MARINE ECOLOGY PROGRESS SERIES Published December 14 Mar Ecol Prog Ser

The deepest chemosynthesis-based community yet discovered from the hadal Zone, 7326 m deep, in the Japan Trench

Katsunori ~ujikura'l: Shigeaki ~ojima~,Kensaku ~arnaki~,Yonosuke ~aki~, James Hunt1, Takashi Okutani4

'Japan Marine Science and Technology Center, 2-15 Natsushima-cho. Yokosuka, Kanagawa 237-0061, Japan ' Research Institute, University of Tokyo, 1-15-1 Minamidai. Nakano-ku, Tokyo 164-8639, Japan 3Faculty of Humaniiies and Socieb Science, Iwate University, 3-18-34 Ueda, Morioka-shi, Iwate 020-8550, Japan 4College of Bioresource Sciences, Nibon University, and Scientific adviser at Japan Marine Science and Technology Center, 1866 heino. Fujisawa-shi, Kanagawa 252-8510. Japan

ABSTRACT: A dense commnrty of benthc was discovered by the Japanese ROV 'Kaiko' in the hadal Zone near the bottamaf the Japan Trench, 7326 m deep. The community was dorninated by a new of thyasind halve Maorithyas hadalis. This comrnunity appears to be sustained by chemosynthesis (nutrients bbemg derived from reduced compounds within the sediment) for reasons including: a high concentrath of sulfur contained in the thyasind giils; the existente of numerous bac- tena-like particles in the gdi üssues; a suiiide smell from soft body parts and from coiiected sediment; the anoxic and high mdecontent of the sediments suggested by the dark gray to black color; the occurrence above geologic laults. Thyasinds have also been collected frorn other chemosynthesis- based communities aroud Japan, including the Japan Trench, Sagami Bay and the Nankai accre- tionary pnsm. Until now, the deepest chemosynthesis-based community known occurred on the Sanriku Escarpment of thb Japan Trench, 6437 m deep. This site was dominated by the vesicomyid clam Calyptogena phaseaüTarpuarPUs.The discovery of a chemosynthesis-based comrnunity dominated by thyasirid clams from even cbeper waters suggests a wider vanety of chemosynthesis-based cornmuni- ties exists throughout deep-sea trenches. Additionally, the thyasind species discussed here differs from other thyasinds by having drcular symbionts which appear to exist intracellularly, and by living exposed above the sediment snuiace.

KEY WORDS: Chemosynth&s-based comrnunity . Thyasirid clams . Hadal Zone. Japan Trench

INTRODUCTION Suess et al. 1998), groundwater seeps (Hecker 1985, Barry et al. 1996), hydrocarbon seeps (Kennicutt et al. The discoveries of chemosynthesis-based communi- 1985) and shallow rnethane seeps 150 m deep (Dando ties at both areas and hydrothermal fields et al. 1991). Seep communities typically have high bio- has enhanced our understanding of deep-sea ecosys- mass. Dominant megafaunal species differ from site to tems. Deep-sea cold seep communilies have been site though they are often in the Same families and observed in 24 areas worldwide at depths ranging genera. Descriptions of these communities have been from 400 to 6000 m (Sibuet & Olu 19%31. Examples in- based primarily upon video and still camera images clude zones (Okutani & Egswa 1985, Kulm and/or -obtained specimens. Chemosyn- et al. 1986, Juniper & Sibuet 1987, OIU et al. 1996a,b, thesis-based communities are restricted to areas where sulfide and/or rich water seeps out along geologic faults. The food webs of and cold seep communities rely on the production

Q Inter-Research 1999 Resale of fuii article not permitted 18 Mar Ecol Prog Ser 198): 13-26.1999

35'

40°N

32'

