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Research Article
First record of subterranean rissoidean gastropod assemblages in Southeast Asia (Mollusca, Gastropoda, Pomatiopsidae)
expand article infoJozef Grego
‡ Unaffiliated, Banská Bystrica, Slovakia
Open Access

Abstract

In February 2017 we investigated several caves and karstic springs in Laos for the presence of underground freshwater gastropod species. We report previously unrecorded freshwater gastropod assemblages in the largest cave in Laos, Tham Khon Dôn, and in the third largest cave, Pha Soung, in Khammouane Province, with single finds in Na Li Cave (Khammouane Province), an unnamed cave near Vieng Thong (Bolikhamsay Province) and a small karst spring near Phonsavan (Xianghouan Province). All 15 species recorded and described herein are new to science. Four species are assigned to the new genus Pseudoiglica: P. pseudoiglica sp. n., P. olsavskyi sp. n., P. kameniari sp. n., and P. phonsavanica sp. n. Three species are assigned to the new genus Thamkhondonia: T. moureti sp. n., T. vacquiei sp. n., and T. smidai sp. n. Eight species are assigned to the genus Tricula Benson, 1843: T. valenasi sp. n., T. davisi sp. n., T. spelaea sp. n., T. lenahani sp. n., T. reischuetzorum sp. n., T. phasoungensis sp. n., T. bannaensis sp. n., and T. viengthongensis sp. n.

Keywords

Troglobiont, Stygobiont, Spring, Cave, Laos, Khammouane, Bolikhamsay, Xianghouan

Introduction

The subterranean freshwater molluscan fauna of South and East Asia is virtually unknown. Bole and Velkovrh (1986) mentioned only one subterranean species from South Asia that was recorded in Sri Lanka (Srilankiella horanae Bole & Velkovrh, 1986, nomen nudum according Kabat and Hershler 1993) and 12 probable pomatiopsid taxa from Japan belonging to the genera Akiyoshia Kuroda & Habe, 1954, Cochliopopsis Mori, 1938 and Moria Kuroda & Habe, 1958. Despite the recent increased interest in South and East Asian terrestrial micromolluscs (Maassen 2008, Culver 2012, Páll-Gergely 2014, Páll-Gergely et al. 2015, Inkhavilay et al. 2016, Páll-Gergely et al. 2016, Páll-Gergely et al. 2017), our knowledge of the stygobiont gastropod fauna of the region has not advanced significantly since 1986. Despite the extraordinary high radiation of surface freshwater Pomatiopsidae Bourguignat, 1863, species of the tribe Triculini Annandale, 1924 in the Mekong tributary (Deshayes and Jullien 1876; Crosse and Fischer 1879; Annandale 1919; Brandt 1968, 1970, 1974; Davis 1979, Temcharoen 1971; Brandt and Temcharoen 1974; Strong et al. 2007), the troglobiont and stygobiont species of the area remained unknown. Studies of this extraordinary high diversity in the Mekong Basin of over 90 taxa was driven primarily by parasitological studies of the trematode Schistostoma mekongi Voge, Bruckner & Bruce, 1978, the main intermediate hosts of which are some of the local Triculini species such as Neotricula aperta (Temcharoen, 1971). The assumption that the high diversity of surface Triculini species could extend into the underground stygobiont habitat was confirmed in the present study.

Materials and methods

The material studied was collected in Laos in February 2017 from the localities shown in Figs 1 and 2. Various cave streams, outflows and karstic springs were sampled using microhabitat preference and sampling methods as described by Grego et al. (2017).

Figure 1. 

Map of sampling localities in Laos. 1–3 Khammouane: Tham Khon Dôn Cave 1 Earthquake Dome, Type locality (LT) of Pseudoiglica pseudoiglica gen. n., sp. n., P. olsavskyi n .sp., P. kameniari sp. n., Thamkhondonia moureti gen. n., sp. n., T. vacquiei sp. n., T. smidai sp. n., Tricula valenasi sp. n., T. davisi sp. n., T. spelaea sp. n., T. lenahani sp. n. and T. bannaensis sp. n. 2 Entrance passage 3 Source of Nam Dôn River 4 Khammouane: Tham Pha Soung Cave, Frog Lake, LT of Tricula phasoungensis sp. n. 5 Khammouane Cave Na Li, LT of Tricula reischuetzorum sp. n. 6 Bolikhamsay, 16 km W Vieng Thong, LT of Tricula viengthongensis sp. n. 7, Xianghouan: Ban Nadom Village, LT of Pseudoiglica phonsavanica sp. n.

Samples (fine sand) were screened under a stereomicroscope. They were first screened wet for live animals. Then they were dried and screened again for dry shells that might have been overlooked during the wet screening. Frontal and lateral view images were taken with a digital camera and ImageJ scientific image analysing software was used to take measurements, with additional direct measurements obtained using an eyepiece micrometer.

We followed the shell morphology nomenclature according Davis et al. (1992) and Hershler and Ponder (1998).

Abbreviations

NHMUK Natural History Museum, London, UK

HNHM Hungarian Natural History Museum, Budapest, Hungary

OSUM Ohio State University Museum of Biological Diversity, Columbus, Ohio, USA

H Shell height

W Shell width

BH Height of the body whorl

BW Width of the body whorl

AH Aperture height

AW Aperture width

LT Type locality

MY Million years

Results

Many of the newly-recorded stygobiont Triculini have a convergent shell morphology superficially resembling to that of species of Moitessieriidae Bourguignat, 1863 and Hydrobiidae Troschel, 1857, known from other stygobiont habitats. All the present stygobiont specimens from the cave streams and spring outlets were empty shells collected during the dry season. As it is highly unlikely to find live specimens because of the inaccessibility of habitats during rainy seasons, the two new genera (Pseudoiglica gen. n. and Thamkhondonia gen. n.) were established based only on the shell morphology, without information on the soft parts and without molecular data. I assume, based on shell morphology that all the new species belong to the family Pomatiopsidae Bourguignat, 1863, tribe Triculini Annandale, 1924. The tribe is extremely diverse along the neighboring tributary of the Mekong River and represents the only known Gastropoda similar to the new taxa within the whole region.

Superfamily Truncatelloidea Gray, 1840

Family Pomatiopsidae Bourguignat, 1863

Tribe Triculini Annandale, 1924

Pseudoiglica gen. n.

