Rhiostoma laosense Tongkerd & Inkhavilay, 2023

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

27. Rhiostoma laosense Tongkerd & Inkhavilay sp. nov.

Figs 44 , 52 , 53D

Type material.

Holotype CUMZ 10001/2 (W 28.7 mm, H 14.2; Fig. 52A ). Paratypes CUMZ 10001/1 (23 adults + 2 juveniles), CUMZ 10001/3 (2 shells; Fig. 52B, C ), NHMUK 20220448 (5 shells), and SMF 368683 (5 shells). All paratypes are from the type locality.

Type locality.

Wat Pa Pha, Khamkeut District, Borikhamxay Province, Laos (18°11'17.3"N, 104°56'26.2"E).

Other material examined.

Laos: Tam Mangkorn (cave), Khamkeut District, Borikhamxay Province: CUMZ 4753 (2 shells; Fig. 52D ), 10005 (17 shells). Tad Muang, Hineboun District , Khammouane Province: CUMZ 10045 (33 shells), 10205 (2 shells; Figs 52E, F , 53D ) .

Diagnosis.

Shell large, depressed, and without detached whorl. Breathing device canal-shaped, pointed anteriorly, and attached to preceding whorl. Peristome double; outer lip with wide canal; inner lip continuous without incision. Operculum calcareous, cup-shaped, and densely lamellate.

Differential diagnosis.

This new species is superficially similar to R. morleti and R. prestoni ; the distinguishing characters are the canal-shaped breathing device with outer lip expanded at the base and inner lip continuous without incision, and the operculum with dense lamellae. In contrast, R. morleti has an incomplete tube or canal-shaped breathing device with outer lip forming a canal or nearly closed tube, inner lip with shallow incision or a small hole, and operculum with loose lamellae. In addition, R. prestoni has a canal-shaped breathing device with a wide and shallow incision on the inner lip.

Description.

Shell. Shell large, width 26.7-31.0 mm, height 12.3-15.9 mm, thickened, and flattened to sub-discoidal shape. Apex acute with dark colouration; spire flat. Whorls 4 to 5, convex, increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thin or thick, corneous, dark brown. Shell colour uniformly reddish brown to dark brownish irregular patterns; wide and dark spiral band on periphery. Detached whorl absent. Peristome circular and double; lip thickened, expanded, and multi-layered. Aperture opened sub-laterally. Breathing device canal-shaped, attached to preceding whorl and protruding anteriorly; outer lip protruding, with expanded base and wide canal; inner lip continuous without incision at base of breathing device. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral with dense lamellae (Fig. 52 ).

Radula. Teeth arrangement and shape are very similar to those of R. cambodjense . Central tooth with five cusps; central cusp with similar shape and nearly equal size with two lateral cusps on each side. Lateral teeth are composed of three dull cusps. Marginal teeth each composed of three cusps (Fig. 53D ).

Etymology.

The species name laosense is derived from the type locality Laos, where the type specimen was collected.

Distribution.

This new species is known from multiple localities on the massive limestone karsts in Borikhamxay and Khammouane provinces, southern Laos (Fig. 44 ).

Remarks.

No live specimens of this species were found during our surveys. The record from Laos in Inkhavilay et al. (2019) as " Rhiostoma morleti Dautzenberg & Fischer, 1906" should be referred to as this new species based on a unique canal-shaped breathing device protruding anteriorly.

Figures

Figure 44. Distribution map of Rhiostoma morleti - species group.

Figure 52. Shell of Rhiostoma laosense sp. nov. A holotype CUMZ 10001 / 2 from Wat Pa Pha, Khamkeut, Borikhamxay, Laos B, C paratypes CUMZ 10001 / 3 from type locality D specimen CUMZ 4753 from Tam Mangkorn Cave, Lak Sao, Borikhamxay, Laos E, F specimens CUMZ 10205 from Tad Muang, Khamkeut, Borikhamxay, Laos.

Figure 53. Radula morphology A Rhiostoma cambodjense, specimens CUMZ 4714 from Khao Chakan, Srakeo B Rhiostoma cheliopegma sp. nov., paratype CUMZ CUMZ 3985 / 2 Tham Takien, Khao Chamao, Rayong C Rhiostoma gnomus sp. nov. paratype CUMZ 4717 from Pha Chu, Na Noi, Nan D Rhiostoma laosense sp. nov. specimen CUMZ 10205 from Tad Muang, Khammouane, Laos.