Acinolaemus mueangonensis Changlom, Chan-ard & Dumrongrojwattana, 2019

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 mueangonensis Changlom, Chan-ard & Dumrongrojwattana, 2019

Figs 11 , 12 , 13 B, C , Table 4

Acinolaemus mueangonensis Changlom, Chan-ard & Dumrongrojwattana, 2019: 159–161, fig. 3. Type locality: Tham Mueang On [Mueang On Cave], Mae On District, Chiang Mai Province, Thailand.

Acinolaemus muangonensis [sic] — Changlom et al. 2019: 155 (abstract), 160 (figure caption).

Material examined.

Thailand • 8 shells (Fig. 11 A, B ); Mueang On Cave , Ban Sa Ha Khon, Mae On District, Chiang Mai Province; 18°47'13.2"N, 99°14'16.6"E; S. Panha leg.; CUMZ 15361 [type locality] . • 53 shells (Figs 12 A , 13 B, C ); Phra Phutthabat Doi Khao Nam , Ban Na Subdistrict, Sam Ngao District, Tak Province; 17°14'54.7"N, 98°56'17.1"E; P. Tongkerd leg.; CUMZ 14455.1 . • 1 shell (Fig. 12 B ); same data as preceding; CUMZ 14455.3 . • 10 specimens in ethanol (Fig. 12 C, D ); same data as preceding; CUMZ 14455.4 .

Diagnosis.

Shell concave-conical, brownish and with rectangular reticulated sculpture. Apertural dentitions nine, long and reaching peristome edge: three on parietal wall, three on palatal wall, one basal and one on columella wall. Palatal tubercle continuous with upper palatal plica.

Re-description.

Shell conical, concave-sided, brownish to pale brown; spire low to high, growing regularly, and sometimes slightly bent; last whorl expanded. Shell height 1.8–1.9 mm and shell width 1.7–1.8 mm. Apex blunt; protoconch spirally striated, ~ 2. Whorls ~ 5–6, rounded and convex; suture wide and well impressed. Shell surface sculptured with equidistantly spaced spiral striae, crossed with discontinuous and irregular radial growth lines making rectangular reticulated sculpture throughout shell. Penultimate whorl regularly coiled or sunken into last whorl. Last whorl bluntly shouldered and flattened below periphery. Aperture subcircular; peristome thickened and slightly expanded. Aperture with eight dentitions and with strong knobs reaching peristome edge. Parietal wall with three lamellae: parietal lamella large, outer part low, and inner part strongly developed and high, long deeper inside aperture; infraparietal lamella long and low; angular lamella prominent, high, long deeper inside aperture, and with slightly narrow and low ridge in middle. Palatal tubercle triangularly shaped, situated on peristome edge and connected to upper palatal plica. One or two tiny and low plicae (not reaching peristome edge) may be present in sinulus. Palatal wall with three plicae: upper-, inter-, and lower-palatal plicae ending on peristome edge as prominent tubercles, then continuing as narrow and low, but becoming strong deeper inside aperture. Basal plica low and long deeper inside aperture. Columellar lamella strong, high, and continuing deep inside aperture. Umbilicus widely perforate, ~ ⅓ of shell width, rounded and deep.

Living animal.

Snails are typically stylommatophoran with two pairs of tentacles. Upper tentacles are long, stout, cylindrical tubes, dark greyish, and with dark eye spots on the tip (Fig. 12 C, D ). Lower pairs are very short to knob-shaped and can be seen clearly in full extension in moving snails. Animal with short body, anterior-dorsal side with grey to blackish pigmentation, and posterior body and foot pale greyish to semi-translucent. The snails tend to cover their shell with soil, mud, or dirt.

Distribution.

Acinolaemus mueangonensis has a distribution beyond its type locality in northern Thailand ( Changlom et al. 2019), since during this study several specimens were collected in soil samples taken at the base of limestone cliffs and outcrops in Tak Province, ca 170 km south of the type locality.

Remarks.

The specimens from the type locality examined herein have shell sculpture and apertural dentition similar to the original description and the type specimens ( Changlom et al. 2019), except for the presence of a tiny plica in the sinulus, which was not mentioned in the original description.

The specimens from central Thailand ( Tak population) show the same distinguishing characters as the type specimens: a brownish shell, rectangular reticulated sculpture, and long denticles deep inside the aperture and becoming strong knobs when reaching the peristome edge. In addition, the parietal lamella has a low ridge near the peristome that becomes a tall ridge inside the aperture, and the angular lamella has a high ridge with a narrow and low ridged incision in the middle. However, the central Thailand population (Fig. 12 A, B ) differed from typical specimens (Fig. 11 A, B ) in having a more concave-sided shell, a more elevated and slender spire (height 2.1–2.2 mm and width 1.7–1.8 mm), and the penultimate whorl slightly sunken into the last whorl. Additionally, the central Thailand population has a basal plica instead of a subcolumellar lamella, while a typical shell has a subcolumellar lamella instead of a basal plica. However, the distinction between the subcolumellar lamella and the basal plica is sometimes difficult and not a reliable character for species distinction and is subject to intraspecific variability. Therefore, we provisionally recognise the central population as conspecific with A. mueangonensis , since no concrete difference in morphology could be observed. DNA sequence data are needed to resolve this ambiguity.

Figures

Figure 11. Acinolaemus mueangonensis from the type locality in Chiang Mai Province. A. Specimen CUMZ 15361 with enlarged aperture from three different angles; B. Protoconch and earlier whorl sculptures. An asterisk (*) indicates small lamella and plica situated in posterior chamber.

Figure 12. Acinolaemus mueangonensis from Tak Province. A. Specimen CUMZ 14455.1 under light microscope showing brownish shell and plicae; B. Specimen CUMZ 14455.3 with enlarged aperture from two different angles; C, D. Living snails CUMZ 14455.4: C. Snail crawling on Hypselostoma pendulum; D. Crawling on wet paper towel (shell width ~ 1.8 mm). An asterisk (*) indicates small lamella and plica situated in posterior chamber.

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.