Rhiostoma lannaense 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

12. Rhiostoma lannaense Tongkerd & Tumpeesuwan sp. nov.

Figs 18 , 30 , 32E

Rhiostoma sp. 1- Tumpeesuwan 2001: 59-64, figs 4.19-4.21 (in part).

Type material.

Holotype CUMZ 4500 (cW 17.1 mm, cH 8.1 mm, dL 35.0 mm; Fig. 30A ). Paratypes CUMZ 3910 (40 shells), CUMZ 4350 (1 shell), CUMZ 4701 (1 shell; Figs 30B , 32E ), CUMZ 10037 (8 adults + 30 juveniles), CUMZ 10038 (5 shells), NHMUK 20220440 (5 shells), and SMF 368675 (5 shells). All paratypes are from the type locality.

Type locality.

Ban Ping Klong (village), Chiangdao District, Chiang Mai Province, Thailand (19°30'48.6"N, 99°03'21.1"E). Small limestone hills covered by dry deciduous forest.

Other material examined.

Thailand: Tham Sam Ta, Muang, Maehongsorn: CUMZ 4804. 1 km. from the junction to Tham Mae Ra Na, Pang Mapha, Maehongsorn: CUMZ 4440, 4464. Pang Mapha, Maehongsorn: CUMZ 4341. Tham Mae Lana, Pang Mapha, Maehongsorn: CUMZ 4702 (Fig. 30C, D ), 10042, 10043 (Fig. 30E ). Tham Pha Mon, Pang Mapha, Maehongsorn: CUMZ 4343. Tham Phadeang, Pang Mapha, Maehongsorn: CUMZ 10040. Wat Pa Tham Wua, Pang Mapha, Maehongsorn: CUMZ 10036. Tham Tabtao, Chai Prakarn, Chiang Mai: CUMZ 3912, 10041 (Fig. 30F ). Chaiprakarn, km. 43 reach Chiang Dao, Chiang Mai: CUMZ 4738. Km. 93+ 200 m. Tham Klap, Pingkong, Chiang Dao, Chiang Mai: CUMZ 3908, 10039. Pa Sak Ngam, Doi Saket, Chiang Mai: CUMZ 4754. Huai Nam Dang, Mae Tang, Chiang Mai: CUMZ 10033 .

Diagnosis.

Shell small to medium, thin, and flattened to depressed shell. Detached whorl long, slender, curved, and descending. Breathing device tubular. Shell colour with dark brown zigzag patterns. Operculum calcareous, tall cup-shaped with loose lamellae.

Differential diagnosis.

Rhiostoma lannaense sp. nov. differs from R. marioni , R. thachi , and R. jalorensis in having a whitish shell with a dark brown zigzag pattern and longer detached whorl. In contrast, these three species exhibit uniformly brownish to dark brown shells and generally without a colour pattern and a relatively shorter detached whorl. In addition, R. thachi has a broader expanded aperture on the palatal side, while this new species has a thickened but not expanded lip.

This new species is superficially similar to R. tigrina sp. nov., but it can be distinguished by having very long and curved detached whorl, aperture opened ventrally, and tall cup-shaped operculum with looser lamellae. In contrast, the latter species has a shorter detached whorl, aperture opened sub-ventrally and low cup-shaped operculum with denser lamellae. Although these two species are mainly distributed in northern Thailand, the COI barcoding shows high genetic diversity (Appendix 1: Table A1 ) and divided R. lannaense sp. nov. as a distinct clade from other northern species, which suggests they are distinct species (Fig. 3 ).

Description.

Shell. Shell small to medium, cW 14.5-19.3 mm, cH 7.3-10.1 mm, thin, and nearly flattened to sub-discoidal shape; detached-whorl length 19.5-28.5 mm. Apex acute with dark colouration; spire convex to nearly flat. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and slender. Shell surface with fine growth lines. Periostracum corneous and transparent. Shell colour with brown to dark brown zigzag pattern and faded on ventral shell surface; narrow black spiral band on periphery. Detached whorl long, ~3 × longer than aperture width, curved, and descending. Peristome circular and double; lip thickened, slightly expanded and multi-layered. Aperture opened sub-ventrally to ventrally. Breathing device tubular and its tip sometimes attached to preceding whorl; outer lip forming a short to long and closed tube, and located just behind apertural lip; inner lip with deep incision or small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, tall cup-shaped, and multispiral with loose lamellae (Fig. 30 ).

Radula. Teeth arrangement and shape are similar to those of R. housei . Central tooth with large pointed central cusp; two lateral cusps on each side with small pointed tips. Lateral teeth consisting of four cusps; central cusp large, with dull tip, and flanked by two inner cusps and one small outer cusp. Marginal teeth each consisting of three pointed cusps (Fig. 32E ).

Etymology.

The specific name lannaense is derived from the historical name of the Lan Na Kingdom, which flourished approximately from the 13th to 18th centuries. It refers to the distribution range of this new species in the northern part of Thailand, which is the approximate centre of the Lan Na Kingdom.

Distribution.

This new species has a narrow distribution range in a few localities in Chiang Mai and Maehongsorn provinces (Fig. 18 ).

Remarks.

There are two morphotypes occurring in this species. The typical morphotype has a long and curved detached whorl, and the tubular breathing device does not reach the preceding whorl. The shorter morphotype has a short, detached whorl, and the tubular breathing device reaches the preceding whorl (Fig. 30D ). These two morphotypes are sympatric, and also have brownish zigzag colour patterns and a tall cup-shaped operculum with loose lamellae. However, the shell morphologies together with COI barcoding suggest these are conspecific.

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 18. Distribution map of Rhiostoma housei - species group.

Figure 30. Shell of Rhiostoma lannaense sp. nov. A holotype CUMZ 4500 from Pingkong, Chiang Dao, Chiang Mai B paratype CUMZ 4701 from type locality C, D paratypes CUMZ 4721 from type locality E specimen CUMZ 10043 from Pang Mapha, Maehongsorn F specimen CUMX 10041 from Chai Prakan, Chiang Mai

Figure 32. Radula morphology A Rhiostoma housei specimen CUMZ 3982 from Wat Tham Srivilai, Saraburi B Rhiostoma hainesi specimen from Chanthaburi C Rhiostoma jalorensis specimen CUMZ 3994 from Wat Tham Sua, Krabi D Rhiostoma ebenozostera sp. nov. paratype CUMZ 4704 from Tham Pha Poo, Loei E Rhiostoma lannaense sp. nov. paratype CUMZ 4701 from Chiang Dao, Chiang Mai F Rhiostoma tigrina sp. nov. paratype CUMZ 4497 / 2 from Mae Sai, Chiang Rai.