Fig. 1. Bathyrnetnc mapaad location of the 35ON OE ROV 'Kaiko' dive # 87 (A) in the Japan Trench, off northern Japan. Locations of other cold seep comnuinities are also shown. 1: Kuriie Trench, 2: landward slope, Japan Trench, 3: Sannku Escarp- ment, Japan Trench. 4: Kashirna Sea- mount. Japan Trench, 5 off Hatsushirna Island site, Sagami Bay.6: Okinoyama 125"E 130°E Bank, Sagarni Bay, 7: off Toi Town site, Suruga Bay, 8: Ryuyo Submarine Canyon, Nankai accretionary piism, 9: Tokai Thrust, Nankai accretionary pnsm. 1Q. Dai-ni Tenryu Submarine Canyon. Nankai accre- tionary prism, 11: Yukie Ridge, Nankai accretionary prism, 12: Dai-ssn Teciryu Submarine Canyon, Nankai accretionary prism, 13: Tenryu Submarine Canyon, Nankai accretionary prism. 14: Zen* kee, Nankai accretionary pnsm. 15: Dai-ichi Minami Muroto Knoll, Nankai accretionary pnsm, 16: Kuroshha Knoii, Ryukyu Islands area

of organic matter via bacterial chemoautolithotrophy slope of Ihe Japan Trench at a depth of 6437 m (Ogawa (Jannasch & Mottl 1985). The megafauna living at cold et al. 1996) (Fig. 1). Distribution of chemosynthesis- seeps is generaiiy considered closely related to mega- based co~mmunities in the hadal Zone is very difficult fronl hydrothermal vent communities. The to sscestain, as submersible dive time is limited and principal bivalve families from chemosynthesis-based expensive. Thus each new chemosynthesis-based communities are Vesicomyidae, , Solemyi- community discovered provides valuable data for com- dae, Lucinidae, and Thyasindae. Members within panng faunal compositions between chemosynthesis- these families have symbiotic in their gills based communities. This is important in order to under- (Fiala-Medioni & Felbeck 1990, Fisher 1990, Nelson & stand the biogeography and biodiversity of chemo- Fisher 1995). synthesis-based communities in the . Dunng The Japan Trench was formed by the subduction of a recent survey using the ROV 'Kaiko' in the Japan the beneath the North Amencan or Trench, a dense aggregation of thyasirid clams was (Seno & Sakurai 1996). A broad-based discovered at a depth of 7326 m.' The purpose of this study of geologic and biologic features within the paper is to present this chemosynthesis-based commu- Japan Trench was started under a Japan-French coop- nity as the deepest in the world to date. erative program (the KAIKO Project) using surveys from the French manned submersible 'Nautile' (KAIKOI 'During the review process of this paper, several aggrega- Research Group 1986, Juniper & Sibuet 1987, Ohta & tims of Maorithyas hadalis were found at sliyhtly deeper Laubier 1987, Sibuet et al. 1988). The deepest chemo- sites, 7336 m and 7434 m, near the present study area using the ROV 'Kaiko'. Although these comrnunities could not be synthesis-based community known prior to this report fully investigated for this manuscnpt, initial observations was discovered during a dive of the Japanese sub- suggest lbese communities were similar in nature to those mersible 'Shinkai 6500', on the Sannku Escarpment reported here Fujikura et al.. Deepest chemosynthesis-based community 19

MATERIAL AND METHODS at 7326 m (Fig. 2). The cornrnunity was dominated by a single thyasirid species Maorithyas hadalis (Okutani et A sea floor survey near the bottom of the northern al. 1999) (Fig. 3). Aggregations were circular, approxi- Japan Trench was conducted on August 5, 1998, mately 0.7 to 1 m in diameter. Three aggregations using the ROV 'Kaiko' of the Japan Marine Science were observed in the survey area, which was approxi- and Technology Center. The dive site was located at mately 400 m2.The total numbers of living bivalves in 40" 02.85' N latitude, 144" 16.50' E longitude, and the each aggregation were approximately 50, 80 and 100. bottom depth ranged from 7300 to 7330 m (Fig. 1).This Burrow trails of bivalves were not present in or around site was chosen after a multichannel seismic survey the thyasirid aggregations. Almost all living specimens suggested the existence of a geologic . Observa- were buried with about 20% of shell exposed in a ver- tions were made with a high quality color TV camera tical position in the muddy sediments. Dead or dis- (PHOTOSEA W3IOOXD), 3 additional TV cameras solving shells were scattered sparsely within the aggre- (PHOTOSEA TV2200XD), and 35 mm photographs gations (Fig. 2A). Two living specimens and many (SUBSEA OFFSHORE LIMITED ABERDEEN CI801T). disarticulated shells were collected. Most of the col- Positioning of the 'Kaiko' was recorded by a Super lected M. hadalis were of the Same size, approxi- Short Base Line (SSBL) using a GPS. Bivalve speci- mately 3 cm in shell length. The thyasirid aggregations mens were collected by the ROV manipulator Water occurred on a muddy seafloor made of soft debns temperature and salinity were measured by a CTD deposited by downslope movements or by bottom cur- (SEABIRD SBE-9). The presence of symbiotic bactena in the gills of the bivalves was deter- rnined by transmission electron micro- scopy (TEM). Gilis were excised on board ship and sliced into 2 mm sec- tions in a droplet of 8% glutaralde- hyde, and preserved in cacodylate buffer. Thin-sections were viewed by TEM (JEOL JEM-1210). Gill tissue was frozen at -80°C for sulfer analy- sis. After defrosting, the tissue was diced into 3 mm sections and desic- cated at 60°C for 12 h, and coated by carbon vapor deposition. Gill tissue was not cntical point dried. Elemental analyses were conducted using an Energy Dispersive X-ray Spectro- meter (OXFORD LINK ISIS 200I), and detector (JEOL SUPERMINI CUP). , Thyasirid mantle tissue, gill tissue from the shallow-living short-necked clam Ruditapes phiiippinarum which lives in non-chemosynthesis-based communities, and gill tissue contain- ing symbiotic bactena from the vesi- comyid clam Calyptogena soyoae were also analyzed to compare the composition of dominant inorganic elements (mainly sulfur) of bivalves from the Japan Trench.

