Rhiostoma samuiense Tomlin, 1932

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

2. Rhiostoma samuiense Tomlin, 1932

Figs 9B-F , 10 , 12 , 13 , 14 , 16B, C

Rhiostoma samuiense Tomlin, 1932: 227, 228, pl. 26, with 2 figures. Type locality: Kaw Samui Island in the Gulf of Thailand [Samui Island, Surat Thani, Thailand]. Hemmen et al. 1999: 15, fig. 17. Patamakanthin 2001: 222, 223, fig. d1-d3. Tumpeesuwan 2001: 53-58, figs 4.16-4.18. Kongim et al. 2013b: 16, fig. 2i.

Rhiostoma chupingense Tomlin, 1938: 73, p1. 2, figs 1, 2. Type locality: Bukit Chuping, Perlis, Malaysia. Berry 1963: pl. 4, fig. 28. Hemmen et al. 1999: 26, figs 18.1-18.3. Patamakanthin 2001: 222, 223, figs b1, b2, c1-c3. Tumpeesuwan 2001: 29-34, figs 4.4-4.6. Tarruella and Doménech 2010: 189, fig. 1c. Kongim et al. 2013b: 16, fig. 2d. Sutcharit et al. 2019: 19, fig. 4c. New synonym

Rhiostoma "spec. 1"- Hemmen et al. 1999: 12, fig. 12.

Rhiostoma "sp. 2"- Hemmen et al. 1999: 13, 14, figs 13-16.

Rhiostoma sp.- Patamakanthin 2001: 222, 223, figs a1-a4.

Rhiostoma sp. 5- Tumpeesuwan 2001: 82-84, figs 4.31, 4.32.

Type material.

Tomlin (1932) had obtained several living specimens, and the original description included an illustration and one set of shell measurements. There are two lots from the Melvill-Tomlin collection, which are considered the syntypes NMW 1955.158.01104 (1 shell; Fig. 12A ) and NMW 1955.158.01105 (6 shells; Fig. 12B ). Lectotype (designation in Sutcharit et al. (2019)) NHMUK 1938.10.25.1 (Fig. 13A ) of Rhiostoma chupingense Tomlin, 1938, from Bukit Chuping, Perlis, Malaysia; paralectotypes NMW 1955.158.01101 (3 shells), NMW 2.1981.118.02703 (1 shell).

Other material examined.

