Rhiostoma breviocollar 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

18. Rhiostoma breviocollar Tongkerd & Tumpeesuwan sp. nov.

Figs 34 , 39 , 42D

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

Rhiostoma sp. 8- Tumpeesuwan 2001: 93-95, figs 4.38, 4.39.

Type material.

Holotype CUMZ 4490 (cW 22.8 mm, cH 11.2 mm, dL 8.2 mm; Fig. 39A ). Paratypes CUMZ 3975 (139 adults + 10 juveniles), CUMZ 4366 (21 shells), CUMZ 4491 (6 shells; Figs 39B-D , 42D ), CUMZ 10159 (4 adults + 4 juveniles), NHMUK 20220443 (5 shells), and SMF 368678 (5 shells). All paratypes are from the type locality.

Type locality.

Small isolated limestone hill at Wat Khao Smokon, Ban Mi District, Lopburi Province, Thailand (14°54'25.9"N, 100°30'21.9"E).

Other material examined.

Thailand: Khao Noi, Muang, Nakhonsawan: CUMZ 4352. Khao Patavi, Tubtan, Uthai Thani: CUMZ 3804, 3900, 3978, 4358, 4492 (Fig. 39E ), 4493 (Fig. 39F ), 4772. Khao Tee Hin, Ban Mi, Lopburi: CUMZ 4367. Wat Tham Tambon, Chai Ba Dan, Lopburi: CUMZ 3888, 3983, 4445, 4466, 4788, 4841, 10019, 10157 .

Diagnosis.

Shell small to medium, thick, and depressed; detached whorl shorter than apertural width. Shell colour whitish and with brownish zigzag patterns. Breathing device incomplete tube shape.

Differential diagnosis.

Rhiostoma breviocollar sp. nov. can be distinguished from R. anceyi sp. nov. by having a thin corneous periostracum and incomplete tube as a breathing device. In contrast, R. anceyi sp. nov. has a thickened and brownish periostracum and a short tubular breathing device.

The COI barcoding suggests a close relationship between this new species and R. housei (Fig. 3 ). However, this new species differs by having flattened to a sub-discoidal shell, short to lacking detached whorl and incomplete tube as breathing device. In contrast, R. housei has a discoidal shell, long detached whorl, and a tubular breathing device.

Description.

Shell. Shell small to medium, cW 21.6-25.5 mm, cH 10.0-12.8 mm, thick, and flattened to sub-discoidal shape; detached-whorl length 3.0-7.5 mm. Apex acute; spire nearly flattened to slightly elevated. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and stout. Shell surface with irregular growth lines. Shell colour with brownish zigzag or blotch patterns; ventral side with paler colour patterns; narrow dark brown spiral band on periphery. Periostracum thick or thin, corneous, and transparent. Detached whorl very short (sometimes nearly absent) and slightly descending. Peristome circular and double; lip thickened, expanded, and multi-layered. Aperture opened sub-laterally. Breathing device incomplete tube-shaped and its tip sometimes attached to preceding whorl; outer lip forming a short and nearly closed tube; inner lip with deep incision. Operculum calcareous, cup-shaped, and multispiral (Fig. 39 ).

Radula. Teeth shapes are similar to those of R. asiphon ; only minor variations occur in the central tooth with low triangular-shaped central cusp. Lateral teeth composed of three or four cusps, innermost lateral cusp very small to nearly absent (Fig. 42D ).

Etymology.

The species name breviocollar comes from the two Latin words brevis meaning short or shorter, and collum meaning neck. Thus, it refers to a very short to absent detached whorl, a distinguishing character of this species.

Distribution.

This nominotypical species is recorded from several localities in Lopburi and Uthai Thani provinces, central Thailand (Fig. 34 ).

Remarks.

Rhiostoma housei and R. furfurosum sp. nov. have overlapping distribution ranges with R. breviocollar sp. nov., while the COI barcoding indicates they are distinct species (Fig. 3 ). However, additional morphometric analyses will be needed to identify differences in shell form.

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 34. Distribution map of Rhiostoma asiphon - species group.

Figure 39. Shell of Rhiostoma breviocollar sp. nov. A holotype CUMZ 4490 from Khao Samorkon, Ban Mi, Lopburi B-D paratypes CUMZ 4491 from type locality E, F specimens from Khao Patavi, Tubtan, Uthai Thani E specimen CUMZ 4492 and F specimens CUMZ 4493.

Figure 42. Radula morphology A Rhiostoma asiphon CUMZ 4826 from Angthong Islands, Surat Thani B Rhiostoma strubelli specimen CUMZ 10200 from Vieng Phuka, Luang Namtha, Laos C Rhiostoma breviocollar sp. nov. paratypes CUMZ 4491 from type locality D Rhiostoma anceyi sp. nov. paratype CUMZ 4494 / 2 from type locality E Rhiostoma furfurosum sp. nov. paratype CUMZ 4705 / 2 from type locality F Rhiostoma platymorpha sp. nov. paratype CUMZ 4763 from type locality.