VENUS 69 (3–4): 123–133, 2011 ©Ma lacological Society of Japan

The Bivalve Divariscintilla toyohiwakensis n. sp. (Heterodonta: Galeommatidae) from Japan, a Commensal with a Mantis Shrimp

Hiroyoshi Yamashita1, Takuma Haga2** and Jørgen Lützen3* 1Association of Conservation Malacology, 3-1-26-103 Matsugaoka, Kugenuma, Fujisawa, Kanagawa 251-0038, Japan 2Department of Geology and Paleontology, National Museum of Nature and Science, 3-23-1 Hyakunincho, Shinjuku-ku, Tokyo 169-0073, Japan 3Biological Institute, University of Copenhagen, Universitetsparken 15, DK-2100, Copenhagen Ø, Denmark

Abstract: We describe the morphology of a new species of the bivalve family Galeommatidae, Divariscintilla toyohiwakensis n. sp., which lives in the burrows of the mantis shrimp Acanthosquilla acanthocarpus at Oshinden, Nakatsu, Oita Prefecture, Kyushu, Japan. An unpaired median tentacle and three paired tentacles issue from the mid-mantle fold, which partially covers the shell. A single flower-like organ originates on the anterior surface of the visceral mass. A byssal adhesive gland is observed on the most posterior part of the foot. This is the first record of the genus in Japan and the North Pacific Ocean.

Keywords: Acanthosquilla acanthocarpus, flower-like organ, Lysiosquilloidea, morphology, new species, North Pacific

Introduction

Species of the predominantly tropical/subtropical family Galeommatidae occur chiefly in marine coastal waters. Most species are cryptic and live attached to the undersurfaces of stones, shale and dead coral pieces. Other species are associated with burrowing in that they hide in their host’s burrows. Three species of Ephippodonta Tate, 1889 live commensally with thalassinidean shrimps (Tate, 1889; Matthews, 1893; Lützen & Nielsen, 2005) and one, Ambuscintilla praemium Iredale, 1936, lives in burrows made by a prawn. A few species of Scintillona Finlay, 1927 occur in galleries made by crabs (Morton, 2008). Eleven species belonging to Divariscintilla Powell, 1932 and a small number of other genera are known or suspected to live in the burrows of mantis shrimps (Mikkelsen & Bieler, 1992). While excavating the burrows of the mantis shrimp Acanthosquilla acanthocarpus (Claus, 1871) on a Japanese tidal beach, Mr. Taichi Wada (T.W.) discovered a species of Divariscintilla that is new to science and represents the first record of the genus in Japan and the Pacific Ocean north of Australia. The present paper describes the shell, the morphology of the soft parts, and some aspects of the host association of the new species.

* Corresponding author: [email protected] ** Present address: Marine Biodiversity Research Program, Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan 124 H. Yamashita et al.

Materials and Methods

The bivalves were sampled at extreme low water from the burrows of Acanthosquilla acanthocarpus at Oshinden (part of the Suo-nada Sea, Seto Inland Sea; northwestern coast of Kyushu), in Nakatsu, Oita Prefecture, Japan. The burrow of A. acanthocarpus in sandy mud sediment sufficiently solid that it can be excavated with a shovel while maintaining its form and revealing the bivalves’ habitat (Fig. 1C). Live bivalves were maintained in a Petri dish filled with seawater at room temperature for behavioral studies. For histological studies, six specimens collected on 1 July 2007 were preserved in 70% ethanol and five specimens collected on 27 August 2007 were preserved in 4% formaldehyde. Two formalin-preserved bivalves (3.5 mm and 3.6 mm in shell length) were postfixed and decalcified in Bouin’s fluid, embedded in araldite, cut into 2-µm-thick sagittal and transverse section series, and stained with toluidine blue. Fifteen specimens collected on 22–23 July 2009 were selected for conchological observations and processed according to Fukuda et al. (2008) to remove the valves from the soft parts, of which three sets were preserved in pure ethanol for future molecular work. Twenty-six living specimens collected on 22–23 July 2009 were anaesthesized by immersion in 7.5% MgCl·6H20 in freshwater solution for one hour, fixed in 10% buffered seawater-diluted formalin for 48 hours, rinsed in tap water, and preserved in 70% ethanol. For scanning electron microscopy, specimens were passed through an ethanol to t-buthyl alcohol series, freeze-dried on a JFD-310 (JEOL) freeze drier device, and coated with gold. Scanning electron micrographs were produced on a JSM-T330A (JEOL) at the Department of Geology and Paleontology, National Museum of Nature and Science.

