Benthic associations in bathyal and hadal depths off the Pacific coast of north eastern Japan: physiognomies and site factors

Prog. Oceanog. Vol. 24, pp. 331-339, 1990.

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Benthic associations in bathyal and hadal depths off the Pacific coast of north eastern Japan: physiognomies and site factors MASUOKI HORIKOSHIl, TOSHImKO FUJITA2 and SUGURU OHTA 2

1Chiba University, 1-33 Yayoicho, Chiba City 260, Japan ZOcean Research Institute, University of Tokyo, 1-15-1, Minamidai, Nakanoku, Tokyo 164, Japan Abstract - Off north-eastern/apart, the continental slope is rather narrow. It slopes steeply down to a depth of 100Ore,but then forms a flat terrace between 1000 and 3000 m. Beyond 3000m the slope drops steeply down to the axis of the Japan Trench (7500m). Only one community type occurs in the upper bathyal zone (200-600m), an Ophiura association whose physiognomy is a "tu~' of ophiuroids. The middle bathyal zone (600-1200m) is occupied by two types of community. The predominant community is an association characterized by an asteroid, Ceramaster, and a demersal fish, Sebastolobus, which has no characteristic physiognomy. The second is a cornatulid community, which occurs on the shoulder of the slope as a comatulid "park". The lower bathyal zones (12003000m) is inhabited by two distinct associations one of which has two facies. Most of the terrace area is occupied by an amphiurid association. In the plateau-like areas where there is a higher flow regime, there are sporadic, or sometimes rather dense stands of a coelenterate - the Radicipes facies. Whereas at the deeper site currents are slower and the sediment is muddy, a Megayoldia Ascorhynchus facies occurs in which a bivalve and a gigantic pycnogonid axe found associated with amphiurids. The other community-type is a tubicolous-polychaete association which consists of a dominant polychaete species with a rather thin sandy tube which is associated always with a caprellid. This association is found on a shoulder-like topography, where bottom currents are expected to be constant and sediment is sandy. In the axis of the Japan Trench, where the sediment is soft mud, a small globular bivalve, Kelliella, is numerically dominant, which is associated with a holothurian, Elpidia, and a demersal fish, Careproctus. Nearby where the topography is a gentle slope, the community is typified by a dominant isopod, Storthyngura sp., accompanied by a bivalve, Spinula, and an actiniarian, Galatheanthemum.

1. INTRODUCTION P h y s i o g n o m y and site factor are ecological characteristics of terrestrial plant communities which have been well studied (e.g. WrlrrrArmR, 1956; SPURS and BARNES, 1973), but have seldom been documented in the deep sea, except for the hydrothermal vent and cold seepage c o m m u n i ties. After years of experience of deep-sea trawling and bottom photography on board the R/Vs Ry~f-a, Hakuh~ Maru and Tansei Maru, and the submersible Shinkai 2000 (Suv~nmo, IWALI s a m o and H o ~ o s m , 1960; Sua,~mRO, OKADA,HORmosm and IwnI, 1962; HoRmosI-n, 197 la, b; HoRmoSill, OHTA, SHIRAYAMAand TSUCHIDA, 1983), we have recognised that several distinct types of communities occur at bathyal depths along the northeastern coast of Japan, and within the axis of the Japan Trench in the hadal zone. Some of these have characteristic physiognomies which are as distinctive as those of terrestrial plant associations.

331

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Each benthic community or association occupies a peculiar biotope, where there is a certain "location factor". The senior author has already discussed location factor in shallow water environments (HORII
Physiognomies and site factors of benthic associations

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FIo.1. Localities of the tTawling and photographic stations, off the northeastern Pacific coast of Honshfl Island, Japan, showing the topography consisting of a narrow continentalslope (200-120Ore), a deep-sea terrace (1200-ca.3000m) and the axis of the Japan Trench (deeper than 7000m: stippled). St. 1: RyOf~ Maru, JEDS-2 (1959); St.2: Ry~f~ Maru, JEDS--4 (1961 ); Sts. 3-5: Hakuh~ Maru, KH-672 (1967); Sts. 6-10: Hakuh6Maru, KH-69-2 (1969); Sts. 11-12: Hakuh~Maru, KH-81-4 (1981); Sts. 13-19: Tar~eiMaru, KT-84-9 (1984); St.20: Shinkai2000(1984); Sts. 21-26: TanseiMaru, KT-875 (1987); St.27: Tansei Maru, KT-88-7 (1988).

