Basilissopsis Hakuhoae, a New Abyssal Seguenziid Gastropod from the Oceanward Slopes of the Japan and Kurile Trenches

VENUS 66 (3-4): 113-118, 2008

Basilissopsis hakuhoae, a New Abyssal Seguenziid Gastropod from the Oceanward Slopes of the Japan and Kurile Trenches

Yukito Kurihara1 and Suguru Ohta2* 1 Department of Geology, National Museum of Nature and Science, 3-23-1, Hyakunin-cho, Shinjuku-ku, Tokyo 169-0073, Japan; [email protected] 2Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan

Abstract: A new seguenziid gastropod, Basilissopsis hakuhoae, is described from 5473-5762 m depths of the oceanward slopes of the Japan and Kurile Trenches, western North Paciﬁc Ocean. This new species is characterized by its large trochiform shell with a shoulder spiral, a bicarinate periphery, and secondary spiral riblets crossing with sigmoidal axial riblets allover the shell surface. Because the combination of these shell characters would be unique in Seguenziidae, the generic placement of this new species is by no means deﬁnitive.

Keywords: Basilissopsis hakuhoae, new species, Seguenziidae, abyssal Gastropoda

Introduction

The family Seguenziidae is a group of small vetigastropods, typically characterized by elaborate microsculpture and complex labral notches and emarginations (Hickman, 1998). It is a typical deep-sea inhabitant with a worldwide distribution on fine sedimentary substrata. Twenty- one genera and approximately 150 species are currently recognized in the Recent fauna (Marshall, 1991; Poppe et al., 2006). In the course of Cruise KH-01-2 of the R/V Hakuho-Maru of the Ocean Research Institute, the University of Tokyo (now operated by Japan Agency for Marine-Earth Science and Technology), two specimens of an undescribed large seguenziid were collected from the oceanward slopes of the Japan and Kurile Trenches in the western North Pacific Ocean. This new species is described and illustrated herein on the basis of the shell, and comparisons are made with other seguenziid taxa.

Materials and Methods

Two specimens of the new species were collected from the oceanward slopes of the Japan and Kurile Trenches (Fig. 1: St. TD-8 and XR-12) at depths of 5473-5762 m, by means of ORE type beam trawl of 4 m span during Cruise KH-01-2 of the R/V Hakuho-Maru of the Ocean Research Institute, the University of Tokyo between September and October, 2001. These sampling stations are shallower than 6000 m, but lie on the oceanward slope of the trench between the trench axis and the trench marginal swell. One of the two specimens is a dead shell but the other is live-collected with the animal body tightly withdrawn into the shell. The former is preserved as a dry specimen, and the latter in 99% ethanol. We observed the shell only and not the operculum, radula or gross anatomy. For SEM observation, a JEOL JSM5310LV SEM at Saitama Museum of Rivers was used in low vacuum mode without gold coating. Measurements, terminology and the suprageneric classification

*Present address: 2-4-12-303 Fujimi, Chiyoda-ku, Tokyo 102-0071, Japan 114 Y. Kurihara & S. Ohta

140°E 144°E 148°E

44°N

HOKKAIDO Kushiro

3000 m

6000 m Kurile Trench42°N

6000 m St.XR-12

HONSHU 40°N

Miyako St.TD-8

Fig. 1. Map showing the sampling 38°N km stations. St. TD-8, type locality

Japan Trench 0 50 100 and XR-12, locality of paratype of Basilissopsis hakuhoae n. sp.

adopted in this study follow those of Marshall (1991). The type specimens described are deposited in the Department of Zoology, National Museum of Nature and Science, Tokyo (NSMT).

