Aenigmatoconcha eunetis Pholyotha & Panha

Pholyotha, Arthit, Sutcharit, Chirasak, Tongkerd, Piyoros & Panha, Somsak, 2021, Systematic revision of the limestone karst-restricted land snail genus Aenigmatoconcha (Eupulmonata: Helicarionidae), with description of a new species, European Journal of Taxonomy 767, pp. 55-82 : 72-77

Aenigmatoconcha eunetis Pholyotha & Panha sp. nov.

urn:lsid:zoobank.org:act:98AD360F-93A3-4A8E-BE1A-5C991DBC358F

Figs 1 , 2F–H , 7E–F , 9 , 10D

Diagnosis

Shell medium-sized and pale yellowish white. Aperture ovate-lunate in shape and vertically open. Genitalia with very long and slender penis with many tiny conical penial pilasters inside. Radular teeth arranged in wide-angle U-shaped row, teeth with oblong shape, monocuspid.

Etymology

The specific name ʻ eunetis ʼ is from the Greek word meaning ʻspousesʼ, honouring the authors of genus Aenigmatoconcha , who are married.

Material examined

Holotype THAILAND • 1 sh (width 13.9 mm, height 7.4 mm); Uthai Thani Province, Lan Sak District, Limestone outcrops at Tham Namthip Bureau of Monks; 15°26′00.3″ N, 99°35′18.7″ E; CUMZ 7931 .

Paratypes THAILAND • 1 sh, 7 sp; same collection data as for holotype; CUMZ 7933 • 12 sp; same collection data as for holotype; CUMZ 7932 • 2 sh; same collection data as for holotype; NHMUK .

Other material

THAILAND • 1 sp; Uthai Thani Province, Lan Sak District, Limestone outcrops at Hup Pa Tat; 15°22′36.5″ N, 99°37′49.5″ E; CUMZ 7934 • 6 sp; same collection data as for preceding; CUMZ 7935 • 3 sh, 27 sp; Uthai Thani Province, Nong Chang District, Limestone outcrops at Wat Khao Bang Kraek; 15°18′07.9″ N, 99°41′04.5″ E; CUMZ 7936 .

Description

SHELL ( Fig. 7E–F ). Depressed to globosely depressed, medium-sized (width 12.6–14.2 mm, height 6.1– 7.1 mm), thin and translucent. Shell surface smooth, and polished, and pale yellowish white. Whorls: 5–5½, regularly increasing in size, separated by shallow suture. Spire rather elevated; varix usually present; last whorl well-rounded. Aperture very obliquely oval-lunate in shape; peristome simple. Columellar margin simple and slightly expanded near umbilicus. Umbilicus open and deep.

EXTERNAL FEATURES ( Fig. 2F–H ). Living snails with reticulated skin and pale yellowish to slightly dark grey body. Five well-developed mantle lobes: left and right shell lobes translucent, same colour as body, covered by tiny whitish dots, right shell lobe smaller than left shell lobe. Dorsal lobes broad and crescent-shaped: right dorsal lobe larger than both anterior and posterior left dorsal lobes. Caudal fossa present; caudal horn raised and rather large, and same colour as body.

GENITALIA ( Fig. 9 ). Atrium (at) enlarged and very short. Penis (p) very long and slender with rather thick penial sheath (ps) covering from atrium to almost middle of penis. Inner sculpture of penis with small conical penial pilasters (pp), and three prominent longitudinal folds along entire length of penis chamber. Epiphallus (e1 + e2) approximately half of penis length: e1 elongate and slender, and e2 very short and bulbous. Inner sculpture of e1 nearly smooth with small, thin, and longitudinal folds, and e2 densely papillate. Epiphallic caecum (ec) straight, short, and approximately as long as e2; penial retractor muscle (prm) thin and attached at tip. Flagellum (fl) small, rather short, and approximately as long as e2. Vas deferens (vd) very long, thin, and convoluted. Vagina (v) short, enlarged, thickened and cylindrical. Gametolytic sac (gs) short and bulbous; gametolytic duct (gd) long, enlarged near vagina then becoming smaller and very slender. Free oviduct (fo) long, cylindrical, and encircled with dense tissue near vagina.

RADULA ( Fig. 10D ). Teeth arranged in wide-angle U-shape with half row consisting of about 97–98 teeth at middle plate. Central tooth symmetrical monocuspid, elongated oblong-shaped with curved cusp, and slightly smaller than lateral tooth. Lateral and marginal teeth undifferentiated, asymmetrical monocuspid and elongated oblong-shaped with curved cusp; outermost teeth gradually becoming smaller.

