Hydrobiologia 530/531: 537–547, 2004. D.G. Fautin, J.A. Westfall, P. Cartwright, M. Daly & C.R. Wyttenbach (eds), Coelenterate Biology 2003: Trends in Research on Cnidaria and Ctenophora.
2004 Kluwer Academic Publishers. Printed in the Netherlands.

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The scyphomedusan fauna of the Japan Trench: preliminary results from a remotely-operated vehicle Dhugal J. Lindsay1,*, Yasuo Furushima1, Hiroshi Miyake1, Minoru Kitamura1 & James C. Hunt2 1

Extremobiosphere Research Center, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0016, Japan 2 Present address: Department of Biology, University of New England, 11 Hills Beach Road, Biddeford, ME 04005, USA (*Author for correspondence: Fax: +81-468-67-9525, E-mail: [email protected])

Key words: ROV, scyphozoa, distribution, physico-chemical parameters

Abstract A series of dives was conducted during April/May 2002 in a variety of water masses over the Japan Trench using the remotely-operated vehicle ROV HyperDolphin. The scyphomedusan fauna was catalogued, resulting in the first records of Atolla vanhoeffeni and Atolla russelli from Japanese waters. A new genus and species referable to the family Paraphyllinidae was also recognized. The ulmarid scyphomedusa Poralia rufescens is by far the most common scyphomedusa in these waters. Its distribution was determined by water mass structure and it was not present in subducted Oyashio Current-derived waters occurring within its usual depth range. Temperature and salinity affected its distribution more than dissolved oxygen concentrations. Distributional information, data on the physico-chemical parameters of the water column, and notes on biological associations with scyphomedusae are introduced.

Introduction The eastern seaboard of Japan is a highly productive area, due to the presence of upwelling and the formation of frontal zones between various water masses. In this area, warm waters of Kuroshio Current origin meet and mix with the colder nutrient-laden waters of the Oyashio Current before turning eastward and flowing into the open ocean. The species composition and distribution patterns of robust organisms such as krill, shrimps, fishes, and copepods, which can be sampled with conventional plankton nets and midwater trawls, have been studied in this area to a certain extent (e.g., Nishikawa et al., 1995). Much attention has focused in recent years on the physical and biological properties of these water masses, with a special edition of the Journal of Oceanography (Vol. 54, No. 5, 1998) devoted to

the topic. Very little information, however, is presently available on the gelatinous macroplanktonic community. Two dives with a crewed submersible off the Boso Peninsula (Pe´re`s, 1959), and a series of dives with the Mir submersibles (Vinogradov & Shushkina, 2002 and references therein), are the only exceptions. A cruise (YK0206) on the R/V Kaiyo was designed to characterize the upper water column fauna and identify the physical parameters affecting vertical distributions of the macroplanktonic gelatinous organisms. Particular attention was paid to scyphozoan medusae and their associated fauna.

Materials and methods A transect that included Oyashio-derived cold waters, Kuroshio-derived warm waters, and their

538

Figure 1. Satellite map of surface water temperatures over the Japan Trench on 29 April 2002 with XCTD (grey circles) and ROV HyperDolphin (HD) Dive sites (white circles) superimposed.

respective mixing and frontal zones was run along the eastern seaboard of Japan using the R/V Kaiyo. Water masses were identified using satellite imagery and XCTD (eXpendable Conductivity, Temperature and Depth profiling system) probes, and dive surveys using the ROV HyperDolphin were carried out to determine the vertical distributions of the macroplanktonic gelatinous organisms (Fig. 1). Dives analyzed during this study were conducted from 22 April to 1 May 2002 in the locations and to the maximum depths outlined below: Dive 98, 41 00¢ N, 144 41¢ E, 1034 m; Dive 100, 38 56¢ N, 143 06¢ E, 1000 m; Dive 101, 40 26¢ N, 144 32¢ E, 2000 m; Dive 103, 38 20¢ N, 143 55¢ E, 1002 m; Dive 104, 39 30¢ N, 144 15¢ E, 1212 m; Dive 105, 39 52¢ N, 144 22¢ E, 1000 m. All dives were conducted during daylight hours with the exception of the latter halves of dives 98 and 103, where the ROV surfaced at 20:31 and 20:35, respectively. Sunset on these two days was at 18:09 and 18:13, respectively. The ROV HyperDolphin was equipped with a high-definition camera integrating an ultra sensitive super HARP (High gain Avalanche Rushing Photo-conductor) tube. Camera sensitivity was F1.8 at 2 lux, gain was variable at 4 – 200 times,