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FIo.2. Distribution of distinct types of benthic communities, or associations, and facies of an association, in the bathyal depths off northeastern, Pacific coast of Honshfi Island, Japan. Chain-dot lines indicate the boundaries between the upper and middle bathyal zones (A), and the middle and lower bathyal zones (B) respectively.

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FIo.3. Distribution of distinct associations and facies of an association on projected profiles of the submarine topography off the northeastern Pacific coast of Honshfl Island.

Physiognomies and site factors of benthic associations

PLATE 1. Physiognomies of, and trawl catch from several distinct benthic communities in bathyal depths off northeastern part of Japanese main island, HonshQ. (Fig.1: Upper bathyal zone, 2a,b-3: Middle bathyal zone, 4-6: Lower bathyai zone.) Fig. 1. Ophiura association: St.14, 280m (photo by Ohta an Fujita); Fig.2. Ceramaster-Sebastolobus association; a, Sebastolobus macrochir: St.16, 720m (Ohta and Fujita); b, Ceramasterjaponicus: St.16,720m (ditto); Fig.3. Comatulid Association: St.8,1050m (Nakai and Otobe); Fig.4. Radicipes facies of arnphiurid association: St.7, 1550m (ditto); Fig.5. Ditto with dense growth of Radicipes: St.18,1290m (Ohm and Fujita); Fig.6. Trawl catch from tubicolous polychaete association: St.2, 1700m (I-Iorikoshi).

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M. HoP.~OSmet al.

There are two distinct communities within the lower bathyal zone between 1200 and 3000m, and two different subdivisions of a community (facies). Most of the lower bathyal terrace is occupied widely by an amphiurid association (13 stations out of 15). Innumerable arm-tips of amphiurid ophiuroids are seen protruding out of sediment surface (PI.1, Fig.4). There are two distinctive facies of this community, one inhabiting topographically higher plateau-like areas which are gently sloping, as well as on steeper slopes where the average flow regime is probably enhanced. This facies is typified by sporadic growth of a colonial coelenterate, Radicipes pleurocristatus Stearns, but occasional dense stands of the coelenterate occurs (PI. 1, Fig.5) and hence this is termed the Radicipes facies. Another inhabitant of this facies is an epizoic ophiuroid Asteronyx loveni Mtiller and Troschel which clings to the coelenterate (Furrrn and OnTA, 1988).

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F~o.4. I3~p-sea caprellid amphipod collected at the St.2 (JEDS-4: 1650-1690m). Drawn from a coloured sketch made immediately after capture. The mouth parts, the antennae and the gnathopods were vermilion in colour (stippled).

Physiognomies and site factors of benthic associations

337

Deeper down where bottom currents are slower and so sediments are more muddy, another facies of the amphiurid association occurs associated with the protobranch bivalve Megayoldia sp. (M. lischkei Smith sensu SCAgLATO, 1981, non SMITH, 1885) and a gigantic pycnogonid, Ascorhynchus japonicus Ives; the Megayoldia-Ascorhynchus facies. The occurrence of this facies in the deeper areas of the deep-sea terrace, is not a bathymetric zonation, because elsewhere the same facies occurs at much shallower depths (HoRmosm, unpublished data). The other distinct benthic community in the lower bathyal zone is a tubicolous-polychaete association. The dominant polychaete of this association has a tube constructed of sand grains rather than mud, so it requires sand grains to be available in its environment. This community occurs at a station located on the shoulder of a break in topography (see Fig.3: diamond mark), where the hydrodynamic regime is probably more dynamic and prevents the deposition of silt and clay. Further evidence for this site factor is the association of a sizeable population of caprellid amphipods (Fig.4), an epibiotic, seston feeding animal (HoRIKOSm, 1987) with this community at both stations (stations 2 and 4) where it occurred. Within the axis of the Japan Trench, at least two distinct benthic communities occur. One occurs at stations A and B (Fig.5) where a narrow gorge is filled with soft muddy sediment, and is entirely flat. The second community was found at station X (Fig.5) in a different site situated just on the eastern, ocean side of the deepest narrow basin. Here the axis is somewhat broader, the bottom is gently sloping, and is covered by less muddy sediment. The species compositions of the two benthic communities were remarkably different (Table 1). At stations A and B a small, globular bivalve, Kelliella sp. (Vesicomya in Russian literature) was numerically dominant and an elasipod holothurian, Elpidia gracialis kurilensis Baranova and Belyaev was dominant in terms of biomass. Two polychaete species also occurred in low numbers, and a brotulid fish, Careproctus amblystomopsis Andriaqshev, was exclusive to this community. TABLE1. Benthic communities with the axis of the Japan Trench. Flat Axis