Distribution

Aenigmatoconcha eunetis sp. nov. occurs in a few isolated limestone hills in Uthai Thani Province.This new species lives on limestone karsts, where snails tend to hide themselves in rock crevices and shelter during the daytime.

Remarks

Aenigmatoconcha eunetis sp. nov. clearly differs from all congeners in having 1) the longest penis, 2) penial internal sculpture consisting of longitudinal folds and small conical penial pilasters, and 3) inner sculpture of epiphallus (e2) densely papillae. For comparison, A. clivicola has oblique trapezoidshaped penial pilasters, A. mitis has oblique wrinkled penial pilasters with a longitudinal fold, and A. sumonthai has very small conical penial pilasters without longitudinal fold. In addition, for the inner sculpture of epiphallus (e2), A. clivicola has irregularly oblique trapezoid and small conical papillae. Aenigmatoconcha mitis and A. sumonthai have loose papillae arranged in oblique rows, but the former has relatively fewer rows and papillae are significantly larger in size than the latter. Moreover, radular teeth of only A. eunetis sp. nov. are arranged in wide-angle U-shaped row and oblong-shaped teeth, while radular teeth of other Aenigmatoconcha species are arranged in V-shaped rows and spatulate in shape.

Discussion

Each of the four Aenigmatoconcha species forms a well-defined clade in the COI phylogeny. Aenigmatoconcha can be divided into two groups based on the genital characters ( Figs 1–2 ; Table 2 ). Group I ( Fig. 2 ) has a long epiphallic caecum and short penial caecum, and contains one species, A. clivicola , which is restricted to northeasthern Thailand ( Fig. 1 ). Group II ( Fig. 2 ) has a short epiphallic caecum and no penial caecum; this group contains A. mitis , A. sumonthai , and A. eunetis sp. nov. This group occurs from central to southern Thailand along the Tenasserim Range ( Fig. 1 ). The COI tree showed very low nodal support for the relationships among the two Aenigmatoconcha groups and the other two helicarionid genera. Although the phylogenetic relationships among Aenigmatoconcha , Sophina and Chalepotaxis remain unresolved, the genital characters of both groups of Aenigmatoconcha were clearly distinct from Sophina and Chalepotaxis . In the phylogenetic tree of the 28S gene, moreover, these three genera were confirmed as different genera, but the systematic position of two Aenigmatoconcha species ( A. clivicola and A. sumonthai ) is unresolved ( Sutcharit et al. 2020a). Further research should include on more genes and more taxa of the Southeast Asian helicarionoids to better understand the phylogenetic relationships and morphological evolution of these groups.

Aenigmatoconcha can be distinguished from almost all other Southeast Asian helicarionoid genera by their unique milky to pale whitish-horny and umbilicate shell. Other helicarionoids with a whitish shell include Macrochlamys psyche Vermeulen et al., 2019 , Sarika lactoconcha Pholyotha & Panha, 2020 , and Sarika consepta (Benson, 1860) . However, the relatively medium to large-sized shells (shell width larger than 15 mm), narrowly perforate umbilicus, genital structure, and radula with triangular teeth of these three species clearly differentiate them from all Aenigmatoconcha species ( Vermeulen et al. 2019; Pholyotha et al. 2020 a, 2020b). The greatest similarity in shell characters occurred in Sophina and Chalepotaxis , but their genitalia are obviously different. The distinctive characters between Aenigmatoconcha and Chalepotaxis have been reported ( Tumpeesuwan & Tumpeesuwan 2017; Sutcharit et al. 2020a). However, Tumpeesuwan and Tumpeesuwan (2017) did not report the presence of a “penial sheath” and a “flagellum” in A. clivicola (the type species). In fact, they stated that the absence of a penial sheath in Aenigmatoconcha is a diagnostic difference between Aenigmatoconcha and Chalepotaxis . Yet, in the present work, the four Aenigmatoconcha species clearly exhibited a well-developed penial sheath and flagellum. Hence, the only difference between Chalepotaxis and Aenigmatoconcha is the presence of a flagellum in the latter ( Páll-Gergely et al. 2016). In addition to the genitalia without a dart apparatus, the presence of well-developed anterior and posterior left dorsal lobes is a significant character of Aenigmatoconcha , while Sophina has an undivided left dorsal lobe and a dart apparatus (Blanford & Godwin-Austen 1908; Sutcharit et al. 2020a). However, information on the mantle lobe morphology of Chalepotaxis was not available for comparison.

