Rhiostoma prestoni (Bavay & Dautzenberg, 1909)

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

24. Rhiostoma prestoni (Bavay & Dautzenberg, 1909)

Figs 48 , 49

Pterocyclos prestoni Bavay & Dautzenberg, 1909b: 248, 249. Type locality: Binh-Lu, Tonkin [Binh Lieu, Quang Ninh Province, Vietnam]. Bavay and Dautzenberg 1909a: 283, 284, pl. 11, figs 1-3. Kobelt 1913: 969, 970, pl. 112, figs 5-7. Zilch 1956: 172.

Pterocyclos prestoni var. depicta Bavay & Dautzenberg, 1909b: 249. Type locality: Phong-Tho, Tonkin [Phong Tho, Lai Chau Province, Vietnam]. Bavay and Dautzenberg 1909a: 284, pl. 11, fig. 4. Zilch 1956: 173. Kobelt 1913: 970, pl. 112, figs 8-9. New synonym.

Pterocyclos fruhstorferi Kobelt, 1909: 82. Type locality: Chiem-hoa, Tonkin [Chiem Hoa, Tuyen Quang Province, Vietnam]. Kobelt 1911: 739, 740, pl. 107, figs 7-9. Zilch 1956: 172.

Type material.

Syntype MNHN-IM-2000-30754 (1 shell; Fig. 48A ) from Binh-Lu (figured in Bavay and Dautzenberg 1909a: pl. 11, figs 1-3) and syntype RBINS 525354 (1 shell; Fig. 48B ) from Binh Lu. Syntype MNHN-IM-2000-30755 (1 shell; Fig. 48C ) of Pterocyclos prestoni var. depicta Bavay & Dautzenberg, 1909, from Phong-Tho (figured in Bavay and Dautzenberg 1909a: pl. 11, fig. 4) and syntype RBINS 525355 (3 shells; Fig. 48D-F ) from Phong-Tho. Lectotype (designation in Zilch (1956)) SMF 130353 (1 shell; Fig. 49A ) of Pterocyclos fruhstorferi Kobelt, 1909, from Chiem-hoa, Tonkin, paralectotype SMF 130354 (1 shell; Fig. 49B ) and paralectotype SMF 130355/2 (2 shells; Fig. 49C, D ).

Other material examined.

Vietnam: Trinh-Loung, Tonkin: SMF 130356. Muong-Hum, Tonkin: SMF 130357, 130358 (Fig. 49E ). Ba-Xot, Tonkin: NHMW 71770/R/9406 (1 shell). Not-Son, Tonkin: NHMW 71770/R/9405 (1 shell). Tuah-Tuong, Tonkin: NHMW 71770/R/9404 (1 shell). Khe-So, Tonkin: NHMW 47842 (2 shells). Muong Bo, Tonkin: NHMW 71770/R/9407 (1 shell). Bo Xat, Tonkin: NHMW 71770/R/9403 (1 shell).

Diagnosis.

Shell large, flattened to depressed shell, and without detached whorl. Breathing device canal-shaped, pointed anteriorly, and attached to preceding whorl. Peristome double, outer lip with wide canal, inner lip with wide and shallow incision. Operculum calcareous, low cup-shaped.

Differential diagnosis.

Rhiostoma prestoni has a shell morphology that resembles that of R. morleti , but it differs in having the canal-shaped breathing device completely attached to the preceding whorl, while R. morleti has an incomplete tube or a canal-shaped breathing device not attached to the preceding whorl.

Description.

Shell. Shell medium to large, width 22.5-29.1 mm, height 7.8-12.0 mm. Shell thickened, and flattened shape. Apex acute; spire flat to slightly elevated. Whorls 4 to 5, convex, increasing regularly; suture wide and deep; last whorl rounded, stout and wider than penultimate whorl. Shell surface with fine growth lines. Periostracum thick, corneous, and brownish. Shell colour varies from uniformly brownish to brownish zigzag patterns; brownish peripheral band present or absent. Detached whorl absent. Peristome circular and double; lip thickened and slightly expanded. Aperture opened sub-laterally. Breathing device canal-shaped and attached to preceding whorl, causing it to appear without a detached whorl; outer lip protruding, expanded at base and forming wide canal; inner lip with a wide and shallow incision. Umbilicus widely opened. Operculum calcareous, cup-shaped, and multispiral (Figs 48 , 49 ).

Distribution.

