VENUS 65 (3): 193-202, 2006

Description of the Bivalve Litigiella pacifica n. sp. (: Galeommatoidea: ), Commensal with the Sipunculan Sipunculus nudus from the Ryukyu Islands, Japan

Jørgen Lützen1 and Takeharu Kosuge2 1Department of Cell Biology and Comparative Zoology, Biological Institute, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen Ø, Denmark; [email protected] 2Ishigaki Tropical Station, Seikai National Fisheries Research Institute, 148-446 Fukai-ota, Ishigaki, Okinawa 907-0451, Japan

Abstract: In the Ryukyu Islands, Japan, Litigiella pacifica n. sp. lives attached to the body of the burrowing sipunculan Sipunculus nudus. The morphology of the shell and the soft parts are described and compared with other bivalve commensal with the same sipunculan. The new is hermaphroditic, and the filamentous sperm cells are stored in paired seminal receptacles.

Keywords: Litigiella pacifica, Sipunculus nudus, morphology, reproduction, commensalism

Introduction

Sipunculus nudus Linnaeus, 1766 is a large sipunculan that inhabits inter- and subtidal sandy or muddy bottoms and is distributed world-wide (Stephen & Edmonds, 1972). Four species of galeommatoidean bivalves have been found to attach to S. nudus: Litigiella cuenoti (Lamy, 1908), Fronsella ohshimai Habe, 1958, F. philippinensis Habe & Kanazawa, 1981 and Pseudopythina nodosa Morton & Scott, 1989. Of the four species, L. cuenoti has been recorded from southwest- ern Europe and the Mediterranean Sea, and the other three from temperate to tropical regions of the Western Pacific: F. ohshimai from Japan (Habe, 1964); F. philippinensis from the Philippines and the Ryukyu Islands, southern Japan (Habe & Kanazawa, 1981; Kosuge & Kubo, 2002); and P. nodosa from Hong Kong (Morton, 1988). When sampling intertidal crabs in Ishigaki Island in the southern Ryukyu Islands, we encountered another commensal with S. nudus. Although it is similar to L. cuenoti, differences in shell characters have led us to describe it as a new species. The present paper deals with the species’ morphology and histology, reproduction and way of life.

Material and Methods

The host worms were dug up at low tide on the mud flats at Nagura Bay on Ishigaki Island, Ryukyu Islands. Ten specimens of Litigiella pacifica were collected on 21-23 May, 20 June, 13 August, and 7 November 2000. One more bivalve was collected on the same host on 6 July 2001 at Awase tidal flat, Okinawa City, Okinawa Island. The bivalves were preserved and decalcified in Bouin’s fluid. The of a few specimens was removed to study the overall morphology. The preserved specimens were processed in one of two ways. Six were embedded in Paraplast and cut into 8-μm thick serial sections stained with hematoxylin and eosin (H+E). Four others were embed- ded in Araldite and cut into 2-μm thick sections stained with toluidine blue. Shell length and shell height are abbreviated as SL and SH, respectively. For comparison, the following material was also examined: empty shells of the European L. cuenoti collected at Laredo, N. Spain in 1977, and the holotypes of two other bivalves associated 194 J. Lützen & T. Kosuge with Pacific S. nudus, namely Fronsella ohshimai Habe, 1958 (NSMT-Mo 49819) and F. philip- pinensis Habe & Kanazawa, 1981 (NSMT-Mo 59497).