Diagnosis

The diagnostic features of the genus are the same as those of the type species, Pseudoiglica pseudoiglica sp. n. The elongated smooth shell shape with open umbilicus differs from all known genera of Pomatiopsidae in tributaries of the Mekong River.

Etymology

Named for the shell morphology, which is convergent with the subterranean moitesieriid genus Iglica Wagner, 1910 from the western Balkans.

Pseudoiglica pseudoiglica sp. n.

Figures 3–6

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments on bank of cave river (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180001).

Paratypes: type locality (NHMUK 20180016 – 1 specimen; HNHM 102769 – 1 specimen; coll. Grego F0871 – 6 specimens); Laos, Khammouane Province, Tham Nam Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., temporary side rivulet sediment at entrance passage 1.5 km from the main entrance, dry sand on the cave floor; J. Grego leg. 11 February 2017 (coll. Grego F0863 – 1 specimen).

Other material

Laos, Khammouane Province, 2 km WNW of Ban Na village, Pha Soung Cave, Frog Lake, 155 m a.s.l., J. Grego leg. 09 February 2017, 17°33.052'N; 104°52.410'E (coll. Grego F0888).

Measurements

Holotype: H 3.08 mm; W 1.32 mm; BW 0.95 mm; BH 1.50 mm; AH 0.81 mm; AW 0.75 mm; H/W 2.33; AH/AW 1.08; W/BW 1.39; H/BH 2.05; H/AH 3.80; W/AW 1.76. Paratype 1: H 3.05 mm; W 1.31 mm; BH 1.00 mm; BW 1.55 mm; AH 0.87 mm; AW 0.75 mm; H/W 2.33; AH/AW 1.16; W/BW 1.31; H/BH 1.97; H/AH 3.51; W/AW 1.76.

Diagnosis

This new species is similar to the syntopic Pseudoiglica kameniari sp. n., from which it differs by its more slender, elongated shell with a more prominent umbilicus and less elongated aperture situated further to the right of the columellar axis. It differs from syntopic P. olsavskyi sp. n. by its markedly larger and more conical shell shape and proportionally larger aperture. Pseudoiglica phonsavanica sp. n. (Xianghouan Province) has more a robust shell with more prominent body whorl and a differently shaped aperture.

Description

The milky yellowish silky shell has six tumid convex whorls with a deep suture and a blunt apex. The surface is smooth and shiny. The shell is elongated, almost cylindrical, slightly tapering towards a blunt apex, the umbilicus is tiny, open. In frontal view, the lateral aperture protrudes against the rest of the teleoconch. The aperture is ovoid, separated from the body whorl by a gap. The peristome margin is blunt, equally thick all the way around and slightly reflexed outwards. The outer lip is sinuous in lateral view and slightly scooped forward at its lower end.

Etymology

See the etymology of the genus Pseudoiglica gen. n.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as in the related source of the Nam Dôn River and from sediments in Tham Pha Soung Cave.

Ecology

Empty shells of the new species were extracted from the side stream sandy sediments of an underground river inside the cave Tham Khon Dôn about 3 km from the main entrance situated above the source of Nam Dôn River (Fig. 1A). The cave Tham Khon Dôn is situated under the massif of Mount Pha Kouankaohong (Fig. 2C) north of Ban Na Village. It represents the largest explored cave system in Laos, with a length of known passages of ca. 42 km. The cave is morphologically diverse with large domes and passages modeled by phreatic corrosion representing the cave multiple genetic horizons formed during the past 11 million years (since Late Miocene-Lower Pliocene) of its natural history possibly driven by a hydrothermal, H2S speleogenesis (Mouret 2005). Despite the remarkable length of the cave system, its water passages are accessible only for a limited length near the entrance. The major part of the underground river comprises mostly unexplored submerged cave passages. The main sampling site was located at the bottom of Earthquake Dome (Fig. 2B), named after the sounds of the earthquake experienced here by the first explorers (Claude Mouret and Jean-Francois Vacquié pers. com.). The dome floor is covered by very large flat limestone slabs approx. 0.8–1.2 m thick fallen from the horizontally flat ceiling that reflects the horizontal beds of Carboniferous/Permian Khammouane Limestone. The fallen slabs fragmented the underground stream into several lakes and helped to create the sedimentation zones in which empty shells could be deposited during high water flows. A few shells were also found in the sand floor of the entrance passage approximately 1.5 km from the entrance close to the junction with a temporary side stream, as well as in the sand deposited directly at the cave entrance at the source of the Nam Dôn River (Fig. 1A). The character of the material deposited in the cave sediments and the freshwater shell assemblages suggested their autochthonous origin rather than the allochthonous influence of horizontal surface waters. The occurrence of tiny terrestrial gastropod shells of mainly soil and leaf litter dwelling families such as Vertiginidae Fitzinger, 1833 (Hypselostoma sp., Angustopila sp., Krobylos sp., Paraboysidia sp.) and Diapheridae Panha & Naggs, 2010 (Sinoennea sp.) in the underground river sediments suggests a stronger influence of vertically circulating surface karst waters. The presence of the same species in the sediments of nearby Tham Pha Soung Cave indicates possible communication between the two caves through phreatic waters under Ban Na polje or during the rainy seasons. Surface Triculini species inhabiting the Nam Dôn River are larger, with a stronger periorostracum and different shell morphology. It appears that they are not penetrating deeply into the dark cave system, probably because of a lack of their main algal food. The new species probably inhabits the so far unexplored submerged cave passages of the Tham Khon Dôn system.

Figure 2. 

Photos of sites where subterranean gastropods were found. A Khammouane: main entrance of Tham Khon Dôn Cave with source of Nam Dôn River (2 in Fig. 1) B Earthquake Dome in cave Tham Khon Dôn (1 in Fig. 1) C Mount Pha Kouankaohong with entrance of Tham Khon Dôn cave at its foot D Bolikhamsay, travertine cascades below the LT of Tricula viengthongensis sp. n. (6 in Fig. 1) E Khammouane, one of the main entrances of Tham Pha Soung Cave F Tham Pha Soung Cave, sampling at Frog Lake (4 in Fig. 1). (Photos: Ondrej Kameniar, Mário Olšavský and Jozef Grego).

Figures 3–12. 

Representatives of the genus Pseudoiglica gen. n. 3–6 Pseudoiglica pseudoiglica sp. n. (3–4 holotype NHMUK 20180001 5–6 paratype 1 coll. Grego F0871) 7–8 P. olsavskyi sp. n. (holotype NHMUK 20180003) 9–10 P. kameniari sp. n. (holotype NHMUK 20180002) 11–12 P. phonsavanica sp. n. (holotype NHMUK 20180004).