the signal to noise ratio was 43 dB, and resolution was 800 TV lines. The zoom lens had a minimum focal length of 5.5 mm and a 5· zooming ratio. There were five 400-W SeaArc HMI/MSR metal halide lamps. Two were situated on the port swinging boom arm, and one on the starboard swinging boom arm. These arms were usually opened such that the lights optimized the field of view of the high-definition camera, but were sometimes moved to optimize lighting when making observations of individual organisms in situ. The remaining two lights were forwardpointing and fixed to the frame of the vehicle. Video footage was recorded continuously and simultaneously with depth/time overlay on Sony BCT-D124L Digital BetaCam tapes via an analogue composite signal, and without overlay on BCT-124HDL HDCAM tapes via a native digital signal at 1080i and 30 frames. Physico-chemical data were collected using a SeaBird SBE19 CTD (Conductivity, Temperature and Depth profiling system) and an SBE13 oxygen sensor attached to the vehicle on all dives. CTD and dissolved oxygen were correlated to the presence of a given animal by matching the timecode on the CTD series to the timecode on video.

539 Water mass profiles were plotted using linear interpolation, adding CTD data from Dive 99 (41 00¢ N, 144 42¢ E, 1206 m). Specimens were collected for positive identification using a suction sampler. A motor-driven fan slurps the specimen and the water surrounding it into either a 90 mm diameter intake hose and thence through an 80 mm diameter intake hole into a single clear acrylic canister of 300 mm diameter and 285 mm height, or into one of three gate valve samplers. The gate valve sampler, developed at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) according to a concept outlined by Dr William Hamner, allows collection of larger and more fragile organisms. This sampler uses gate valves, which are sold commercially and designed to control irrigation for agriculture. The gate valve sampler consists of an elongated box-shaped Plexiglas body (35 mmL
18 mmB
18 mmH) which is sealed by a metal plate that blocks the box transversely. This plate slides open and closed and acts as a gate to control the water flow. The plate slips between two rubber rings that act to seal the 103 mm diameter hole. It can be used directly as an aquarium, thereby reducing stress to the captured animal. The frontal wake created by the pressure of forward movement of the sampler is minimized by applying gentle suction with the suction sampler to draw a specimen into the gate sampler before the gates are closed. This system works efficiently and effectively, and we have developed an array of three gate valve samplers with box-shaped bodies for the ROV HyperDolphin system. Sampling of voucher specimens allowed recognition of species in the HDTV video record based on their external macromorphology and behaviour. Animals collected in the single cannister of the suction sampler were transferred to shipboard aquaria, phototanks, or planktonkreisels (Hamner, 1990) for behavioural observation and positive identification while those sampled with the gate samplers were observed within the original collection device. Still digital photographs of specimens were taken with a Nikon D1H digital camera with a macro lens (AF Micro Nikkor 105 mm 1:2.8 D) and recorded in TIFFRGB format at an image size of 2000 · 1312 pixels. Illumination was provided by National Ref Lamp colour-balanced flood bulbs (PRF-500WB) which lit the specimen from the side before a black felt backdrop. Observations of the live animals were

also made in the laboratory under a Nikon SMZ-U dissecting microscope and recorded on DV tape. Sketches were made in cases where the still image and video equipment were unable to resolve or record internal or fine structures to our satisfaction.

