken by the Shinkai2000.(a)Distribution of live provannid gastropods,Proyanna glabra(arrow) ,and two species of vesicomyid clams,Calyptogena soyoae and C.okutanii,at the cold-seep site at the Off Hatsushima Island site(OHI)in Sagami Bay.Provanna glabra aggregated on the sediments and shell surfaces of Calyptogena soyoae or C.okatanii.(b)The orange seabed near Calyptogena aggregations at the Off Hatsushima Island site(OHI)in Sagami Bay .Note the large aggregation of turrid gastropods, sagamiana(arrow).(c)A dense biological community composed of the turrid gastropod Phymorhynchus buccinoides(arrow1) ,small brown provannid gastropod OenopotaProvanna glabra(arrow2),and mytilid bivalve Bathymodiolus japonicas(arrow3)on rock outcrops within the orange seabed at the Off Hatsushima Island site(OHI)in Sagami Bay.Numerous whitish egg cases(arrow4)of Phymorhynchus buccinoides were found attached to the surface of Brathymodiolus japonicas .(d)Distribution of live turbinid gastropods.Cantrainea jamsteci(arrow1),near the filamentous bacterial mat(arrow2)areas on sediments associated with hydrothermal vents at the Minami-Ensei Knoll(MEK)in the Okinawa Trough . Fujikura et al.

Paralomis multispina,were encountered frequently in Thirty-seven specimens of M.enseiensis were found in and around the Calyptogena beds. specimens of the undescribed Bathymodiolus species. The nautiliniellid polychaete Shinkai sp.has been ob served in the mantle cavity of Calyptogena solidissima Minami-Ensei Knoll(MEK) (T.Miura,personal communication). A total of27benthic species were recorded here(Table 2).The integrated population density of two Bathymodiolus mussel species,B.japonicus and B. DISCUSSION aduloides, was very high(Table2).Locally,the maxi- mum population density values for these mussels Deep-sea chemosynthesis-based communities are consid reached644inds./m2.These mussels formed dense ag ered widely to have richer concentrations of biomass and gregations and were found attached via their byssi to higher densities of megafauna than other deep-sea areas rock outcrops in the vicinity of the vents.The population (Gage&Tyler1991).The mean population densities of density of B.japonicus was probably larger than that of Provanna glabra,Calyptogena soyoae and C.okutanii at B.aduloides, because the ratio between B.japonicus and OHI,and of Bathymodiolusjaponicus at MEK were very B.aduloides from collected specimens was13:2.The high,with values greater than10inds./m2(Table2). turbinid snail Cantrainea jamsteci also occurred at rela However,other megafaunal species occurred at densities tively high densities.C.jamsteci crawled over the shell from0.1to8.2inds./m2.Ohta(1983)estimated the surfaces of Bathymodiolus mussels and was especially mean population densities of benthic animals from concentrated near filamentous bacterial mat areas on the photosynthetically-based communities between950and sediments(Fig.2d).The maximum population density 1340m depth in Suruga Bay based on photographs taken of C.jamsteci was127inds./m2. by an underwater camera unit.Mean population densities Two species of limpets,Bathyacmaea secunda and of the holothurian Peniagone japonica were conspicu Lepetodorilus japonicus,and four species of crustaceans, ously higher(7.9inds./m2)than those of other benthic Alvinocaris leurokolos,Lebbeus washingtonianus, megafauna(less than0.1inds./m2).According to data Paralomis jamsteci and Shinkaia crosnieri,were com from the U.S. crewed submersible Alvin during surveys monly found on and within the Bathymodiolus aggrega of the continental slope south of New England from500 tions, and these population densities ranged from0.3to to1800m depth,almost all megafaunal species occurred 2.8inds./m2(Table2).Two species of bivalve, at low population densities of less than0.02inds./m2 Calyptogena solidissima and Bathymodiolus sp.,formed However,three megafaunal species(the sea urchin aggregates in the coarse sediments.Provanna glabra, Echinus affinis,a cerianthid sea anemone,and the brittle which was the dominant species at OHI,was present at star Ophiomusium lymani)were present at high density much lower population densities,approximately1/3000 values of0.3,0.8and2.4inds./m2, respectively(Grassle of those at OHI.P.glabra crawled over the shell sur et al.1975).Except for a few predominant species at faces of Bathymodiolus mussels. OHI and MEK,almost all of the megafaunal species at Population densities of the primitive barnacle these two chemosynthesis-based communities had simi Neolepas sp.,the limpet Puncturella parvinobilis,and lar population density values to the dominant species in four commensal polychaete species,Mytilidiphila deep-sea photosynthetically-based communities at simi okinawaensis,M.enseiensis,Brachipolynoe pettiboneae lar depth. and Shinkai sp.,could not be determined because the The most dominant animal with respect to popula number of individuals could not be counted in the photo tion density was Provanna glabra at OHI(Table2).At graphs.Only six specimens of Neolepas sp.were col MEK,P.glabra was not abundant,however,the gastro lected from the shell surfaces of C.solidissima and only pod Cantrainea jamsteci was present at relatively high four specimens of P.parvinobilis occurred on the shell population densities.P.glabra has a taenioglossate surfaces of Bathymodiolus mussels.The commensal radula(Okutani et al.1992)indicative of omnivory polychaetes Mytilidiphila okinawaensis and and/or scavenging.In contrast with P.glabra,C.