RESULTS Fig. 2. (A) Photograph of an aggregation of Maorithyas hadalis (Thyasiridae), During the surveY, 3 dense bi- showing dead sheiis scattered over the bed. (B)Video still of black sediment sus- valve aggregations were discovered pended after core sampling. The blackish color suggests a reducing environment 20 Mar Ecol Prog Ser 190: 17-26, 1999

and Scotoplanes-like holothunans were also common outside the thyasind aggregations. The densities of the Eurycopidae-like isopods and ihe Scotoplanes-like holothurians within the thyasirid aggregations were higher than without the aggregations. Vesicomyid clams, spp. (Mytilidae) and vestimen- tiferan (Pogonophora) tube worms, which are the pnn- cipal members of most chemosynthesis-based commu- nities, were not found within these Maorithyas hadalis dominated communities. TEM analysis of Maonthyas hadalis gill tissue showed densely packed assemblages of round bacte- ria-like particles, 0.3 to 0.7 microns in diameter, within vacuoles in cells of the gill tissues (Fig. 4). The distrib- ution of these assemblages was not even, but rather concentrated near the ciliated gill epithelium. Elemental analysis showed sulfur (K = 2.3 KeV) was the dominant inorganic element in the gill tissues of Maorithyas hadalis (Fig. 5). The sulfur peak in M.

Fig. 3. Maonthyas hadalis (Thyasindae) coiiected from the Japan Trench, 7326 m deep. Scale divisions = 1 mm. (A) Ex- ternal surface of a valve. (B)Inner surface of a valve rents. A rock (approx. 40 cm in diameter) was in the Center of 1 aggregation (Fig. 2A). The surface layer of mud near the thyasind aggregations was olive-gray colored, while the surface Sediment within the thyasind aggregations was dark gray to black (Fig. 2B), sug- gesting an anoxic environment with high sulfide levels. No bacterial mats were Seen in or around the thyasirid aggregations. A sulfide smell was easily de- tectable from the soft parts of M. hadalis and collected Sediments. In situ water temperature was 1.7g°C; sal- inity was 3.47 %. Within the thyasirid aggregations, there were 2 tubes ap~roximatelv- - 1 cm in diameter and 8 cm in lenath." Fig. 4. Gill section (TEM) of Maonthyas 11adaljs (Thyasindae). *hece were presumably , *dditionally, there (A) ba: particles resembling bactena; bc: structures similar to bactenocytes; mv: microvilli on the external surface of the to7 Cm were ~ur~copidae-likeisopods approximatel~ ~~11.White scale bar = 0.5 pm. (B) mb: membrane bound in length, and Scoto~lanes-like holothurians a~Proxi- vacuoles A structure sirnilar to bactenocytes is surrounded by mately 8 to 9 cm lony. The Eurycopidae-like isopods a membrane Fujikura et al.: Deepest chemosynthesis-based cornrnunity 21

3000 Gill of Thyasiridae gen. sp. Gill of Calyptogena soyoae I

3 1 2000- , e* - C 7. - z d - P - ?

