Borniopsis mortoni sp. n. (Heterodonta, Galeommatoidea, Galeommatidae sensu lato), a new bivalve commensal with a synaptid sea cucumber from Japan

Abstract The Galeommatoidea is a bivalve superfamily that exhibits high species diversity in shallow waters. Many members of this superfamily are associated commensally with burrowing marine invertebrates in benthic sediments. The genus Borniopsis is known only from eastern Asia and exhibits high host diversity (e.g., mantis shrimps, crabs, holothurians, sipunculans and echiurans). A new species, Borniopsis mortoni sp. n., is described from mud flats at the mouth of the Souzu River, southwestern Shikoku Island, Japan. This species has elongate-ovate shells covered by a tan to dark brown periostracum, and lives attached by both its foot and byssal threads to the body surface of the synaptid sea cucumber Patinapta ooplax. Several individuals of Borniopsis mortoni are often found on the same host, but sometimes more than 10 individuals can occur together. Borniopsis mortoni is one of the smallest species in this genus. Probably, its small body size is an adaptation to the mode of life in a narrow host burrow. Until now, only two other Borniopsis species were known to have commensal associations with synaptids. Thus, this is the third example of a synaptid-associated species from this genus. In addition, we briefly review the galeommatoideans commensal with apodid sea cucumbers.


Introduction
The Galeommatoidea is a superfamily of small bivalves that exhibits tremendous diversity in the intertidal zone (Bouchet et al. 2002, Paulay 2003, Lützen and Nielsen 2005. Many members of this superfamily are commensals associated in highly specific relationships with benthic invertebrates that burrow in soft sediments (Boss 1965, Morton and Scott 1989, Li et al. 2012. Most commensal galeommatoideans live attached directly onto the host's body surface or the walls of its burrow (Boss 1965, Morton and Scott 1989. The genus Borniopsis was established for B. tsurumaru Habe, 1959 as the type species with a second species, B. ariakensis Habe, 1959, from Japan (Habe 1959). Morton and Scott (1989) described four Pseudopythina (Borniopsis) species from Hong Kong. After this publication, Borniopsis was frequently synonymized with Pseudopythina (e.g., Lützen et al. 2004). However, Huber (2015) noticed that these East Asian Pseudopythina species are distinct from the type species P. macandrewi (P. Fischer, 1867), which occurs only in the eastern Atlantic Ocean, in dentition, seminal receptacles, and demibranchs and thus he transferred them to Borniopsis together with some species of Byssobornia, Squillaconcha and Kellia (Huber, 2015), although B. fujitaniana (Yokoyama, 1927) was recently reassigned to the genus Tellimya (Goto et al., 2016). As a result, at least nine species currently belong to this genus -B. tsurumaru, B. ariakensis, B. macrophthalmensis (Morton & Scott, 1989), B. maipoensis (Morton & Scott, 1989), B. nodosa (Morton & Scott, 1989), B. ochetostomae (Morton & Scott, 1989), B. subsinuata (Lischke, 1871), B. yamakawai (Yokoyama, 1922) and B. sagamiensis (Habe, 1961) (Huber 2015). All of these species are known only from eastern Asia. Those Borniopsis species, for which the biology is known, are host-specific commensals associated with burrowing invertebrates (mantis shrimps, crabs, holothurians, sipunculans, echiurans and probably tanaids) (Kuroda 1937, Morton 1972, 1988, Morton and Scott 1989. Host animals are different among the Borniopsis species, except for one species pair (B. tsurumaru and B. ariakensis), suggesting that this group diversified by repeated host shifts among various invertebrates in eastern Asia. This assumption is partially confirmed by molecular phylogeny .
In this study, we describe a new species of Borniopsis, which was collected from the synaptid sea cucumber Patinapta ooplax (von Marenzeller, 1881) on mud flats at the mouth of Souzu River, southwestern Shikoku Island, Japan. Patinapta ooplax is a small earthworm-like holothurian that burrows in muddy sediments in the intertidal zone. We compared its morphology and host associations with the other Borniopsis species. In addition, we reviewed galeommatoideans associated with apodid sea cucumbers.

Materials and methods
An undescribed species of Borniopsis was found attached to the synaptid sea cucumber Patinapta ooplax in the mud flats at the mouth of the Souzu River, Ainan-cho, Ehime Prefecture, southwestern Shikoku Island, Japan (32°57'N, 132°33'E) on 20 May 2000. We collected the specimens of this bivalve species during spring low tides in 2000, 2001, 2003, 2006, 2007, 2009, 2012, and 2013. Seven specimens collected on 11 March 2012 and two specimens collected on 15 March 2013 were preserved in 100% ethanol and brought back to the laboratory and observed under a binocular dissecting microscope for description. One specimen collected on 12 April 2009 was bleached to remove the periostracum for observation of shell surface sculpture. We deposited the holotype and two paratypes in the Museum of Zoology, University of Michigan (UMMZ), and two paratypes in National Museum of Nature and Science, Tokyo (NSMT). We also observed specimens of B. tsurumaru (SBMNH 149526), B. ariakensis (SBMNH35056), B. ochetostomae (SBMNH 149525) and B. maipoensis (SBMNH 35126) loaned from the Santa Barbara Natural History Museum. All the loaned specimens were collected from the mud flats of Hong Kong.