Pseudoiglica olsavskyi sp. n.

Figs 7–8

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments on cave river bank (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180003). Paratypes: type locality (NHMUK 20180149 – 1 specimen; coll. Grego F0872 – 3 specimens); Laos, Khammouane Province, Tham Nam Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., temporary side rivulet sediment at entrance passage 1.5 km from the main entrance, dry sand on the cave floor; J. Grego leg. 11 February 2017 (coll. Grego F0864 – 3 specimens); Laos, Khammouane Province, 3 km NW of Ban Na Village, sand on the bottom of Nam Dôn River source at 149 m a.s.l.; J. Grego leg. 07 February 2017, 17°33.20'N; 104°52.38'E (coll. Grego F0854 – 4 specimens) (Fig. 2A).

Measurements

Holotype: H 2.45 mm; W 0.85 mm; BW 0.52 mm; BH 1.00 mm; AH 0.61 mm; AW 0.45 mm; H/W 2.88; AH/AW 1.36; W/BW 1.63; H/BH 2.45; H/AH 4.02; W/AW 1.89

Diagnosis

The tiny, elongated cylindrical shell of P. olsavskyi sp. n. with a proportionally very small aperture distinguishes this species from all other known members of the genus, which all have larger shells.

Description

The tiny milky shell has six flattened convex whorls with a deep suture and a blunt apex. The surface is smooth and shiny. The shell is cylindrically elongated, slightly narrowing towards the apex. Umbilicus is slit-like. In frontal view, the aperture protrudes laterally from the shell periphery outline. Aperture is ovoid, separated from the body whorl by a weak furrow. The peristome margin is sharp, equally thick all the way around and very slightly reflexed outwards. The outer lip is sinuous in its lateral profile and its lower end scooped backward.

Etymology

Named after my friend Mario Olšavský, a geologist and speleologist from Banská Bystrica, Slovakia, who actively participated in sample collection in the cave Tham Khon Dôn.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as from the related source of Nam Dôn River

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Pseudoiglica kameniari sp. n.

Figures 9–10

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments on bank of cave river (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180002). Paratypes: type locality (NHMUK 20180017 – 1 specimen; HNHM 102770 – 1 specimen; OSUM 42383 –1 specimen; coll. Grego F0873 – 11 specimens).

Measurements

Holotype: H 2.85 mm; W 1.21 mm; BW 0.85 mm; BH 1.50 mm; AH 0.95 mm; AW 0.75 mm; H/W 2.36; AH/AW 1.27; W/BW 1.42; H/BH 1.60; H/AH 3.00; W/AW 1.61.

Diagnosis

Similar to syntopic Pseudoiglica pseudoiglica sp. n., from which it differs by its smaller, more robust shell with a less prominent umbilicus and more elongated aperture situated more towards the columellar axis. It differs from the syntopic P. olsavskyi sp. n. by its larger and more conical shell shape and proportionally larger aperture. It differs from P. phonsavanica by its less robust shell, less prominent body whorl and the different shape of the aperture.

Description

The milky yellowish shell has five convex whorls with a deep suture and a blunt apex. The shell has a smooth and shiny surface and is elongated-conical. Umbilicus is slit-like. In frontal view, the aperture aligns with the shell periphery outline. Aperture is ovoid, attached to the body whorl by a very weak furrow. The peristome margin is a blunt callous, equally thick all the way around and slightly reflexed outwards. The outer lip is weakly sinuous in lateral profile.

Etymology

Named after Ondrej Kameniar, young speleologist and biologist, friend from Ľubochňa, Slovakia, who actively participated in our 2017 field trip to Laos.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as from the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Pseudoiglica phonsavanica sp. n.

Figs 11–12

Type locality

Laos; Xianghouan Province, Ban Nadom Village, 18 km SE of Phonsavan, 3 km N of Ban Kaua cement factory at highway 1D (9 km ENE of Xiang Khouang), small spring at eastern foot of limestone hill, 19°23.142'N; 103°17.630'E, 1196 m a.s.l., fine sand directly at spring zone.

Type material

Holotype: type locality: J. Grego leg. 22 February 2017 (NHMUK 20180004).

Measurements

Holotype: H 3.05 mm; W 1.30 mm; BW 1.00 mm; BH 1.71 mm; AH 0.93 mm; AW 0.79 mm; H/W 2.35; AH/AW 1.18; W/BW 1.30; H/BH 1.78; H/AH 3.28; W/AW 1.65.

Diagnosis

Similar to Pseudoiglica kameniari sp. n. (Khammouane Province), from which it differs by its more robust shell with a more prominent umbilicus and a proportionally smaller aperture. The robust shape differentiates the species from all other members of the genus.

Description

The light orange, silky shell has five tumid convex whorls with a weak suture and a blunt apex. The smooth shell surface is covered by sparse rusty incrustations. The shell is elongated conical, with a prominent body whorl. Umbilicus is slit-like. In frontal view, the aperture is aligned with the shell periphery outline. Aperture is ear-shaped, separated from the body whorl by a weak sulcus. The peristome margin is blunt, not reflexed and slightly callous internally. The labral lip has a straight profile in lateral view, scooped backward from the columellar axis. The elongate ellipsoidal spiral operculum is light yellowish corneous with submarginal nucleus.

Etymology

Named after the city of Phonsavan, Laos, capital of Xianghouan Province, which is the closest large city to the type locality.

Distribution

Only known from the type locality.

Ecology

The locality is a small karstic spring rising at the foot of a rounded cone-shaped limestone hill at the boundary between limestone beds and a sandy slate substrate just a few meters above the road. The spring is connected to a small waterworks to supply water to the nearby village Ban Nadom. The water supply seems to be permanent throughout all seasons.

Remarks

The body whorl of P. phonsavanica sp. n. is proportionally larger than that of all other species of the genus. The more teardrop-shaped aperture suggests that this geographically distant species could represent a new genus distinct from Pseudoiglica gen. n. Anatomical and molecular data are needed to confirm such a possible distinction.

Thamkhondonia gen. n.