Malaysia: Bukit Chuping, Kangar, Perlis: CUMZ 3844, 3996 (Figs 9C , 13B ), 4746, 4768. Sungai Jernih Kangar, Kuala Perlis, Perlis: CUMZ 3850, 3997 (Fig. 13F ), 4832 . Myanmar: Lampane Cave , Ngawun, Tanintharyi: CUMZ 10214 . Thailand: Kaw Samui: NMW 2.1981.118.20704 ex. T. Pain coll. (Fig. 12C ). Khao Marong, Bang Sapan, Prachuap Khiri Khan: CUMZ 3865, 3874, 3986 (Fig. 13C ), 4477, 4739, 4854, 4861, 4900, 10079, 10086, 10089. Ban Na Sarn, Surat Thani: CUMZ 4305, 4395. Khao Chongchang, Ban Na Sarn, Surat Thani: CUMZ 4306. Tham Kha Min, Ban Na Sarn , Surat Thani: CUMZ 4310. Wat Na Sarn , Ban Na Sarn , Surat Thani: CUMZ 3842. Donsak, Surat Thani: CUMZ 4308, 4309. Khao Chang, Donsak, Surat Thani: CUMZ 3866. Khao Pra, Donsak, Surat Thani: CUMZ 4777. Khao Sai, Donsak, Surat Thani: CUMZ 3992, 4759. Km 8+ 500m, Donsak , Surat Thani: CUMZ 10094. Tham Khiriwong , Donsak , Surat Thani: CUMZ 3811, 10078. Wat Tham Po Ngam , Donsak , Surat Thani: CUMZ 3873. Kra Dae , Kanchanadit , Surat Thani: CUMZ 3869, 4790 (Fig. 14E ). Tham Khuha , Kanchanadit , Surat Thani: CUMZ 4424, 4481. Wat Tham Petpanomwang , Kanchanadit , Surat Thani: CUMZ 3833, 3835, 3895, 3991, 4361, 4362, 4392, 4396, 4398, 4471, 4472, 4473, 4474, 4475, 4712 (Fig. 14A ), 4713 (Figs 7C , 9D-F , 14B-D ), 4721, 4741, 4782, 4785, 4793, 4802, 4812, 4875, 10065, 10066, 10067, 10069. Hin Lad Waterfall , Koh Samui , Surat Thani: CUMZ 3864, 4373, 4413, 4834, 4893. Koh Samui , Surat Thani: CUMZ 4307, 4332, 4333, 4334, 4335, 4397, 10073. Koh Tan , Koh Samui , Surat Thani: CUMZ 3863, 3890, 4382, 4415, 4432, 4707 (Fig. 12F ), 4791 (Fig. 12E ), 10070. Koh Wang Nok , Koh Samui , Surat Thani: CUMZ 3385, 3891. Na Muang Waterfall , Koh Samui , Surat Thani: CUMZ 4800, 3862, 3989 (Figs 9B , 12D , 16B ), 4780. Wat Tham Yai , Tha Cha Na , Surat Thani: CUMZ 3990 (Fig. 13E ), 4476, 4722, 4769, 4787, 4857, 4863, 4891, 4896, 10084, 10093, 10095. Wat Wichit Dittharam , Tha Cha Na , Surat Thani: CUMZ 10085. Wat Khao Hauy Hang , Hauy Yod , Trang: CUMZ 3840, 3846, 3945, 3993, 4774. Samet Chun Waterfall , Kanom , Nakhon Si Thammarat: CUMZ 4773. Tham Khao Krot , Kanom , Nakhon Si Thammarat: CUMZ 3944, 4000, 10087, 10092. Tham Wang Thong , Kanom , Nakhon Si Thammarat: CUMZ 3820, 4311, 4388, 4708 (Fig. 13D ), 4781, 10091. Thongnian , Kanom , Nakhon Si Thammarat: CUMZ 10090. Wat Ao Sadet , Kanom , Nakhon Si Thammarat: CUMZ 10088. Tham Nam Wang Srithammasokkarach , Lan Saka , Nakhon Si Thammarat: CUMZ 4855. Wat Tham Khao Dang , Ron Phiboon , Nakhon Si Thammarat: CUMZ 10068. Wat Tham Khao Dang , Hintok , Ron Phiboon , Nakhon Si Thammarat: CUMZ 3838. Khao Chang (Sikeed), Sichon , Nakhon Si Thammarat: CUMZ 3894, 3999, 4786 (Fig. 14F ). Khao Hu Thu , Khao Noi , Sichon , Nakhon Si Thammarat: CUMZ 3871. Khao Nam Lark (Sikeed), Sichon , Nakhon Si Thammarat: CUMZ 3893, 3898, 4757. Wat Tham Pra Chaison , Khao Chaison , Patthalung : CUMZ 10096. Khao Pu-Khao Ya , Si Banphot , Patthalung : CUMZ 3995, 4304, 4375, 4399, 4811, 10076, 10077. Khao Roop Chang , Padang Besa , Songkhla: CUMZ 3849, 4479, 10097. Tham Srikaesorn , Rattaphum , Songkhla: CUMZ 4488 .

Diagnosis.

Shell thick and nearly flattened. Detached whorl medium to long in length, curved, descending, and twisted. Breathing device knob-shaped. Shell colour uniformly brown to purplish or sometimes with brownish zigzag pattern. Operculum calcareous, and tall cup-shaped.

Differential diagnosis.

Riostoma samuiense can be distinguished from R. haughtoni in having a brown to purplish shell, knob-shaped breathing device, and medium detached-whorl length. In comparison, R. haughtoni has a smaller shell, brown to whitish shell colour, a short detached whorl, and notch-shaped breathing device.

Description.

Shell. Shell small, cW 15.1-18.9 mm, cH 8.1-10.7 mm, thick, and nearly flattened to sub-discoidal shape; detached-whorl length 3.5-14.5 mm. Apex acute with dark colouration. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and slender. Shell surface with fine growth lines. Periostracum thick, corneous, dark brown, and usually worn down on ventral surface. Shell colour uniformly brown to purplish (sometimes with brownish zigzag pattern); peripheral band usually absent (rarely present). Detached whorl medium to long, ~ 2 × apertural width, curved, descending, and sometimes twisted. Peristome circular and double; lip thickened, expanded, and multi-layered. Aperture opened sub-laterally to ventrally. Breathing device knob-shaped; outer lip protruding, with small knob; inner lip with indistinct incision. Umbilicus widely opened and deep. Operculum calcareous, tall cup-shaped, and multispiral (Figs 12 - 14 ).

Radula. Teeth arrangement and shape are very similar to R. haughtoni . Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of three to four cusps. Marginal teeth each composed of three cusps (Fig. 16B, C ).

Distribution.

