Bull. Natl. Mus. Nat. Sci., Ser. A, 49(2), pp. 75–80, May 22, 2023
DOI: 10.50826/bnmnszool.49.2_75

Culeolus tenuis (Urochordata, Ascidiacea) Newly Found oﬀ
Fukushima, Japan in the Japan Trench
Teruaki Nishikawa1,* and Takashi Kuramochi2
1

Department of Zoology, National Museum of Nature and Science,
4–4–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan
2
Hayama Shiosai Museum, 2123–1 Isshiki, Hayama, Miura,Kanagawa 240–0111, Japan*E-mail: nishikawateruaki@gmail.com(Received 17 March 2023; accepted 22 March 2023)

Abstract The ascidian genus Culeolus (family Pyuridae) is known exclusively from deep-seaﬂoors, being anchored by a long, thin hard stalk. A specimen of Culeolus sp. sensu Kuramochi etal. (1999), collected in 1998, was identiﬁed as C. tenuis Vinogradova, 1970, and described indetail, including some taxonomic and ecological remarks. The specimen had been one of 6 individuals observed in situ from the remote-operated submersible “KAIKO”, supported by the JAMSTEC R/V “KAIREI” (KR98-07 #79 Leg. 1), at depths of 5,726 to 5,792 m oﬀ Fukushima Prefecture, Japan, on the land-slope side of the Japan Trench. The species has been previously recorded
from the NW Paciﬁc, including the Japan Trench oﬀ Sendai, Miyagi Prefecture. The examined
specimen was characterized by 6 branchial folds with 3 elongated lobed gonads on each side, andone of the left gonads within the ﬁrst intestinal loop. The habitat ﬂoor oﬀ Fukushima was composed of yellowish sand mixed with black manganese-coated pebbles of olivine basalt, the bottom
water likely being oxygen-rich.
Key words: Culeolus tenuis, Culeolus, stalked ascidian, deep sea, Japan Trench, manganese precipitation, oxygen-rich environment.

Introduction
The ascidian genus Culeolus (family Pyuridae), recorded exclusively from deep-sea ﬂoors,
being anchored by a long, thin hard stalk,includes ca. 30 nominal species, of which three
are known from the vicinity of Japan: C. herdmani Sluiter, 1904 from Sagami Bay, 400 m deep(Tokioka, 1953, pp. 289–289), C. murrayi Herdman, 1881 from the Japan Trench oﬀ Sendai,
Tohoku District, 6,156 to 6,207 m (Vinogradova,1970, pp. 498–502), and C. tenuis Vinogradova, 1970 from the Japan Trench oﬀ Sendai,
6,156 to 6,207 m (Vinogradova, 1970, pp. 491–493). In addition, Kott (2002, Tab. 1) listed C.
annulatus Sluiter, 1904 and C. quadrula Sluiter,© 2023 National Museum of Nature and Science

1904 from “Japan”, but such occurrences areapparently without any literature basis.
Kuramochi et al. (1999) made in situ observations in 1998 of 6 similar ascidians at depths of
5,726 to 5,792 m oﬀ Fukushima Prefecture,
Japan, on the land-slope side of the Japan
Trench, from the remote-operated submersible“KAIKO” (JAMSTEC), with a brief descriptionof a single collected specimen referred to as
Culeolus sp. It is here identiﬁed as C. tenuis
Vinogradova, 1970, previously recorded fromabyssal or hadal ﬂoors in the NW Paciﬁc, from
5,027 to 6,282 m depth (Vinogradova, 1970;
Sanamyan and Sanamyan, 1998, 2006; Fig. 1). A
detailed description of the specimen is given,
with some remarks on taxonomic and ecologicalconsiderations. This is the second record of C.
tenuis from the Japan Trench.

