Rhiostoma rhothonotaphrosa Tongkerd & Panha, 2023

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

3. Rhiostoma rhothonotaphrosa Tongkerd & Panha sp. nov.

Figs 10 , 15 , 16D

Rhiostoma sp. 6 - Tumpeesuwan 2001: 85-89, figs 4.33-4.35.

Rhiostoma sp. 9 - Tumpeesuwan 2001: 96-99, figs 4.40, 4.41.

Type material.

Holotype CUMZ 4709 (cW 21.0 mm, cH 11.8 mm, dL 22.7 mm; Fig. 15A ). Paratypes CUMZ 3858 (25 adults + 12 juveniles), CUMZ 4710 (2 shells; Figs 15B, C , 16D ), CUMZ 4823 (5 shells), NHMUK 20220438 (5 shells), and SMF 368673 (5 shells). All paratypes are from the type locality.

Type locality.

Tham Sri Thong, Khlong Had District, Srakeo Province, Thailand (13°28'43.6"N, 102°16'53.8"E). Limestone outcrop with dry deciduous forest, surrounded by sugarcane and tapioca farms.

Other material examined.

Thailand: Khao Wong Kot, Kaeng Hang Maeo, Chanthaburi: CUMZ 4852, 4874, 4877, 10174, 10176. Khao Cha Ang Oun, Bo Thong, Chonburi: CUMZ 4363, 4364. Khao Chamao-Khao Wong, Khao Chamao, Rayong: CUMZ 3808, 3857, 3923, 4337, 4368, 4711 (Fig. 15D, E ), 10177, 10178. Khao Wong, Khao Chamao, Rayong: CUMZ 4725. Limestone near Khao Chamao Khao Wong, Khao Chamao, Rayong: CUMZ 3851. Khao Chakan, Khao Chakan, Srakeo: CUMZ 3806, 4369, 4370, 4735, 4820, 4829, 4882, 4895 (Fig. 15F ), 4898, 10173, 10179. Tham Mahasombat, Khao Chakan, Srakeo: CUMZ 3830. Khao Siwa, Khlong Had, Srakeo: CUMZ 4371. Tham Leam, Khlong Had, Srakeo: CUMZ 3852. Tham Phet Po Thong, Khlong Had, Srakeo: CUMZ 4856, 4878, 10172. Wat Khao Pha Pheung, Khlong Had, Srakeo: CUMZ 10180. Ta Phraya, Srakeo: CUMZ 4862. Tham Khao Phlapphueng Thong, Wang Som Bun, Srakeo: CUMZ 10175 .

Diagnosis.

Shell thick and flattened. Detached whorl long, curved, and descending. Breathing device notch-shaped. Shell colour whitish with brownish zigzag pattern. Operculum calcareous, tall and cup shaped.

Differential diagnosis.

This new species differs from R. samuiense from peninsular Thailand by having a notch-shaped breathing device and white shell with brownish zigzag colour pattern. In contrast, R. samuiense has a knob-shaped breathing device and generally monochrome brown to purplish shell. Although this new species has a notch-shaped breathing device similar to R. haughtoni , the distinct characters are longer detached whorl and dark apex, while R. haughtoni has a shorter detached whorl and without dark colour on apex. In addition, R. rhothonotaphrosa sp. nov. differs from R. housei and R. simplicilabre by having a notch-shaped breathing device and relatively longer and descending detached whorl. In contrast, these two species possess tubular breathing devices and a relatively shorter detached whorl.

Description.

Shell. Shell medium, cW 19.8-24.9 mm, cH 9.7-12.5 mm, detached-whorl length 10.0-18.5 mm, thick, and sub-discoidal shape. Apex acute with dark colouration; spire slightly convex. Whorls 5 to 6, convex, increasing regularly; suture depressed and deep; last whorl rounded and slender. Shell surface with fine growth lines. Periostracum thick and brown. Shell with brownish zigzag pattern or sometimes with uniformly brownish colour; ventral surface with paler colour; with narrow dark brown spiral band on periphery. Detached whorl long, nearly 2 × longer than apertural width, curved and descending. Peristome circular and double; lip thickened, slightly expanded, and multi-layered. Aperture opened sub-ventrally. Breathing device notch-shaped; outer lip protruding, with narrow groove; inner lip with shallow to deep incision. Umbilicus widely opened and deep. Operculum calcareous, tall cup-shaped, and multispiral (Fig. 15 ).

Radula. Teeth arrangement and shape are almost identical to R. haughtoni . Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of three or four cusps. Marginal teeth each composed of three cusps (Fig. 16D ).

Etymology.

The specific name rhothonotaphrosa comes from two Greek words, rhothon meaning nose and taphros meaning ditch or trench. It refers to the notch-shaped breathing device of this new species.

Distribution.

This new species is known from several localities in eastern Thailand near the Cambodia border in Srakeo, Chanthaburi, Rayong, and Chonburi provinces.

Remarks.

Four species, R. hainesi , R. cambodjense , R. rhothonotaphrosa sp. nov., and R. cheliopegma sp. nov., are distributed in eastern Thailand. Our survey throughout their distribution ranges revealed no sympatric populations except for a single locality with adjacent populations of R. cambodjense and R. rhothonotaphrosa sp. nov. These two species inhabit a small limestone outcrop of Khao Chakan, Srakeo Province; the outcrop is ~ 1 km long, 0.5 km wide, and oriented east-west. The new species occupies the northern side of the outcrop, while R. cambodjense commonly occurs on the southern flank of the same mountain. However, these two species are totally different in shell morphology, and their COI barcoding suggests they are not closely related taxa (Fig. 3 ).

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 10. Distribution map of Rhiostoma haughtoni - species group.

Figure 15. Shell of Rhiostoma rhothonotaphrosa sp. nov. A holotype CUMZ 4709 from Klong Had, Srakeo B, C paratypes CUMZ 4710 from type locality D, E specimens CUMZ 4711 from Khao Chamao, Rayong F specimen CUMZ 4895 from Khao Chakan (north site), Srakeo.

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.