Results Order Coronatae Vanho¨ffen, 1892 Family Periphyllidae Haeckel, 1880 Genus Periphylla Haeckel, 1880 Periphylla periphylla (Pe´ron & Lesueur, 1810) Description: Coronate medusae with four interradial marginal sense organs, and 12 tentacles arranged in groups of three between adjacent rhopalia; gonads eight, U-shaped; bell steeply pointed with height roughly equivalent to the combined length of the pedalia and marginal lappets; tentacles thick, more or less straight. Distribution: Observed depth of occurrence ranged from 322 to 750 m. Physico-chemical parameters of the water column habitat were as follows: temperature 2.6–3.8 C, salinity 33.68– 34.20, dissolved oxygen 0.7–5.1 ml/l, sigma-t 26.87–27.21 kg/m3. The observed distribution of Periphylla periphylla is outlined in Figure 2. All records of Periphylla occurred during daylight hours, except for a single record at 350 m depth at 19:51 on Dive 103. Comments: None of the individuals were observed in association with other organisms. Family Paraphyllinidae Maas, 1903 A new member of this family recognized in Japanese waters awaits a full description and taxonomic treatment (see below). We have yet to identify characters that would allow easy in situ identification at the family level. Waving the manipulator arms of the vehicle to create gentle currents sometimes changed the attitude of this medusa enough to peek under the exumbrella and ascertain whether the four rhopalia were situated perradially or not – the rhopalia count and perradial condition being definitive for the family. The sole genus currently recognized in this family is Paraphyllina Maas, 1903, containing two species – both with 12 tentacles arranged in groups of three (Maas, 1903; Russell, 1956).

540

Figure 2. Horizontal and vertical distributions of the coronate scyphomedusae Periphylla periphylla (purple triangles), Atolla spp. (orange circle), and the undescribed paraphyllinid species (orange triangles) over the Japan Trench during April/May 2002, and correlations with (a) water temperature, (b) salinity, (c) and dissolved oxygen (DO) concentrations. Upturned grey triangles signify latitudes at which XCTD probes were deployed and upturned white triangles where ROV HyperDolphin (HD) dives were conducted.

541 Genus and species undefined Common Japanese name: benimanjukurage (red manju medusa) Description: Paraphyllinidae with 20 tentacles arranged in groups of five, and with four perradial rhopalia; rhopalia with no ocellus or lens present, and no opaque pigmented bulbous collar. Recognizable in situ by reddish brown pigmentation over the whole body such that none of the internal structure is visible, an extremely pronounced coronal groove, four bright white statocysts with five tentacles between each, a shallow conical bell of height equivalent to the combined length of the pedalia and marginal lappets when the medusa is floating in a stationary position, and tentacles folded towards the centre of the subumbrella or extruded only from the opening formed by the extremely long lappets when they are folded in as the medusa is stationary. Macromorphology is shown in Figure 3. Distribution: Observed depth of occurrence was at 880 and 996 m. Physico-chemical parameters of

Figure 3. (a) Side view and (b) oral view of the paraphyllinid coronate scyphomedusae referred to as the red manju jelly (benimanjukurage).

their water column habitat were as follows: temperature 2.73.2C, salinity 34.3134.33, dissolved oxygen 0.70.8 ml/l, sigma-t 27.3227.38 kg/m3. Comments: No organisms have yet been observed in association with this medusa. Family Atollidae Bigelow, 1913 Genus Atolla Haeckel, 1880 Recognizable in situ by dark red, reddish brown to black pigmentation on manubrium, a flattened and often heavily pigmented bell, and by one very long hypertrophied tentacle dragged behind the umbrella as they swim. Several individuals had a single small crustacean associated with the centre of their exumbrella. The observed distribution of the genus Atolla is outlined in Figure 2. Atolla wyvillei Haeckel, 1880 Common Japanese name: murasakikamurikurage (purple coronate medusa) Description: Adults can be recognized by the primrose-shaped stomach base, lack of pigment spots between each gonad and the stomach base, large size, deeply furrowed bell, lack of papillae on lappets, and presence of auriform gonads. Distribution: Below 500 m. Atolla vanhoeffeni Russell, 1957 Common Japanese name: batsukamurikurage (cross coronate medusa) Description: Recognizable in situ by its clear bell, 20 tentacles, cross-shaped stomach base, and presence of eight pigment spots situated between each gonad and the stomach base. Distribution: Positive identification at 484 m depth (Dive 103, 38 20¢ N, 143 55¢ E, temperature 3.78 C, salinity 34.06, dissolved oxygen 1.23 ml/l, sigma-t 27.06 kg/m3). Comments: A specimen captured in Sagami Bay (Shinkai 2000 Dive 1139, 35 01¢ N, 139 22¢ E, depth 711 m, temperature 4.45 C, salinity 34.02, dissolved oxygen 1.50 ml/l, sigma-t 26.96 kg/m3) was used to verify our identification based on the video record and, as it represents an extension of the known geographical range, has been placed in the National Science Museum, Tokyo, under