Diagnosis

The diagnostic features of the genus are the same as those of the type species, Thamkhondonia moureti sp. n. The elongated axially and radially sculptured shell with an ear-shaped aperture differs from that of any known genus of Pomatiopsidae from tributaries of the Mekong River. The shell shows some resemblance to those of species in the triculinid genus Paraprosothenia Annandale, 1919 and the marine genus Attenuata Hedley, 1918, but differs from both by the characteristic shell sculpture consisting of spiral cords and axial ribs.

Etymology

Named after the type locality inside the Tham Khon Dôn Cave situated under the massif of Pha Kouankaohong near village Ban Na in Khammouane Province.

Thamkhondonia moureti sp. n.

Figs 13–14

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments on cave river banks (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180005). Paratypes: type locality (NHMUK 20180018 – 2 specimens; HNHM 102771 – 2 specimens; OSUM 42384 – 2 specimens; coll. Grego F0874 – 33 specimens); Laos, Khammouane Province, Tham Nam Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l, temporary side rivulet sediment at entrance passage 1.5 km from the main entrance, dry sand on the cave floor; J. Grego leg. 11 February 2017 (coll. Grego F0865 – 2specimens); Laos, Khammouane Province, 3 km NW of Ban Na Village, sand on the bottom of Nam Dôn River source at 149 m a.s.l.; J. Grego leg. 07 February 2017, 17°33.20'N; 104°52.38'E (coll. Grego F0855 – 1 specimen) (Fig. 2A).

Measurements

Holotype: H 4.48 mm; W 1.42 mm; BW 0.80 mm; BH 1.55 mm; AH 1.01 mm; AW 0.91 mm; H/W 3.15; AH/AW 1.11; W/BW 1.78 H/BH 2.89; H/AH 4.44; W/AW 1.56.

Diagnosis

Compared to the most closely related syntopic species, Thamkhondonia vacquiei sp. n. and T. smidai sp. n., T. moureti sp. n. has a much higher and more slender shell with much coarser spiral sculpture and a proportionally smaller aperture.

Description

The milky whitish, elongated turritiform shell has nine slightly convex whorls with a weak suture. The surface sculpture consists of 4–5 coarse spiral cords crossed by very fine axial ribs. The aperture is oval ear-shaped and extends beyond the shell periphery outline; the peristome is sharp and expands only at its columellar side. The lateral profile of the labral lip is straight and a characteristic axial varix is parallel to the labral lip. The umbilicus is closed.

Etymology

Named after my friend, a renowned French geologist and speleologist Claude Mouret (Magnac-Bourg, France), who in 2006 discovered and for the first time explored the Tham Khon Dôn cave system, the largest cave in Laos, and led annual expeditions to explore caves of Khammouane. Without his substantial help, the sampling of the type locality in cave Tham Khon Dôn would not have been possible.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as in the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Figures 13–22. 

Representatives of the genus Thamkhondonia gen. n. 13–14 Thamkhondonia moureti sp. n. (holotype NHMUK 20180005) 15–18 T. vacquiei sp. n. (15–16 holotype NHMUK 20180006 17–18 paratype 1 coll. Grego F0875) 19–22 T. smidai sp. n. (19–20 holotype NHMUK 20180007 21–22 paratype 1 coll. Grego F0876).

Thamkhondonia vacquiei sp. n.

Figs 15–18

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek; Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments on cave river banks (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180006). Paratypes: type locality (NHMUK 20180019 – 5 specimens; HNHM102772 – 5 specimens; OSUM 42385 – 5 specimens; coll. Grego F0875 – 274 specimens); Laos, Khammouane Province, 3 km NW of Ban Na Village, sand on the bottom of Nam Dôn river source at 149 m a.s.l.; J. Grego leg. 07 February 2017, 17°33.20'N; 104°52.38'E (coll. Grego F0856 – 1 specimen) (Fig. 2A).

Measurements

Holotype: H 3.35 mm; W 1.51 mm; BW 1.25 mm; BH 1.65 mm; AH 0.95 mm; AW 0.90 mm; H/W 2.22; AH/AW 1.06; W/BW 1.21; H/BH 2.03; H/AH 3.53; W/AW 1.68; Paratype 1: H 3.00 mm; W 1.40 mm; BH 1.20 mm; BW 1.55 mm; AH 0.90 mm; AW 0.85 mm; H/W 2.14; AH/AW 1.06; W/BW 1.17; H/BH 1.94; H/AH 3.33; W/AW 1,65.

Diagnosis

Thamkhondonia vacquiei sp. n. differs from the two syntopic species T. moureti sp. n. and T. smidai sp. n. by its shorter and more robust shell shape with a proportionally larger aperture and by its much finer and more numerous spiral sculpture.

Description

The whitish translucent, conical shell has six convex whorls with a weak slightly wavy suture. The surface sculpture consists of 11–12 weak spiral cords crossed by fine axial ribs. The aperture is oval ear-shaped with an indication of a posterior canal that extends slightly beyond the shell periphery outline. The peristome is sharp and expands only on the columellar side. The lateral edge of the labral lip is sinuous and a weak axial varix is present parallel to the labral edge. The umbilicus is slit-like.

Etymology

Named after my friend Jean-Francois Vacquié (Castelnau d’Estretefonds, France), a French speleologist who participated in the explorations of Tham Khon Dôn Cave and supported our activities during the 2017 field trip.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as in the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Thamkhondonia smidai sp. n.

Figs 19–22

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments at cave river bank (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180007). Paratypes: type locality (NHMUK 20180020 – 2 specimens ; HNHM 102773 – 2 specimens; OSUM 42383 – 2 specimens; coll. Grego F0876 – 34 specimens); Laos, Khammouane Province, 3 km NW of Ban Na Village, sand on the bottom of Nam Dôn river source at 149 m a.s.l.; J. Grego leg. 07 February 2017, 17°33.20'N; 104°52.38'E (coll. Grego F0902 – 2 specimens) (Fig. 2A).

Measurements

Holotype: H 3.62 mm; W 1.45 mm; BW 1.15 mm; BH 1.45 mm; AH 1.05 mm; AW 0.75 mm; H/W 2.50; AH/AW 1.15; W/BW 0.36; H/BH 2.16; H/AH 3.45; W/AW 1.59. Paratype 1: H 3.45 mm; W 1.38 mm; BH 1.10 mm; BW 150; AH 1.00 mm; AW 0.87 mm; H/W 2.50; AH/AW 1.15; W/BW 0.36; H/BH 2.18; H/AH 3.45; W/AW 1.59.

