Borniopsis mortoni, Goto, Ryutaro & Ishikawa, Hiroshi, 2016

Goto, Ryutaro & Ishikawa, Hiroshi, 2016, Borniopsismortoni sp. n. (Heterodonta, Galeommatoidea, Galeommatidaesensu lato), a new bivalve commensal with a synaptid sea cucumber from Japan, ZooKeys 615, pp. 33-45 : 34-39

Taxon classification Animalia Veneroida Galeommatidae

Borniopsis mortoni sp. n. Figs 1, 2, 3, 4

Material examined.

Holotype (Figs 1, 2): UMMZ 305035 (SL 4.1 mm, SH 2.8 mm). Paratype 1 (Fig. 3): UMMZ 305036-1 (SL 3.9 mm, SH 2.7 mm), paratype 2: UMMZ 30536-2 (SL 2.4 mm, SH 1.8 mm), paratype 3: NSMT-Mo 78968 (SL 3.7 mm, SH 2.7 mm) and paratype 4 (Fig. 1B): NSMT-Mo 78969 (SL 3.3 mm, SH 2.3 mm). Non-type specimens: four individuals of Borniopsis mortoni (SL 2.4, 3.3, 3.8, 3.3 mm). All specimens were collected 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). Comparative species (Fig. 4): Borniopsis tsurumaru , SBMNH 149526; Borniopsis ariakensis , SBMNH 35056; Borniopsis ochetostomae , SBMNH149525; and Borniopsis maipoensis , SBMNH 35126.

Type locality and habitat.

Mud flats at the mouth of the Souzu River, Ainan-cho, Ehime, southwestern Shikoku Island, Japan (32°57'N, 132°33'E).

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). Sculpture consisting of fine, dense and a few strong, widely spaced commarginal growth striae visible even on periostracum and very faint dense radial striae only visible underneath periostracum (Fig. 1B). Hinge of each valve consisting of a single stout cardinal tooth in front of umbo and well-developed oblique internal ligament posterior to umbo (Figs 2F, G; 3E, F). Soft parts (Figs 1, 2): Mantle not reflected, 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.

Patinapta ooplax ( Echinodermata: Holothuroidea: Apodida : Synaptidae ) (Fig. 1 C–F).

Host association.

Borniopsis mortoni attaches to the body surface of Patinapta ooplax by both its foot and byssal threads (Fig. 1 C–F). Individuals were patchily distributed in the mud flats. Within the particular patches we sampled, approximately 70% of synaptids served as hosts for Borniopsis mortoni . Several Borniopsis mortoni often occured on one the same host. At the maximum, more than 10 individuals were attached to a single host. Two Patinapta ooplax infested by Borniopsis mortoni also harbored the endoparasitic eulimid gastropod Hypermastus lacteus (A. Adams, 1864).

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 subfamily-level groupings are ill-defined when a range of characters and taxa are considered ( Ponder 1998). Indeed, the molecular analyses conducted by Goto et al. (2012) 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 Borniopsis mortoni , both Borniopsis tsurumaru and Borniopsis 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 Borniopsis mortoni ) and Protankyra bidentata (Woodward & Barrett, 1858) (host for both Borniopsis tsurumaru and Borniopsis ariakensis ) ( Morton and Scott 1989, Lützen et al. 2004). Borniopsis mortoni always attaches directly onto the body surface of the host (this study), whereas Borniopsis 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, Goto et al. 2012). Furthermore, the number of bivalves per host is much higher in Borniopsis mortoni (several to more than 10) than Borniopsis tsurumaru and Borniopsis ariakensis (usually one) ( Lützen et al. 2004, Goto, Ishikawa, and Hamamura, personal observations).

The shells of Borniopsis tsurumaru and Borniopsis ariakensis are much larger (up to 11-12 mm in SL) than those of Borniopsis mortoni (up to 4.1 mm) ( Morton and Scott 1989) (Fig. 4A, B). Probably, this corresponds with the size of the host because Protankyra bidentata is much larger than Patinapta ooplax . The shells of Borniopsis tsurumaru and Borniopsis ariakensis are thicker and more inflated than those of Borniopsis mortoni ( Lützen et al. 2004; this study). In addition, the shells of Borniopsis mortoni are always covered by a dark brown periostracum, whereas those of Borniopsis tsurumaru and Borniopsis ariakensis are often whitish, although some are dark brown. The umbones of Borniopsis tsurumaru and Borniopsis ariakensis are more protruding than those of Borniopsis 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 Borniopsis tsurumaru and Borniopsis 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 Borniopsis ochetostomae and Borniopsis maipoensis in having an elongate ovate shell covered by a brownish periostracum (Fig. 4C, D). However, Borniopsis ochetostomae is much larger (up to 11 mm) than Borniopsis mortoni and its beak is located more centrally than that of Borniopsis mortoni ( Morton and Scott 1989, Jespersen et al. 2002, this study) (Fig. 4C). On the other hand, Borniopsis maipoensis is rather smaller (up to 3 mm) and more rounded than Borniopsis mortoni ( Morton and Scott 1989, this study) (Fig. 4D). In addition, Borniopsis maipoensis has two distinct papillae on the dorsal surface of the foot ( Morton and Scott 1989), whereas we did not observe such papillae on Borniopsis mortoni (Fig. 1A). And lastly, the hosts for these three species are different - Borniopsis mortoni (holothurian hosts), Borniopsis ochetostomae (echiuran hosts) and Borniopsis maipoensis (probably tanaid hosts) ( Morton and Scott 1989, this study).