Brachipolynoe pettiboneae were collected from within jamsteci is a so-called•gspecialist•hgrazer based on its the mantle cavities of Bathymodiolusjaponicus and/or B. rhipidoglossate radula(Okutani&Fujikura1990).Bac aduloides.Another commensal polychaete,Mytilidiphila terial mats and films are formed on the surfaces of sub enseiensis, was found within the mantle cavity of an strata such as debris,rocks and the shells of undescribed Bathymodiolus species buried in sediments. Bathymodiolus mussels. C.jamsteci was concentrated

26 Population densities of chemosynthetic communities

Table3.Population densities of different species at other chemosynthesis-based communities .

SpeciesC・mmunityfieldTypepoP糊 鼎sityS・urce

騰 鼎 一1∫3誰v・urSegmenUuandeFucaVent32-4259Sarrazin&Junipeち1999

B鷺"細5孟 ∫ 髄eεvou「SegmenちJuandeFucaVent305-108226Sa「 「azin&Junipeち1999

Colyρ'086η αsp.PeruvianactivemarginSeep39 .501uetal.,1996a

Colyμ08ε ηαsp.BarbadosaccretionaryprismSeepO .007-0.10501uetal.,1997

VesicomyidclamAMiddleValley,JuandeFucaRidgeVent5Grahan&Juniper ,1996

VesicomyidclamBLogatchevarea,Mid-AtlanticRidgeVent3-4Gebruketal .,2000

βo∫加 〃ω4'01配5sp.ASouthernBarbadosprismSeep139 .501uetaL,1996b

Bo'勿 〃ω4∫01〃5sp.BSouthernBarbadosprismSeep175-37001uetal .,1996b

Bo'乃 遡04'01配3sp.CLouisianaSlopeSeep467-829MacDonaldetal .,1990 Crustacea

λ1v'ηocor'∬p.Logatchevarea,Mid-AtlanticRidgeVent1-5Gebruketal ,.2000

approximately4times the maximum value of P.glabra

at OHI.The population densities of vesicomyid clams

varied from0.007to39.5inds./m2 among different com

munity fields.The ratio between C.okutanii and C.

soyoae in the OHI community is approximately2.87:1

(K.Ito,unpublished data).To calculate the mean popu lation densities of the two Calyptogena species at OHI,

we applied this ratio,and the resulting densities for C.

okutanii and C.soyoae were94.6and33.0inds./m2,re

spectively.These values are higher than those from other

community fields,such as at the Peruvian active margin, the Barbados accretionary prism,the Middle Valley in

the Juan de Fuca Ridge and the Logatchev area in the

Mid-Atlantic Ridge.Henry et al.(1992),Lallemant et

al.(1994)and Sibuet&Olu(1998) suggested that bio near the filamentous bacterialBathymodiolus mat areas japonicus on sediments contains and symbiontsalong the shell that surfaceslogical of Bathymodiolusproduction at cold mussels seeps whereis related filamentous to the intensity bacteria also occur.This suggests that chemosynthetic bacteria provide an important food source for C.jamsteci.The conid whelk Oenopota sagamiana had a relatively high population density at OHI and was concentrated over the orange areas of the seabed.The radula of O.sagamiana is toxoglossate(Okutani&Fujikura 1992).Generally,the feeding habits of gastropods with this radula type are carnivorous.Based on observation of the gill tissue of this whelk,however,Endow et al.(1992)suggested that it performs intracellular digestion of material filtered from ambient water.We have no data on the relationship between feeding habits and the orange-colored deposits.

are associated with methanotrophic bacteria,and thus the of the fluid flux from the sea bottom.Calyptogena clams

distribution of B.japonicus is strongly influenced by the contain intracellular symbiotic chemoautotrophic sulfur

methane concentration in vent and seep areas(Fujiwara oxidizing bacteria in their gills(Nelson&Fisher1995). et al.2000).The community at MEK was dominated by At cold-seep fields,hydrogen sulfide in the pore water of B.japonicus with respect to population density,but this sediments is produced by sulfate reducers(bacteria) mussel was not as common at OHI.The methane con using the sulfate from overlying bottom water and meth

centration ranged from0.029to2.5mmol/kg above the ane(presumably of microbial origin)(Masuzawa et al.