Diagnosis

Thamkhondonia smidai sp. n. differs from syntopic T. moureti sp. n. by its smaller shell with less coarse and more numerous spiral sculpture and from T. vacquiei sp. n. (syntopic) by its longer and more slender shell shape with coarser spiral cords.

Description

The whitish translucent, elongate shell has seven convex whorls with a weakly wavy suture. The shell surface sculptured by 5–6 spiral cords crossed by very fine axial ribs. The oval ear-shaped aperture has a weak posterior canal and extends slightly beyond the shell periphery outline; the peristome is blunt and reflexed at the columellar side. The lateral edge of the labral lip is weakly sinuated and an axial varix is present parallel to the labral lip. The umbilicus is closed.

Etymology

Named after the famous Slovak speleologist Branislav Šmída, Bratislava, Slovakia, who actively participated in our 2017 biospeleology survey of Laos.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as in the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Remark

The shell morphology of T. smidai sp. n. is an intermediate between that of T. moureti sp. n. and T. vacquiei sp. n.

Tricula Benson, 1843

Nore

Based on their shell morphology the species described below are provisionally placed in the genus Tricula until anatomical and molecular data can be obtained.

Tricula valenasi sp. n.

Figs 23–24

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments at cave river banks (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180008). Paratypes: type locality (coll. Grego F0877 – 3 specimens).

Measurements

Holotype: H 3.25 mm; W 1.65 mm; BW 1.05 mm; BH 1.25 mm; AH 1.15 mm; AW 0.95 mm; H/W 1.97; AH/AW 1.2; W/BW 1.57; H/BH 2.60; H/AH 2.83; W/AW 1.74.

Diagnosis

Tricula valenasi sp. n. is similar to syntopic T. lenahani sp. n., from which it differs by its elongate shell with a less open umbilicus and an aperture more prominent at the shell periphery outline. It differs significantly from syntopic T. davisi sp. n. by its longer, more elongated shell shape, its more inflated whorls and its proportionally smaller aperture with a straight columellar margin rather than the columellar sinuation characteristic of T. davisi sp. n. From T. bollingi Davis, 1968 it differs by the aperture shape, which extends beyond the shell periphery outline and by its more open umbilicus and more blunt apex. It can be distinguished from T. burchi Davis, 1968, by its more slender and more conical shape with a smaller aperture, a more prominent umbilicus and a less blunt apex.

Description

The whitish shell has five rounded convex whorls with a semi-deep suture and a blunt apex. The surface is smooth and shiny. The shell is narrow-conical with prominent body whorl. Umbilicus is tiny opened. In frontal view, the outer part of the aperture protrudes from the shell periphery outline. Aperture is ovoid ear-shaped, separated from the body whorl by a very weak furrow and an adapical gap. The peristome margin is blunt, slightly reflexed outwards. The outer lip is slightly callous and has a straight lateral profile.

Etymology

Named after my friend Liviu Valenas (Nuremberg, Germany), an avid speleologist born in Romania, who spent 10 years exploring the cave system Pha Soung, which, with its currently documented 20.4 km, is the third largest cave system in Laos.

Distribution

Only known from the type locality.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Figures 23–48. 

Representatives of the genus Tricula, Benson, 1843. 23–24 Tricula valenasi sp. n. (holotype NHMUK 20180008) 25–28 T. lenahani sp. n. (holotype NHMUK 201800010) 29–32 T. spelaea sp. n. (holotype NHMUK 20180011) 33–36 T. davisi sp. n. (holotype NHMUK 20180009) 37–38 T. reischuetzorum sp. n. (holotype NHMUK 20180012) 39–42 T. phasoungensis sp. n. (39–40 holotype NHMUK 20180013 41–42 paratype 2 coll. Grego F0881) 43–44 T. bannaensis sp. n. (holotype NHMUK 20180014) 45–48 T viengthongensis sp. n. (45–46 holotype NHMUK 20180015 47–48 paratype 1 coll. Grego F0904).

Tricula lenahani sp. n.

Figs 25–28

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments of cave river banks (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 201800010). Paratypes: type locality (NHMUK 20180021 – 5 specimens; HNHM 102774 – 5 specimens; OSUM 42391 – 5 specimens; coll. Grego F0879 – 103 specimens); Laos, Khammouane Province, Tham Nam Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., temporary side rivulet sediment at entrance passage 1.5 km from the main entrance, dry sand on the cave floor; J. Grego leg. 11 February 2017 (coll. Grego F0868 – 1 specimen); Laos, Khammouane Province, 3 km NW of Ban Na Village, sand on the bottom of Nam Dôn River source at 149 m a.s.l.; J. Grego leg. 07 February 2017. 17°33.20'N; 104°52.38'E (coll. Grego – 6 specimens) (Fig. 2A).

Measurements

Holotype: H 2.72 mm; W 1.71 mm; BW 0.96 mm; BH 1.70 mm; AH 1.15 mm; AW 0.93 mm; H/W 1.59; AH/AW 1.24; W/BW 1.78; H/BH 1.60 H/AH 2.37; W/AW 1.84.

Diagnosis

This species is similar to the syntopic Tricula valenasi sp. n., but differs from it by its more robust, shorter shell with a more open umbilicus as well as by its sinuated labral margin profile. It differs from syntopic T. spelaea sp. n. by its more inflated shell shape, and a different arrangement of the whorls, a larger umbilicus and a different shape of the aperture. It can be distinguished from syntopic T. davisi sp. n. by its more slender, less inflated shell and the shape of the columellar peristome. From T. bollingi Davis, 1968 it differs by its general shell shape and the position of the aperture, and by its more open umbilicus and blunter apex.

Description

The whitish, semi-translucent shell has five convex whorls with a deep suture. The surface is smooth with fine, whitish, inconsistent axial bands. The shell is ovate-conical with whorls smoothly tapering towards the apex. The aperture is ear-shaped; the peristome expands outwards especially at the columellar side. The lateral edge of the labral lip is characteristically sinuated, as well a week sinuation is present at apical inner peristome. The umbilicus is open, partly obscured by the reflected columellar margin.

Etymology

This species in named after my ever helpful friend Peter Lenahan, an avid caver from New York City, USA, for his great support during the field trip and for his indispensable help to Ban Na village by supporting construction of a new well and tap water supply for the villagers.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as in the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n..

Tricula davisi sp. n.