Abbreviations: FMNH – Field Museum of Natural History, Chicago; NSMT – Department of Zoology, National Museum of Nature and Science, Tokyo (formerly National Science Museum, Tokyo); SEM – scanning electron microscopy; SH – shell height; SL – shell length.

Systematics

Superfamily Galeommatoidea J. E. Gray, 1840 Family Galeommatidae J. E. Gray, 1840 Genus Divariscintilla Powell, 1932 Type species: Divariscintilla maoria Powell, 1932, by original designation.

Divariscintilla toyohiwakensis n. sp. (Figs. 1A–E; 2–4)

Description Shell: Shell small, ranging 2.1–4.1 mm (M ± SD: 3.71 ± 0.46, n = 16) in SL and 1.8–3.3 mm (M ± SD: 2.86 ± 0.21, n = 16) in SH, roundly triangular, equivalve, thin, laterally compressed, and slightly inequilateral with beaks in front of midline (Fig. 2A–B). Very shallow indentation (vi in Fig. 2A) occurs at midventral margin in both valves. Numerous close-set very fine commarginal striae at external valve margins. Shell colorless, translucent. Brownish digestive gland visible through shell in living specimen (Fig. 1). Very thin, almost transparent periostracum covers valves. Shell surface smooth and glossy (Fig. 1A, C–E); interior bears weak radial ribs along edges, resulting in formation of finely denticulate margin, especially posteriorly (Fig. 2A). Shell capable of complete closure, not gaping at ends. Adductor muscle scars indistinct and subequal (Fig. 2A). Pallial line entire and indistinct (Fig. 2A). Hinge line short. One cardinal tooth in each valve; lateral teeth absent (Fig. 2D). Cardinal teeth abutting, not interlocking. Shallow socket (cs in Fig. 2D) on anterior part of left hinge plate accommodating right cardinal tooth. A New Species of Divariscintilla from Japan 125

Fig. 1. Divariscintilla toyohiwakensis n. sp. and its in situ occurrence. A. Living individual with soft parts extended. B. Camera lucida drawing from left. C. Longitudinal section of burrow of Acanthosquilla acanthocarpus; arrow and dotted line indicate a colony of D. toyohiwakensis n. sp. and sea bottom surface, respectively. D. Close-up of a colony. E. Live individual hanging down from wall of aquarium. F. Host, Acanthosquilla acanthocarpus. Abbreviations: amf, anterior mid-mantle fold; bag, byssus adhesive gland; bms, burrow of mantis shrimp; by, byssus; ec, excurrent siphon; ft, foot; ic, incurrent aperture; imf, inner mantle fold; mf, point of mantle fusion; pd, prodissoconch II; pfe, posterior extension of foot; pmf, posterior mid-mantle fold; pt1–pt3, pallial tentacle pairs 1–3; sh, shell; ut, unpaired tentacle. A, D–E are not scaled. 126 H. Yamashita et al.

Fig. 2. Valves of Divariscintilla toyohiwakensis n. sp., scanning electron micrograph. A. Internal view of left valve. B. External view of left valve. C. Dorsal view of right valve. D. Internal view of umbonal area of conjoined valves. E. Close-up of prodissoconch II. Arrowheads indicate demarcation between prodissoconch II and dissoconch. A–C and E, holotype (NSMT-Mo 77209); D, paratype (NSMT-Mo 77210). Abbreviations: aam, anterior adductor muscle scar; apr, anterior pedal retractor muscle scar; cs, cardinal tooth socket; ct, cardinal tooth; el, external ligament; il, internal ligament; ny, nymph; pam, posterior adductor muscle scar; pe, periostracum; pl, pallial line; ppm, pedal protractor muscle scar; ppr, posterior pedal retractor muscle scar; re, resilifer; rh, ridge of hinge plate; vi, ventral indentation.