Systematic Account

Superfamily Galeommatoidea J. E. Gray, 1840 Family Lasaeidae J. E. Gray, 1847

Genus Litigiella Monterosato, 1909

Type species: Erycina cuenoti Lamy, 1908 [=Lepton glabrum P. Fischer, 1873], original desig- nation. Diagnosis: Shells ovate, partly transparent, anterior region longer than posterior. Valves rather compressed, somewhat produced anteroventrally. Hinge flat, broad, each valve with single narrow protruding cardinal and distinct anterior and posterior laterals. Behind cardinal tooth, single hollow and oblique oblong resilifer. Ctenidia with single (inner) demibranch. Seminal receptacles paired. Remarks: The genus Litigiella was established by Monterosato (1909) to accomodate Erycina cuenoti, described the year before by Lamy (1908). Monterosato (1875) synonymized Lepton glabrum P. Fischer, 1873 with L. cuenoti, but the hinges are somewhat dissimilar and the identity of L.glabrum cannot be verified as the types no longer exist (personal communication, Philippe Bouchet, Muséum national d’Histoire naturelle, Paris). Pelseneer (1911) transferred the species to Montacuta Turton, 1822 (as M. glabrum) and included it in his own M. perezi Pelseneer, 1909. Montacuta Turton, 1822, as defined by its type species, M. substriata (Montagu, 1808) has neither posterior laterals nor cardinal teeth, seminal receptacles are absent, and the sperm are totally differ- ent from those of the present species (Oldfield, 1961). A second species of Litigiella, L. corgani, was established by van Aartsen (1996) based on empty shells. The hinge and resilifer in Litigiella are a close match with those of Brown, 1827, but species of this genus have one and a half demibranchs per ctenidium, widely different sperm cells (Ó Foighil, 1985), and no seminal recepta- cles. Because the definitions of the traditional families of the Galeommatoidea are still insufficient, we have followed Hoeksema et al. (1995) in including Litigiella in the Lasaeidae in its wider sense (Coan et al., 2000), which encompasses also the Kellidae and Montacutidae.

Litigiella pacifica n. sp. (Figs. 1-5)

Type locality: Nagura Bay, Ishigaki Island, Ryukyu Islands. Type material: Two cleaned shells (SL 4.9×SH 3.9 mm and SL 5.9×SH 4.5 mm) were col- lected 26 June 2005. The larger one has been selected as the holotype and deposited in the National Science Museum, Tokyo (NSMT-Mo 73728). The smaller one is kept as a paratype in the Zoological Museum of the State Natural History Museum, Copenhagen, Denmark. Host attachment: The bivalves attach to the skin of the host by means of several very thin bys- sus threads. They are usually located on the body near the posterior end. Sometimes two or three were found on the same host specimen.

Description Shell: The shell is relatively thin and translucent at the centre where the yellowish brown peri- ostracum has worn away (Fig. 1). The surface has both many fine and coarse incremental lines. Overlying the occur patches of dark brownish deposits, especially dorsally and pos- teriorly. The shell is ovate, the margins everywhere gently rounded, the small orthogyrate umbones Litigiella pacifica n. sp. from the Ryukyu Is. 195

Fig. 1. Litigiella pacifica n. sp. Cleaned shell of holotype. Right valve seen from outside (A) and inside (B), and left valve seen from outside (C) and inside (D). located well behind the middle. Both hinges have a single cardinal, larger in the right than in the left valve, and distinct anterior and posterior lateral teeth (Figs. 2C, D). The internal resilifer is long and located on the margin of the posterior lateral. Soft parts: The powerful foot has several densely ciliated grooves on its ventrolateral surfaces. The byssus gland is located in the posterior portion of the foot. A duct leads from it into a groove running along the ventral margin of the foot and opens behind its anterior tip. The gill axis of the single (inner) demibranch is almost vertical. There is no vestige of an outer demibranch. The gill is attached to the mantle at the base of a small exhalent aperture. The inner (muscular) mantle folds have fused for a considerable distance below the exhalant aperture. The much larger anterior aperture is a combined inhalent and pedal opening. None of the apertures form siphons. The middle (sensory) mantle fold is of modest size and bears no papillae. Immediately above the muscular fold and running along the anterior part of the mantle periphery there are two glandular non-ciliary tracts. The globular cells of the smaller and more dorsal of these tracts are clearly mucous-producing. The larger, ventral tract consists of columnar cells that are packed with minute eosinophilous granules and are interspersed by supporting cells. In the region of the exha- lant aperture there is a corresponding, but shorter, glandulo-ciliary tract. It is probably concerned with the rejection of waste matter. All along the periphery of the mantle there are powerful bundles 196 J. Lützen & T. Kosuge