Taxonomy

Family Seguenziidae Verrill, 1884 Genus Basilissopsis Dautzenberg & Fischer, 1897

Basilissopsis hakuhoae n. sp. (Figs. 2, 3)

Type Material: Holotype, NSMT-Mo 76732, fresh dead shell (Figs. 2A-D, 3): oceanward slope of the northern Japan Trench off Miyako, 39˚15.54Ń, 144˚45.37É to 39˚17.01Ń, 144 ˚42.46É, 5762-5733 m, R/V Hakuho-Maru, Cruise KH-01-2, St. TD-8, 29 September 2001. Paratype, NSMT-Mo 76733 (Figs. 2E-H): oceanward slope of the southern Kurile Trench off Kushiro, 41˚37.67Ń, 146˚54.19É to 41˚37.08Ń, 146˚52.92É, 5473-5484 m, R/V Hakuho- Maru, Cruise KH-01-2, St. XR-12, 22-23 September 2001. Diagnosis: A large and fragile-shelled species of Basilissopsis, with a shoulder spiral, a bicarinate periphery, and secondary spiral riblets crossing with sigmoidal axial riblets allover the shell surface. Description: Shell large for genus (largest diameter 9.2 mm), trochiform, broader than high, fragile, rather thin, umbilicate, spire 0.74-0.80 height of aperture; white, thin, nacreous within. Protoconch missing. Remaining teleoconch of 5.75 whorls; shoulder gently sloping, broad, very shallowly convex; side narrow, shallowly concave: periphery with two gently angulating New Seguenziid from Japan and Kurile Trenches 115

Fig. 2. Basilissopsis hakuhoae n. sp. Apertural, dorsal, apical and basal views (A-D) of holotype, NSMT-Mo 76732. Apertural, dorsal, apical and basal views (E-H) of paratype, NSMT-Mo 76733. 116 Y. Kurihara & S. Ohta

Fig. 3. SEM images of Basilissopsis hakuhoae n. sp., holotype, NSMT-Mo 76732: Dorsal (A) and lateral (B) views of apical part; enlarged view (C) of a part of body whorl showing the microsculpture; Dorsal view (D) of aperture. Scale bars = 1 mm.

spiral cords, rendered shallowly serrate by axial riblets; base weakly convex. Primary sculpture consisting of three similar, distinctly angulate spiral cords and of collabral axial riblets; intersections rather sharply nodular. Shoulder spiral slightly upturned, submedian. Abapical peripheral spiral with summit covered by succeeding whorls. Axial riblets crisp, rounded, prosocyrt and prosocline or widely sigmoidal, collabral, rather evenly developed over spire, 93 in number on body whorl, 76 on penultimate whorl, obsolete on base; interspaces with faint collabral growth lines. Secondary sculpture consisting of thread-like spiral cords, 11 on shoulder, four on side, one on periphery. Basal spiral cords 20, of which innermost two prominent, narrowly elevated; others broad, flattened. Umbilicus deep, smooth inside, diameter 9-13% of shell diameter. Aperture subrhomboidal. Outer lip thin, simple within; posterior notch narrow; basal notch broad, shallow; no peripheral notch. Parietal glaze thin. Inner lip thin, straight, simple, slightly reflexed. Measurements: See Table 1. Remarks: Because the combination of a relatively large and fragile shell, bicarinate periphery and a submedian spiral cord would be unique in the family, the generic placement of the new species is by no means definitive. The shell form and the pattern of sculpture are somewhat similar to those of some species of Basilissopsis and Ancistrobasis, such as A.regina Marshall, 1983 from bathyal depths off New Zealand, which was later referred to Basilissopsis by Marshall (1991). However, species of Basilissopsis and the closely related Ancistrobasis differ from the new species in having a smaller and stouter shell (maximum diameter usually less than 5 mm), monocarinate or rounded periphery, and more remarkable external ornaments. In its relatively large and fragile shell with a bicarinate periphery, the new species resembles Basilissa lampra Watson, 1879, the type species of Rotellenzia Quinn, 1987, from abyssal depths in the North New Seguenziid from Japan and Kurile Trenches 117

Table 1. Measurements of Basilissopsis hakuhoae n. sp. Abbreviations, H. height; D. diameter; TW. teleoconch whorls (number); AH. aperture height as percentage of shell height; UD. umbilicus diameter as percentage of shell diameter; AA. apical angle. Specimens Register # H (mm) D (mm) H/D TW AH (%) UD (%) AA ( °) Holotype NSMT-Mo 76732 6.8 9.2 0.74 5.75 43 13 84 Paratype NSMT-Mo 76733 7.0 8.8 0.80 5.75 49 9 90