Internal ligament (il in Fig. 2D) opisthodetic, short and stout. Resilifers distinct in both valves (re in Fig. 2D). External ligament (el in Fig. 2D) amphidetic, supported by nymphae. Nymphae present at both anterior and posterior margins of umbo. Nymph of right valve prominent, overlying that of left valve (ny in Fig. 2D, E). Rather long and transparent ridges (rh in Fig. 2D) present in hinge plate laterally to opaque cardinal tooth region in both valves. Prodissoconch II brownish-yellow, 320–400 µm long (n = 5), and abruptly demarcated from dissoconch in few specimens observed that were not corroded (Fig. 2E). Prodissoconch I not confirmed. Soft parts: Shell more or less internalized by profuse development of mid-mantle fold that reaches maximum extension anteriorly and posteriorly (Figs. 1B; 3A, C). Anterior edges of extended inner mantle folds form wide and hood-like incurrent region; posterior edges fused except for short excurrent siphon (Fig. 1B). Numerous small, rounded papillae (rp in Fig. 3A) A New Species of Divariscintilla from Japan 127

Fig. 3. Scanning electron micrograph of Divariscintilla toyohiwakensis n. sp., freeze-dried, paratype (NSMT- Mo 77215). A–C. Left, ventral and dorsal views, respectively. D. Close-up of posterior extension of foot showing byssal threads and byssal adhesive gland (outlined in B). E. Close-up of unpaired tentacle (outlined in C). F. Close-up of pallial tentacle #2 (outlined in A). Abbreviatioins: amf, anterior mid-mantle fold; bag, byssal adhesive gland; bg, byssal groove; by, byssus; ec, excurrent siphon; ic, incurrent aperture; imf, inner mantle fold; pfe, posterior extension of foot; pmf, posterior mid-mantle fold; pt1–pt3, pallial tentacle pairs 1–3; rp, rounded papillae; ut, unpaired tentacle. spread at regular intervals all over part of mantle reflected over shell. When extended, mantle displays total of seven tentacles (Figs. 1A–B; 3A–C, E–F) comprising relatively small anterior pallial pair, pair of longer “cephalic” tentacles, single long pallial tentacle above excurrent siphon, and pair of posterior pallial tentacles, arising ventrally to siphon (Fig. 1B). All tentacles have papillose surface and lack any coloration. Unpaired tentacle surface with stratiform petal composition (Fig. 3F), paired tentacles bear globular papillae on surface (Fig. 3F). Central core of each tentacle made up of prominent longitudinal muscle fibers and large nerve trunk, which in retracted tentacle follows convoluted course among muscle fibers (e.g., pt in Fig. 4C). 128 H. Yamashita et al.