76T. Nishikawa and T. KuramochiTaxonomic AccountCuleolus tenuis Vinogradova, 1970
(Figs. 1–2; Table 1)
Culeolus tenuis Vinogradova, 1970, pp. 491–493, textﬁgs. 1–2; Sanamyan and Sanamyan, 1998, p. 214;
Sanamyan and Sanamyan, 2006, pp. 329–331.
Culeolus sp.: Kuramochi et al., 1999, pp. 1–3, photos 1–3.
Material examined (Deposited in National Museum ofNature and Science, Tsukuba). NSMT-Pc 5353, landslope side of Japan Trench oﬀ Fukushima Prefecture,
Japan (36°23′86.72″N; 143°45′59.96″E), 5,784 m, T.
Kuramochi [from remote-operated submersible “KAIKO”
supported by JAMSTEC R/V “KAIREI” (KR98-07 #79Leg. 1)], 15 July 1998.

Description. Body proper pale pink, gonads
visible through tunic, with a long brownish stalk(in situ observation), but body proper later
becoming brown (Kuramochi et al., 1999, p. 2,photos 1–3); brownish coloration persisting forseveral years following formalin-ﬁxation and70%-ethanol preservation, subsequently becoming pale yellow (same preservative) by early2023. Body length 68 mm, width 22 mm, anddorsoventral depth 35 mm; a long blackishbrown stalk, ca. 365 mm (length), 2.5 to 3 mm
(diameter), distal end ﬁxed in bottom substrate,
proximal end running posteriad along anterior
half of mid-ventral line of body (Fig. 1A, St);

stalk comprising a dark brown, hard, wire-likebut relatively fragile shaft, covered with thin,almost transparent tunic layer; body tunic thin,tough, semi-transparent whitish, entire surface
densely covered with pale to moderately darkbrownish papillae, up to 0.2 mm high (coloration
becoming pale yellow by early 2023); bothsiphons indistinct, branchial aperture terminal,
widely opened with many protruding branchedtentacles, atrial aperture situated at posterior oneﬁfth of body, very wide with anal end of intestine
exposed on left; a distinct low U-shaped crest
widely opened dorsally, running from antero-lateral aspect to posteroventral aspect of atrial aperture on each side to unite at mid-ventral line (Fig.
1A). Mantle very thin, white (almost transparent), body musculature a sparse network of verythin ﬁbers, except around apertures (furnisheddensely with circular ﬁbers). Branchial tentacles
28 in total, including both second- and thirdorder branches (sensu Tokioka, 1953); ciliatedgroove C-shaped, opening to right (Fig. 1D),
many distinct dorsal languets following posteriorly, with six branchial folds on each side (formula: L. D. 3(14)2(17)2(15)4(15)4(12)4(6)10 V.;
R. D. 2(13)3(17)2(16)4(14)4(12)4(7)8 V). Intestinal loop simple, U-shaped, elongated longitudinally; stomach extensively covered with complicatedly branched hepatic lobes (Fig. 1C). ThreeFig. 1. Map showing the localities of Culeolus tenuis Vinogradova, 1970, so far known (open circles) or newly
recorded in the present study (solid circle).

Culeolus tenuis from the Japan Trench

77

Fig. 2. Culeolus tenuis Vinogradova, 1970 from oﬀ Fukushima Prefecture in the Japan Trench. A: Left side ofbody proper with branchial tentacles projecting from branchial aperture (Br) and anal end visible throughatrial aperture (At), B: Inner view of right mantle of body, C: Inner view of left mantle of body; D. Dorsal
tubercle and vicinity, E: Proximal (＝posterior) terminal of right dorsal-most gonad, directed to atrial aperture
end, to show openings of oviduct (Od) and sperm duct (Sd); F: A horny polygonal spicule from pericardium.
Mantle musculature and spicules in B and C, and in B-E, respectively, omitted. Abbreviations: At: atrial aperture, Br: branchial aperture, En: endocarp, Ep: pericardium, Od: oviduct, Ov: ovarian eggs, Sd: sperm duct, St:proximal end of stalk along anterior mid-ventral half of body proper, Te: testicular follicles. Scales for A, B,and C 10 mm, for D and E, 5 mm, and for F, 1 mm.