This species is mainly distributed in Lai Chau, Quang Ninh, and Tuyen Quang provinces of northern Vietnam (Fig. 44 ).

Remarks.

Rhiostoma prestoni has long been classified as a member of the Pterocyclos (see Kobelt 1911; Zilch 1956). The operculum stuck inside the last whorl of the syntype is a calcareous cup shape and multispiral with elevated lamellae (Figs 48 , 49 ); these are the distinguishing characters of the genus Rhiostoma (Fig. 5 ). Therefore, we have reclassified this species as a member of Rhiostoma .

We examined specimens labelled as R. prestoni in the RBINS ex. Dautzenberg collection from northern Vietnam that show a high variation in shell colour, from monochrome yellowish to brownish zigzag pattern. However, the shape of the breathing device has less variation than the shell characters and colour patterns. Therefore, we retain R. prestoni as a distinct species until the DNA phylogeny becomes available and confirms their status and relationship.

The nominal species, Pterocyclos fruhstorferi Kobelt, 1909, was described based on specimens from northern Vietnam. The unique name-bearing type was subsequently designated in Zilch (1956: 172). The lectotype has a shell form similar to that of R. prestoni , and the only noticeable difference is the brownish zigzag colour pattern. In contrast, the syntype of R. prestoni has a monochrome yellowish brown shell colour. Therefore, we agree with Kobelt (1911) in recognising P. fruhstorferi as a junior subjective synonym of R. prestoni .

The specimen figured in Do et al. (2015: 122, fig. 4b) as Pterocyclos prestoni Bavay & Dautzenberg, 1909 exhibits a corrugated shell surface with nodules arranged on growth lines over the entire shell, and with canal-shaped breathing device protruding apically and attached to preceding whorl. These characters clearly differ from the typical R. prestoni and represent an undescribed species. However, this specimen has no operculum for comparison; therefore, the generic placement is still provisional.

Finally, the 'var. Rhiostoma prestoni depicta ' was identified based on the distinct shell colour having brownish zigzags or blotches. However, the shell pattern alone cannot be considered the distinguishing character for recognising species or subspecies, since several species of the genus Rhiostoma have high variation in shell colour and pattern.

Figures

Figure 5. Comparison of breathing device and operculum characteristics among the five genera that closely resemble Rhiostoma. The specimens and opercula represented herein are the type specimen or topotype of the respective type species of each genus; Pterocyclos: P. rupestris Benson, 1832 (lectotype UMZC I. 102070, no locality data), Spiraculum: S. hispidum Pearson, 1833 (specimen NHMUK 1906.4.4.72, from Khasi, Teria Ghat), Rhiostoma: R. haughtoni Benson, 1860 (syntype UMZC I. 103445, from Moulmein), Cyclotus: C. variegatus Pearson, 1833 (specimen NHMUK 1896.6.7.34 - 35, from Palawan), Opisthoporus: Cyclostoma taylorianum Pfeiffer, 1851 = junior synonym of Pterocyclos biciliatum Mousson, 1849 (specimen NHMUK 20210200 for shell, from Basan, Sarawak) and Pterocyclos tener Menke, 1856 (NHMUK ex. Cuming collection for operculum, from Cochin China), and Ptychopoma: Cyclophorus chinensis Moellendorff, 1875 (lectotype SMF 39415, from Lushan bei Kiukiang, China). Red arrows indicate the expansion of outer lip at upper junction of the peristome. Blue arrows indicate breathing tubes (apertural tube in Rhiostoma and sutural tubes in Spiraculum and Opisthoporus).

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

Figure 48. Shell of Rhiostoma prestoni A syntype MNHN-IM- 2000 - 30754 from Binh-Lu B syntype RBINS / MT / 525354 from Binh-Lu C-F originally labelled as " Pterocyclus prestoni depictus Bavay & Dautzenberg, 1908 " C syntype MNHN-IM- 2000 - 30755 from Phong-Tho and D-F syntype RBINS / MT / 525355 from Phong Tho. Photograph: M Caballer and V Heros, MNHN (A, C).

Figure 49. Shell of Rhiostoma prestoni A-D originally labelled as " Pterocyclos fruhstorferi Kobelt, 1909 " A lectotype SMF 130353 from Chiem-hoa, Tonkin B paralectotype SMF 130354 from type locality and C, D paralectotype SMF 130355 from type locality E specimen SMF 130358 from Muong-Hum.