Figs 29–32

Type locality

Laos; Khammouane Province; Ban Na village 20 km NNE of Thakhek; Tham Khon Dôn Cave, 161 m a.s.l.; 17°33.82'N; 104°52.30'E; Earthquake Dome 3 km from the south entrance, sand sediments at cave river banks (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180009). Paratypes: type locality (NHMUK 20180022 – 3 specimens; HNHM 102775 – 3 specimens; OSUM 42387 – 3 specimens; coll. Grego F0878 – 76 specimens); Laos, Khammouane Province, 3 km NW of Ban Na Village, sand on the bottom of Nam Dôn River source at 149 m a.s.l.; J. Grego leg. 07 February 2017, 17°33.20'N; 104°52.38'E (coll. Grego F0858 – 6 specimens) (Fig. 2A).

Measurements

Holotype: H 2.72 mm; W 1.81 mm; BW 1.23 mm; BH 1.88 mm; AH 1.5 mm; AW 1.05 mm; H/W 1.50; AH/AW 1.29; W/BW 1.47; H/BH 1.45; H/AH 2.01; W/AW 1.72.

Diagnosis

This new species is similar to syntopic Tricula lenahani sp. n., from which it differs by its more oval, inflated shell with more inflated whorls, a closed umbilicus and the shape of the aperture. The aperture of T. davisi sp. n. has a characteristic sinuation at the columellar peristome and a straight labral peristome, distinguishing it from T. lenahani sp. n., which has a sinuated labral peristome and a different columellar peristome, as well from the syntopic T. spelaea sp. n., the peristome of which lacks significant sinuation on both sides. From T. bollingi Davis, 1968 and T. burchi Davis, 1968 it differs by its shell and aperture shapes.

Description

The shell is rounded oval-conical with four slightly inflated whorls with elevated spire and a deeper suture. The surface is milky whitish and smooth with faint growth lines. The aperture is oval ear-shaped, the peristome slightly callous attached to the body whorl and expanding only towards the columella. The labral lip lateral profile is straight, while a characteristic deep sinuation is present at the apical inner lip. The last whorl is broadening towards the aperture and from lateral view is curved upward. The umbilicus is closed.

Etymology

Named after George M. Davis, (Washington D.C., USA) who contributed much to the molecular phylogeny and taxonomy of the Mekong River Pomatiopsidae.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as from the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Tricula spelaea sp. n.

Figs 33–36

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments on cave river bank (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180011). Paratypes: type locality (Grego F0880 – 3 specimens); Laos, Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Pha Soung Cave, 17°33.052'N; 104°52.410'E, 155 m a.s.l., sandy sediment on the bottom of Frog Lake at central part of the cave (coll. Grego F0886 – 2 specimens).

Measurements

Holotype: H 2.80 mm; W 1.65 mm; BW 1.05 mm; BH 1.80 mm; AH 1.10 mm; AW 0.95 mm; H/W 1.70; AH/AW 1.16; W/BW 1.57; H/BH 1.56; H/AH 2.55; W/AW 1.74.

Diagnosis

This new species is similar to syntopic Tricula lenahani sp. n., from which it differs by its more conical shell with less inflated whorls, a closed umbilicus and the different shape of the aperture. It differs significantly from syntopic T. valenasi sp. n. by its shorter, more inflated shell shape, more flattened whorls, straight lateral labral profile and closed umbilicus. From the T. bollingi Davis, 1968 it differs by its shell and aperture shape, position of the aperture and a more blunt apex. From T. burchi Davis, 1968, it differs by its more conical shape with a smaller aperture. The labral lateral profile of T. spelaea sp. n. is straight, and no sinuation is present on its columellar side.

Description

The shell shape is conical with five somewhat flattened but still convex whorls with a weak suture and blunt apex. The surface is milky whitish and smooth. The aperture is oval tear-shaped; the peristome slightly callous expanding outwards. The labral lip lateral profile is straight, and no sinuation is present at the apical inner lip. The umbilicus is closed.

Etymology

Named after its type locality inside the Cave Tham Khon Dôn in Khammouane.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well from the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Tricula reischuetzorum sp. n.

Figs 37–38

Type locality

Laos; Khammouane Province, Cave Tham Na Li 8 km E of Thakhek on road AH131, bottom of cave river Nam Xiangliap, 17°27.20'N; 104°54.54'E.

Type material

Holotype: type locality: J. Grego leg. 16 February 2017 (NHMUK 20180012).

Measurements

Holotype: H 2.68 mm; W 1.29 mm; BW 0.86 mm; BH 1.61 mm; AH 0.93 mm; AW 0.79 mm; H/W 2.08; AH/AW 1.18; W/BW 1.50; H/BH 1.66; H/AH 2.88; W/AW 1.63.

Diagnosis

The shell is similar to that of Tricula valenasi sp. n. (Tham Khon Dôn Cave), from which it differs by being smaller, less elongate and with more inflated whorls, a more closed umbilicus and a more elongated aperture situated more towards the columella. It differs significantly from T. lenahani sp. n. (Tham Khon Dôn Cave) by its overall shell shape, the position of aperture and the closed umbilicus. From T. bollingi and T. burchi it differs by general shell and aperture shape and the position of the aperture.

Description

The milky white, narrow oval-conical shell with four convex whorls and deep suture has a blunt apex. The shell surface is smooth and shiny. The shell is narrow-conical. Aperture is ovoid elongated and its lower part slightly angled towards the columella. The aperture is attached to the body whorl by a weak furrow. The peristome margin is sharp, somewhat darker stained. The outer lip is slightly sinuated laterally. Umbilicus is closed.

Etymology

Named after active researchers of the Balkan stygobiont gastropod fauna, Peter L. and Alexander Reischütz (Horn, Austria), who brought our attention to the Na Li cave.

Distribution

Only known from the type locality.

Ecology

The shells were found in the sandy sediment inside the cave Tham Na Li close to the river outlet. The about 300m long cave passage was formed by the river Nam Xiangliap under the limestone hill, and thus the cave habitat has a direct contact with surface waters.

Tricula phasoungensis sp. n.

Figs 39–42

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Pha Soung Cave, 33.052'N; 104°52.410'E, 155 m a.s.l., sandy sediment on the bottom of Frog Lake at central part of the cave (Fig. 2F).

Type material

Holotype: type locality: J. Grego leg. 09 February 2017 (NHMUK 20180013).