Fig. 2A-B. Litigiella cuenoti (Lamy), Laredo, N. Spain. Hinge of right (A) and left valve (B). 2C-D. Litigiella pacifica n. sp., holotype. Hinge of right (C) and left valve (D). al, anterior lateral tooth; ca, cardinal tooth; re, resilifer; pl, posterior lateral tooth. Arrows indicate the insinuation of the dorsal shell margin in L. cuenoti. Scale represents 500 μm. of radial muscles. There are two pairs of extremely small labial palps with hardly any detectable ridges on the opposed surfaces. In some specimens, the lateral sides of the visceral mass a number of small bulg- es containing digestive diverticulae protrude into the mantle cavity (Fig. 4A). The kidneys are of ordinary structure. They consist of two glandular sacs that intercommunicate immediately between the posterior foot retractors and the pericardial cavity. Each kidney is lobed dorsally and ventrally. Within the lumen of the kidney in most of the sectioned specimens occur several solid excretory concretions in the shape of whitish pellets (Fig. 5C). They are visible from outside, when the shell is removed. The heart and pericardial cavity are of normal position and structure. Reproductive organs: The gonad is an ovotestis (Figs. 3A, 5B). The compact main portion of the gonad occupies the posterior part of the visceral mass and extends forward on each side of the digestive diverticulae and to the right side of the intestine and style sac. From this part a number of branching lobes spread further onto the left, right and ventral surfaces of the digestive gland. Litigiella pacifica n. sp. from the Ryukyu Is. 197

Fig. 3. Litigiella pacifica n. sp. A. Morphology of soft parts of hermaphrodite, seen from left side, valve and mantle of that side removed. B. Spermatozoon as seen in the light microscope, showing acrosome, nucleus (black) and midpiece. aa, anterior adductor; af, anterior foot retractor; bg, opening of byssus gland; cg cerebral ganglion; dg, digestive gland; ga, gill axis: k, kidney; lp, labial palp; ov, ovotestis (testis black, ovary white); pa, posterior adductor; pe, pericardium; pf, posterior foot retractor; r, rectum; rd, right inner demibranch; rmm, radial mantle muscles; rs, seminal receptacle; ss, style sac; st, stomach; vg, visceral ganglion. Arrows indicate water current through mantle cavity and gills. Left and right mantle are fused between asterisks. Scales represent 1 mm (A) and 2 μm (B).

Fig. 4. Litigiella pacifica n. sp. A. Horizontal section through male (SL 5.2 mm). B. Horizontal section through immature specimen (SL 2.5 mm) showing ciliated epithelium of empty seminal receptacle and renal aperture. f, foot; k, kidney; m, mantle; p, protrusions of digestive gland (dg) into mantle cavity; pf, posterior foot retractor; r, rectum, ra, renal aperture; rs, seminal receptacle; sb, suprabranchial chamber; ss, style sac; t, testis. 8-μm paraplast sections stained with H+E. Scales represent 200 μm (A) and 50 μm (B).

Although there is no separation between male and female parts, the former predominates in, or is restricted to, the posterior portion of the ovotestis. In the few specimens showing full or almost full sex reversal, even this posterior part becomes invaded by ovarian tissue. The sperm-oviducts are short and ciliated and open far back in the suprabranchial chamber near the nephidiopores and openings of the seminal receptacles. The smallest specimen (SL 2.5 mm) was immature. The gonad of five specimens (SL 3.5-4.6 198 J. Lützen & T. Kosuge

