©Ma lacological Society of Japan

Short Notes

A New Deep-Sea Protobranch Bivalve from Southwestern Japan

Takashi Okutani Japan Agency for Marine-Earth Science & Technology, 2-15 Natsishima-cho, Yokosuka, Kanagawa 237-0061, Japan; [email protected]

Amo ng diverse benthos specimens collected smoothly convex; postero-ventral margin gently by the submersibles and ROVs of the Japan ascending; postero-dorsal margin straight, forming Agency for Marine-Earth Science & Technology acute angle at meeting point with ascending postero- (JAMSTEC), many interesting mollusks have been ventral margin. Blunt radial ridge running from discovered. The bivalves and some gastropods umbo to posterior tip defines escutcheon. Internal associated with chemosynthetic environment have surface white, with weak luster. Adductor scars been promptly described (see Sasaki et al., 2005), indistinct. Pallial sinus moderate. Hinge plate but many more specimens are remained unstudied. strong. Anterior arc with 11 strong teeth; 3–4 This paper describes a unique new species of the proximal teeth small and obsolete. Posterior arc deep-sea family Neilonellidae collected by the with 13–14 strong teeth and 3–4 small proximal ROV Hyper-Dolphin from a depth of 641 m on the teeth. No resilium present between anterior and Kuroshima Knoll, south of Ishigaki Island, posterior arcs. southwestern Japan. Measurements in mm (SL × SH × SW): Holotype (NSMT-Mo 77381; ex-JAM STEC 055153), 8.9 × Abbreviations: JAMSTEC – Japan Agency for 5.5 × 4.6. Pararype #1 (NSMT-Mo 77382a; ex- Marine-Earth Science & Technology; NSMT – JAMSTEC 055154), 6.5 × 4.3 × 3.2; paratype #2 National Museum of Nature & Science, Tokyo; SH – (NSMT-Mo 77382b; ex-JAMSTEC 055155), 6.5 × shell height; SL – shell length; SW – shell width. 4.3 × 3.1. Etymology: The species name is after the type locality, Kuroshima Knoll. Remarks: The memb ers of the family Neilonellidae Family Neilonellidae Schileyko, 1989 usually have a shell covered by a smooth olive- colored periostracum, and seldom with strong Neilonella kuroshimaensis n. sp. commarginal riblets (e.g. Coan et al., 2000). In (Fig. 1) contrast, this new species has strong riblets and straw-colored periostracum, both rather rare among Material examined: Four conjoined specimens, the family. including the holotype and two paratypes, from the This species resembles Neilonella kirai Habe, Kuroshima Knoll, south of Ishigaki Island, 1953 from Tosa Bay in having regular commarginal Okinawa chain (ROV Hyper-Dolphin Dive #291: riblets. However, N. kirai has a more inflated shell April 14, 2004), 24°07.8´N, 124°11.6´E, 641 m. with a short anterior portion, and a distinctly Description: Shell elongate oval, equivalve, upturned posterior rostrum. N. kirai measures SL inequilateral. Surface covered by pale yellow 10.0 mm × SH 6.4 mm × SW 4.7 mm (Habe, periostracum, ornamented by regularly spaced, 1953). Another species, N. dubia Schepman, 1932 rather sharp commarginal riblets. Umbo low, from the Flores Sea (Siboga St. 52: 9°3´.4S, situated about 40% anteriorly, almost orthogyrous. 119°56´7E, 959 m) has commarginal lamellae that Prodissoconch indistinct. Antero-dorsal margin are denser than those in the present species and almost straight; anterior to ventral margins weakly undulating in the middle. It also has more 204VENUS 69 (3), 2011

ROV. However, this species may not be endemic to chemosynthetic environment, as no protobranch is known to be directly associated with vent/seep communities. Additional notes: Two conjoined valves and odd valves of a very young stage (SL3.1–2.7 mm × SH2.2–1.6 mm × SW1.5–1.2 mm) in the R/V Hakuho-Maru collection from a depth of 576–594 m off Amami-Oshima Island (St. OT-06 of KH-05- 1: May 13, 2005: 28°32.2´–31.1´N, 127°01.8´– 01.5´E), NSMT-Mo 77387, are presumed to be conspecific with this new species, and are here described: Commarginal riblets about 30 in number. Postero-dorsal margin more straight or even slightly convex, postero-dorsal tip not upturned. Hinge plate very strong relative to size of shell. Anterior arc with five developed teeth and 3–4 small proximal teeth. Posterior arc with six strong teeth and 3–4 small proximal teeth.