542 registration number NSMT-Co 1401. Another individual, also collected from Sagami Bay (Shinkai 2000 Dive 1055, 35 01¢ N, 139 22¢ E, depth 557 m, temperature 5.35 C, salinity 34.56, dissolved oxygen 1.87 ml/l, sigma-t 27.28 kg/m3), has been stored at JAMSTEC under registration number MP1-4 024273. Atolla russelli Repelin, 1962 Common Japanese name: girishajuujikurage (Greek cross medusa) Description: Recognizable in situ by deep red bell and skirt pigmentation (largely obscuring gonads), bell vaulted with deep radial grooves, 16–20 tentacles, and squarish Greek cross-shaped stomach. An individual with the characteristics outlined below was sampled. Mature female; bell diameter 19 mm; 20 tentacles; 20 deep radial grooves on bell; eight gonads hanging freely in the subumbrellar cavity surrounding the stomach but hidden behind the deeply red-pigmented dermis. Distribution: Sampled at 900 m depth on HyperDolphin dive 103 (38 20¢ N, 143 55¢ E, temperature 2.95 C, salinity 34.37, dissolved oxygen 0.68 ml/l, sigma-t 27.39 kg/m3, 20 tentacles). Other positive records where the quality of the HDTV imagery left no doubt as to species identification occurred at 1146 m (Dive 104, 39 30¢ N, 144 15¢ E, temperature 2.69 C, salinity 34.44, dissolved oxygen 0.64 ml/l, sigma-t 27.47 kg/m3, 20 tentacles). Comments: As this record represents an extension of the known geographic range, the specimen has been placed in the National Science Museum, Tokyo, under registration number NSMT-Co 1402. Family Nausithoidae Bigelow, 1913 Genus Atorella Vanho¨ffen, 1902 The genus is recognizable in situ by the presence of six tentacles, six rhopalia, and a coronal groove. Atorella sp. Common Japanese name: mutsuashikamurikurage (six legged coronate medusa) Description: Recognizable in situ by a combination of characters including six tentacles, six rhopalia, eight cream-tan gonads, and a heavily red–brown pigmented stomach wall.

Distribution: This species was found in Suruga Bay at a depth of 1038 m with physico-chemical parameters as follows: temperature 3.22 C, salinity 34.45, dissolved oxygen 1.42 ml/l, sigma-t 27.43 kg/m3. Comments: This medusa is provisionally listed here as a possible inhabitant of waters off Sanriku, as water masses with these physico-chemical properties occur in the Kuroshio Current extension area.

Order Semaeostomeae Agassiz, 1862 Family Ulmaridae Haeckel, 1879 Subfamily Poraliinae Larson, 1986 Genus Poralia Vanho¨ffen, 1902 Poralia rufescens Vanho¨ffen, 1902 Common Japanese name: ringokurage (apple medusa) Description: Poraliinae with 30 marginal tentacles and 15 rhopalia arranged in the following sequence, where T ¼ tentacle and S ¼ statocyst: STTTSTTTTSTTSTSTSTSTTSTTSTSTTSTTSTTSTTTSTTSTT. All lappets similar in length and rectangular in outline, with deep cleft in rhopalial lappets and shallow cleft in tentacular lappets; radial canals wide, unbranched (29 of 31 [33] total; one canal branched into two distally, and two canals joined into one distally), leading to a wide peripheral ring canal and usually continuing into each lappet to meet the tentacle bases or rhopalia. Disk diameter 90 mm. Stomach circular, occupying just under half disk diameter. Juvenile specimen? with no visible gonads. Marginal tentacles orange, tapering to a white tip. Exumbrella covered in nematocyst warts. Epidermis reddish– brown except for whitish oral surface of the six oral arms, which are V-shaped in cross section. Partially digested shrimplike animal of 9 mm carapace length, round eyes of 1.5 mm diameter and with reduced rostrum found in stomach. Distribution: The observed distribution of Poralia rufescens is outlined in Figure 4. Physicochemical parameters of their water column habitat were as follows: temperature 2.73.6 C, salinity 34.0234.41, dissolved oxygen 0.63.8 ml/l, sigma-t 27.0927.43 kg/m3. Comments: A single species of lysianassid amphipod was commonly associated with Poralia medusae over the Japan Trench. They were usually

