Rhiostoma asiphon Moellendorff , 1893

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

14. Rhiostoma asiphon Moellendorff, 1893

Figs 33 , 34 , 35 , 42A

Rhiostoma asiphon Möllendorff, 1893: 142. Type locality: Insel Samui, Golf von Siam [Samui Island, Surat Thani Province, Thailand]. Möllendorff 1894: 152, 153, p1. 16, figs 16, 17. Fischer and Dautzenberg 1904: 427. Kobelt 1902: 176. Kobelt 1911: 760, 761, pl. 111, figs 4-8. Zilch 1956: 174. Benthem Jutting 1960: 11. Chan 1996: 5, fig. 3. Kongim et al. 2013b: 16, fig. 2c.

Rhiostoma sp. 7- Tumpeesuwan 2001: 90-92, figs 4.36, 4.37.

Type material.

Lectotype (designation in Zilch (1956)) SMF 130509/1 (Fig. 35A ) from Golf von Siam: Insel Samui. Paralectotypes SMF 130510/5 (5 shells), SMF 130511/2 (2 shells), SMF 130512/1 (1 shell).

Other material examined.

Thailand: Siam: NHMW 6657 ex. Gerstenbrandt collection (1 shell). Insel Samui, Golf von Siam: NHMW 40488 ex. Möllendorff collection (1 shell). Koh-Samui, Gulf of Siam: NHMW Rüsnov collection (2 shells). Koh Samsao, Angthong Islands, Koh Samui, Surat Thani: CUMZ 3839, 4756. Koh Wua Ta Lab, Angthong Islands , Koh Samui, Surat Thani: CUMZ 3872, 4365, 4400, 4715, 4730, 4767 (Fig. 35D-F ), 4825, 4826 (Figs 33 , 35B-C , 42A ) .

Diagnosis.

Shell thick and depressed; detached whorl short. Shell uniformly brown to purplish, sometimes with yellowish colour. Breathing device notch-shaped.

Differential diagnosis.

Rhiostoma aiphon is superficially similar to R. samuiense in having a uniformly brownish to purplish shell and notch-shaped breathing device. However, this species can be distinguished by having an extremely short detached whorl, while R. samuiense possesses a medium to long and descending detached whorl.

Description.

Shell. Shell small to medium, cW 18.4-22.7 mm, cH 9.4-12.5 mm, thickened, and sub-discoidal shape; detached-whorl length 2.5-8.0 mm. Apex acute, earlier whorls with dark colouration; spire nearly flat. Whorls 4 to 5, convex, increasing regularly; suture wide and very shallow; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum brownish to thin, corneous, and translucent. Shell colour uniformly brown to purplish; with thin and narrow brownish peripheral band sometimes present. Detached whorl shorter than apertural width to nearly absent. Peristome circular and double; lip thickened, expanded, and multi-layered. Aperture opened sub-laterally. Breathing device notch-shaped; outer lip protruding, with shallow notch; inner lip with shallow incision to indistinct. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 35 ).

Radula. Taenioglossate radula arranged in inverted V-shaped rows. Central tooth with well-developed central cusp and three lateral cusps on each side; central cusp large and tall with blunt tip; lateral cusps triangular, tapering in size and with pointed tips. Lateral teeth consisting of four cusps; central cusp large, blunt tip, and flanked by pointed tips of two inner cusps and one outer cusp. Inner and outer marginal teeth each composed of three cusps; central cusp large with dull tip, flanked by small pointed tips of one inner cusp and one outer cusp (Fig. 42A ).

Distribution.

The previous record of this species was only from the type locality "Insel Samui, Golf von Siam" ( Möllendorff 1893, 1894). The records of " R. asiphon " on the Malay Peninsula (Thailand and Malaysia) are based on misidentifications and excluded herein. Therefore, this species is probably endemic to the Ang Thong Islands in Surat Thani Province. These isolated limestone islands are located ~30 km to the west of Samui Island, Surat Thani Province, and this is considered the type locality of this species (Fig. 34 ).

Remarks.

Under the name Rhiostoma asiphon [non Möllendorff 1893], this species has been reported from several places on the Malay Peninsula (see Benthem Jutting 1960; Berry 1963; Chan 1996) and also reported as " Pterocyclus [sic] Pterocyclus asiphon " from several localities in southernmost Thailand ( Tumpeesuwan 2001). We examined these specimens and revisited all mentioned localities, but found no samples that could be provisionally identified as R. asiphon s. s. On the other hand, numerous specimens of Pterocyclos diluvium , common snails in those localities, were collected (see Sutcharit et al. 2014). Their shell morphology is superficially similar to R. asiphon . However, the distinguishing characters of the genus Pterocyclos are an outer lip expanded at the base, forming a nearly closed tube; an apertural groove is located inside the aperture near the apertural lip; and a calcified plate-shaped operculum is present (Fig. 2 ). Therefore, we conclude that the nominal species name R. asiphon in Benthem Jutting (1960), Berry (1963), and Chan (1996) and Pterocyclus asiphon in Tumpeesuwan (2001) and Tumpeesuwan and Panha (2003) are erroneously identified and should be recognised as Pterocyclos diluvium .

Of the paralectotypes SMF 130510/5 (5 shells), four of them are identical to the lectotype specimen (Fig. 35A ). The exception is a single specimen exhibiting canal-shaped breathing device that is expanded at the base near the suture, similar to the distinct characters of either R. morleti or R. prestoni . However, this is beyond the range of individual variation, and the recent topotype specimens (~100 shells) included no shells exhibiting these peculiar characters. Therefore, we excluded that specimen from our attention in the type series.

Figures

Figure 2. Schematic of shell morphology and terminology of the Rhiostoma A apical view with shell terminology and measurements of coiled-whorl width and detached-whorl length B operculum showing side and cross-section views C apertural views showing various shell shapes as recognised in this revision: flattened, discoidal, sub-discoidal and heliciform.

Figure 33. Live snails of the species group III. Rhiostoma asiphon specimen CUMZ 4826 from Koh Wua Ta Lab, Ang Angthong, Surat Thani (topotype), A life habitus and B fully extending body underwater. All figures are not to scale. Photographs: P. Tongkerd

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

Figure 35. Shell of Rhiostoma asiphon A lectotype SMF 130509 from Insel Samui, Golf von Siam B, C specimens CUMZ 4826 from Koh Wua Ta Lab, Angthong Islands, Surat Thani with dark shell form D-F specimens CUMZ 4767 from Koh Wua Ta Lab, Angthong Islands, Surat Thani with yellowish shell form.

Figure 42. Radula morphology A Rhiostoma asiphon CUMZ 4826 from Angthong Islands, Surat Thani B Rhiostoma strubelli specimen CUMZ 10200 from Vieng Phuka, Luang Namtha, Laos C Rhiostoma breviocollar sp. nov. paratypes CUMZ 4491 from type locality D Rhiostoma anceyi sp. nov. paratype CUMZ 4494 / 2 from type locality E Rhiostoma furfurosum sp. nov. paratype CUMZ 4705 / 2 from type locality F Rhiostoma platymorpha sp. nov. paratype CUMZ 4763 from type locality.