Rhiostoma thachi Huber, 2018

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

10. Rhiostoma thachi Huber, 2018

Figs 18 , 28

Rhiostoma thachi Huber in Thach, 2018: 17, 18, figs 74-80. Type locality: Thakhek, Central Laos (close to Vietnam borderline).

Rhiostoma haughtoni - Thach 2018: 17, figs 66, 67 (misidentified).

Type material.

Holotype MNHN-IM-2000-34050 (Fig. 28A ) from Thakhek, Central Laos.

Other material examined.

Laos: Tam Jung (cave), Vang Vieng District, Vientiane Province: CUMZ 3941 (8 shells; Fig. 28B-E ), CUMZ 10201 (5 shells).

Diagnosis.

Shell medium, thick and depressed shell. Detached whorl long, curved and descending, and sutural ridge absent. Breathing device very long, tubular. Shell colour uniformly brown to dark brown along dorsal and ventral sides. Peripheral band absent. Apertural lip much expanded at palatal side, double, with many concentric lines.

Differential diagnosis.

Rhiostoma thachi can be distinguished from R. jalorensis from peninsular Thailand in having a very long detached whorl, without peripheral band, and lip much expanded at palatal side. In comparison, R. jalorensis has a shorter detached whorl, with peripheral band, and lip evenly thickened and expanded.

Description.

Shell. Shell medium, cW 17.2-21.4 mm, cH 8.8-11.9 mm, thick, and nearly flattened to sub-discoidal shape; detached-whorl length 19.5-27.0 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thin corneous, and transparent. Shell colour uniformly brown to dark brown; ventral shell surface with paler colour; peripheral band absent. Detached whorl long, ~ 2 × longer than apertural width, curved and descending, and sutural ridge inconspicuous. Peristome circular and double; lip brownish, thickened, much expanded on palatal side, and multi-layered. Aperture opened sub-laterally to ventrally. Breathing device tubular, and may be attached to preceding whorl; outer lip forming a long and complete tube, located just behind apertural lip; inner lip with hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 28 ).

Distribution.

This species is known from the type locality, Khammouane Province, and a limestone mountain in Vientiane Province (Fig. 18 ).

Remarks.

The empty shells from Vang Vieng District, Vientiane Province, Laos, are relatively bleached and have no operculum. Instead, they have a long, curved and descending detached whorl; tubular-shaped breathing device; aperture opened ventrally; apertural lip brownish, thickened and expanded. However, these unique shell characters suggest they are conspecific with R. thachi from Central Laos. No living specimens of this species were found during this survey. Therefore, more sampling effort and molecular phylogenetic analyses are required to demonstrate the relationship between this species and other species in the R. housei group.

The specimen mentioned as R. haughtoni in Thach (2018: 17, figs 66, 67) is absolutely different from the type specimens from Myanmar (Fig. 11A, B ). In addition, this specimen agrees well with all shell characters and was also collected from an area near the R. thachi type locality. Therefore, this specimen is misidentified and should be recognised as R. thachi .

Figures

Figure 11. Shell of Rhiostoma haughtoni A lectotype UMZC I. 103445. A from Moulmein B paralectotype UMZC I. 103445. B C specimen NHMUK 88.12.4.1985 - 1986 figured in Gude (1921, fig. 22) from Damaltha, Burma D-F specimens CUMZ 10048 from Dhammatat Cave, Mon, Myanmar.

Figure 18. Distribution map of Rhiostoma housei - species group.

Figure 28. Shell of Rhiostoma thachi A holotype MNHN-IM- 2000 - 34050 from Thakhek, Laos B-E specimen CUMZ 3941 from Vang Vieng, Vientiane, Laos (E with malform). Photograph: M Caballer and V Heros, MNHN (A).