543

Figure 4. Horizontal and vertical distributions of the ulmarid scyphomedusae Poralia rufescens (red circles), Deepstaria enigmatica (white circle), and Tiburonia granrojo (yellow circles) over the Japan Trench during April/May 2002, and correlations with (a) water temperature, (b) salinity, and (c) dissolved oxygen (DO) concentrations. Upturned grey triangles signify latitudes at which XCTD probes were deployed and upturned white triangles where ROV HyperDolphin (HD) dives were conducted.

544 attached at the bases of the oral arms but sometimes elsewhere on the subumbrella and rarely on the exumbrella. Between one and six amphipods were associated with each medusa. The above description of the Japanese morphotype is based on a specimen collected in Suruga Bay on Shinkai 2000 Dive 1335 at a depth of 867 m. It was in much better condition than the four specimens taken over the Japan Trench. Although Poralia sp. has been reported previously from Japanese waters (Hunt & Lindsay, 1999), this is the first identification to species level. The specimen has therefore been placed in the National Science Museum, Tokyo, under registration number NSMT-Co 1403. Subfamily Deepstariinae Larson, 1986 Genus Deepstaria Russell, 1967 Deepstaria enigmatica Russell, 1967 Common Japanese name: diipusutariakurage (deepstar medusa) Description: Recognizable in situ by the lack of marginal tentacles, the presence of five slender oral arms that do not extend beyond the bell margin, the bell being transparent white throughout except for some brown pigmentation around bell margin, and the thin anastomosing gastrovascular canals. Distribution: Sampled at 669 m depth on HyperDolphin dive 98 (41 00¢ N, 144 41¢ E, temperature 2.93 C, salinity 34.13, dissolved oxygen 3.54 ml/l, sigma-t 27.20 kg/m3). Comments: two anuropid isopods inhabited the inner surface of the bell close to the oral arm bases, as previously reported for other Japanese specimens (Lindsay et al., 2000). Although Deepstaria enigmatica has been reported previously from Japanese waters (Lindsay et al., 2000), this is the first specimen collected and it has therefore been placed in the National Science Museum, Tokyo, under registration number NSMT-Co 1404. Subfamily Tiburoniinae Matsumoto, Raskoff & Lindsay, 2003 Genus Tiburonia Matsumoto, Raskoff & Lindsay, 2003 Tiburonia granrojo Matsumoto, Raskoff & Lindsay, 2003 Common Japanese name: yubiashikurage (fingerfoot medusa)

Description: Recognizable in situ by the lack of marginal tentacles, the presence of four to seven thick, blunt oral arms that extend beyond the bell margin to a length of no more than the height of the bell, pigmentation red–brown throughout, nematocyst warts covering all surfaces, and thin, anastomosing gastrovascular canals. Distribution: Sampled at 1019 m depth on HyperDolphin dive 99 (41 00¢ N, 144 42¢ E, temperature 2.66 C, salinity 34.36, dissolved oxygen 0.65 ml/l, sigma-t 27.40 kg/m3). The observed distribution of T. granrojo is outlined in Figure 4. Comments: No organisms were observed in association with these medusae. Although Tiburonia granrojo has been reported previously from Japanese waters (Matsumoto et al., 2003), this is the first specimen collected and it has therefore been placed in the National Science Museum, Tokyo, under registration number NSMT-Co 1405.

Discussion Order Coronatae Vanho¨ffen, 1892 The genera Periphylla and Atolla are arguably the best recognized of the midwater scyphomedusae. Only one species in the genus Periphylla is currently considered valid but, based on the morphotypes we have observed and/or sampled from submersibles and the current trend towards resurrection of previously described scyphozoan species based on molecular data (Schroth et al., 2002), we believe that there may be at least one cryptic species within this nominal species. Periphylla has been reported previously from the 250–1000 m depth layer in the Kurile–Kamtchatka region (Vinogradov & Shushkina, 2002), corresponding most closely with HyperDolphin Station 98 in our survey, and from 504 m in Sagami Bay (Hunt and Lindsay, 1999). No evidence of vertical migration was found for any of the scyphomedusan species observed in this study. However, this may be an artifact of the early retrieval time of the ROV (i.e. only 2 h 20 min after sunset) during the two dives that contained a nighttime component. Atolla wyvillei Haeckel, 1880, is the only currently recognized species of Atolla reported previ-

