Acinolaemus corusticorus Tongkerd & Panha, 2025

Tongkerd, Piyoros, Janjai, Teerangkul, Pholyotha, Arthit, Gojšina, Vukašin, Panha, Somsak & Sutcharit, Chirasak, 2025, The microsnail genera Clostophis and Acinolaemus (Eupulmonata, Pupilloidea, Hypselostomatidae) from central Thailand, with description of three new species, ZooKeys 1258, pp. 35-71 : 35-71

Acinolaemus corusticorus Tongkerd & Panha sp. nov.

Figs 13 D, E , 14 , 15 , Table 4

Type material.

Holotype. Thailand • height 2.2 mm, width 1.9 mm (Fig. 14 A, B ); Phra Phutthabat Doi Khao Nam , Ban Na, Sam Ngao District, Tak Province; 17°14'54.7"N, 98°56'17.1"E; P. Tongkerd leg.; CUMZ 15363.1 . Paratypes. Thailand • 2 shells (Fig. 15 A, B ); same data as for holotype; CUMZ 15363.2 . • 45 shells (Fig. 13 D, E ); same data as for holotype; CUMZ 14455.2 . • 2 shells; same data as for holotype; NHMUK 20250360 . • 2 shells; same data as for holotype; SMF .

Diagnosis.

Shell concave-conical, colourless, and with rectangular reticulations. Nine apertural dentitions, reaching peristome edge: three on parietal wall, palatal tubercle, four on palatal wall, and two on columellar wall. Palatal tubercle continuous with upper palatal plica.

Description.

Shell concave-conical, colourless; spire high and growing regularly; last whorl expanded. Shell height 2.1–2.3 mm and shell width 1.8–1.9 mm. Apex blunt; protoconch ~ 2 whorls with fine spiral striae. Whorls 4–5, rounded and convex; suture wide and well impressed. Shell surface sculptured with equidistantly spaced spiral striae, crossed with irregular radial growth lines making a rectangular reticulated sculpture throughout shell. Penultimate whorl regularly coiled; last whorl bluntly shouldered and flattened below periphery. Aperture subovate; peristome thickened and slightly expanded. Aperture with nine or ten dentitions and with more or less strong knobs reaching peristome edge. Parietal wall with three lamellae: parietal lamella large, strongly developed, and high and long deeper inside aperture; infraparietal lamella long and low; angular lamella relatively smaller than parietal lamella, long, low near peristome edge, somewhat sinuous, and higher deeper inside aperture. Palatal tubercle strongly developed with triangular shape and continuous with upper palatal plica. Palatal wall with four plicae: upper-, inter-, lower-, and infra-palatal plicae connecting with peristome in the form of prominent tubercles then continuing as narrow and low, becoming strong and high inside aperture. Inter- and lower-palatal plicae much larger than upper palatal plica, infrapalatal plica smallest or may be absent (Fig. 15 A ). Basal plica may be present with small and low ridge. Columellar wall with two lamellae: columellar lamella strong and distinct, continuing deep inside aperture; subcolumellar lamella small, low. Umbilicus widely perforate, ~ ⅓ of shell width, rounded and deep.

Differential diagnosis.

This new species can be distinguished from A. cryptidentatus from northern Thailand by having major dentitions (on parietal, palatal, and columellar walls) that become strong knobs when reaching peristome edge, palatal tubercle continuous with upper palatal plica, and without tiny plicae inside the sinulus. In comparison, A. cryptidentatus possesses dentitions that do not reach the peristome edge, a palatal tubercle situated between upper- and inter-palatal plicae, and with two tiny plicae inside the sinulus.