Papillae on extensible mid-mantle fold and especially tentacles contain up to 60 µm long goblet cells producing substance that stains intensely with toluidine blue (Fig. 4C). Each gland cell contains individual secretory compartments that are discharged at tip of cell as minute rounded droplets. Single flower-like organ (fo in Fig. 4A–B) issues from anterior surface of visceral mass just ventral to and directed towards labial palps (Fig. 4A). Organ consists of angular stalk and head composed of fewer than dozen glandular subunits, each of which opens to exterior via single pore (Fig. 4B–C). Total length of organ ca. 300 µm, diameter of head ca. 135 µm. Foot slender, anterior portion extended proboscis-like (Figs. 1A–B, E; 3B). crawls actively (Fig. 1A–B). Deep byssal groove starts posterior to level of pedal ganglia (anterior to middle of foot) and runs almost to foot’s posterior tip (Fig. 3B, D). Tangled byssal threads issuing from posteriormost part of groove (Fig. 3D) used for hanging animal on to walls of host’s burrow (Fig. 1D–E). Profuse amount of mucous substance produced by other glands of groove evident in sections. Posterior part of foot well extended and byssal adhesive gland observed at dorsal part of posterior tip of foot (Figs. 1A; 3D). Byssus adhesive gland not verified from sections, however, clearly distinguished from other part of foot in live animal by opaque whitish coloration (Fig. 1A) and adhesive to touch with forceps. Foot, mid-mantle and tentacles can become completely withdrawn between closed valves (Fig. 1D). Gills eulamellibranchiate, homorhabdic, comprising two pairs of unpleated demibranchs, both of which have descending and ascending lamellae (Fig. 4A). Outer demibranch much shorter than inner one and makes no anterior contact with labial palps, resulting in formation only of oralward currents along ventral marginal food groove on inner demibranch and where both demibranchs unite with visceral mass. Hypobranchial glands represented by glandular patches and stained heavily with toluidine blue. Each placed on protrusion of suprabranchial cavity directed towards anus. Labial palps of medium size, inner and outer palps of equal size, each with 7–8 broad lamellae. Digestive gland envelops oesophagus (Fig. 4C) and is mainly distributed anterior to stomach and style sac, extending into base of foot. Both sectioned specimens hermaphroditic, with reproductive organs situated in posterior part of visceral mass, dorsally and posterior to stomach. Though male and female components form individual compartments, each split up into many profusely branched lobes that are intricately intermixed with each other. Posteriorly, sperm cells become concentrated within distinct but relatively small space near common genital ducts. Oocytes, less than fully mature, measured 50 µ m in diameter in both specimens.a Sperm tozoa with long tail filaments. Elongated sperm head measures ca. 4.5 µm in length and 1.2 µm in maximum diameter. Midpiece could not differentiated from oblong nucleus. Hemispherical and slightly tilted acrosome clearly present at tip of nucleus. Reproduction: A few specimens collected 22–23 July 2009 brooded D-shape larvae and eggs in the gills. During brooding, the excurrent siphon closes narrowly. Type locality: Oshinden (33º36´N, 131º13´E) in Nakatsu, Oita Prefecture, Kyushu, Japan. Type material (size: SL × SH): Holotype – NSMT-Mo 77209, 3.9 × 2.9 mm, conjoined valves, gold-coated, dry (Fig. 2A–C, E). Paratypes – NSMT-Mo 77210, one specimen, 3.8 × 2.9 mm, conjoined valves, dry, soft parts in 70% ethanol; NSMT-Mo 77211, one specimen, 4.1 × 3.3 mm, conjoined valves, dry, soft parts in 70% ethanol; NSMT-Mo 77212, 3.9 × 2.9 mm, conjoined valves, dry, soft parts in 70% ethanol; NSMT-Mo 77213, one specimen, 4.0 × 3.0 mm, conjoined valves, dry, soft parts in 70% ethanol; NSMT-Mo 77214, 3.9 × 3.2 mm, conjoined valves, dry, soft parts in 70% ethanol; NSMT-Mo 77215, six specimens, intact and shell partly removed, freeze-dried, gold-coated; NSMT-Mo 77216, two specimens, conjoined valves and soft parts in pure ethanol; NSMT-Mo 77217, six conjoined valves, dry, soft parts in 70% ethanol; NSMT-Mo 77218, 10 specimens, intact, in 70% ethanol; FM NH 328146, 10 specimens, intact, in 70% A New Species of Divariscintilla from Japan 129

Fig. 4. Divariscintilla toyohiwakensis n. sp. A. Scanning electron micrograph of left view of freeze-dried animal, mantle removed, paratype (NSMT-Mo 77215). B. Scanning electron micrograph of freeze-dried flower- like organ (outlined in A), paratype (NSMT-Mo 77215). C. Histological section through the flower-like organ. Abbreviations: bg, byssus groove; dg, digestive gland; fo, flower-like organ; ft, foot; gc, goblet cells; id, inner demibranch; lp, labial palp; mo, mouth opening; od, outer demibranch; oe, oesophagus; pt, pallial tentacle. 130 H. Yamashita et al. ethanol; FMNH 328147, one specimen, conjoined valves and soft parts in pure ethanol. All the type specimens were collected from the type locality by T. W. and H. Y., July 22–23, 2009. Distribution: Known only from the type locality. Habitat: Burrows of the mantis shrimp in sandy mud tidal flat in low tidal zone. Etymology: Named after “Toyohiwake” in Japanese, the mythical God and ancient place name of northeastern Kyushu including type locality. Remarks on host relationships: In the type locality there are many burrows of the mantis shrimp A. acanthocarpus (Fig. 1F) at the extreme low tidal zone where the substratum is a sandy mud (Fig. 1C). The burrows are nearly vertical, with a maximum length of more than 25 cm. The diameter of the burrow’s opening on the surface is ca. 12 mm. The bivalve commensals occurred in ca. 70% of the observed burrows. Each bivalve attaches to the wall of the burrow, and hangs by several byssal threads that emanate from the foot’s posterior tip (Fig. 1E). The ventral margin of the hanging shell is thereby turned obliquely towards the burrow’s opening (Fig. 1C–E). Attachment to the wall could be facilitated by a coating produced by the host, which the byssal threads stick to or even penetrate. At low tide the bivalves occurred in all levels of the burrow between 7 and 23 cm depth while beyond 25 cm depths observations were inaccurate (Fig. 1C). In each burrow 10 to 20, and up to 30 bivalves of many sizes were crowded together (Fig. 1C–D). In some of the burrows an undescribed vitrinellid gastropod occurred together with D. toyohiwakensis n. sp. A similar case of co-occurrence of Divariscintilla species and vitrinellids in mantis shrimp burrows was reported from the western Atlantic Ocean by Bieler & Mikkelsen (1988) and Mikkelsen & Bieler (1989). Recent reclassification has placed the Vitrinellidae in the synonymy of the family Tornidae (Bouchet & Rocroi, 2005).