545 ously from Japanese waters (Kramp, 1961). In situ identification to species level of members of the genus Atolla is often difficult, especially in smaller specimens, without sampling or extensive high definition video footage being taken. Many observational records allowed the elimination of some species from consideration (e.g. not vanhoeffeni or russelli, with 30 tentacles, no papillae, probably wyvillei; transparent, tentacle number between 20 and 24, either vanhoeffeni or a young wyvillei but viewing angle did not allow observation of the eight pigment spots) but did not allow for positive identification of the specimen. The shape and length of the radial septae are usually not discernable in situ, therefore confounding identification based on presently available taxonomic keys. Atolla vanhoeffeni Russell, 1957, and A. russelli Repelin, 1962, are both recorded for the first time from Japanese waters, although A. russelli has been observed in north-east Pacific waters (M. Arai, personal communication) The present specimen of A. russelli differs in several ways from the original description by Repelin (1962), and correlates more closely with the description of A. russelli by Pages et al. (1992) of a single specimen from the Benguela Current. The radial septae are straighter than those shown in the original figure of A. russelli (Repelin, 1962), curving in slightly at the ends as described by Kramp (1968) and Pages et al. (1992). Contrary to previous descriptions, the tips of the radial septae in the present specimen extend just past the edge of the coronal muscle. Although the gonads hang freely in the subumbrellar cavity surrounding the stomach, as described by Repelin (1962) and differing from any other currently recognized species of Atolla, they are not lobed but rather rectangular in shape. Pages et al. (1992) also reported gonad shape to be rectangular but Kramp’s (1968) specimen was too damaged to ascertain their shape or structure. The tentacle number in the present specimen was 20, not 16–18 as reported in previous studies. However, tentacle number has been shown to not always be a reliable character for distinguishing between different species of Atolla (Mauchline & Harvey, 1983). We therefore record this species as A. russelli pending the sampling of at least one other specimen from Japanese waters and re-examination of Repelin’s original material to determine whether the present

specimen instead warrants the erection of a new species. Most of the observations of individuals of Atolla with clear exumbrellas (i.e., Atolla vanhoeffeni) occurred at depths shallower than 500 m. Juveniles of A. wyvillei Haeckel, 1880, were also sampled and they too have clear bells. It is necessary to obtain close-up video footage either of the dorsal or ventral surface to enable distinction from A. vanhoeffeni based on the lack of pigment spots and shape of the stomach base. Tentacle count is greater in juvenile A. wyvillei, gonad size is smaller relative to bell diameter, and there is little dark pigmentation around the rim of the bell, in contrast with A. vanhoeffeni (personal observation). Atolla parva also has a clear non-pigmented bell but should be distinguishable based on tentacle count, lappet shape and gonad size/morphology. Atolla parva has yet to be recorded from Japanese waters. Species of Atolla have been reported previously in Sagami Bay, with nine individuals at depths between 555 and 775 m, and a single individual at 1167 m depth (Hunt & Lindsay, 1999). Records of Atolla also occur in Pe´re`s (1959) from 38 07¢ N, 142 16¢ E, at depths of 580–890 m and temperatures of 3–5 C, but no photographic plates were published so the identifications cannot be checked. The genus Atorella contains only one species with eight gonads, Atorella octogonos (Mills et al., 1987). The Atorella individual observed in Suruga Bay had eight gonads but also had a heavily pigmented stomach wall, unlike the transparent stomach of Atorella octogonos. The only other record of Atorella from Japanese waters is for the polyp of Atorella japonica at depths of 40–60 m off Okinoshima Island, Japan Sea (Kawaguti & Matsuno, 1981). The morphology of the adult medusa of Atorella japonica is unknown. Order Semaeostomeae Agassiz, 1862 A single morphotype of Poralia has been recognized in Japanese waters. Only one species is currently recognized in this genus, Poralia rufescens Vanho¨ffen, 1902, but evidence suggests that multiple species are contained under this name (Larson, 1986). At least two undescribed species have been distinguished (Larson et al., 1991; Larson et al., 1992; Wrobel & Mills, 1998) but descriptions