Acknowledgements: I am indebted to the diving teams of ROV Hyper-Dolphin and the crew of the Fig. 1. Neilonella kuroshimaensis, n. sp. Holotype, supporting vessel of JAMSTEC. Thanks are due to NSMT-Mo 77381 (SL = 8.9 mm). Dr. H. Saito, NSMT, who permitted me to examine the Hakuho-Maru collection under his care. teeth (18 + 22). An Atlantic species, N. striolata (Brugnone, 1876), bears a certain similarity to the References present species, particularly in having commarginal sculpture, but N. striolata has a broader posterior Allen, J. A. 1978. Evolution of the deep-sea area with an acute posterior end. protobranch bivalves. Philosophical Cox et al. (1969) and Allen & Hannah (1986) Transactions of the Royal Society of London treated Neilonella Dall, 1881 as a synonym of 234: 387– 401. Saturnia Seguenza, 1877. However, subsequent Allen J. A. & Hannah, F. J. 1986. A reclassification authors such as Habe (1977), Allen (1978), Coan et of the Recent genera of the subclass (: ). Journal of al. (2000), and Kurozumi & Tsuchida (2000) Conchology 32: 225 – 249. among others, still retain Neilonella as distinct, an Coan, E. V., Valentich Scott, P. & Bernard, F. R. opinion followed in this paper. The generic 2000. Bivalve seashells of western North characters in shell morphology may be the hinge America. Marine bivalve mollusks from arctic armature and pallial sinus. According to Habe Alaska to Baja California. Santa Barbara (1977) Neilonella has taxodont teeth that are Museum of Natural History Monograph, 2: minimized below the umbo, and lacks a pallial viii + 764. sinus. Cox et al. (1969) stated (as Saturnia in Cox, L. R. & 24 authors. 1969. Bivalvia. Vol. 1. In: Malletiidae) that the genus is characterized by a Moore, R. C. (ed.), Treatise on Invertebrate “strong” concentric sculpture, and weak pallial Paleontology, Part N, p. xxxviii + N1-N489. Geological Society of America & University sinus. Coan et al. (1969) stated that the pallial sinus of Kansas. is large. These comments suggest that the pallial Habe, T. 1953. Descriptions of twelve new sinus of Neilonella may vary from “lacking” to Japanese shells. Venus (Japanese Journal of weak (shallow) or “large” (deep). Malacology) 17: 130 – 144. The type lot was collected from bathyal depth Habe, T. 1977. Systematics of Mollusca in Japan. (641 m) near the Kuroshima Knoll during the Bivalvia and Scaphopoda. xiii+372 pp. investigation of vent fauna using JAM STEC’s Zukan-no-Hokuryukan, Tokyo. Short Notes 205

Sasaki, T., Okutani, T. & Fujikura, K. 2005. 南西日本の漸深海底の Molluscs from hydrothermal vents and cold seeps in Japan: A review of taxa recorded in ハトムギソデガイ科の１新種 twenty recent years (1984 – 2004). Venus 64: 87 – 133. 奥谷喬司

(Accepted October 8, 2010) Neilonella kuroshimaensis n. sp.スダレハトムギソデ ガイ（新種・新種） 殻長 8．9 mm （ホロタイプ） 。殻表は同心円状の 肋があるところはキラハトムギソデガイに似ている が、殻頂は前から 40％の所にあり、後部は嘴状に 反り上がらない。また殻皮は同科の他種がオリー ブ色であるが、本種はむしろ藁色。石垣島南方の 黒島海丘の水深 641 m から無人潜水探査機 （ROV）ハイパードルフィンによって採集された。 また、白鳳丸が奄美大島沖の水深 576～594 mから 採集した殻長 3.1 mm 以下の幼殻は本種と思われ る。