Description. Shell (Figs 1-3): Shell small (up to 4.1 mm), thin, slightly inflated, shape elongate-ovate and equivalve; inequilateral, longer anteriorly. Anterior and posterior margins rounded, ventral margins slightly rounded. Umbo small. Beak small, prosogyrate, situated 2/3 of way toward posterior. Each valve covered by tan to dark brown periostracum with black deposits, often eroded around beaks (Figs 2, 3). Shell surface underneath periostracum smooth and whitish with pearly luster (Fig. 1B). without prominent tentacles, edges narrowly extend beyond margin of shell, with numerous short papillae regularly arranged. Both anterior and posterior adductor muscles elongate-ovate, subequal and situated in the middle of dorsal and ventral margin. Ctenidia with gill axis nearly vertical, flat, consisting of single demibranch with both ascending and descending lamellae, joined anteriorly to inner and outer labial palps. Labial palps leaf-shaped. Foot slender, of moderate size, with small heel; byssal glands located just in front of heel. Gonads situated from middle to posterior in visceral mass just below umbo. Stomach and digestive gland large, occupying anterior part of visceral mass.
Distribution. Only known from the type locality.
Host association. Borniopsis mortoni attaches to the body surface of P. ooplax by both its foot and byssal threads (Fig. 1C-F). Individuals were patchily distributed in the mud flats. Within the particular patches we sampled, approximately 70% of synaptids served as hosts for B. mortoni. Several B. mortoni often occured on one the same host. At the maximum, more than 10 individuals were attached to a single host. Two Etymology. The specific name is dedicated to Dr. Brian Morton who has made great contributions to marine biology, marine ecology and malacology. He discovered many interesting commensal galeommatoidean species from Hong Kong, some of which now belong to the genus Borniopsis.
Remarks. The genus Borniopsis has been variously assigned to the Kelliidae (Morton and Scott 1989), Lasaeidae (Bieler et al. 2010), and the subfamily Montacutinae within Galeommatidae sensu lato (Huber 2015). However, these family-or subfamilylevel groupings are ill-defined when a range of characters and taxa are considered (Ponder 1998). Indeed, the molecular analyses conducted by  showed that each of these groups is actually polyphyletic. In this study, we assigned the genus Borniopsis tentatively to Galeommatidae sensu lato, which Ponder (1998) defined by the same diagnosis that was applied to the superfamily Galeommatoidea, as did Huber (2015). Further taxonomic assignment of this genus within Galeommatoidea (or Galeommatidae sensu lato) should be delayed until its family-level (or subfamily-level) classification is revised.
As with B. mortoni, both B. tsurumaru and B. ariakensis have a symbiotic relationship with synaptid sea cucumbers (Morton 1988, Morton and Scott 1989, Lützen et al. 2004, Kai and Henmi 2008. However, the particular host species differ between them -Patinapta ooplax (host solely for B. mortoni) and Protankyra bidentata (Woodward & Barrett, 1858) (host for both B. tsurumaru and B. ariakensis) Scott 1989, Lützen et al. 2004). Borniopsis mortoni always attaches directly onto the body surface of the host (this study), whereas B. tsurumaru can attach to the body surface of the host, or the wall of the host's burrow, or to the carapace of commensal crabs living in the same burrows (Morton 1988, Morton and Scott 1989, Lützen et al. 2004, Kai and Henmi 2008. Furthermore, the number of bivalves per host is much higher in B. mortoni (several to more than 10) than B. tsurumaru and B. ariakensis (usually one) (Lützen et al. 2004, Goto, Ishikawa, and Hamamura, personal observations).