Pacific Basin near Midway Island. However, the single known species of Rotellenzia clearly differs from the new species in having a more depressed shell with obscure external ornaments lacking a submedian spiral. Basilissa soyoae Okutani, 1964 from bathyal depths off Torishima in the Izu Islands superficially resembles the new species. However, B. soyoae is easily distinguished by its smaller shell with a stronger shoulder angulation, single remarkable peripheral keel and distinct tooth on the columella. Etymology: Named after the R/V Hakuho-Maru. Distribution: Known only from the type locality (northern Japan Trench, 5733-5762 m) and that of the paratype (southern Kurile Trench, 5473-5484 m).

Acknowledgements

We would like to thank the crew and colleague scientists who participated in the KH-01-2 Cruise of R/V Hakuho-Maru for their cooperation and help in collecting the samples. Drs. T. Kase and K. Hasegawa (National Museum of Nature and Science) and Mr. B. A. Marshall (Museum of New Zealand Te Papa Tongarewa) read the manuscript and provided helpful comments. Dr. T. Sasaki (University Museum, University of Tokyo) provided some photocopies of references. Mr. M. Sekine (formerly of the Saitama Museum of Rivers) helped us with SEM observation.

References

Hickman, C. S. 1998. Superfamily Seguenzioidea. In: Beesley, P. L., Ross, G. J. & Wells, A. (eds.), Mollusca: The southern Synthesis, pp. 692-693, CSIRO Publishing, Melbourne. Marshall, B. A. 1991. Mollusca: Gastropoda: Seguenziidae from New Caledonia and the Loyalty Islands. In: Crosnier, A. & Bouchet, P. (eds.), Résultats des Campagnes MUSORSTOM, vol. 7, Mémoires du Muséum national d’Histoire naturelle A 150: 41-109. Poppe, G. T., Tagaro, S. P. & Dekker, H. 2006. The Seguenziidae, Chilodontidae, Trochidae, Calliostomatidae and Solariellidae of the Philippine Islands. Visaya, Supplement 2: 1-228.

(Received March 27, 2007 / Accepted October 25, 2007) 118 Y. Kurihara & S. Ohta

日本海溝および千島海溝海側斜面から得られたホウシュエビス科腹足類の１新種，ハクホウリュウグウエビス

栗原行人・太田秀

要旨

東京大学海洋研究所の調査船白鳳丸 KH-01-2 次航海にて日本海溝北部および千島海溝南部の海溝海側斜面の水深 5473-5762 m より得られたホウシュエビス科の１新種，Basilissopsis hakuhoae n. sp.（ハクホウリュウグウエビス：和名新称）を記載した。本新種は本科としては比較的大型かつ脆弱な殻を持つこと，周縁に２本の螺肋を持つこと，螺層中央のやや下位に１螺肋を持つことから本科の既知種とは明らかに異なっている。こうした形質の組み合わせは本科において類例がなく，属位は暫定的なものである。本新種は殻の外形と彫刻パターンの点ではニュージーランドの漸深海帯から知られる Basilissopsis regina (Marshall, 1983) といった Basilissopsis 属および Ancistrobasis 属の種に類似するが，Basilissopsis 属および Ancistrobasis 属の種は，より小型で頑丈な殻を持つこと，１本の螺肋を伴う周縁あるいは丸い周縁を持つこと，より顕著な表面彫刻を持つこと，の 3 点で本新種とは異なる。ミッドウェー付近の北太平洋の深海帯から知られる Rotellenzia lampra (Watson, 1879) は２本の螺肋のある周縁を伴う比較的大型で脆弱な殻を持つ点で本新種に似るが，より扁平な殻形と不明瞭な表面彫刻を持つことから本新種とは異なる。伊豆諸島鳥島沖の漸深海帯から知られるソウヨウリュウグウエビス Basilissa soyoae Okutani, 1964 は，殻が小型であること，肩部および周縁にそれぞれ１本づつの竜骨を持つこと，および軸唇の下方に明瞭な牙状突起を持つこと，の３点から本新種とは容易に区別できる。