Pseudoiglica pseudoiglica, Grego, Jozef, 2018

Grego, Jozef, 2018, First record of subterranean rissoidean gastropod assemblages in Southeast Asia (Mollusca, Gastropoda, Pomatiopsidae), Subterranean Biology 25, pp. 9-34 : 11-13

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.