seabed at OHI(Tsunogai et al.1996),and from2.63to 1992).Concentrations of hydrogen sulfide ranging from

7.0mmoles/kg at MEK(Chiba et al.1993).The higher 0.05to0.6mmol/kg are suitable for C.soyoae,and con

methane levels at MEK may support the higher popula centrations over1mmol/kg are detrimental(Hashimoto

tion densities of B.japonicus. et al.1995b).Methane flux of fluid seepage was esti Megafaunal population densities at other mated at less than10 12•}1mol/year at OHI by geochemical chemosynthesis-based communities are listed in Table3. analysis(Tsunogai et al.1996).To understand the fac The maximum density recorded for Provanna variabilis tors controlling population densities of C.soyoae,it is is4259inds./m2 at the Endeavour Segment on the Juan important to accumulate data on the methane flux and

de Fuca Ridge(Sarrazin&Juniper1999),which is production of hydrogen sulfide by sulfate reducers.

27 Fujikura et al.

A few species conspicuously predominated in abun Off Hatsushima site,Sagami Bay,Japan.JAMSTEC Journal dance during the present study;they involved Provanna of Deep Sea Research,11:227-241(in Japanese with Eng glabra,Calyptogena okutanii,and C.soyoae at OHI,and lish abstract). Bathymodiolus japonicus at MEK. Whittaker(1975) Fujiwara,Y.,K.Takai,K.Uematsu,S.Tsuchida,J.C.Hunt suggested that conspicuously large populations of only a and J.Hashimoto 2000.Phylogenetic characterization of few species are characteristics of biological communities endosymbionts in three hydrothermal vent mussels:influ under severe environmental conditions.This is the case ence on host distributions.Marine Ecology Progress Series, for chemosynthesis-based communities,since these are 208:147-155. sites where hydrogen sulfide,methane,heavy metal con Gage,J.D.and P.A.Tyler 1991.Deep-Sea Biology:A Natural centrations,and extreme fluctuations in water tempera History of Organisms at the Deep Seafloor.Cambridge Uni ture(particularly at hydrothermal-vent fields)occur versity Press,New York,504pp. (Lalou&Brichet1982;Juniper&Sibuet1987; Gebruk,A.V.,P.Chevaldonn,T.Shank,R.A.Lutz and R.C. Tunnicliffe et al.1990;Lutz &Kennish,1993;Massoth Vrijenhoek2000.Deep-sea hydrothermal vent communities et al.1994).The conspicuousness of only a few of the Logatchev area(14•‹45•ŒN,Mid-Atlantic Ridge):di megafaunal species may reflect the severe environment verse biotopes and high biomass.Journal of the Marine Bio at chemosynthesis-based community fields. logical Association of the United Kingdom,80:383-393.

Grahan,A.J.andS.K.Juniper 1996.Clam distribution and

Acknowledgments.We would like to express our sin subsurface hydrothermal processes at Chowder Hill(Middle cere gratitude to Drs.Hiroshi Hotta and Yoshihiro Valley),Juan de Fuca Ridge.Marine Ecology Progress Se

Fujiwara(JAMSTEC).We wish to extend our thanks to ries,130:105-115.

Drs.Dhugal Lindsay(JAMSTEC)and James Hunt Grassle,J.F.,H.L.Sanders,R.R.Hessler,G.T.Rowe andT. (University of New England)for editing assistance and McLellan1975.Pattern and zonation:a study of the bathyal their useful comments,and Drs.Kenji Ito(National Ag megafauna using the research submersible Alvin.Deep-Sea ricultural Research Center)and Tomoyuki Miura Research,22:457-481. (Miyazaki University)for providing useful information. Hashimoto,J.and T.Okutani1994.Four new mytilid mussels Also,we are indebted to the operations team of the associated with deep sea chemosynthetic communities crewed submersible Shinkai2000and the ROV Dolphin around Japan.Venus,53:61-83

3K,and the captains and crews of the research vessels Hashimoto,J.,S.Ohta,K.Fujikura and T.Miura1995a.

Natsushima and Kaiyo.Finally,we thank the reviewers Microdistribution pattern and biogeography of the for comments that improved the manuscript. hydrothermal vent communities of the Minami-Ensei Knoll

in the Mid-Okinawa Trough,Western Pacific.Deep-Sea Re

search I,42:577-598.

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