partly matured, longitudinally elongated gonads
on each side, each comprising several lobes discernible as simple or complicated endocarps, twogonads in tight contact with each other along
dorsal aspect of decending limb of gut loop, athird gonad occupying ﬁrst intestinal loop on left

(Fig. 1C), all three in close contact with one
another along epicardium on right (Fig. 1B),
each gonad furnished distally (near atrial siphon)with a free elongated end of sperm duct, accompanied by almost sessile end of oviduct (Fig.
1E); ovarian eggs up to 0.6 mm in diameter; non-78

T. Nishikawa and T. Kuramochi
Table 1.

Known localities of Culeolus tenuis Vinogradova, 1970, listed from north to south, also see Fig. 1

Latitude (N)Longitude (E)Depth (m)Number of individuals

Source54°58′
48°15′
46°08′
45°18′
45°18′
44°43′
38°11′
36°24′
33°18′

165°44′169°39′155°16′156°00′156°00′153°49′143°56′143°46′149°46′

6,074–5,300
6,272–6,282
5,070–5,090
5,035–5,210
5,030–5,031
5,027
6,156–6,207
5,784
6,096

123
1
102*
32
11

Sanamyan & Sanamyan, 1998
Vinogradova, 1970
do.
do.
Sanamyan & Sanamyan, 2006
Vinogradova, 1970
do.
present study
Vinogradova, 1970*) Part of Vinogradovaʼs 10 specimens listed immediately above.