* ,,,,1,,,,1,,"1""L-- II I II IIIIIl 0 „„l",,l,,"l,,,,IIIII~IIII 0 2 4 6 0 2 1 6 E\ERG\(kc\) E\ERCi'(krV) 3000 ,

C 1 Gill of R~rdirupesphilippinururn Mantle of Thyasiridar gen. sp.

2UOO- - --1 d 3-1 " 1000+ hiNa;iE 3,,) P ('I - \,"I 5 n 1'11 i 0 "," ",lr,rIIIIIr 0 2 J 6 C 2 4 6 ESERGY(kcVJ EYERCl (kc\ )

Fig. 5. Elemental composition analysis. as determined by an Energy Dispersive X-ray Spectrometer. (A) Güis from thyasind clam Maorithyas hadalis coilected from the Japan Trench. 7326 m. (B)Gills of the vesicomyid clam, Calyptogena soyoae collected from a deep-sea chemosynthesis-based community in Sagami Bay, 1200 m. (C) Gills from the short-necked clam, Ruditapesphilippinarum collected from a non-chemosynthesis-based community area in shallow water. (D)The rnantle of thyasind clam M. hadalis hadalis was at almost the Same level as that in Calyp- nec 1988). Thus the particles in M. hadalis appear togena soyoae collected from a bathyal chemosynthe- more similar to symbionts in C. phaseoliforrnis than to sis-based community in Sagami Bay, and both species other thyasirid species; (3) a (H,S) had much higher sulfur peaks than that of Ruditapes smell was easily detected from the soft parts of M. philippinarum collected from a non-chemosynthesis- hadalis and collected sedirnents; (4) the sediments based community. Conversely, the sulfur peak from within the aggregations were locally dark gray to mantle tissue of M. hadalis was very low. black, suggesting an anoxic and high sulfide environ- ment; (5) many species belonging to the family Thya- siridae have symbiotic bactena in their gills (Dando DISCUSSION & Southward 1986, Southward 1986, Distel & Wood 1992); (6) thyasirid bivalves have been reported from Dense aggregations of the thyasind clam, Maori- cold seeps at vanous depths along active and passive thyas hadalis, were found at depths greater than 7300 m margins of the Atlantic, East Pacific and West Pacific near the bottom of the Japan Trench. This biological (Dando & Southward 1986, Southward 1986, community is probably sustained by chemosynthetic Zonenshayn et al. 1987, Mayer et al. 1988, Lallemand processes, for reasons that include: (1) the concentra- et al. 1992, Fujikura et al. 1995, Olu et al. 199613); tion of sulfur in gills of M. hadalis was much higher (7) thyasind species have been collected from cold than in the gills of Ruditapes philippinarum from a seeps around Japan including: Conchocele disjuncta non-chemosynthesis-based community, while almost from the off Hatsushima site at 830 to 1230 m depth the Same level as in the gills of Calyptogena soyoae and from the Okinoyama Bank at 1040 to 1180 m depth from a chemosynthesis-based community (Fig. 5); in Sagami Bay (Fujikura et al. 1995), Conchocele sp. (2) the existence of numerous particles in the gill tis- from the Yukie Ridge at a depth of 2050 m in the sue which appeared be to symbiotic bacteria (Fig. 4). Nankai accretionary pnsm (Lallemand et al. 1992), and Symbiotic bactena in a few thyasirid species are rod- Parathyasira kaireiae from the landward slope at 5790 m shaped, 0.18 to 0.5 pm in diameter and 0.5 to 2.0 pm in and from the Sannku Escarpment at 6390 m in the length (Fisher 19901, while symbionts in Calyptogena Japan Trench (Okutani et al. 1999) (Table 1). phaseoliformis from the Japan Trench were round with Chemosynthesis-based communities in subduction diameters from 0.6 to 1.0 pm (Fiala-Medioni & Le Pen- zones are distnbuted along geologic faults via tectonic 22 Mar Ecol Prog Ser 190: 17-26, 1999