Fig. 5. Litigiella pacifica n. sp. A, B, D. Horizontal 2-μm araldite sections stained with toluidine blue trough hind body of hermaphrodite (SL 4.1 mm) with seminal receptacles (rs). C. 8-μm paraplast section stained with H+E through seminal receptacle and kidney with renal concretions. ac, zone of acrosomes of oriented sperm cells; k, kidney; nu, nuclei of sperm cells; op, opening of seminal receptacle; ov, ovary; ovt, ovotestis; pf, posterior foot retractor; sb, suprabranchial chamber; t, testis. Scales represent 100 μm (A-C) and 20 μm (D). and 6.5 mm) were hermaphroditic with oocytes up to 80 μm in diameter and perhaps therefore mature, while three other specimens (SL 5.2-5.8 mm) were entirely males, and one (SL 5.1 mm) was entirely female. None of the sectioned specimens were brooding eggs or embryos. Paired semi- nal receptacles are embedded in the visceral mass and the kidneys (Fig. 5). They are ovoid or spher- ical with a largest diameter of 125-175 μm and, because of their superficial location, can be seen Litigiella pacifica n. sp. from the Ryukyu Is. 199 from outside in decalcified specimens (Fig. 3A). The receptacle wall consists of a simple cuboidal epithelium, seen in empty receptacles to be ciliated (Fig. 4B). Each receptacle communicates with the suprabranchial chamber by means of a short, slightly convoluted ciliated duct that opens on a 200 μm long papilla close to the opening of the sperm-oviduct (Figs. 3A, B). The receptacles were present in a 2.5 mm large juvenile specimen , but were small (25×40 μm) and lacked sperm cells. A 3.5 mm long predominantly female bivalve (with 65 μm large oocytes) had fully developed but empty receptacles; all other specimens (SL 4.1-6.5 mm), whether male, female, or hermaphrodite, had both receptacles packed with sperm cells that were neatly oriented with their heads pointed towards the receptacle epithelium (Fig. 3D). The sperm have 9 μm long filamentous heads consist- ing of an 8 μm long and 0.6 μm wide nucleus terminating in a small pointed acrosome. No other type of sperm cell was found in the receptacle or in the ovotestis.

Discussion

Comparisons The shape and dimension of the valves of L. pacifica are very similar to those of L. cuenoti. The SL and SH of the holotype (5.9×4.5 mm) almost match those of Lamy’s specimen (6.0×4.5 mm). The maximum size of 12 Okinawan specimens is 6.5 mm, compared to 6.0 mm (Lamy, 1908), 8.0 mm (Pelseneer, 1909) and and 8.5 mm (Hoeksema et al., 1995, as L. glabra) in European speci- mens. The dentition of the two species matches each other (Fig. 2), but a consistent difference noted in all our studied shells is that the dorsal shell margin below and immediately in front of the umbo does not show the distinct insinuation present in the European species (Figs. 2A, B; Hoeksema et al., 1995). The left and right cardinals are equal in size in L. cuenoti, while in L. pacifica the right one is significantly larger. L. pacifica and L. cuenoti distinguish themselves from L. corgani in having two lateral teeth in the right valve rather than one. L. pacifica resembles the specimens of L. cuenoti studied anatomically by Pelseneer (1909; 1911: pl. XIV, figs. 9, 0, and pl. XV, figs.1, 2, 7, as Montacuta glabra) in that both have only one demibranch, small lateral protrusions of the visceral mass invaded by parts of the digestive glands (in L. cuenoti also parts of the ovary), and paired spherical seminal receptacles (vesicula seminalis of Pelseneer). Fronsella ohshimai and F. philippinensis (originally referred to the Montacutidae) are two other commensals with S. nudus. F. ohshimai differs from the commensal species of Litigiella in that the anterior lateral is absent and thus satisfies the definition of Fronsella Laseron, 1956 (Habe, 1958). Although the description of the hinge of F. philippinensis matches that of the two species of Litigiella reasonably well, it was not illustrated and the ventral shell margin is angular rather the regularly curved (Habe & Kanazawa, 1981). Pseudopythina nodosa (originally referred to the Kellidae), a third commensal, has no posterior laterals and the short ligament is seated in a deep resilifer immediately below the umbo (Morton & Scott, 1989).

