Rhiostoma cheliopegma Tongkerd & Tumpeesuwan, 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

25. Rhiostoma cheliopegma Tongkerd & Tumpeesuwan sp. nov.

Figs 44 , 50 , 53B

Rhiostoma sp. 4- Tumpeesuwan 2001: 76-81, figs 4.28-4.30 (in part).

Type material.

Holotype CUMZ 3985/1 (W 18.2 mm, H 10.3; Fig. 50A ). Paratypes CUMZ 3985/2 (13 shells; Figs 50B, C , 53B ), CUMZ 4470 (55 adults + 14 juveniles), CUMZ 4853 (10 shells; Fig. 50F ), NHMUK 20220446 (5 shells), and SMF 368681 (5 shells). All paratypes are from the type locality.

Type locality.

Isolated limestone hill at Tham Takien, Khao Chamao District, Rayong Province, Thailand (13°05'30.0"N, 101°36'27.7"E).

Other material examined.

Thailand: Khao Cha-Ang Oun, Bo Thong, Chonburi: CUMZ 3809, 3818, 3861, 3984, 4377, 4448, 4461, 4755/1, 4886, 10053, 10054, 10055, 10057, 10058, 10060. Khao Cha-Ang, Bo Thong, Chonburi: CUMZ 4755/2, 4824, 4847, 4865. Khao Ha Yot, Bo Thong, Chonburi: CUMZ 4833, 10052, 10059, 10082. Khao Tham Mi, Bo Thong, Chonburi: CUMZ 10081. Tham Khao Prathun, Bo Thong, Chonburi: CUMZ 4860. Wat Pluang Thong, Bo Thong, Chonburi: CUMZ 4354, 4355, 4357, 4360. Tham Khao Bost, Khao Chamao, Rayong: CUMZ 4884/2. Tham Neramit Bureau of Monks , Khao Chamao, Rayong: CUMZ 4468, 4876. Wat Ma Diea, Tham Khao Loy, Khao Chamao, Rayong: CUMZ 4467, 4469, 4815, 4818 (Fig. 50D ), 4871 (Fig. 50E ), 10083. Wat Tham Wattana Mongkon, Khao Chamao, Rayong: CUMZ 4864 .

Diagnosis.

Shell small, depressed, and without detached whorl. Peristome multi-layered; lip thickened; breathing device notch-shaped. Shell colour usually uniformly reddish to dark brown. Operculum calcareous, thick, tall cup-shaped, and multispiral.

Differential diagnosis.

This new species is superficially similar to R. cambodjense , and the two species are distributed in nearby geographical areas in eastern Thailand. Rhiostoma cheliopegma sp. nov. differs from R. cambodjense by a sub-discoidal shell, a uniform or blotched reddish brown colour pattern, apertural lip thickened, and expanded tall cup-shaped operculum. For comparison, R. cambodjense has a heliciform shell, uniformly brownish or yellowish shell, apertural lip thin, and low cup-shaped operculum. In addition, the COI barcoding in this study suggests separating these two species (Fig. 3 ).

Description.

Shell. Shell small, width 17.1-20.0 mm, height 5.8-11.5 mm, thickened, and sub-discoidal shape. Apex acute with dark colouration; spire slightly elevated. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded. Shell surface nearly smooth with fine growth lines. Periostracum thin, corneous, and transparent. Shell colour usually uniformly reddish to dark brownish (rarely with albinistic forms), sometimes with unclear patterns; with narrow dark brown spiral band on periphery. Detached whorl usually absent. Peristome circular and double; lip thickened, 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 opened and deep. Operculum calcareous, thickened, tall and cup-shaped, and multispiral (Fig. 50 ).

Radula. Teeth arrangement and shape are similar to those of R. cambodjense . Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth have four cusps: central cusp large, triangular, and flanked by two small inner cusps and one outer cusp. Marginal teeth each consisting of three pointed cusps (Fig. 53B ).

Etymology.

The species name cheliopegma is derived from two Greek words cheilos meaning lip or rim, and pegma meaning thickened. This name refers to the strongly thickened apertural lip, which is the distinct character of this species.

Distribution.

This new subspecies is known from several isolated limestone hills in Chonburi and Rayong provinces, eastern Thailand (Fig. 44 ).

Remarks.

Despite the COI barcoding seeming to show two subclades (Chonburi and Rayong populations) within this species (Fig. 3 ). We assign them as conspecific due to the indistinguishable shell form and breathing device and their geographical proximity in eastern Thailand. Therefore, more effort should be paid to additional molecular markers, and morphometric analyses are required to determine whether these distinct subclades should be recognised as a distinct taxon.

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 44. Distribution map of Rhiostoma morleti - species group.

Figure 50. Shell of Rhiostoma cheliopegma sp. nov. A holotype CUMZ 3985 / 1 from Tham Takien, Khao Chamao, Rayong B, C paratypes CUMZ 3985 / 2 from type locality D, E from Tham Khao Loi, Khao Chamao, Rayong D specimen CUMZ 4818 and E specimen CUMZ 4871 F paratype CUMZ 4853 from type locality with albinistic shell form.

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.