Table 1. Endernic species from deep-sea cold seeps around Japan. 'Not enough biological data. 1: Okutani & Egawa (1985), 2: Okutani & Metivier (1986). 3: Juniper & Sibuet (1987), 4: Ohta & Laubier (1987), 5: Sibuet et al. (1988), 6: Hashimoto et al. (1989), 7: Miura & Laubier (1989). 8: Lallemand et al. (1992), 9: Fujioka et al. (1993), 10: Okutani et al. (1993). 11: Fujikura et al. (1995), 12: Ogawa et al. (1996), 13: Okutani et al. (1996), 14: Ashi (1997), 15: Iwai & Mornma (1997), 16: Okutani et al. (1997), 17 Kojima & Ohta (199?a), 18 K0jin7a & Ohta (1997b), 19: Kuramoto & Joshima (1998), 20: Matsumoto et al. (1998), 21: Fujikura (unpubl data), 22: Fujikura et al. (unpubl. data), 23: Okutani et al. (unpubl.data)

Nankai accretionary pnsm Sagami Bay Nankai accretionary pnsm Dai-ni Tenryu Submarine Canyon ' 19.22 Okinoyama Bank ". l7 Tenryu Submarine Canyon 2.3.4. 600 m 1040-1180 m 3760-3840 m Pogonophora Annelida Cnidana Unidentified vestimentifera Nicomache ohtai Unidentified actinian Na tsushima bifurca ta Annelida Calyptogena soliddisima Pogonophora Serpulid polychaeta Ryukyu Islands area Lameilibrachia sp. Mollusca Kuroshima KnoL1 ' 20,22. 23 Unidentified vestimentifera Calyptogena nautilei 680-810 m Mollusca Calyptogena kaikoi Pogonophora Ba lhyacmaea nipponica Calyptogena laubien Unidentified vestimentifera Margarites shinkai Zenisu Ridge '4 Moliusca Provanna glabra 4020 m Vesicomya sp. Acharax johnsoni Moliusca Calyptogena solidissima Bathyrnodiolus japonicus Calyptogena spp. Sagami Bay Ba thymodiolus platifrons Kurile Trench '', ' off Hatsushima Island site '. 6. l7 Bathymodiolus aduloides 4980-5780 m 830-1230 m Conchocele disjuncta Moiiusca Annelida Calyptogena soyoae Calyptogena phaseoliformis Protonlystldes hatsushimaensis Calyptogena okutanii JapanTrench Nicomache ohtai Suruga Bay Kashima Seamount 3,4,

Shinkai sagamiensis off Toi Town site 'I0,21 5130-5770 m Natsushima bifurca ta 1490 m Cnidana Nautiliniellidae gen. sp. Pogonophora Unidentified actinian Unidentified polychaeta Unidentified vestimentifera Annelida Poyonophora Mollusca Tube-dwelling polychaetes Lameilibrachia sp. Calyptogena fausta Mollusca Unidentified vestimentifera Nankai accretionary prism Calyptogena phaseol~formjs

Mollusca Yukie Ridge ls Arthropoda Serradonta vestirnentifericola 1940-2180 m Caprellid amphipod Bathyacrnaea nipponica Mollusca landward slope 3. 4. 5. 23 Margantes shinkai Solemya sp. 5640-5940 m Provanna glabra Conchocele sp. Cnidana Oenopota sagamiana Calyptogena svnilaris Unidentified actinian Phymorhynchus buccinoides Calyptogena fausta Annelida Acharax johnsoni Tokai Thrust l6 Nautilina calyptogenicola Bathymodiolus japonicus 2080 m Tube-dwelling polychaetes Bathymodiolus platifrons Pogonophora Mollusca L ucinoma yoshidai Unidentified vestirnentifera Parethyasira kaireiae Conchocele djsjuncta Mollusca Calyptogena phaseoliformis Calyptogena soyoae Calyptogena similans Arthropoda Calyptogena okutanü Dai-ichi Minami Muroto Knoll 'I5. 22 Caprellid amphipod Arthropoda 3620 m Sannku Escarpment 9f 23 Neolepas sp. Mollusca 6270-6440 m Alvin ocaris longirostris Calyptogena nautilei Cnidaria Lebbeus sp. Dai-san Tenryu Submarine Canyon 22.23 Unidentified actinian Nankai accretionary prism 3760 m Annelida Ryuyo Submarine Canyon '13.'H Moliusca Tube-dwelling polychaetes 1000 m Vesicomya sp. Moilusca Mollusca Calyptogena na utilei Pare thyasira kaireiae Calyptogena okutanii Calyptogena kaikoi Calyptogena phaseoliforn~is Calyptogena nankaiensis Calyptogena la ubieri Calyptogena sp. Calyptogena sp. Arthropoda Caprellid amphipod Present study site 7326 m Mollusca Maorithyas hadalis Fujikura et al.: Deepest chemosynthesis-based cornmunity 23

