Acinolaemus rhamphodontis 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 rhamphodontis Tongkerd & Panha sp. nov.

Figs 16 , 17 , Table 4

Type material examined.

HoIotype. Thailand • height 1.3 mm (including tuba), width 1.4 mm (Fig. 16 A, B ); Phra Phutthabat Doi Khao Nam , Ban Na subdistrict, Sam Ngao District, Tak Province; 17°14'56.4"N, 98°56'16.3"E; Tongkerd leg.; CUMZ 14449 . Paratypes. Thailand • 3 shells (Fig. 17 A – C ); same data as for holotype; CUMZ 14450 . • 1 adult + 1 juvenile in ethanol (Fig. 17 D – F ); same data as for holotype; CUMZ 14451 (COI accession number PV 698334 – PV 698335 ) . • 37 adults + 3 juveniles (COI accession nos. PV 698334 , PV 698335 ); same data as for holotype; CUMZ 14452 . • 2 shells; same data as for holotype; NHMUK 20250361 . • 2 shells; same data as for holotype; SMF .

Diagnosis.

Shell concave-conical, with long and descending tuba, peristome expanded. Apertural dentitions eight, all longer inside aperture: infra parietal, parietal, angular, upper- and lower-palatal, palatal tubercle, basal, subcolumellar lamella, and columellar lamella. Umbilicus wide.

Description.

Shell concave-conical, colourless; spire depressed and growing regularly; last whorl broadly expanded. Shell height 1.3–1.4 mm (including tuba) and shell width 1.3–1.5 mm. Apex large and rounded; protoconch ~ 2 whorls and with prominent spiral striae. Whorls ~ 4 (excluding tuba), rounded and convex; suture wide, well impressed, and deep. Shell surface sculptured with equidistantly spaced spiral striae (14–16 on body whorl in frontal view), crossed with discontinuous narrow radial growth lines making a rectangular reticulated sculpture throughout shell. Penultimate whorl regularly coiled; last whorl rounded; tuba long, ~ ¼ whorl or less, strongly descending and curving. Aperture subovate, open ventrally to subventrally; peristome thin, weakly expanded, and with thin depression area on parietal side. Apertural dentitions eight and all knob-shaped when reaching peristome edge. Parietal wall with three lamellae: parietal lamella long, outer part low, and then gradually becoming strong, thickened, and high inside aperture; infraparietal lamella evenly low; angular lamella strong. Palatal tubercle prominent, triangular, situated at peristome lip, and continuous with upper palatal plica. Palatal wall with two plicae: upper- and lower-palatal plicae connecting with peristome edge in form of prominent knobs, then continuing as narrow and low deeper inside aperture, eventually becoming stronger and higher at innermost ends. Basal plica narrow, and low. Columellar wall with two lamellae: columellar lamella prominent with tall ridge; subcolumellar lamella very small and low. Umbilicus widely opened, occupying ~ ½ of shell width and showing all preceding whorls.

Living animal.

Snail typically stylommatophoran with two pairs of tentacles. Upper tentacles are long, stout, cylindrical tubes, colourless to translucent, with dark eye spots on the tip. Lower pairs are very short to knob-shaped (difficult to observe in living snails). Animal with short body, anterior-dorsal side brownish while posterior body and foot paler to translucent. The snails tend to cover their shell with soil, mud, or dirt (Fig. 17 D – F ).

Distribution.

This new species is known only from the type locality, where the empty shells and one living snail were found in the soil, leaves, and twig litter at the base of a limestone cliff. The type locality is an island located in the reservoir of Bhumibol Dam, ca 45 km northwest of Tak Town. This island is ca 400 m long and 200 m wide and aligned north-south; the dry shells and the specimen were collected from the eastern slope of the island. The hills are low, with scattered land used by temples and with some exposed limestone rocks and cliffs. The vegetation on this island is dominated by low, dry, dipterocarp forest.

Differential diagnosis.

The shell of A. rhamphodontis sp. nov. is most similar to A. dayanum from Myanmar, and A. cryptidentatus , A. mueangonensis and A. ptychochilus from Thailand. They all share a rectangular reticulated shell sculpture, many long dentitions that reach the peristome edge where they form small denticles (except in A. cryptidentatus ) and have a palatal tubercle ( Thompson and Upatham 1997; Changlom et al. 2019; Tongkerd et al. 2024). The differences are that A. rhamphodontis sp. nov. has a long and descending tuba and a spire sunken into the last whorl. In contrast, the other four species have no tuba, and have a conical spire with straight to curved sides.

Acinolaemus rhamphodontis sp. nov. differs from A. carcharodon Vermeulen et al., 2007 and A. pyramidalis ( Vermeulen et al., 2007) from the Mekong Delta limestone hills in Vietnam, and A. rectus Vermeulen et al., 2019 from Cambodia in having a long and descending tuba, long dentitions that reach the peristome edge, and a rectangular reticulated shell surface. In comparison, these three species possess a very short tuba, prominent radial ridges, and dentitions that are short and situated inside to deep inside the aperture. Additionally, A. carcharodon has a sunken spire and four dentitions (parietal, hooked angular, palatal, and basal), A. pyramidalis has a conical spire and four to five dentitions (parietal, angular, two palatals, and a very inconspicuous columellar), and A. rectus has a conical spire, last whorl rounded, tuba slightly ascending and five dentitions (parietal, angular, two palatals, and columellar) ( Vermeulen et al. 2007, 2019).

This new species has a long descending tuba similar to several species in the Clostophis sankeyi species group. It differs by having rectangular reticulated shell sculpture, with eight apertural dentitions (parietal, infra parietal, angular, two palatals, basal, and two columellar) reaching the peristome edge, and with a palatal tubercle present. Furthermore, A. rhamphodontis sp. nov. has 14–16 dash-like spiral striae on the last whorl and the penultimate whorl not sunken, whereas C. sankeyi , C. proboscideus , C. yoga , and C. udayaditinus have 18–20 or more continuous spiral striae on the last whorl, and the penultimate whorl sunken into the last whorl. Clostophis yoga also has a narrower umbilicus, <1 / 3 of the shell width, than the new species ( Benson 1860; Páll-Gergely et al. 2020; Páll-Gergely and Hunyadi 2022; Sutcharit et al. 2025). Finally, A. rhamphodontis sp. nov. also differs from C. rhynchotes sp. nov. by having a rectangular reticulated shell surface and dash-like spiral striae on the last whorl, while C. rhynchotes sp. nov. has a weak reticulated shell surface and continuous spiral striae on the last whorl.

Etymology.

The specific name rhamphodontis is from two Greek words: rhamphos meaning ‘ curving beak’ and odontos meaning ‘ tooth’. Together they refer to the tuba that curves downward and the many dentitions in the aperture.

Remarks.

The presence of a rectangular reticulated shell sculpture and the many dentitions reaching the peristome edge where they form small denticles and extend inside the aperture, clearly position A. rhamphodontis sp. nov. within the genus Acinolaemus ( Thompson and Upatham 1997) .

Figures

Figure 16. Acinolaemus rhamphodontis sp. nov., holotype CUMZ 14449 from Tak Province. A. Shell with enlarged aperture from two different angles; B. Protoconch and earlier whorl sculptures.

Figure 17. Acinolaemus rhamphodontis sp. nov., from Tak Province. A. Paratype CUMZ 14450 with enlarged aperture; B. Juvenile paratype CUMZ 14450 from the type locality; C. Natural broken shell with dentition on parietal and palatal walls as seen from inside the shell; D – F. Specimen CUMZ 14451 of living snails crawling on wet paper towel (different angles of same individual; shell width ~ 1.4 mm).