gonadal endocarps absent from mantle and intestinal surface facing peribranchial cavity. Flat,yellowish or white (almost transparent) polygonal horn-shaped spicules up to 1.5 mm long discernible densely in walls of branchial sac, pericardium (not heart), intestine (including hepaticlobes), and gonads (Fig. 1F).
Remarks. The present specimen is closely
similar to the original description of C. tenuis
given by Vinogradova (1970) for 40 syntypespecimens (holotype not designated) from 6 stations in the NW Paciﬁc at depths of 5,027 to
6,282 m; in particular, in the proximal end of thestalk running along the anteroventral aspect ofthe body, the tunic crest U-shaped and opening
dorsally, the dense visceral distribution of horny
spicules, 6 distinct branchial folds, a simple ciliated groove, simple U-shaped intestine, and 3elongated, (almost always) lobed and endocarpbearing gonads on each side, opening near theatrial aperture. The position of the ventral-mostgonad on the left (i.e., within the ﬁrst intestinal
loop or not), seems to be a signiﬁcant diagnostic
character within the genus, but the originaldescription of C. tenuis lacked that detail. An
attempt (by TN) to examine the syntypes of C.
tenuis, deposited at the P. P. Shirshow Institute of
Oceanology of Russian Academy of Sciences,
Moscow (Vinogradova, 1970, p. 490), wasunsuccessful. However, Sanamyan and Sanamyan (2006) described “two intact specimens” ofC. tenuis, collected at [R/V ʻVityazʼ] “St. 5621,5030–5031 m” and “identiﬁed as Culeolus tenuis
by Vinogradova” (p. 330) [part of Vinogradovaʼs(1970) 10 syntypes from the ʻVityazʼ station 5621
(K. Sanamyan, personal communication; seeTab. 1)], as having three elongated lobed gonads
on each side, with one of the left gonads includedin the ﬁrst intestinal loop. In gonadal number andarrangement, therefore, the present specimen
from the Japan Trench is identical to thosedescribed by Sanamyan and Sanamyan (2006),
supporting the identiﬁcation of the specimen collected oﬀ Fukushima. The description by Sanamyan and Sanamyan (1998) of a single non-type
specimen of C. tenuis from the Aleutian Trench
noted only two gonads (instead of three) on theright side, whereas the number and arrangementof left gonads matched the description in Sanamyan and Sanamyan (2006) of specimens fromthe Kurile-Kamchatka Trench, as well as thepresent Japan Trench specimen. The taxonomic
status of the Aleutian specimen should be reconsidered following examination of further material.
Sanamyan and Sanamyan (2006) noted specimens as having the tunic surface “almost smooth,without papillae” (p. 330), Vinogadova (1970)
having earlier described the tunic surface insome specimens as being furnished with irregularly-shaped ﬂat papillae at the posterior endof the body (p. 493). However, the entire bodysurface of the present specimen was densely
covered with minute papillae. Any taxonomic
signiﬁcance of this diﬀerence is presently questionable, and requires further investigation.
Although Monniot and Monniot (1991, p. 421)
regarded C. sluiteri Ritter, 1913 from the NW
Paciﬁc as “peut-être synonyme de” (perhaps aCuleolus tenuis from the Japan Trenchsynonym of) C. tenuis, they overlooked the presence of only “ﬁve prominent (branchial) folds oneach side” (Ritter, 1913, p. 464) in the former
species, compared with six in C. tenuis, as notedby Sanamyan and Sanamyan (1998, 2006). Thus,C. tenuis can be regarded as valid, being distinct
from C. sluiteri. Although Sanamyan and Sanamyan (1998, 2006) questioned the validity of C.
nadejdae Sanamyan, 1992 as a species distinct
from C. tenuis, the former is so far known only
from the Sea of Okhotsk, in 1,050–1,040 m depth(Sanamyan, 1992, pp. 188–190), and is clearly
distinguishable from C. tenuis by the number andarrangement of left gonads [total of 2–5, all inthe ﬁrst intestinal loop in the former; 3 (one inthe loop) in the latter], as correctly noted bySanamyan and Sanamyan (2006).
Ecological Considerations
The in situ observations by Kuramochi et al.
(1999) of 6 individuals of C. tenuis, made at
depths of 5, 726 to 5,792 m oﬀ Fukushima in theJapan Trench from the remote-operated submersible “KAIKO” over a period of 2 hours, indicatedthat individuals occurred sparsely on the sandand gravel sea ﬂoor, and at no time on neighboring rocky substrata. The long stalk projected
almost vertically from the sea ﬂoor, the bodyproper moving slightly in the water column withthe dorsal surface upward (i.e., longitudinal axis
horizontal). A video is available at https://www.
godac.jamstec.go.jp/jedi/static_player/j/KAIKO
0079BCSV10_00503890 (accessed on Mar. 2,2023). The sea ﬂoor comprised yellowish sandmixed with manganese-coated black pebbles ofolivine basalt. As stated above, the live bodycolor of the collected individual was initiallypale pink, becoming brown on board soon aftercollection, and later pale yellow in preservative.
The chemical processes behind the color changes
remain to be determined, but it is likely that thebody was also coated with manganese (possiblywith iron as a ferromanganese crust), whichbecomes brown depending upon oxygen concentration. In any case, because manganese or ferro-79

manganese in deep water is precipitated by oxidation (Usui et al., 2020), the habitat of C. tenuis
can be regarded as oxygen-rich, although the insitu measurements taken of dissolved oxygen
content are now unavailable (JAMSTEC DataManagement Oﬃce, personal communication).
Acknowledgements
Our sincere thanks go to Dr. Karen Sanamyan(Far-Eastern Branch of the Russian Academy ofSciences) for indicating the location of Sanamyan and Sanamyanʼs (2006) C. tenuis specimens, as well as providing helpful comments on
the manuscript, Dr. Tohru Iseto (JAMSTEC DataManagement Oﬃce) for specimen collection,
environmental data, and the in situ video taken
during the KR98-07 #79 cruise, late Dr. Mutsuo
Hattori (JAMSTEC) for much help in sampling,and Dr. Graham S. Hardy (Ngunguru, New Zealand) for assistance with English.
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