CDP 1 00 500 1000 1500

Fig. 6. Geoloqc structure O ~pper~lope I Mid-slope Terrace Trench Axis recorded by a rnultichan- I nel seismic reflection sys- I Sea Bottom Thyasirid clam tem on the landward 1 ,!, .;, , , Sanriku slope of the Japan Trench 5000 -E - - (rnodified from von Huene - & Culotta 1989). Bold solid lines below the sea bot- B torn show geologic faults. 0 looOO Thyasirid aggregations oc- curred above a geologic fault. One CDP (cornrnon decollew depth point) = 25 rn 15000

compression or extension. Seismic data for the region served near Calyptogena aggregations (Ohta & Laubier (von Huene & Culotta 1989) revealed that the thyasirid 1987). aggregations occurred above a geologic fault near the Species occurnng in chemosynthesis-based cornrnu- bottom of the Japan Trench (Fig. 6). Fluids seep out of nities are classified into 3 types: endemic, colonists, these faults, providing nutrients for chemosynthesis- and vagrants (Carney 1994). Endemic species are com- based species. Seep fluid is generaliy characterized by pletely restricted to cold seep environments; colonist high methane concentrations and is often low in sul- species are more abundant in cold seep areas than in fide (Sibuet & Olu 1998). Sulfide is produced in sedi- surrounding areas; vagrant species may profit from the ments from rnicrobial Sulfate reduction (Masuzawa et organic enrichment around seeps, but remain common al. 1992). Sulfide for these thyasirid aggregations may beyond the influence of seep areas. Using these defin- be produced in this way, using methane as an energy itions, species within the thyasirid aggregations may source. be classified. Maorithyas hadalis is an endernic spe- The distnbution of this comrnunity seems more rest- cies, while the Eurycopidae-like isopods and Scoto- ncted than seep communities in other areas, such as planes-like holothurians are colonists. There were not Sagami Bay (Hashimoto et al. 1989, Fujikura et al. enough of the tube-dwelling species to afford classifi- 1995), the Nankai accretionary prism (Juniper & Sibuet cation. Sibuet & Olu (1998) suggested that species 1987, Ohta & Laubier 1987, Sibuet et al. 1988), Mon- richness of chemosynthesis-based communities tends terey Bay (Barry et al. 1996), the Peruvian active mar- to decrease with increasing depth. Data from the pre- gin (Olu et al. 1996a) and the Barbados prism (Olu et sent study Support this idea as only 1 endemic species al. 199613). Detailed observations show that the newly occurred at this deepest known site. A Summary of discovered communities differ in other aspects as weil. data on endemic species collected from cold seep sites Each aggregation of Maorithyas hadalis was very around Japan, however, is inconclusive (Table 1). Of small and contained no other commonly endemic the 3 major regions surveyed sufficiently for comment, chemosynthetic taxon (for example vestimentiferan it does appear that the number of endemic species tube worms, vesicomyids and Bathymodiolus bivalves, generally decreases with depth. There are more or provannid gastropods). In shallower (from 4981 to endemic species reported frorn Sagami Bay than from 5938 m) cold seep communities in the Japan Trench the Nankai accretionary prism, and more from both (dominated by Calyptogena phaseoliformis), numerous these areas than from the Japan Trench. Moreover, the actinians, buccinid and rissoacean gastropods, tube- number of endemic species from the best studied indi- dwelling polychaetes, capreilid amphipods, and swim- vidual sites within each region generally decreases rning holoihurians (Pem'agone sp.) are consistenily asso- with depth. The exceptions listed in Table I appear in ciated with Calyptogena aggregations (Juniper & areas which are not well studied. Therefore these Sibuet 1987, Ohta & Laubier 1987, Sibuet et al. 1988), areas need to be more fully explored before a conclu- but they were not observed in these thyasirid aggrega- sive Statement regarding endemic diversity at chemo- tions. The tubes observed at this site were curved and synthetic sites can be made for all cold seep sites sparsely distributed, while those tubes commonly ob- around Japan. served within Calyptogena-dominated sites are straight Five bivalve families have some species known to be and densely packed. The Eurycopidae-like isopods endemic to chemosynthetic environments, including and Scotoplanes-like holothunans were similar to the the Solemyidae (Metivier & von Cosel 1993, Fujikura Storthyugura-type isopods and Scotoplanes globosa ob- et al. 1995, Olu et al. 1996a,b), Mytilidae (Cavanaugh 24 Mar Ecol Prog Ser 190: 17-26, 1999