Discussion

Comparison with other species In a phylogenetic analysis of the family Galeommatidae, Bieler & Mikkelsen (1992) recognize a species groupp com rising six species of Divariscintilla, and one species each of Parabornia Boss, 1965 and Phlychtaenachlamys Popham, 1939. Seven of the eight species have one or more flower-like organs protruding from the visceral mass below the mouth. The function of these organs is debatable (Mikkelsen & Bieler, 1989). The extent to which the mid-mantle fold covers the shell varies from complete coverage except for a small umbonal foramen in Divariscintilla yoyo Mikkelsen & Bieler, 1989, Divariscintilla troglodytes Mikkelsen & Bieler, 1989, and Phlychtaenachlamys lysiosquillina Popham, 1939, to the condition in the other species where the fold is retractable and may cover a smaller or larger part of the shell. The bipartite foot and elongate posterior foot extension with both byssus and byssal adhesive gland are important features of Divariscintilla, and this “hanging apparatus” may be present also in some species of Phlychtaenachlamys and Ceratobornia Dall, 1899 (Mikkelsen & Bieler, 1992). In some species (Divariscintilla maoria Powell, 1932 and Divariscintilla cordiformis Mikkelsen & Bieler, 1992) the shell has a ventral notch, a feature shared by species of a few related galeommatid genera in which the soft anatomy and/or biology is less well known (Cornet, 1982; Coney, 1990). The present new species clearly belongs to those species in which the mantle fold is capable of complete retraction into the mantle cavity. Among them, it closely resembles Divariscintilla octotentaculata Mikkelsen & Bieler, 1992 in shell shape, dentition, mantle coverage and moderate number of tentacles. However, although the ventral shell margin in D. octotentaculata is slightly convex, in the present species there is a shallow but consistent indentation, perhaps reminiscent of the shell notch of other species (Fig. 2A–B). The new species has one pair of tentacles less than D. octotentaculata, but on the other hand possesses a distinct unpaired tentacle in front of the excurrent siphon, represented in D. octotentaculata by a mere knob. A notable difference between A New Species of Divariscintilla from Japan 131 the two is the presence in D. toyohiwakensis n. sp. of a flower-like organ, which is absent in all examined specimens of D. octotentaculata (Mikkelsen & Bieler, 1992). The tentacles of D. toyohiwakensis n. sp. have a central core of a longitudinal muscle and nerve cord as has been described in other species of the genus (Judd, 1971; Mikkelsen & Bieler, 1889, 1992). An axial haemocoelic compartment that functions to extend the retracted tentacles occurs in some other species of Divariscintilla and in P. lysiosquillina and presumably also occurs in the present species, but was no longer demonstrable in the strongly retracted tentacles. Goblet cells on tentacles and mantle papillae have also been described in P. lysiosquillina (see Popham, 1939). Because these cells are associated with the papillae on the mantle and tentacles in D. toyohiwakensis n. sp., they probably also occur in other species of Divariscintilla. They were not reported by Mikkelsen & Bieler (1989, 1992), but a layer of subepithelial gland cells were found in the so-called fusiform tentacles of D. maoria by Judd (1971). In the many small pallial tentacles of Galeomma layardi Deshayes, 1856, Morton (1973) observed that a strongly basophilic secretion from the subepithelial gland cells accumulated in vacuoles immediately under the squamous epithelium. These tentacles undergo regular autotomy and Morton (1973) suggested that the secretion was released when the tentacles autotomized. It is generally believed that the various types of tentacle-like extensions of the mantle, besides being sensory and reacting to touch, are also defensive, and that the substance they produce may be noxious serve to deter would-be predators or even the host mantis shrimp (Judd, 1971; Morton, 1973). Species of Divariscintilla are believed to be simultaneous hermaphrodites (Mikkelsen & Bieler 1989, 1992) and both sectioned specimens of the present species contained female as well as male products. However, the single examined specimen of P. lysiosquillina was a male (Popham, 1939) and at least six other species of galeommatids studied by Jespersen & Lützen (2006) were alternate hermaphrodites. The ova are spawned into the suprabranchial chamber as in other species of the genus and family, but how sperm is introduced into the brood chamber is open to speculation. Mikkelsen & Bieler (1992) described a quasi-copulatory behavior in two species of Divariscintilla in which the animal reached with the tip of its foot deeply into the mantle cavity of its partner, perhaps transferring at the same time a mass of sperm. Sperm ultrastructure in species of the Galeommatidae has been studied by Eckelbarger et al. (1990) and Jespersen & Lützen (2006). The sperm of 19 species are of the ent-aquasperm type with a more or less hemispherical acrosome that is tilted with respect to the long axis of the moderately elongated nucleus. Although the material did not permit us to study the ultrastructure of the sperm in D. toyohiwakensis n. sp., the typical tilted acrosome could be recognized. The sperm head is slightly shorter and thicker than in the spermatozoa of D. yoyo (see Eckelbarger et al., 1990).