Reproduction The present species is so close to L. cuenoti, that the two may be considered together. As the sex in the studied specimens of L. pacifica did not correlate to size, it is possible that the species under- go repeated sex reversals. Neither our specimens nor the few examples of L. cuenoti containing soft parts and studied by Lamy (1908) were brooding ova, embryos, or larvae within the suprabranchial chamber. In spite of this, there can be little doubt that both species are ovoviviparous like all other galeommatoideans, storing sperm in seminal receptacles, and Pelseneer (1911) moreover stated that L. cuenoti incubates the ova in the gills, having probably observed this in his own specimens. Pelseneer (1909, 1911) considered the receptacle to be a seminal vesicle, an expression normally reserved for a vesicle for deposition of autosperm, and that from the fact that the receptacles were full in all specimens producing sperm and in the single female, which may have had a past as a 200 J. Lützen & T. Kosuge male, it is difficult to deny that the receptacles contain the bivalve’s own sperm and that the species may reproduce by self-fertilization. This is unlikely, however, since autofertilizers normally pro- duce relatively small amounts of sperm (Charlesworth & Charlesworth, 1981), while in the present species plenty were present not only in the males, but also in some of the hermaphrodites. Also, in other hermaphrodites having receptacles, it has been shown beyond reasonable doubt that they con- tain only allosperm (Lützen et al., 2001). On the other hand, since the bivalves sometimes occurred singly on a host, it is possible that it may at least have the potential for autofertilization. The slender filiform heads of the sperm are packed as closely together as possible and oriented towards the receptacle’s epithelium in the same way sperm are stored in Pseudopythina macrophthalmensis Morton & Scott, 1988, Peregrinamor ohshimai Shôji, 1938 and Mysella bidentata (Montagu, 1803) (Jespersen et al., 2001; Lützen et al., 2001; Jespersen & Lützen, 2001). This allows the maximum number of sperm to be deposited within a limited space. Renal concretions in bivalves often consist of calcium phosphate, and more rarely of breakdown products of purine nucleotides (Lucas & Hignette 1983; Secchi & Martoja, 1986). They usually accumulate within the cells of the kidney and their presence in the lumen of the kidney of L. pacifi- ca is unusual, although similar concretions have also been found in the kidney of the commensal bivalve (Montagu, 1808) by one of the present authors (JL).

Host and distribution The host of both L. cuenoti and L. pacifica is the large burrowing sipunculuan Sipunculus nudus. If L. corgani is associated with a host, this is not known. Live shells of L. cuenoti attached to its host have only been found in the Bay of Arcachon, W. France (Lamy, 1908; Pelseneer, 1909). Many shells of L. cuenoti, most of them empty ones, have been found from a range of western European and Mediterranean localities extending from the Bay of Arcachon around the coasts of the Iberian Peninsula and far into the Mediterranean Sea (Hoeksema et al., 1995, as L. glabra), and these records all fall within the geographical range of S. nudus which is a Lusitanian/Mediterranean species. The sipunculan has a world-wide distribution which besides western Europe and the Mediterranean includes the coastal waters of the western North Atlantic, the Red Sea and the Indian and Pacific Oceans (Stephen & Edwards, 1972). Fox (1979: 29) reported the presence on the North Carolina coast of an unidentified bivalve found with S. nudus. Additionally, it has turned out that the species recorded as “Pseudopythina sp.” from Awase tidal flat in Okinawa Island in association with S. nudus by Yamashita et al. (2005), is conspecific with L. pacifica. The Japanese name “Yuntaku-shijimi” provided by Yamashita et al. (2005) with it is adopted in the present paper for this species to avoid possible confusion in future.

Position on host Lamy (1908) recorded three large and two juvenile specimens of L. cuenoti attached to a single host, but the part of the body which this species attaches to was not noted. We found all specimens attached, singly, or a few together, to the lateral sides of the posterior part of the hosts’ body, in contrast to the positions of Fronsella ohshimai and F. philippinensis, both of which are attached singly to the posterior pole (Habe, 1964; Habe & Kanazawa, 1981). S. nudus does not construct a permanent burrow but, earthworm-fashion, moves more or less continuously forward through the sediment ingesting on its way sand and organic material (Ruppert & Fox, 1988). The two species of Fronsella are therefore located at the only place where they are protected from abrasion from the surrounding sediment. In the case of the present species, we cannot explain how it avoids being detached when its host moves through the sediment. Pseudopythina nodosa is attached to S. nudus almost everywhere on the trunk, but the shell is more compressed, offering less resistance when the worm is moving, and it further clamps itself strongly to the skin by apposition of the anterior margins of the valves which are internally nodulose to improve the grip (Morton, 1988). Litigiella pacifica n. sp. from the Ryukyu Is. 201

Acknowledgements

We are very grateful to Mr. Dick F. Hoeksema, Watertoren 28, 4336 KC Middelburg, The Netherlands, for having donated some of the specimens of L. cuenoti that were used in redescribing the shell of this species (Hoeksema et al., 1995). Dr. Philippe Bouchet, Muséum national d’Histoire naturelle, Paris, kindly tried to locate the shells of P. Fischer’s Lepton glabrum. The study was supported by grants to JL from the Danish National Science Research Council (N. 51-00-0278) and the Carlsberg Foundation (ANS- 0375/30).

