Rhiostoma haughtoni Benson, 1860

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

1. Rhiostoma haughtoni Benson, 1860

Figs 9A , 10 , 11 , 16A

Rhiostoma haughtoni Benson, 1860: 96, 97. Type locality: ad cavernam Damatha, non procul ab urbe Moulmein [Dhammathat Cave, Mawlamyine, Mon State, Myanmar]. Pfeiffer 1865: 39, 40. Stoliczka 1871: 150. Kobelt and Möllendorff 1897: 115. Kobelt 1902: 177, 178, fig. 38. Kobelt 1911: 759, 760, pl. 110, fig. 14; pl. 111, figs 14-16. Gude 1921: 128, 129, fig. 22. Wenz 1938: 462, fig. 1166. Zilch 1956: 174. Rees 1964: 67, pl. 4, fig. 14. Preece et al. 2022: 77, 78, fig. 29e, f.

Pterocyclos (Rhiostoma) haughtoni - Hanley and Theobald 1870: 3, pl. 5, fig. 10.

Pterocyclus [sig] ( Rhiostoma ) Rhiostoma haughtoni - Nevill 1878: 262.

Pterocyclos haughtoni - Reeve 1863: Pterocyclos pl. 5, species 30.

Type material.

The W.H. Benson collections are mainly held in UMZC, Cambridge, and most are considered the primary type specimens. However, Benson’s type specimens were not very explicit, and difficult to specify due to mislaid labels and subsequent substitution of the original labels (see Naggs 1997 and Preece et al. 2022 for more detail). However, in the original description of R. haughtoni , Benson stated that he had received several specimens, and one was alive. The W.H. Benson ex. R. MacAndrews collection contains one lot of two specimens. One specimen with a mummified soft body remaining inside the shell corresponds with Benson’s statement ( Benson 1860: 96). This specimen lot was designated as the lectotype UMZC I.103445.A (Fig. 11A ) by Preece et al. (2022: 78), and another shell, UMZC I.103445.B (Fig. 11 B ), becomes the paralectotype.

Other material examined.

Myanmar: Damaltha, Burma: NHMUK 88.12.4.1985-1986 (specimen with “x” was figured in Gude 1921, fig. 22; Fig. 11C ), SMF 130508. Moulmein, Tenasserim, Burma: NHMUK 75.06.5.5 (4 shells), NHMUK 1903.7.1.1566 (2 shells), NHMUK ex. E.R. Sykes coll. Acc. 1825 (1 shell), NHMUK ex. Cuming coll. (1 shell), NHMUK ex. Oldham coll. (1 shell). Damatha, Burma: NHMUK ex. McAndrew coll. (4 shells). Dhammatat Cave, Mawlamyine District, Mon State: CUMZ 10048 (Figs 9A , 11D-F , 16A ), CUMZ 10049.

Diagnosis.

Shell thick and depressed. Detached whorl short, curved and slightly descending. Breathing device notch-shaped. Shell colour uniformly brownish or with dark zigzag pattern.

Differential diagnosis.

In addition, the shell morphology of R. haughtoni differs from R. housei in having a notch-shaped breathing device, and a short detached whorl, while R. housei bears a tubular breathing device and the detached whorl is medium in length.

Description.

Shell. Shell small to medium, cW 16.9-20.4 mm, cH 8.7-10.9 mm, thickened, and sub-discoidal to discoidal shape; detached-whorl length 3.0-8.0 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded. Shell surface with fine growth lines. Periostracum thick or thin, corneous, and transparent. Shell colour uniformly brownish, sometimes with dark brown zigzag pattern; ventral shell surface paler in colour; with or without a dark brown spiral band on periphery. Detached whorl shorter than apertural width, curved and descending. Peristome circular and double; lip thickened, slightly expanded and multi-layered. Aperture opened sub-laterally. Breathing device notch-shaped; outer lip protruding, with narrow groove; inner lip with shallow to deep incision. Umbilicus widely open and deep. Operculum calcareous, tall cup-shaped, and multispiral (Fig. 11 ).

Radula. Taenioglossate radula arranged in inverted V-shaped rows. 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, elongate, and flanked by pointed tip of two small inner cusps and one small outer cusp. Inner and outer marginal teeth each composed of three cusps; central cusp large and with dull tip, and flanked by pointed tip of one inner cusp and one outer cusp (Fig. 16A ).

Distribution.

This species is currently known only from the type locality in Mon State, southern Myanmar (Fig. 10 ).

Remarks.

Rhiostoma haughtoni is often regarded as a junior synonym of R. housei from central Thailand ( Habe 1965; Abbott 1989). However, R. housei exhibits significant intra-specific variation in shell morphology across its range. Furthermore, recent allozyme studies have suggested that R. housei is likely to represent a complex of cryptic species ( Prasankok et al. 2011). In this study, the topotypic specimens of R. haughtoni clearly show a significant genetic distance from R. housei , proving it to be clearly distinct from R. housei .

The shell of R. haughtoni can vary from monochrome brownish or whitish without a peripheral band (Fig. 11D ) to brownish zigzag patterns or blotches and thin peripheral band (Fig. 11E, F ). The length of the detached whorl and shape of breathing device are relatively conserved, at least in the topotype population.

Figures

Figure 7. A-C Internal anatomy of Rhiostoma housei specimen CUMZ 3982 from Tham Sriwilai, Saraburi, A inside view of lung cavity shows an arrangement of the kidney, circulatory system and the terminal part of the digestive tract B male genitalia and C female genitalia D spermatophore of Rhiostoma samuiense specimen CUMZ 4713 from Kanchanadit, Surat Thani with its cross sections showing spermatophore shape in each position. Abbreviations: an, anus; bc, bursa copulatrix; fc, faeces; go, genital opening; h, heart; kd, kidney; mc, mantle collar; od, oviduct; pg, prostate gland; re, rectum; sr, seminal receptacle; ut, uterus; v, vein; vd, vas deferens.

Figure 9. Live snails of the species group I A Rhiostoma haughtoni specimen CUMZ 10048 / 2 from Dhammathat Cave, Myanmar B-D Rhiostoma samuiense B specimen CUMZ 3989 from Samui Island, Surat Thani C specimen CUMZ 3996 from Perlis, Malaysia (topotype of Rhiostoma chupingense) D specimen CUMZ 4713 of " proboscis " morphotype from Surat Thani E, F copulation pair of Rhiostoma samuiense specimen CUMZ 4713 from Surat Thani shows unsuccessfully transferred spermatophore (female on the right in E and on the left in F), and the red arrows indicate a spermatophore illustrated in Fig. 7 D. All figures are not to scale.

Figure 10. Distribution map of Rhiostoma haughtoni - species group.

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 16. Radula morphology A Rhiostoma haughtoni specimen CUMZ 10048 / 2 from Dhammatat Cave, Mon, Myanmar B, C Rhiostoma samuiense B specimen CUMZ 3989 from Samui Island, Surat Thani and C specimen CUMZ 3996 from Bukit Chuping, Perlis, Malaysia D Rhiostoma rhothonotaphrosa sp. nov. paratype CUMZ 4710 from Klong Had, Srakeo.