Kelliella Elpidia Macellicephala sp.A Careproctus Storthyngura Spinula vityazi

Gentle Slope St.X 6700-7400m

St.A 7420-7440m

St.B 7420-7450m

1426 638 27

3606 324 3

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l

21

20

91

2

6

11

Macellicephala sp.B

5

Galatheanthemum

4

The second community consisted of a few individuals ofElpidia andKelliella was absent. The dominant species was an isopod crustacean, Storthyngura sp., and a paleotaxodont bivalve, Spinula vityazi Filatova was fairly common. Another species of the hadal polychaeteMacelicephala sp.B. and a sheathed sea-anemone, Galatheanthemum sp., were exclusive to this community. In an undisturbed sediment sample collected by an USNEL box-corer at station B, there were sand and silt lamina, probably produced by turbidity currents. Within the laminae 20cm below the sediment surface we found a remnant of Galatheanthemum tube together with a valve of Spinula vityazi. This may imply that these organisms may have lived on the neighbouring slope

338

M. Homa~osmet al.

of less muddy sediment and had been transported onto the flat axis of the trench by turbidity current. 4. DISCUSSION Hitherto there have been few descriptions of deep-sea benthic communities and their physiognomies. I-I~ezEN and HOLHSTe~ (1971) compiled a fascinating book of deep-sea photographs, but their interests were directed towards individual organisms, and they only occasionally indicated a suggestive name, e.g. "feather garden" (Fig.2.40). However, several of their photographs illustrate physiognomies of several benthic communities dominated either by an asteroid (Fig.2.41), or holothurians (Figs. 1.49 and 2.59) or an ophiuroid (Fig.2.60), but no comment was made either about the benthic community or the physiognomy. TnORSON(1968) is the only person who previously depicted physiognomies of shallow and deep-sea benthic communities; his representations are schematic (compare his pictorial presentation of the communities occurring in shallow water (Figs. 98 and 128), deep water (Fig.110) and deep-sea environments (Figs.136 and 146)). SOKOLOVA(1959) discussed the relationship between very large-scale topography of the ocean and benthic communities, but made no mention of any actual associations or their physiognomies. Every facies of a given association will be situated at sites where there is a specific location (or site) factor. HomKosm (1962a, 1970) has described several distinct deep-sea associations of macrobenthos found at different sites around a deep-sea knoll and within submarine canyons, where the topography and the bottom sediments provide each site with a specific character, the clearest examples are the sites where the benthic associations are dominated by epibenthic seston feeding organisms. The comatulid association described here occurs on the shoulder of a slope, and also the Radicipes facies of amphiurid association was found wherever there were plateaulike eminences on the deep-sea terrace. At these localities, the hydrodynamic (or flow) regime is more vigorous and transports in enough seston to support the suspension-feeders (see also FojrrA and OnTA, 1988). Similar conditions are known for the stalked crinoids (HomKosm, 1960; Roox, 1985; FoJrrA, OHTA and OJI, 1987). In their intensive survey of a hexactinellid sponge, P h e r o n e m a carpenteri (Wyville Thomson), Rtce, TrrtrasroN and New, (1989) found dense populations of the sponge occur in the Porcupine Seabight (NE Atlantic) where the interaction of the physiography of the seabight and its local topography, with the water masses, tidal movements and bottom currents, creates a characteristic site factor (location factor). 5. ACKNOWLEDGEMENTS Our thanks are due to the captains and crews of the R/Vs Ry~f~ Maru of the MeteorologicalAgency, Japan, Hakuh~ Maru and Tansei Maru of the Ocean ResearchInstitute,Universityof Tokyo, as well as to the operational members of the submersibleShinkai 2000 of the Japan Marine Science and TechnologyCenter (JAMSTEC). We aregratefulto Messrs S. Nakai andH. Otobe,OceanResearchInstitute,UniversityofTokyofortheireffortson taking We are much obliged to Dr Marlin Angel for his kind revision of our English manuscript. Part ofthis workwas supportedby the Grant-in-aidfor CooperativeResearch(A),MinistryofEducation,Science and Culture,Japan: "Studies on Offshore and Deep-sea Faunas in the West Pacific and Indian Oceans" (Proj. No. 00534033). This work was also supportedin part by fundsfrom the CooperativeProgram(Nos. 86178a, 87128, 88) provided by the senior author (M.H.) by Ocean Research Institute,Universityof Tokyo. b~tt~mph~t~gra~hsintheear~estperi~d~ftheph~t~gra~hic~perati~n~nb~ardtheR~VHakuh~Maru.

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339

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