References

Aartsen, J. J. van, 1996. Galeommatacea & Cyamiacea. Part II. La Conchiglia 28: 27-53. Charlesworth, D & Charlesworth, B. 1981. Allocation of resources to male and female functions in hermaphrodites. Biological Journal of the Linnean Society 15: 57-74. Coan, E. V., Scott, P. V. & Bernard, F. R. 2000. Bivalve Sea Shells of Western North America. 764 pp. Santa Barbara Museum of Natural History, Santa Barbara. Fischer, P. 1873. Les Fonds de la Mer 2: 65-112. Paris. Fox, H. 1979. Reproductive adaptations and life histories of the commensal leptonacean bivalves. Ph. D. dissertation, University of Carolina, Chapel Hill. 207 pp. Habe, T. 1958. Description of five new bivalves from Japan. Venus 20: 173-180. Habe, T. 1964. Two commensal bivalves from the west coast of Kyushu, Japan. Venus (Japanese Journal of Malacology) 23: 137-139. Habe, T. & Kanazawa, T. 1981. A new commensal bivalve from the Philippines (Montacutidae). Venus (Japanese Journal of Malacology) 40: 123-125. Higo, S., Callomon, P. & Goto, Y. 1999. Catalogue and Bibliography of the Marine Shell-bearing of Japan. 749 pp. Elle Scientific Publications, Osaka. Hoeksema, D. F., Aartsen, J. J.van, Keukelaar-Van der Berge, T., van Nieulande, F. A. D. & Simons, G. F. 1995. On the identity and distribution of Litigiella glabra (P. Fischer, 1873). La Conchiglia 27: 17-22. Jespersen, Å., Kosuge, T. & Lützen, J. 2001. Sperm dimorphism and spermatozeugmata in the commensal bivalve Pseudopythina macrophthalmensis (Galeommatoidea, Kellidae). Zoomorphology 120: 177- 189. Jespersen, Å. & Lützen, J. 2001. Ultrastructure of the seminal receptacle and the dimorphic sperm in the commensal bivalve Mysella bidentata (Veneroida: Galeommatoidea; Montacutidae). Acta Zoologica, Stockholm 82: 107-115. Kato, M. 1998. Morphological and ecological adaptatations in montacutid bivalves endo- and ectosymbiotic with holothurians. Canadian Journal of Zoology 76: 1403-1410. Kosuge, T. & Itani, G. 1994. A record of the crab associated bivalve, Pseudopythina macrophthalmensis from Iriomote Island, Okinawa, Japan. Venus (Japanese Journal of Malacology) 53: 241-144. Kosuge, T. & Kubo, H. 2002. Records of the sipunculid-associated bivalve Fronsella philippinensis from the Ryukyu Islands, south western Japan. The Chiribotan 33: 41-46. (in Japanese) Lamy, E. 1908. Description d’une coquille nouvelle de la côte Atlantique francaise. Journal de Conchyliologie, Paris 56: 35-37. Lucas, A. & Hignette, M. 1983. Les concretions renales chez les bivalves marins: Etudes anciennes et recentes. Haliotis 13: 99-113. Lützen, J., Takahashi, T. & Yamaguchi, T. 2001. Morphology and reproduction of Nipponomysella subtruncata (Yokoyama), a galeommatoidean bivalve with the sipunculan Siphonosoma cumanense (Keferstein) in Japan. Journal of the Zoological Society, London 254: 429-440. Monterosato, T. A. di. 1875. Nuova rivista della conchiglie mediterranee. Atti dell’Accademia di scienze, lettere ed arti, Palermo II (5): 11-50. Monterosato, T. A. di. 1909. Note sur l’Erycina cuenoti. Journal de Conchyliologie, Paris 56: 253-255. Morton, B. 1988. Partnerships in the Sea: Hong Kong’s Marine Symbioses. 124 pp. Hong Kong University Press, Hong Kong. Morton, B. & Scott, P. H. 1989. The Hong Kong Galeommatacea (Mollusca: ) and their hosts, with descriptions of new species. Asian Marine Biology 6: 129-160. Ó Foighil, D. 1985. Fine structure of Lasaea subviridis and Mysella tumida sperm. Zoomorphology 105: 125-132. Oldfield, E. 1961. The functional morphology of Kellia suborbicularis (Montagu), Montacuta ferruginosa 202 J. Lützen & T. Kosuge

