In Sagami Bay and Hyiarothermalism at the Minami-Ensei Knoll(MEK)
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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). 詩 ＼ 諜:穿 〆_ The cold-seep community at OHI was concentrated 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