Amphidromus thachi Huber, 2015

Figs 6J, 12C, 17, 18A-D, 19

Amphidromus thachi Huber, 2015: 29-30, figs 1-8. Type locality: outskirts of Nha Trang area, about 30 km southeast of Nha Trang city (Cam Lam District, Khanh Hoa Province, central Vietnam), at some distance from the village and the National Road No 1A. Thach 2017: 47-48, pl. 53, fig. 668. Thach 2018: pl. 70, figs 838, 839. Thach 2021: 79.

Amphidromus thachi krisi Thach, 2018: 63-64, pl. 70, figs 833-837. Type locality: Lac Duong District, Lam Dong Province, South Vietnam. Thach 2021: 79.

Material examined.

Vietnam: Dextral, holotype of " Amphidromus thachi ", RBINS MT.3381 (Fig. 17A) .

Other material examined.

Vietnam: 1D + 1S specimens, fin de la route de Hon Ba (chalets de Yersin), Commune de Suoi Cat, Province de Khanh Hoa , Vietnam, MNHN- IM-214-6873 (Fig. 17B, C); 1D specimen, réserve de Hon Ba, près du chalet de Yersin, Commune de Suoi Cat, Province de Khanh Hoa , Vietnam, MNHN- IM-214-6874; 3D + 1S specimens, Vinh Thanh town, Binh Dinh Province , NMNS-8764-266- NMNS-8764-269 (Fig. 17D); 1D + 1S specimens, Buon Don District, Dak Lak Province , NMNS-8764-270, NMNS-8764-271 (Fig. 17E); 1S specimen, Da Lat city, Lam Dong Province, NMNS-8764-272 (Fig. 17F); 2D specimens, Krong Bong, Dak Lak Province, NMNS-8764-273, NMNS-8764-274; 2D specimens, Lac Duong District, Lam Dong Province , NMNS-8764-264, NMNS-8764-265 (Fig. 17G, H).

Diagnosis.

Shell medium and chirally dimorphic. Aperture obliquely elliptical with prominent anterior notch; columella bending anteriorly. Parietal callus, lip and columella whitish or with dark brown. Genitalia with appendix.

Differential diagnosis.

Amphidromus thachi is unique compared to all Vietnamese species reported by Schileyko (2011) in having a distinct shell shape, possessing an obliquely elliptical aperture with a prominent anterior notch, a columella bending anteriorly, and whitish or dark brown parietal callus, lip and columella. This type of shell form is similar to that of Pseudopartula Pfeiffer, 1856 (Benthem Jutting 1950). Amphidromus thachi is also recognised by a distinct clade in the molecular phylogeny (Fig. 2), with the closest p -distance to A. asperoides sp. nov. in both COI (12.69%) and 16S (6.22%) (Table 2).

Description.

Shell medium (height 25.0-30.0 mm, width 17.0-18.5 mm), chirally dimorphic, thin to slightly thickened, and conical. Spire short conical with white or pale colouration; apex acute without black spot on tip. Whorls 6-7 little convex to smooth; suture wide and shallow; last whorl well rounded to slightly elongated and with less prominent umbilical hump. Periostracum thin corneous; varices absent. Shell colour uniform whitish to pale cream; subsutural band opaque white. Parietal callus thickened, whitish and translucent or dark to dark brown. Aperture elliptical to obliquely elliptical with prominent anterior notch; inner side of outer wall whitish; peristome thickened, slightly expanded not reflected; lip whitish or with dark to dark brown. Columella whitish or dark, shortly straight then bending anteriorly. Umbilicus imperforate.

Radula. Teeth arranged in anteriorly pointed V-shaped rows. Central tooth monocuspid and spatulate with truncated cusp. Lateral teeth bicuspid; endocone slightly smaller than ectocone, curved, with wide notch and dull cusp; ectocone large with curved to dull cusp. Lateral teeth gradually transformed to asymmetric tricuspid marginal teeth. Outermost teeth with small and curved cusp on ectocone; endocone and mesocone with curved cusps (Fig. 12C).

Genital organs. Atrium relatively short. Penis slender, conical, and short, ~ 1/2 of vaginal length. Penial retractor muscle thickened and inserting on epiphallus close to penis. Epiphallus long, slender tube, almost same diameter as penis. Flagellum short, extending from epiphallus and terminating in weakly coiled. Appendix short, slender tube, similar length with flagellum, and ~ 1/2 of epiphallus length. Vas deferens slender tube passing from free oviduct and terminating at epiphallus-flagellum junction (Fig. 18A, B). Internal wall of penis corrugated, exhibiting series of thickened and smooth surfaced longitudinal penial pilasters forming fringe around penial wall, and with nearly smooth wall around base of penial verge. Penial verge short conical with smooth surface (Fig. 18C).

Vagina slender, cylindrical, and ~ 2 × longer than penis. Gametolytic organ relatively short than other congeners: gametolytic duct shorter to slightly longer than vagina, cylindrical tube, then tapering to short, slender tube terminally; gametolytic sac globular shape. Free oviduct short; oviduct compact, enlarged to form lobule alveoli (Fig. 18A, B). Internal wall of vagina possessing smooth longitudinal ridges near genital orifice; ridges becoming stronger and corrugated vaginal pilasters with swollen, irregular shaped and deep crenelations (Fig. 18D).

Living specimens with soft body morphology generally similar to A. ingens . Animals with whitish to creamy body covered with reticulated skin. Foot broad and long with uniform whitish to creamy colouration to posterior tail. Head with whitish or sometimes with yellowish colour. Upper tentacles drumstick-shaped, greyish to brownish, with dark eyespots on tentacular tips; lower tentacles short and greyish in colour (Fig. 6J).

Haplotype network.

There was a total of six COI haplotypes (Fig. 19A) and five 16S haplotypes (Fig. 19B) of A. thachi in this study, and the highest numbers of mutational steps in the COI and 16S minimum spanning networks are 26 and eight, respectively.

Distribution.

The distribution range of this species covers Binh Dinh, Dak Lak, Khanh Hoa, and Lam Dong provinces, Vietnam.

Remarks.

This species was originally described by Huber (2015) from outskirts of Nha Trang, Vietnam. Later, Thach (2018) described another subspecies from Lac Duong, Lam Dong, Vietnam as A. thachi krisi, which was different from the nominotypical subspecies in having a totally white lip. Based on this study, the specimens having a totally white lip from Lac Duong, Lam Dong constitutes a distinct clade from the remaining specimens with totally or partially dark lip, and the mutational steps between these two morphs with different lip colours are 26 and eight in the COI and 16S haplotype networks, respectively (Fig. 19). More specimens from wider distribution range will be needed to assess the taxonomic status of these A. thachi subspecies.

Two dissected specimens were found to have different lengths of the gametolytic duct. The specimen XM2 from Krong Bong, Dak Lak, Vietnam has a shorter gametolytic duct (Fig. 18A) than the specimen VCD2 from Buon Don, Dak Lak, Vietnam (Fig. 18B).