Angustopila pallgergelyi Dumrongrojwattana, Chuenit & Wongkamhaeng, 2021

Dumrongrojwattana, Pongrat, , Sirilan Chuenit, Wongkamhaeng, & Koraon, 2021, A new species of the world’s smallest cave snail of the genus Angustopila Jochum, Slapnik & Páll-Gergely in Jochum, et al., 2014 (Gastropoda: Hypselostomatidae) from eastern Thailand, Raffles Bulletin of Zoology 69, pp. 102-108 : 104-106

Angustopila pallgergelyi Dumrongrojwattana, Chuenit & Wongkamhaeng , new species

( Figs. 2, 3A, B)

Type material. Holotype. 1 shell ( ZRCBUU 0699 ), THAILAND, Sa Kaeo Province, Klong Haad District, Tham Phet Pho Thong (Cave), an isolated limestone hill; 13°24′49.4″N 102°19′38.0″E; 280 m a.s.l., coll. S. Chuenit, 5 July 2018 . Paratypes. 3 shells ( ZRCBUU 0700 ) and 2 shells ( ZRC.MOL.020945) same collecting data as holotype, coll. P. Dumrongrojwattana, 15 September 2019 ; 7 shells ( ZRCBUU 0730 ) and 2 shells (THNHM-Iv-17645), same collecting data as holotype, coll. P. Dumrongrojwattana, 12 December 2020 .

Measurements. Holotype. SH = 0.64 mm, SW = 0.76 mm, AH = 0.27 mm, AW = 0.27, SW/SH = 1.19, and AW/AH = 1.01. Paratypes. SH = 0.59– 0.71 mm (0.66± 0.03 mm), SW = 0.76– 0.82 mm (1.25± 0.03 mm), AH = 0.26– 0.31 mm (0.28± 0.02 mm), AW = 0.25– 0.31 mm (0.28± 0.02 mm), SW/SH = 1.19–1.35 (1.25±0.08), and AW/AH = 0.90–1.08 (0.98±0.06). See Tables 1 and 2.

Type locality. Thailand, Sa Kaeo Province, Klong Haad District, Tham Phet Pho Thong (Cave), an isolated limestone hill; 13°24′49.4″N 102°19′38.0″E; 280 m a.s.l. (locality no. 10 in Fig. 1) .

Etymology. This new species is named in honour of Dr.Barna Páll-Gergely, the Hungarian malacologist, in appreciation of his significant contributions to the study of microsnails.

Description. Shell minute, lenticular, white, apex blunt, spire lenticular with deep sutures ( Figs. 2A, 3A, B); protoconch about one whorl, surface finely reticulate ( Fig. 2B, C); teleoconch 4–4¼ rounded whorls, irregularly spaced growth lines crossed by rows of fine spiral threads forming fine reticulate sculpture ( Fig. 2D); aperture broadly kidney-shaped, peristome slightly thickened, slightly expanded, slightly prosocline, detached from the body whorl; apertural teeth well developed, one parietal and one palatal directly opposite ( Fig. 2E); umbilicus deep, relatively narrow.

Differential diagnosis. Angustopila pallgergelyi is similar to A. singuladentis by having a white shell, nearly smooth protoconch sculpture, and the teleoconch sculpture having irregular growth lines crossed with fine spiral threads, peristome slightly thickened and slightly expanded. The new species differs from A. singuladentis in some significant ways. The aperture is kidney-shaped (vs. circular in A. singuladentis ); apertural teeth consist of parietal and palatal teeth (vs. only parietal tooth in A. singuladentis ), and the whorl number ranges from 4–4.5 (vs. 3.5 in A. singuladentis ) ( Table 3). The apertural teeth of A. pallgergelyi consist of parietal and palatal teeth, and share this characteristic with only A. dominikae , A. huoyani , and A. tamlod . This new species can be distinguished from these three species by having teleoconch sculpture irregular growth lines crossed with fine spiral threads and the umbilicus deep and relatively narrow. In contrast, the other species have spiral threads and narrow umbilicus.

Ecology. Living snails live in the dark and moisture zone of caves, gliding on the wall or hidden among coral-like stalactite ( Fig. 3A, B). All type specimens were collected from soil sampled from the dark zone of Phet Pho Thong Cave, Sa Kaeo, approximately 300 metres from the entrance at the type locality. We assume that this new species is a cave-dweller, similar to A. tamlod .

Distribution. The new species is only known from the type locality.

Remarks. All specimens were collected inside the cave in Sa Kaeo Province, eastern Thailand, about 300 metres from the entrance. Among the previously described species, A. dominikae was the smallest known land snail ( Pall-Gergely et al., 2015), with the shell height of the holotype being 0.86 mm. Angustopila pallgergelyi , new species, is even smaller (shell height of the holotype is 0.64 mm and shell heights of the paratypes 0.59–0.71 mm), and is therefore now the world’s smallest known terrestrial snail ( Fig. 4).