1983, Hashimoto & Okutani 1994, Ohno et Table 2. Vertical ranges of the pnncipal families of which occur al. 1996, Olu et al.. 1996b), Lucinidae (Fisher in chernosynthetic environments or have chemoautotrophic symbionts in & Hand 1984, Corselli & Basso 1996), the giils Thyasindae (Dando & Southward 1986, P P - P P Southward 1986, Zonenshayn et al. 1987, Farnily Shallowest habitat Deepest habitat Mayer et al. 1988, Fujikura et al. 1995, Solemyidae Eelgrass beds 4776-4949 rn Okutani et al. 1999) and Vesicomyidae near Woods Hole Aleutian Subduction Zone (Cavanaugh 1983, Okutani & Egawa 1985, (Cavanaugh 1983) (Suess et al. 1998) Brooks et al. 1987. Fisher et al. 1988, Barry Lucinidae 0.25 rn 1950 m et al. 1996, Ogawa et al. 1996). Members in St. Joseph's Bay, Florida Napoli Mud Volcano these fanlilies occur in both vent and seep (Fisher & Hand 1984) (Corselh & Basso 1996) habitats and depend on chemosynthesis, Thyasiridae 15 m 7326 rn using symbiotic bacteria in their gills (Reid Loch Etive, Scotland Japan Trench & Bernard 1980, Cavanaugh 1983, Fisher & (Southward 1986) (present study) Hand 1984, Dando & Southward 1986, Mytilidae 432 m 3660 rn Fiala-Medioni & Felbeck 1990, Fisher 1990, Kaikata Seamount, Mariana Back-arc Basin Distel & Wood 1992, Dando et al. 1994, Nel- Ogasawara Islands area (Hessler & Lonsdale 1991) (Ohno et al. 1996) son & Fisher 1995). Vertical distribution ranges of these species-rich fanlilies are Vesicomyidae 400 m 6437 m Louisiana Slope Japan Trench summanzed in Table 2. This report extends (Brooks et al. 1987) (Ogawa et al. 1996) the known range of thyasirid clams from chemosynthetic environments, nearly dou- bling it (it is unclear if thyasinds reported deeper than suggested that vesicomyids gain an advantage over 10000 m [Belyaev 19661 had chemosynthetic sym- other seep taxa (Barry et al. 1996) based in Part on bionts). The deepest previously-known report for the their ability to aquire sulfide through a burrowing foot family was from 3840 to 3890 m in the Laurentian Fan separately from oxygen brought in through an incur- (Mayer et al. 1988), and they have been reported from rent Siphon from overlying seawater (Fisher 1990). near the surface at 15 m (Southward 1986), and many Thyasirids are often found in areas with free sulfide depths in between (in Sagami Bay from 830 to 1230 m levels less than 1.0 pM (Fisher 1990). Conversely, sul- (Fujikura et al. 1995), West of Paramushir Island at fide levels from Calyptogena spp. habitats have rela- 770 m (Zonenshayn et al. 1987), the Barbados prism at tively high sulfide levels including: C. magnifica at 1300 m (Olu et al. 1996b), and the Nankai accretionary 35 pM, the highest level at the vent in the Galapagos prism at 2000 m (Lallemand et al. 1992). Rift (Fisher et al. 1988); C. pacifica at 0.09 mM and C. This community also extends the range for all chemo- kilmeri at 10.9 mM to 11.3 mM in the Monterey Bay synthesis-based bivalves, replacing the previous re- (Barry et al. 1996), C. soyoae at 0.05 to 0.6 mmol kg-' in cord of 6437 m for Calyptogena phaseoliformis which Sagami Bay (Hashimoto et al. 1995), some species of was also collected from the Japan Trench (Ogawa et al. vesicomyids (including 2 Calyptogena species) at 50 to 1996). It is interesting to speculate why C. phaseoli- 70 pM in the Nankai accretionary prism at a depth of formis does not occur at these thyasirid aggregation 3950 m (Fiala-Medioni et al. 1993), and Calyptogena sites. One possibility is that the vertical range for C. sp. at 43 to 183 pM in the Laurentian Fan (Mayer et al. phaseoliformis is more restncted as these thyasinds 1988). Only 1 exception, an unknown Calyptogena were collected nearly 900 m deeper than the deepest species from the Nankai accretionary prism occurred C. phaseoliformis. However, unlike any previously at 0.1 PM (Fiala-Medioni et al. 1993). It remains un- reported case for deep-sea thyasirid bivalves, almost clear whether sulfide concentration levels or a thin all living Maonthyas hadalis were observed with 20% near-surface sulfide layer or both are acting to restrict of the shell exposed. The typical case for thyasirids is to C. phaseoliformis from the present site. Future dives live buried completely within the sediment, reaching will examine the underlying sediment more carefully depths 8 to 9 times their shell length (Dando & South- to ascertain how deep the thyasirid bivalves dwell. If ward 1986). This suggests that the available sulfide thyasirids are found well below the surface, as is the layer at the present site is relatively thin and near the normal case for the family, this would suggest low sul- surface. lf so, C. phaseoliformis may be excluded from fide concentration is limiting C. phaseoliformis. the site due to limited sulfide levels, either due to a thin Fisher (1990) comments on the dispute as to whether sulfide layer near the surface, or by very low sulfide or not thyasirid symbionts are intracellular as in all concentrations within the sediment. A thin sulfide other bivalves with chemoautotrophic symbionts. His layer might inhibit Calyptogena spp., as it has been reluctance to accept claims to the contrary (see Herry Fujikura et al.: Deepest che mmosynthesis-based community 25