The shells of B. tsurumaru and B. ariakensis are much larger (up to 11-12 mm in SL) than those of B. mortoni (up to 4.1 mm) (Morton and Scott 1989) (Fig. 4A,  B). Probably, this corresponds with the size of the host because P. bidentata is much larger than P. ooplax. The shells of B. tsurumaru and B. ariakensis are thicker and more inflated than those of B. mortoni (Lützen et al. 2004;this study). In addition, the shells of B. mortoni are always covered by a dark brown periostracum, whereas those of B. tsurumaru and B. ariakensis are often whitish, although some are dark brown. The umbones of B. tsurumaru and B. ariakensis are more protruding than those of B. mortoni (Fig. 4A, B). A molecular analysis is needed to understand whether these three synaptid-associated species are monophyletic or not. In addition, morphological variation of B. tsurumaru and B. ariakensis is apparently continuous (Goto, Ishikawa and Hamamura, pers. obs.) so molecular testing should be employed to investigate whether they can be distinguished genetically or not. The present new species also closely resembles B. ochetostomae and B. maipoensis in having an elongate ovate shell covered by a brownish periostracum (Fig. 4C, D). However, B. ochetostomae is much larger (up to 11 mm) than B. mortoni and its beak is located more centrally than that of B. mortoni Scott 1989, Jespersen et al. 2002, this study) (Fig. 4C). On the other hand, B. maipoensis is rather smaller (up to 3 mm) and more rounded than B. mortoni (Morton and Scott 1989, this study) (Fig. 4D). In addition, B. maipoensis has two distinct papillae on the dorsal surface of the foot (Morton and Scott 1989), whereas we did not observe such papillae on B. mortoni (Fig. 1A). And lastly, the hosts for these three species are different -B. mortoni (holothurian hosts), B. ochetostomae (echiuran hosts) and B. maipoensis (probably tanaid hosts) (Morton and Scott 1989, this study).

Discussion
Apodid sea cucumbers, including members of the families Synaptidae and Chiridotidae, are one of the major hosts for galeommatoideans (Boss 1965, Morton and Scott 1989, Kato 1998, Middelfart and Craig 2004 (Table 1). Including B. mortoni, at least 13 species are known to have commensal associations with apodid holothurians (Table 1). They can be separated into the following four groups: the first group includes Anisodevonia, Austrodevonia, Devonia and Entovalva, which have reduced shells covered by well-developed mantle lobes (Kawahara 1942, Kato 1998, Middelfart and Craig 2004; the second group includes Borniopsis covered in this study (Morton 1988, Morton andScott 1989); the third groups includes Montacuta (Bateson 1923, Fox 1979; and the forth group includes Scintillona (Morton 1957, Ó Foighil andGibson 1984). All of these four groups live attached to the apodid's body surface, except for Entovalva, which lives inside the host's oesophagus (Spärk 1931, Kato 1998. Molecular phylogenies suggested that associations with apodids have evolved repeatedly in the Galeommatoidea .
Borniopsis mortoni lives commensally with the apodid sea cucumber P. ooplax on the temperate coast of western Shikoku Island. On the other hand, another galeommatoidean species, Anisodevonia ohshimai, lives attached to P. ooplax on the subtropical coast of the Ryukyu Islands, southwestern Japan (Kawahara 1942, Kato 1998. Extensive sampling for A. ohshimai has been undertaken in the Ryukyu Islands (Kawahara 1942, Kato 1998, Kosuge 2001. However, B. mortoni has never been found there. Thus, B. mortoni probably does not occur on the subtropical coast of the Ryukyu Islands and may be restricted to more temperate coasts. An alternative hypothesis is that P. ooplax used by Table 1. Galeommatoidean bivalves commensal with apodid sea cucumbers.
A new species of the genus Borniopsis is described herein. This genus is restricted to eastern Asia, and each species apparently utilizes a different invertebrate host ( Table 2), suggesting that its diversification was caused by repeated host shifting. The shell size varies among species (Table 2), which is probably a specialization to each particular commensal lifestyle. Borniopsis macrophthalmensis, B. maipoensis and B. mortoni are the three smallest species in this genus (Table 2). Borniopsis macrophthalmensis is attached to the body surface of highly-mobile intertidal crabs only by fine byssal threads (Morton and Scott 1989). If B. macrophthalmensis had a large, heavy shell, it would easily fall off the host crab when it scurries quickly on the surface of the mudflat. Thus, such a small shell is probably an adaptation for life on a highly-mobile crab. Similarly, another galeomatoidean species, Arthritica japonica Lützen & Takahashi, 2003, which is also attached to the body surface of crabs, has very small shells (up to 2.05 mm) (Lützen and Takahashi 2003). This may represent a morphological convergence to a similar commensal lifestyle and supports our hypothesis of reduction in shell size associated with fast-moving hosts mentioned above. On the other hand, the second species B. maipoensis lives commensally with the tanaid Discapseudes sp. (Morton and Scott 1989), whereas the third species B. mortoni lives with the synaptid sea cucumber P. ooplax. The diameter of these host burrows is very small. Thus, the small-sized shells of B. maipoensis and B. mortoni are probably an adaptation to the mode of life in narrow host burrows.
The mudflats of eastern Asia evidently possess one of the richest burrowing invertebrate faunas in the world. However, burrow associates remain poorly understood in many of them. Thus, further investigation in this area could reveal increased diversity of this distinctive bivalve genus Borniopsis.