Rhiostoma abletti Thach, 2016

Tongkerd, Piyoros, Tumpeesuwan, Sakboworn, Inkhavilay, Khamla, Prasankok, Pongpun, Jeratthitikul, Ekgachai, Panha, Somsak & Sutcharit, Chirasak, 2023, Systematic revision of the snorkel snail genus Rhiostoma Benson, 1860 (Gastropoda, Caenogastropoda, Cyclophoridae) with descriptions of new species, ZooKeys 1142, pp. 1-144 : 1

16. Rhiostoma abletti Thach, 2016

Figs 34 , 37

Rhiostoma abletti Thach, 2016: 37, 38, figs 53, 122-124. Type locality: Northwest of Lai Chau city, on the way going to Paso, Lai Chau Province, North Vietnam. Sutcharit et al. 2019: 5, fig. 1a.

Rhiostoma christae Thach, 2016: 38, figs 51, 130-133. Type locality: Near the road No. 6 to Chieng Ngan, Son La Province, North Vietnam. Sutcharit et al. 2019: 17, fig. 3l. New synonym.

Rhiostoma ninhbien Do, Nguyen & Do, 2020b: 169, 170, fig. 2a-c. Type locality: near Tay Trang international border gate, Na U Commune, Dien Bien District, Dien Bien Province, Vietnam. New synonym.

Rhiostoma marioni - Do et al. 2020b: 168 (in part), fig. 1e, f (not Ancey 1898).

Type material.

Holotype NHMUK 20160307 (Fig. 37A ) from Northwest of Lai Chau city, on the way going to Paso, Lai Chau Province, Vietnam. Holotype NHMUK 20160306 (Fig. 37B ) of Rhiostoma christae Thach, 2016, from Near the road No. 6 to Chieng Ngan, Son La Province (North Vietnam).

Other material examined.

Laos: Hot Springs (Vieng Thong), Meuang Hiam District, Houaphanh Province: CUMZ 10206 (3 shells; Fig. 37E, F ). Near Ban Na Puek, Meuang Hiam District , Houaphanh Province: CUMZ 10207 (2 shells; Fig. 37C, D ) .

Diagnosis.

Shell medium, thin and flattened; detached whorl shorter than apertural width. Shell colour with brownish zigzag patterns. Breathing device with incomplete tube shape and nearly perpendicular to detached whorl.

Differential diagnosis.

This species can be distinguished from R. anceyi sp. nov. by having an incomplete tube breathing device that is nearly perpendicular to detached whorl, and translucent periostracum. In contrast, the new species has a tubular breathing device, and thickened and reddish brown periostracum.

Description.

Shell. Shell medium, width 38.5 mm, height 14.5 mm, thin, and flattened to sub-discoidal shape. Apex acute; spire slightly elevated to flattened. Whorls 5 to 6, convex, increasing regularly; suture wide and depressed; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thick corneous and translucent. Shell colour with dark brown zigzag pattern; with narrow dark spiral band on periphery. Detached whorl shorter than apertural width. Peristome circular and double; lip thickened and slightly expanded. Aperture opened sub-laterally. Breathing device with incomplete tube shape and nearly perpendicular to detached whorl; outer lip forming a short to long nearly closed tube; inner lip with deep incision or small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, low cup-shaped, and multispiral (Fig. 37 ).

Distribution.

This species is known from Lai Chau, Son La and Dien Bien provinces, northern Vietnam. The current study recorded specimens from a few localities in Hua Phan Province, northern Laos (Fig. 34 ).

Remarks.

The holotypes of R. abletti from Lai Chau Province (Fig. 37A ), " R. christae " from Son La Province (Fig. 37B ) and " R. ninhbien " from Dien Bien Province (see Do et al. 2020b: fig. 2a-c) are nearly identical in appearance. They have a depressed shell, with short detached whorl and incomplete tube shape, but are slightly different in the shell colour pattern, which could not be used as a diagnostic trait. In addition, the type localities of these three species are in neighbouring areas of northern Vietnam, less than 150 km apart. Therefore, we recognise R. christae and R. ninhbien as junior subjective synonyms of R. abletti .

The specimens from Hua Phan Province, Laos, tend to have thicker periostracum and more coarse growth lines than the type specimen. However, a short detached whorl and incomplete tube as breathing device suggest they are more closely related to R. abletti .

Figures

Figure 34. Distribution map of Rhiostoma asiphon - species group.

Figure 37. Shell of Rhiostoma abletti A holotype NHMUK 20160307 from Lai Chau Province, Vietnam B holotype of Rhiostoma christae Thach, 2016, NHMUK 20160306 from Son La Province, Vietnam C, D specimens CUMZ 10207 from Ban Na Puek, Meuang Hiam, Houaphanh, Laos E, F specimens CUMZ 10206 from Hot Springs, Meuang Hiam, Houaphanh, Laos.