& Le Pennec 1987 and Reid & Brand 1986) is based with an active methane seep. Mar Ecol Prog Ser ?0:49-63 mainly on inconclusive micrographs showing no clear Dando PR, Ridgway CA, Spiro B (1994) Sulphide 'mining' by membrane separating symbionts from the external lucinid bivalve molluscs demonstrated by stable sulphur isotope measurements and experimental rnodels. Mar environment. We believe symbionts in Maorithyas Ecol Prog Ser 107:169-175 hadalis are indeed intracellular based on our micro- Distel DL, Wood AP (1992) Characterization of the LUsym- graphs showing more complete membrane bound biont of Thyasira flexuosa (Thyasindae: Bivalvia) by use of vacuoles (Fig. 4B).Thus the case for extracellular syrn- polyrnerase chain reaction and 16s rRNA sequence analy- sis. J Bactenol 174:6317-6320 bionts as a distinguishing feature of thyasirids has to Fiala-Medioni A. Felbeck H (1990) Autotrophie processes in be reconsidered. If other members of the family do invertebrates nutrition: bacterial in bivalve mol- prove to have extracellular symbionts, then the intra- luscs. In: Mellinger J (ed) Anima1 nutrition and transport cellular symbionts of M. hadalis are the exception. processes 1,Vol5. Nutrition in wild and domestic animals; The existence of vanous types of chemosynthesis- Kinne RKH. Kinne-Saffran E, Beyenbech KW (eds).Karger, Basel. p 49-69 based communities within the relatively restricted Fiala-Medioni A, Le Pennec M (1988) Trophic structual adap- range of the Japan Trench has implications for biogeo- tations in relation to the bacterial association of bivalve graphic distribution and speciation in the deep sea. molluscs from hydrothermal vents and subduction zones. Deep-sea trenches (deeper than 6000 m) created by Symbiosis 4:63-74 Fiala-Medioni A. 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Editorial responsibiiity: Otto Kinne (Editor), Submitted: February 26, 1999; Accepted: June 25, 1999 OJdendorf/Luhe, Gernlany Proofs received from authorls): Novem her 18, 1999