Tricula phasoungensis, 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 : 25-26

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.