Among Southeast Asian helicarionoid genera with uniform and spatulate-shaped radula, Sophina is the only genus that a dart apparatus present in genitalia, but a flagellum absent. While genitalia of Aenigmatoconcha lack dart apparatus, but contain the flagellum, and genitalia of Chalepotaxis lack both organs. Generally, some characters of genital anatomy, such as a penial caecum, penial verge, dart apparatus, or flagellum, have been hypothesized to evolve repeatedly during the evolution of land snails, and have been noticed in many groups of terrestrial pulmonate snails ( Solem 1966; Hausdorf 1998; Hyman & Ponder 2010; Hirano et al. 2014; Köhler & Criscione 2015; Köhler et al. 2020; Sutcharit et al. 2020a). However, the classification of these three genera based on the presence or absence of the dart apparatus and flagellum is consistent with the molecular phylogeny ( Sutcharit et al. 2020a).

From the East of Tenasserim Range to northeastern Thailand, three Aenigmatoconcha species occurred in a few limestone karsts, while only A. mitis occurs in many limestone karsts from central to western Thailand ( Fig. 1 ). Aenigmatoconcha clivicola is confined to Loei Province, northeastern Thailand, while A. eunetis sp. nov. to Uthai Thani Province, central Thailand, and A. sumonthai to Chumphon Province, southern Thailand. The isolation explains the degree of endemism and the very high genetic divergence among sister lineages (9.7% to 12.0%) within Aenigmatoconcha . Regarding the West of Tenasserim Range, Sophina also reveals a high degree of endemism and localization with a pattern of one outcrop for one lineage in the Salween Basin, Southern Myanmar. These phenomena can be generally observed in karst-restricted animals because a very large number of ecological niches in karst ecosystems promote their evolutionary diversification and evolution of remarkably different lifestyles ( Clements et al. 2006; Foon et al. 2017; Grismer et al. 2020; Sutcharit et al. 2020a). Therefore, our discovery enhances the understanding of karst biodiversity and supplements the information on terrestrial snails in Thailand available for efforts to establish well-planned and knowledge-based conservation procedures for Thai limestone protection in the future.

Figures

Fig. 1. Geographic distribution of four species of Aenigmatoconcha Tumpeesuwan & Tumpeesuwan, 2017 based on specimens examined herein.

Fig. 2. Maximum likelihood tree of Aenigmatoconcha Tumpeesuwan & Tumpeesuwan, 2017 based on the COI gene dataset. Numbers on nodes indicate the ML bootstrap values (left) and Bayesian posterior probabilities (right) values. Pictures of living snails are not to scale.

Fig. 7. Shells of Aenigmatoconcha spp. A–D. A. sumonthai Tumpeesuwan & Tumpeesuwan, 2018. A–B. Specimen CUMZ 7922 from the type locality. C–D. Specimen CUMZ 7937 from the type locality showing the calcareous epiphragm with a small perforation (white arrows). Only shell (C) and living (D) specimens in both lateral and ventral views. E–F. A. eunetis Pholyotha & Panha sp. nov. E. Holotype CUMZ 7931. F. Paratype CUMZ 7933 from Uthai Thani Province.

Fig. 9. Genital system of Aenigmatoconcha eunetis Pholyotha & Panha sp. nov., paratype CUMZ 7933 from Uthai Thani Province. A. General view of genitalia. B. Internal structure of penis; inset showing penial sculpture. C. Internal structure of epiphallus and epiphallic caecum. White arrow indicates the junction between the penis and epiphallus.

Fig. 10. Radula morphology of Aenigmatoconcha spp. A. A. clivicola Tumpeesuwan & Tumpeesuwan, 2017, specimen CUMZ 7929 from Loei Province. B. A. mitis (Pfeiffer, 1863) comb. nov., specimen CUMZ 7687 from Kanchanaburi Province. C. A. sumonthai Tumpeesuwan & Tumpeesuwan, 2018, specimen CUMZ 7922 from Chumphon Province. D. A. eunetis Pholyotha & Panha sp. nov., paratype CUMZ 7933 from Uthai Thani Province. Central tooth in yellow. Scale bars: A–C = 100 μm; D = 50 μm.