Paratypes: type locality (Grego F0887 – 1 specimen); Laos, Khammouane Province; Ban Na village 20 km NNE of Thakhek; Tham Khon Dôn Cave, 17°33.82'N; 104°52.30'E, 161 m a.s.l., Earthquake Dome 3 km from the S entrance, sand sediments on cave river banks (NHMUK 20180023 – 3 specimens; HNHM 102776 – 3 specimens; OSUM 42388 – 3 specimens; coll. Grego F0881 – 60 specimens).

Measurements

Holotype: H 1.91 mm; W 1.20 mm; BW 0.70 mm; BH 1.25 mm; AH 0.82 mm; AW 0.65 mm; H/W 1.59; AH/AW 1.26; W/BW 1.71; H/BH 1.53; H/AH 2.33; W/AW 1.85; Paratype 2: H 2.05 mm; W 1.43 mm; BH 0.75 mm; BW 1.35 mm; AH 1.06 mm; AW 0.84 mm; H/W 1.43; AH/AW 1.26; W/BW 1.91; H/BH 1.52; H/AH 1.93; W/AW 1.70.

Diagnosis

With its small hydrobioid shell it is similar to T. bannaensis sp. n., but differs from it by its more slender shell, its less inflated whorls, its more closed umbilicus and the shape of the aperture. The similar but larger T. lenahani sp. n. differs by having less inflated whorls, different shell and aperture shapes and a narrower umbilicus. Tricula reischuetzorum sp. n. has a more elongate shell with more inflated whorls, a smaller umbilicus and a different shape of the aperture.

Description

The milky whitish shell has four convex whorls with a deep suture and a smooth, shiny surface. The bythinelloid-shaped shell is oval sub-conical with a blunt apex. The aperture is oval; the peristome is slightly callused and outwardly expanded and connected to the body whorl. The lateral edge of the labral lip is very slightly sinuated. The umbilicus is open and conspicuous.

Etymology

Named after the type locality inside the Pha Soung Cave system, which is 20.4 km long and the third longest cave in Laos.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as in the related source of Nam Dôn River.

Ecology

Shells were found in the sandy sediment of a small and shallow “Frog Lake” (Fig. 2F) in the central part of the Pha Soung Cave system. The site is a shallow 20–30cm deep pond at the lowest part of a cave meander passage, which holds the remains of water after seasonal water crossflow. Green-black streaked frogs Rana chloranata (Günther, 1876) washed from the surface inhabit the pond. The Pha Soung cave system (fig. 2E) is 20.4 km long and is situated under the massif of Mount Pha Soung near village Ban Na in SW Khammouane. The cave consists of several floors of old corrosive and phreatic passages developed during the past 11 MY of speleogenesis in the Khammouane limestone of Carboniferous/Permian age. During the rainy season it drains the Ban Na polje towards a closed flat bottom karstic depression in the southern part of the Pha Soung Mountain and then downwards to the Mekong basin. Many of the entrances act not only as sinkholes, but seasonally also as large springs, which indicate a connection to the deep karst phreatic zone also situated under the Ban Na polje.

Tricula bannaensis sp. n.

Figs 43–44

Type locality

Laos; Khammouane Province, Ban Na village 20 km NNE of Thakhek, Tham Khon Dôn Cave, 17°33.82'N; 104°52.30'E,161 m a.s.l., Earthquake Dome 3 km from the south entrance, sand sediments on cave river banks (Fig. 2B).

Type material

Holotype: type locality: J. Grego and M. Olšavský leg. 11–12 February 2017 (NHMUK 20180014). Paratypes: type locality (NHMUK 20180024 – 5 specimens; HNHM 102777 – 5 specimens; OSUM 42389 – 5 specimens; Grego F0904 – 277 specimens).

Measurements

Holotype: H 1.90 mm; W 1.41 mm; BW 0.75 mm; BH 1.15 mm; AH 0.92 mm; AW 0.76 mm; H/W 1.35; AH/AW 1.21; W/BW 1.88; H/BH 1.65; H/AH 2.07; W/AW 1.86.

Diagnosis

The small hydrobioid shell is similar to that of T. phasoungensis sp. n., from which it differs by being more robust and with more inflated whorls, and by its larger umbilicus and larger aperture. Tricula lenahani sp. n. has a larger shell with less inflated whorls and a different shape of the aperture.

Description

The whitish, translucent shell has four inflated convex whorls with a deep suture. The shell is inflated ovoid-conical with an oval aperture and slightly outward reflexed margins. Its inner side is attached to the body whorl by a marginal callus. The labral lip is typically sinuated at its lateral profile. The umbilicus is open and conspicuous.

Etymology

Named after the village Ban Na, where the team enjoyed the hospitality of villagers in our base camp in the local Buddhist temple.

Distribution

Only known from the type locality and nearby sites in Tham Khon Dôn Cave as well as in the related source of Nam Dôn River.

Ecology

The same as Pseudoiglica pseudoiglica sp. n.

Tricula viengthongensis sp. n.

Figs 45–48

Type locality

Laos; Bolikhamsay Province, 16 km West of Vieng Thong, 500 m North of the road from Vieng Thong to Ban Samsok Noy (and Sôp Sang), unnamed cave with entrance above large karst spring with travertine cascades (Fig. 2D), 18°34.080'N; 104°31.79'E, sand sediments on the bottom of cave rivulet.

Type material

Holotype: type locality: J. Grego and B. Šmída leg. 19 February 2017 (NHMUK 20180015) Paratypes: , type locality (coll. Grego F0904 – 3 specimens).

Measurements

Holotype: H 2.64mm; W 1.64mm; BW 0.93mm; BH 1.71mm; AH 1.14mm; AW 1.00mm; H/W 1.6; AH/AW 1.14; W/BW 1.76; H/BH 1.54; H/AH 2.32; W/AW 1.64.

Diagnosis

The small conical shell is similar to that of T. valenasi sp. n. (Khammouane Province) from which it differs by its smaller size and its more blunt apex. From T. lenahani sp. n. (Khammouane Province) it differs by having a smaller shell with a weaker suture and a closed umbilicus. Tricula phasoungensis sp. n. and T. bannaensis sp. n. have a more robust and rounded shell with more convex whorls and an open umbilicus.

Description

The shell of Tricula viengthongensis sp. n. is whitish, semi-translucent with four slightly flattened convex whorls and a weak suture. The shape of the shell is conical with a blunt apex and an oval ear shaped aperture. The marginal lips with a slight callosity are not reflexed and laterally have a straight profile without any sinuation. The umbilicus is closed.