Distribution and hosts D. toyohiwakensis n. sp. is known only from the type locality. The new record extends the known geographical range of the Divariscintilla species-group (New Zealand; E. Australia; Caribbean; Florida) to Japan. All of the seven described species are commensals in the burrows of mantis shrimps. Five Atlantic species are commensal with Lysiosquilla scabricauda (Lamarck, 1818) of the family Lysiosquillidae, whereas the South Pacific D. maoria lives in the burrows of Heterosquilla tricarinata (Claus, 1871) of the family Tetrasquillidae. The host of D. toyohiwakensis n. sp. belongs to a third family, the . All three families are members of the superfamily Lysiosquilloidea, which suggests an intimate ecological and evolutionary relationship between Divariscintilla and the Lysiosquilloidea. 132 H. Yamashita et al.

Acknowledgements

We are grateful to Mr. Taichi Wada (The Minami-Ko Wetland Group, Osaka) who first discovered and offered specimens and pictures (Fig. 1A, E) of D. toyohiwakensis n. sp. We are also grateful to Dr. Hiroyuki Ariyama (Marine Fisheries Research Center, Research Institute of Environment, Agriculture and Fisheries, Osaka Prefectural Government) for his identification of the mantis shrimp host. Drs. Rüdiger Bieler (FMNH), Paula M. Mikkelsen (American Museum of Natural History) and an anonymous reviewer are acknowledged for their valuable comments on the manuscript.

References

Bieler, R. & Mikkelsen, P. M. 1988. Anatomy and reproductive biology of two western Atlantic species of Vitrinellidae, with a case of protandrous hermaphroditism in the Rissoacea. The Nautilus 102: 1–29. Bieler, R. & Mikkelsen, P. M. 1992. Preliminary phylogenetic analysis of the bivalve family Galeommatidae. American Malacological Bulletin 9: 157–164. Boss, K. J. 1965. A new mollusk (Bivalvia, Erycinidae) commensal on the stomatopod Lysiosquilla. American Museum Novitates (2215): 1–11. Bouchet, P. & Rocroi, J. P. 2005. Classification and Nomenclator of Gastropod Families. Malacologia 47: 1–397. Coney, M. 1990. Bellascintilla parmeleeana new genus and species from the tropical eastern Pacific, with a review of the other notched galeommatid genera. The Nautilus 104: 130–144. Cornet, M. 1982. Anatomical description of Vasconiella jeffreysiana (P. Fischer, 1873) (Mollusca, Bivalvia, Leptonacea). Journal of Molluscan Studies 48: 36–43. Eckelbarger, K. J., Bieler, R. & Mikkelsen, P. M. 1990. Ultrastructure of sperm development and mature sperm morphology in three species of commensal bivalves (Mollusca: Galeommatoidea). Journal of Morphology 205: 63–75. Fukuda, H., Haga, T. & Tatara, Y. 2008. Niku-nuki: a useful method for anatomical and DNA studies on shell-bearing molluscs. Zoosymposia 1: 15–38. Iredale, T. 1936. Australian molluscan notes. No. 2. Records of the Australian Museum 19: 267–340, pls. 20–24. Jespersen, Å. & Lützen, J. 2006. Reproduction and sperm structure in Galeommatidae (Bivalvia, Galeommatoidea). Zoomorphology 125: 157–173. Judd, W. 1971. The structure and habits of Divariscintilla maoria Powell (Bivalvia: Galeommatidae). Proceedings of the Malacological Society of London 39: 343–354. Lützen, J. & Nielsen, C. 2005. Galeommatid bivalves from Phuket, Thailand. Zoological Journal of the Linnean Society 144: 261–308. Matthews, J. T. 1893. On the habit of the genus Ephippodonta (Tate). Conchologist 2: 144–145. Mikkelsen, P. M. & Bieler, R. 1989. Biology and comparative anatomy of Divariscintilla yoyo and D. troglodytes, two new species of Galeommatidae (Bivalvia) from stomatopod burrows in eastern Florida. Malacologia 31: 1–21. Mikkelsen, P. M. & Bieler, R. 1992. Biology and comparative anatomy of three new species of commensal Galeommatidae, with a possible case of mating behavior in bivalves. Malacologia 34: 1–14. Morton, B. 1973. The biology and functional morphology of Galeomma (Paralepida) takii (Bivalvia: Leptonacea). Proceedings of the Zoological Society, London 169: 133–150. Morton, B. 2008. The biology of sympatric species of Scintillona (Bivalvia: Galeommatoidea) commensal with Pilumnopeus serratifrons (Crustacea: Decapoda) in Moreton Bay, Queensland, Australia, with a description of a new species. Memoirs of the Queensland Museum – Nature 54: 323–338. Popham, M. L. 1939. On Phlyctaenachlamys lysiosquilla gen. and sp. nov., a lamellibranch commensal in the burrows of Lysiosquilla maculata. British Museum (Natural History), Great Barrier Reef Expedition 1928–29, Scientific Reports 6: 549–587. Tate, R. 1889. New species of marine molluscs from South Australia and Victoria. Transactions of the Royal Society of South Australia, Adelaide 11: 60–66.