The previous records of this species range from the type locality in Samui Island, Gulf of Thailand, to the northern part of peninsular Malaysia, and the southern part of Thailand ( Tomlin 1938; Patamakanthin 2001). The northern limit is around Bang Sapan, Prachuapkhirikhan, extending southward along the east coast of the Gulf of Thailand to northern peninsular Malaysia (Fig. 10 ).

Remarks.

The previous records of both R. samuiense and R. asiphon from "Samui Islands" were inaccurate (see Möllendorff 1893, 1894; Tomlin 1932). This study surveyed entire ranges of the Samui Islands area and found these two species to be allopatric. Rhiostoma asiphon has a distribution restricted to the Angthong Islands (probably the exact type locality), which are the limestone islands ~ 30 km northwest of Samui Island. In contrast, R. samuiense has a narrow range only on the granitic Samui Island (type locality) and its surrounding satellite islands.

Previously, R. chupingense Tomlin, 1938, had long been recognised as a distinct species mainly distributed in northern peninsular Malaysia and southern Thailand ( Berry 1963; Sutcharit et al. 2019). The COI barcoding of the topotype of R. chupingense together with other populations from southern Thailand and the topotype of R. samuiense indicate these specimens cluster in a monophyletic clade and with low genetic distance (Figs 3 , 4 ; Appendix 1: Table A1 ). Therefore, we consider R. chupingense Tomlin, 1938 as a junior subjective synonym of R. samuiense . This new assignment has made the distribution range of R. samuiense much broader on the Malay Peninsula.

Morphological variation occurs in R. samuiense , especially in the length of the detached whorl, which can be divided into three morphotypes:

The “typical” morphotype tends to have medium detached-whorl length, approximately the same as apertural width. The specimens tend to have thickened and brownish periostracum. Currently, this typical morphotype occurs only from the type locality.

The " chupingense " morphotype is similar to the “typical” but has a long detached whorl (~ 2 × longer than aperture width; detached-whorl length 11.0-18.0 mm, aperture width 6.1-8.1 mm) that is curved, descending and turned so that aperture opens sub-laterally. Some populations tend to have obvious brownish zigzag colour patterns. This morphotype has a wide distribution ranging from Perlis, Malaysia, to the southern part of Thailand and one locality in Myanmar.

The “proboscis” morphotype exhibits a markedly long detached whorl (~ 3 × longer than aperture width; detached-whorl length 16.0-24.0 mm, aperture width 7.3-8.4 mm) that is curved, descending, and twisted, which turns the aperture to open ventrally. This morphotype is restricted to a few localities in the northern part of the Khao Luang Mountains.

Figures

Figure 3. Maximum likelihood (ML) tree showing the relationships among Rhiostoma species based on 660 bp of the mitochondrial COI gene sequence. The major nodes are indicated by numbers as the Maximum likelihood (ML) bootstrap values and Bayesian posterior probabilities of Bayesian inference (BI), respectively. Nodes with sufficient support by both ML and BI are indicated by black circles, and nodes with partial support by only ML are indicated by grey circles, and only BI by white circles. Newly described species in this study are highlighted in blue.

Figure 4. A heat map of average interspecific pairwise p-distance based on 660 - bp COI gene fragment sequences between Rhiostoma species. The simplified phylogenetic tree was generated from the Maximum likelihood tree.

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 12. Shell of Rhiostoma samuiense A syntype NMW 1955.158.1104 from Kaw Samui, Siam B syntype NMW 1955.158.1105 form type locality C specimen NMW 2.1981.118.20704 Kaw Samui D specimen CUMZ 3989 from Koh Samui, Surat Thani E, F specimen from Koh Tan, Surat Thani E specimen CUMZ 4791 and F specimen CUMZ 4707.

Figure 13. Shell of Rhiostoma samuiense A lectotype of R. chupingense NHMUK 1938.10.25.1 from Bukit Chuping, Perlis, Malay State B specimen CUMZ 3996 from Bukit Chuping, Perlis, Malaysia (topotype) C specimen CUMZ 3986 from Bang Saphan, Prachuap Khiri Khan D specimen CUMZ 4708 from Khanom, Nakhon Si Thammarat E specimen CUMZ 3990 from Tha Chana, Surat Thani F specimen CUMZ 3997 from Sungai Jernih, Perlis, Malaysia.

Figure 14. Shell of Rhiostoma samuiense of " proboscis " morphotype A specimen CUMZ 4712 from Kanchanadit, Surat Thani B-D specimens CUMZ 4713 from Kanchanadit, Surat Thani E specimen CUMZ 4790 from Kanchanadit, Surat Thani F specimen CUMZ 4786 from Sichon, Nakhon Si Thammarat.

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.