546 are as yet unpublished. The Japanese species exhibits the characters of P. rufescens Vanho¨ffen, 1902, as currently described (Russell, 1962; Mianzan & Cornelius, 1999). Minimum bottom depth along the survey line was always deeper than 4000 m, proving a midwater, rather than a benthopelagic, habitat for this medusa in contrast with findings by Smith (1982) but agreeing with Larson (1986) and Larson et al. (1992). Benthopelagic occurrences of Poralia may well be artifacts of the sea floor occurring within the regular vertical distribution range of this medusa. Distributional records suggest that Poralia inhabits low temperature waters (Figure 4a) of intermediate salinity (Figure 4b) and is not affected by dissolved oxygen concentrations (Figure 4c). Depression of distributional depth seemed to occur as a result of the subduction of low-salinity waters derived from the Oyashio Current system (Figure 4b). The current Poralia morphotype has been reported previously from Sagami Bay at depths of 614, 750, 835, 857 and 1369 m (Hunt & Lindsay, 1999). Accompanying physico-chemical parameters at those depths in Sagami Bay were as follows: temperature 2.5– 4.9 C, salinity 34.24–34.48 and dissolved oxygen 1.1–1.8 ml/l. The same Poralia morphotype also occurs in plate 4 of Pe´re`s (1959) under the misnomer Aequorea where it is reported from 38 07¢ N, 142 16¢ E, at a depth of 700–750 m and temperature of 3–4 C. The lysianassid amphipod may be a good de facto indicator of Poralia occurrence as no free swimming individuals were observed. Records exist for ?Poralia at depths of 900–3000 m and for ?Stygiomedusa from 750 to 1750 m depth in the Kurile–Kamchatka region (Vinogradov & Shushkina, 2002). However, no reference was made to either a Tiburonia or a paraphyllinid form and the authors themselves appended a question mark to their identifications. Accordingly, we are somewhat circumspect as to the true identities of these large reddish–brown medusae. Several observations of associations between anuropid isopods and Deepstaria enigmatica have been reported (Barham & Pickwell, 1969; Lindsay et al., 2000; S.H.D. Haddock, personal communication). Three specimens of Anuropus bathypelagicus Menzies & Dow, 1958, the species reported to occur in association with D. enigmatica (Barham & Pickwell, 1969), have been caught in otter trawls (mesh aperture 18–30 mm) from depths of

600–700 m over the Japan Trench (Saito et al., 2002). Although all three specimens of Deepstaria enigmatica that we have observed to date have contained a pair of anuropid isopods within their bells, we have yet to observe free-living anuropids during over 300 h of submersible surveys in Japanese waters. It is quite possible that the distributions of anuropid isopods could be used to infer distributional patterns of Deepstaria enigmatica, because the medusa we sampled was observed at 669 m depth, agreeing well with the distribution of 600–700 m depth for Anuropus bathypelagicus reported by Saito et al. (2002). No organisms have so far been observed to occur in association with the medusa Tiburonia granrojo – a medusa recently described as the first member of its subfamily based partly upon samples and images collected during the present research survey (Matsumoto et al., 2003). Conclusions The ulmarid scyphomedusa Poralia rufescens is by far the most common scyphomedusa in waters over the Japan Trench. Its distribution was determined by water mass structure and it was not present in subducted Oyashio Current-derived waters occurring within its usual depth distribution range. Temperature and salinity affected its distribution more than dissolved oxygen concentrations. More data are necessary to characterize the physico-chemical parameters determining the distributions of other members of the scyphomedusan fauna in the study area. The discovery of a putatively new genus of coronate scyphomedusa and a new subfamily of ulmarid scyphomedusa points to our general lack of knowledge about gelatinous deep sea organisms. Acknowledgments We are indebted to Drs Jun Hashimoto, Katsunori Fujikura, Yoshihiro Fujiwara, Shinji Tsuchida, and Mr J. Tanada and M. Oyaizu for logistic and technical support. We sincerely thank the captain and crew of the R/V Kaiyo as well as the operations team of the ROV HyperDolphin for their dedicated efforts. This manuscript was improved by the comments of two anonymous reviewers, to

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