Etymology

Named after the city of Vieng Thong (Bolikhamsay Province), the larger settlement closest to the type locality.

Distribution

Only known from the type locality.

Ecology

The unnamed cave is situated immediately above a large karstic spring with travertine cascades (fig. 2D), and drains a limestone ridge NW of the main water outlet. The cave entrance is a 15m vertical abyss continuing upstream the underground river to a siphon lake. The total length is approximately 400 m with several parallel draining passages. The specimens were collected from sandy sediment at the bottom of a cave stream about 40 m upstream from the entrance.

Discussion

The present study indicates that the underground freshwater gastropod species are more widely distributed in the Southeast Asian habitats then was hitherto supposed. The wide diversity of karstic and cave habitats in the region (Culver 2012) together with the extraordinary high diversity of the surface and troglobiont gastropod species (Culver 2012) indirectly predicted rich stygobiont malacocenoses. We believe the habitat preferences and the factors driving their diversity are the same as we assumed for the stygobiont habitats of the Balkans (Grego et al. 2017) and subsequent SE Asian studies could prove their presence also in the local non-karstic springs, wells and adjacent groundwater systems. Additionally, the natural history of the area had not been influenced by the dramatic climate changes over the Late Cenozoic and Holocene and this may have facilitated uninterrupted diversification through evolutionary adaptation towards a very rich subterranean diversity.

The shell shape evolutionary convergence between the studied SE Asian species and stygobiont species from other parts of the world is remarkable. This indirectly confirms our theory (Grego et al. 2017), that the slender elongated shape could be evolved in the species preferably inhabiting habitats with permanently higher water velocity within the small caverns of gravel and cracklings. The shell shape of Iglica Wagner, 1910, Pseudoiglica gen. n. and Thamkhondonia gen. n. species provides lower frontal hydrodynamic resistance by turning the shell in the flow direction avoiding the shear stress caused by turbulences and thus preventing dislodging from the substrate. Gastropods with globose shell would have a problem to stay attached at high water velocity as also the adaptive evolution of gastropod mucus adhesion to the substrate has some limitations. Small species living interstitially among smaller gravel or sand have still elongated but proportionally shorter shells (Paladilhiopsis Pavlović, 1913, Thamkhondonia smidai sp. n., Tricula reischuetzorum sp. n.) to enable movement under the limitation by the smaller space. The species adapted to calmer waters within larger cavities such as cave lakes tend to have more globose shapes (Dabriana Radoman, 1974, Horatia Bourguignat, 1887, Pontohoratia Vinarsky et al., 2014, Motsametia Vinarsky et al., 2014, Tricula Benson, 1843). The species living in habitats with alternating water velocities (calm waters with frequent occasional floods and temporary very high water velocity) have developed their survival strategy by the globose, but more robust shell shape with more thick shell walls, additionally with broadening apertures (sometimes up to a limpet like shape). The expanded aperture has frequently folds and reflexions along the margin to help attachment of the animal on the rocky substrate during the interim periods with high water velocity. (Plagigeyria Tomlin, 1930, Tricula davisi sp. n., Tricula lenahani sp. n., Pseudotricula eberhardi Ponder, 1992)

As in the other well-known hotspots of stygobiont gastropod biodiversity (Pyrenees, Dinarides) (Darwall et al. 2014) the hidden, hardly accessible subterranean habitats only seldom allow researchers to obtain live specimens for anatomical and molecular studies (Glöer and Grego 2015). This hidden habitat was the main reason why the underground species had been overlooked for such a long time.

However, Laos as well as other SE Asian countries are facing the same ecological threats as most tropical countries worldwide: deforestation, drought, erosion and environmental pollution, together with artificial dam construction, which floods valleys and basins with stagnant water. These negative anthropogenic influences could negatively impact the more stable underground aquatic habitats and all of the sensitive stygobiont/troglobiont species could rapidly vanish prior to the scientists having a chance to fully understand their biology and role in the subterranean ecosystem.

Conclusions

This study confirmed the presence of underground freshwater gastropod species in Laos. Species inhabiting similar habitats were mainly known only from North and South America, North Africa, Europe and the Balkans through Turkey and the Caucasus to Central Asia (Bole and Velkovrh 1986, Kabat and Hershler 1993). The localities from Laos are situated between the single so far known locality in Sri Lanka (wells in the village of Pokonwita, south of Horana) and the few localities hitherto known throughout the Japanese Archipelago (Mori 1938; Kuroda and Habe 1957; Habe 1965; Kuroda 1963; Matsumoto 1976; Bole and Velkovrh 1986). The new finds partly fill the zoogeographical gap and also suggest their probable presence in a much wider area than hitherto supposed: from Naga Hills and Arunachal-Pradesh in India through Myanmar, Thailand, Laos and Vietnam to South China (YunNan and GuangXi) and likely northward to the central and east mainland of China and Korea. I hope this study will encourage all people involved in local biospeleological investigation throughout SE Asia to focus more towards the so far underestimated diversity of subterranean Mollusca and thus gather more knowledge about their biology, anatomy and phylogeny in the near future.

Acknowledgements

I thank all the people in the village Ban Na (Laos, Khammouane) for their hospitality and broad support during our stay in south Khammouane, with special thanks to the mayor of the village Khampong Saithavie, his family and Khamlex Khomxayxana for joining us in the cave explorations. I also thank Liviu Valenas and his wife Malivan (Nurnberg, Germany) for the excellent organization of our Khammouane field trip and research program in the caves of Pha Soung massif, to Cloude Mouret and Jean-Francois Vacquié for facilitating the visit to Tham Khon Dôn cave system. To Peter Lenahan (New York, USA) who, besides his intensive caving and research program, managed also to support the building of a permanent tap water supply to the village Ban Na. Special thanks also go to my friends from the Slovak Speleological Society: Branislav Šmída (Bratislava, Slovakia), Mário Olšavský (Banská Bystrica, Slovakia), Ondrej Kameniar (Ľubochňa, Slovakia), who joined our Laos 2017 field trip and patiently contributed to the Mollusca sampling. I also express my gratitude to our friend Gabriel Jakab (Plešivec, Slovakia) who sponsored our field investigations and to Thomas G. Watters (Ohio, USA) for reviewing the manuscript. Special thanks to Robert Cowie (Honolulu, Hawaii, USA) and Andrzej Faniowski (Krakow, Poland) for valuable comments during proofreading of the manuscript.

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