Acinolaemus corusticorus sp. nov. is similar to A. dayanum ( Stoliczka, 1871) from Myanmar and A. mueangonensis from northern Thailand in shell form and sculpture. It differs by having a colourless shell without cervical crest (a swelling or convex ridge on the last whorl behind the expanded lip); parietal lamella has a thick and high ridge; angular lamella has a continuously high ridge and without incision; palatal plicae short (<½ of last whorl length when seen from lateral view; Fig. 13 D, E ), palatal plicae continue from knobs on peristome edge with short, narrow, and low ridges, and then becoming high inside the aperture. For comparison, A. dayanum and A. mueangonensis have palatal plicae continuing from peristome knobs, which are long and narrow but low ridges before becoming folds inside aperture. Acinolaemus mueangonensis possesses a brownish and conical to elevated conical shell; parietal lamella has a low ridge near the peristome then becoming a tall ridge inside; angular lamella has a high ridge with narrow and low ridge in middle; palatal plicae long (> ½ of last whorl length when seen from lateral view; Fig. 13 B, C ). Acinolaemus dayanum has a conical low spire with a cervical crest (a swelling or convex ridge on the last whorl behind the expanded lip); parietal lamella has a low ridge near peristome edge then gradually becoming a tall ridge inside; angular lamella has a high ridge near peristome edge and then gradually becoming a low ridge inside aperture.

Acinolaemus ptychochilus from northern Thailand also clearly differs from A. corusticorus sp. nov. in shape of parietal and angular lamellae, and in having four palatal plicae (upper-, inter-, hooked lower-, and infra-), a small basal plica, and three columellar lamellae; palatal tubercle situated between upper- and inter- palatal plicae. Acinolaemus rhamphodon Thompson & Upatham, 1997 and A. stenopus Thompson & Upatham, 1997 differ from A. corusticorus sp. nov. in having fewer dentitions, a very strong angular lamella, and a weak parietal lamella. Both species also possess two palatal plicae (upper- and lower-), and a columellar lamella. Finally, A. rhamphodon has a hooked columellar lamella, while A. stenopus has an elevated shell with a high spire, and an enlarged angular lamella and palatal plica that nearly enclose the sinulus.

Distribution.

Acinolaemus corusticorus sp. nov. is known only from the type locality. At this locality, the species is sympatric with five other hypselostomatid species: Hypselostoma pendulum ( Panha & Burch, 2002) , H. khaowongensis Panha, 1998 , A. mueangonensis , A. rhamphodontis sp. nov., and Krobylos takensis Panha & Burch, 2004 ( Panha 1998 b; Panha et al 2004).

Etymology.

The specific name corusticorus is from two Latin words co- meaning ‘ together or with’ and rusticor meaning ‘ living in the country’, referring to the new species being sympatric with two other congeners.

Remarks.

Although this new species occurs sympatrically with A. mueangonensis , under the microscope it is evident that they are two distinct species. Acinolaemus corusticorus sp. nov. differs from sympatric congeners by having a colourless, short, and stout shell (Fig. 13 A ), while A. mueangonensis has a much smaller and more slender shell, and with pale brownish to brownish shell colour. In addition, these two species are clearly distinct in number and morphology of apertural dentitions.

Figures

Figure 13. A. Synoptic photo of sympatric Acinolaemus species, specimens sorted from soil sample collected at Tak Province; B, C. Acinolaemus mueangonensis, specimen CUMZ 14455.1 with brownish shell in lateral and apertural views showing long plicae visible throughout the translucent shell; D, E. Acinolaemus corusticorus sp. nov., paratype CUMZ 14455.2 with colourless shell in lateral and apertural views showing short plicae visible throughout the translucent shell.

Figure 14. Acinolaemus corusticorus sp. nov. from Tak Province. A, B. Holotype CUMZ 15363.1 with enlarged aperture from three different angles and B. Protoconch and earlier whorl sculptures.

Figure 15. Acinolaemus corusticorus sp. nov. from the type locality. A. Paratype CUMZ 15363.2 with enlarged aperture from two different angles (without basal plica); B. Paratype CUMZ 15363.2 with enlarged aperture from two different angles (with basal plica).