(Montagu) and M. substriata (Montagu), (Mollusca, Lamellibranchiata). Proceedings of the Malacological Society of London 34: 255-295. Pelseneer, P. 1909. Phylogenie des Lamellibranches commensaux. Bulletin de l’Academie Royale de Belgique (Classes de Science) 12: 1144-1150. Pelseneer, P. 1911. Les Lamellibranches de l’Éxpedition du Siboga. Partie anatomique. Siboga Expeditie, Monograph 53a: 1-125. Ruppert, E. E. & Fox, R. S. 1988. Seashore of the Southeast: A guide to common shallow water invertebrates of the southeastern Atlantic coast. 429 pp. University of Carolina Press. Secchi, F. & Martoja, M. 1986. Existence de populations uricothéliques chez Abra alba et Dreissena polymorpha (Bivalves, Eulamellibranches). Canadian Journal of Fisheries and Aquatic Sciences 43: 2502-2508. Stephen, A. C. & Edmonds, J. J. 1972. The phyla Sipuncula and Echiura. Trustees of the British Museum (Natural History) Publication 517: 1-528. Yamashita, H., Kurozumi, T., Osuga, K., Nawa, J., Osada, H., Maekawa, S. & Kaihatsu, N. 2005. Discovery of some rare molluscan species from the Awase Tidal Flat and their present condition of conservation measure. Venus 64: 71. (abstract; in Japanese)

(Received October 17, 2005 / Accepted May 1, 2006)

琉球列島産スジホシムシに着生するユンタクシジミ（新種）の記載

J. ルッツェン・小菅丈治

要 約

琉球列島の干潟に生息するスジホシムシ Sipunculus nudus Linnaeus, 1766 の体表に着生する二枚貝の一 種を，ウロコガイ上科，チリハギガイ科の新種ユンタクシジミ Litigiella pacifica n. sp. として記載した。 ユンタクシジミの貝殻は薄質白色で，殻表には細かい成長脈が浅く刻まれる。薄い殻皮に被われるが， 殻の中央付近では摩耗し，前後背縁付近では灰褐色に汚損されることが多い。殻の輪郭は長円形で，殻 頂は突出しない。両殻とも 1 主歯を備え，右殻の主歯は左殻より大きく，発達した前後側歯を持つ。 ホロタイプ：NSMT-Mo 73728，殻長 5.9 mm，殻高 4.5 mm。 タイプ産地：沖縄県石垣島名蔵湾。 分布：沖縄島（沖縄市泡瀬干潟）・石垣島名蔵湾。 付記：本種は，石垣島名蔵湾と沖縄本島泡瀬干潟の礫混じりの砂質干潟に生息するスジホシムシの体 表に粘液糸で付着する。スジホシムシの後端付近に付くことが多いが，フィリピンハナビラガイがスジ ホシムシの後端に強く着生する状況と異なり，付着の程度は緩い。スジホシムシ 1 個体の体表に 2 ～3 個体が付くことも珍しくなく，山下他（2005）によって提唱された和名はこの状況を把えて「数名が集 まっておしゃべりをする」という意味の沖縄方言「ゆんたく」を用いたものである。 フランス西岸からスジホシムシに着生する二枚貝として記載された Litigiella cuenoti (Lamy, 1908) は， ユンタクシジミとよく似た形態の殻を持つ。両者の識別点として，L. cuenoti では殻頂の原殻付近が突出 するのに対しユンタクシジミでは突出しない点，主歯の大きさが L. cuenoti では両殻等大であるのに対 し，ユンタクシジミでは右殻の主歯が明らかに大きい点を挙げうる。 組織切片を作製し生殖器官を観察した結果，ユンタクシジミは雌雄同体で複数回の性転換を行う可能 性が示唆された。また，有糸型の精子が一対の貯精器官内に蓄えられていた。