VENUS 65 (3): 193-202, 2006 Description of the Bivalve Litigiella pacifica n. sp. (Heterodonta: Galeommatoidea: Lasaeidae), 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 species 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 mantle 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 Lasaea 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 periostracum 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.