(Received November 19, 2010 / Accepted March 30, 2011) A New Species of Divariscintilla from Japan 133

シマトラフヒメシャコの巣穴に共生するニッポンヨーヨーシジミ （新種）（異歯亜綱：ウロコガイ科）の記載

山下博由・芳賀拓真・Jørgen Lützen

要 約

シャコ類の巣穴に共生することで知られている Divariscintilla属（ヨーヨーシジミ属，和名新称）の新 種が，大分県から発見された。これは同属の日本及び北太平洋からの初めての記録である。

Divariscintilla toyohiwakensis n. sp.ニッポンヨーヨーシジミ（新種，新称） 殻長約 4 mm，殻高約 3 mm，膨らみは弱く，丸い亜三角形で，殻頂は僅かに前方に寄り，腹縁中央は ごく僅かに窪む。白色半透明，薄質で，殻表は平滑で光沢があり，殻の内側には縁部で強まる多くの放射 条がある。両殻に 1主歯があり，側歯を欠く。外套膜は殻を覆い，前部に 2対，後部に 1対と 1本の外 套触角がある。口の直下の内蔵塊前部に 1個の flower-like organ（花状器官：和用語新称）を備える。足 は前部・後部に分かれ，後部は顕著に伸張し，その後端背面に byssal adhesive gland（足糸粘着腺：和用 語新称）があるが，組織切片では腺構造は確認できなかった。足の底面には byssal groove（足糸溝）が前 部から後端まで走っている。フロリダ産の Divariscintilla octotentaculata Mikkelsen & Bieler, 1992に，殻や 軟体外形が近似するが， D. octotentaculataは触角が 1対多く，花状器官を欠く。 ニッポンヨーヨーシジミ は，Acanthosquilla acanthocarpus (Claus, 1871) シマトラフヒメシャコの巣穴中に小集団を形成して生息し， その壁面に足糸で付着している。 タイプ産地：大分県中津市大新田 付記：ヨーヨーシジミ属は，ニュージーランド・西オーストラリアに 1種，フロリダ・カリブ海に 5 種，日本に 1種が分布するが，いずれもトラフシャコ上科の種の巣穴に生息し，同上科との生態・進化 上の密接な関係が示唆される。 種小名 toyohiwakensisは，タイプ産地の大分県中津市を含む豊前・豊後地方の古名「豊日別」に由来す る。属・種の和名は，玩具のヨーヨーのように足糸でぶら下がり上下する生態に着目して命名された Divariscintilla yoyo Mikkelsen & Bieler, 1989と，そこから派生した英名 yoyo clamに由来する。D. yoyoに は，ヨーヨーシジミの和名を与える。