Rhiostoma cambodjense (Morelet, 1875)

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

22. Rhiostoma cambodjense (Morelet, 1875)

Figs 44 , 45B-F , 53A

Pterocyclos cambodjensis Morelet, 1875: 286, 287, pl. 13, fig. 1. Type locality: Battambang, Cambodje [Cambodia]. Breure et al. 2018: 232, figs 173, 174.

Opisthoporus pulchellus Morlet, 1889: 154, 188-189, pl. 6, fig. 5. Type locality: Mount. Sisophon (Siam) [Serei Saophoan, Banteay Meanchey, Cambodia]. Fischer 1891: 100. Kobelt and Möllendorff 1897: 119. Morlet 1904: 376, 377, pl. 21, figs 6, 6a. Fischer and Dautzenberg 1904: 427. New synonym.

Opisthoporus cambodjensis - Fischer 1891: 100. Fischer and Dautzenberg 1904: 427.

Cyclotus (Siphonocyclus) pulchellus - Kobelt 1902: 210. Kobelt 1911: 815, pl. 121, figs 1, 2.

Rhiostoma cambodjense - Kobelt 1902: 177. Kobelt 1911: 763, 764, pl. 113, figs 14-16. Sutcharit et al. 2019: 16, fig. 3i.

Type material.

Syntype NHMUK 1893.2.4.766 (1 shell; Fig. 45B ) from Battambang. Syntype MNHN-IM-2000-9828 (1 shell; Fig. 45C ) of Opisthoporus pulchellus Morlet, 1889, from Mont Sisophon (Siam).

Other material examined.

Cambodia: Thammaban Khiri Temple, Serei Saophoan (Sisophon), Banteay Meanchey: CUMZ 10203/2 (Fig. 45D ). Wat Kirirom Phnombak, Serei Saophoan (Sisophon), Banteay Meanchey: CUMZ 10203/1. Thailand: Tham Khao Chakan, Khao Chakan, Srakeo: CUMZ 3810, 3953, 3854, 3856, 3860, 4714 (Figs 45E, F , 53A ), 4734, 4813, 10122, 10123, 10124. Wat Khao Panom Sarapee, Khlong Had, Srakeo: CUMZ 10212.

Diagnosis.

Shell small, heliciform, detached whorl absent or inconspicuous, and breathing device notch-shaped. Peristome slightly thickened but not expanded, lip thickened, Shell colour yellowish to dark brown with zigzag patterns. Operculum calcareous, low cup-shaped.

Differential diagnosis.

Rhiostoma cambodjense can easily be distinguished from all other known Rhiostoma species (except R. gnomus sp. nov.; for further comparison, see under that species) in having heliciform shell and narrow umbilicus. Although this species is superficially similar to Cyclotus with a heliciform shell, the distinguishing characters are a notch-shaped breathing device, outer lip not expanded, and cup-shaped operculum with elevated lamella and straight lateral fringe. In contrast, the heliciform Cyclotus tend to have an expanded outer lip, a concave operculum without elevated lamella, and a deep groove on the lateral fringe (Table 2 ).

Description.

Shell. Shell small, width 16.5-18.7 mm, height 10.7-12.9 mm, thick or thin, and heliciform shape. Apex acute with dark colour; spire elevated. Whorls 4 to 5, convex, and increasing regularly; suture wide, shallow and with or without white subsutural band; last whorl rounded. Shell surface nearly smooth with fine growth lines. Periostracum thin, corneous, and transparent. Shell colour uniformly brownish, black, or with brown zigzag patterns, with narrow dark spiral band on periphery. Detached whorl usually absent (sometimes very short). Peristome circular and double; lip slightly thickened and expanded. Aperture opened laterally. Breathing device notch-shaped; outer lip protruding, with narrow groove; inner lip with shallow incision. Umbilicus narrowly opened and deep. Operculum calcareous, low cup-shaped, and multispiral (Fig. 45B-F ).

Radula. Taenioglossate radula arranged in inverted V-shaped row. Central tooth with well-developed central cusp and two lateral cusps on each side; central cusp large with pointed tip; lateral cusps triangular and tapering in size. Lateral teeth consisting of four cusps; central cusp large, triangular 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 and pointed tip, and flanked by pointed tips of one inner cusp and one outer cusp (Fig. 53A ).

Distribution.

The previous records of this species were only from Cambodia ( Morelet 1875; Morlet 1889; Kobelt 1911). The recent record of this species is from Serei Saophoan in Banteay Meanchey Province and Battambang Province, Cambodia, and Srakeo Province in Thailand (Fig. 44 ).

Remarks.

Rhiostoma cambodjense was described based on a specimen collected from Battambang in Cambodia ( Morelet 1875). The syntype NHMUK 1893.2.4.766 (Fig. 45B ) seems to have an eroded periostracum and outer shell surface, where only a dark apex and traces of brownish colour pattern remain on the upper shell surface. However, the calcareous and low cup-shaped operculum with elevated lamellae present on the syntype clearly show the characters of the genus Rhiostoma .

The original description of Opisthoporus pulchellus Morlet, 1889 includes an illustration of a specimen without an operculum. In this study, we re-visited the type locality and found that the specimens have a calcareous and low cup-shaped operculum, diagnostic of the Rhiostoma . In contrast, the syntype of Opisthoporus pulchellus Morlet, 1889 has a heliciform shell without a detached whorl, and a breathing device with notch shape. In addition, the type locality is Sisophon, whereas that of R. cambodjense is Battambang. These two localities are very close to each other (~70 km), and the type specimens are almost identical in shell characters except the brownish shell colour. Therefore, we consider Opisthoporus pulchellus Morlet, 1889 as a junior subjective synonym of R. cambodjense .

Figures

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

Figure 45. Shell of Rhiostoma spp. A Rhiostoma cochinchinensis probable syntype NHMUK 20170354 from Cochinchina B-F Rhiostoma cambodjense B syntype NHMUK 1893.2.4.776 from Battambang, Cambodje C syntype of " Opisthoporus pulchellus Morlet, 1889 " MNHN-IM- 2000 - 9828 from Mount. Sisophon D specimen CUMZ 10203 / 2 from Thammaban Khiri, Serei Saophoan (Sisophon), Cambodia E, F specimens CUMZ 4714 from Khao Chakan, Srakeo. Photograph: A Robin and V Heros, MNHN (C).

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.