Unveiling the Diversity of the Semislug Genus Durgella Blanford, 1863 (Eupulmonata: Helicarionidae) from Thailand and Myanmar, with Description of Two New Species

© 2024 Academia Sinica, Taiwan Open Access Unveiling the Diversity of the Semislug Genus Durgella Blanford, 1863 (Eupulmonata: Helicarionidae) from Thailand and Myanmar, with Description of Two New Species Arthit Pholyotha1, Chirasak Sutcharit1, Ngwe Lwin2, and Somsak Panha1,3,* 1Animal Systematics Research Unit, Department of Biology, Faculty of Science, Chulalongkorn University, Bangkok, Thailand. E-mail: [email protected] (Pholyotha); [email protected] (Sutcharit) 2Fauna and Flora International, Sanchaung Township, Yangon, Myanmar. E-mail: [email protected] (Lwin) 3Academy of Science, The Royal Society of Thailand, Bangkok, Thailand. *Correspondence: E-mail: [email protected] (Panha) Received 14 February 2023 / Accepted 21 February 2024 / Published 11 June 2024 Communicated by Benny K.K. Chan Durgella is a terrestrial semislug genus in the family Helicarionidae and currently comprises nine species recorded from Thailand and Myanmar. Two species, D. concinna and D. rhaphiellus, have been described based only on shell information, while the taxonomy of the remaining seven species is comprehensively treated herein using comparative morphology. Revised species descriptions are given for D. levicula, D. erratica, D. siamensis, and D. libas; D. birmanica (previously placed in the Megaustenia) is moved to this genus; and two species, D. pentata Pholyotha & Panha, sp. nov. and D. nulla Pholyotha & Panha, sp. nov., are described as new to science. Based on our findings, the combination of shell characters including shape, size, aperture, and umbilicus; the number of mantle extensions; and the genitalia, especially the penis, epiphallus, and dart apparatus, can be used to distinguish these nine species. Among these nine recognised species, only D. nulla Pholyotha & Panha, sp. nov. has no dart apparatus. Key words: Comparative morphology, Durgellinae, Indochina, Land snail, Systematics, Taxonomic revision BACKGROUND The semislug genus Durgella Blanford, 1863, also known as ‘dancing semislugs’, is the type genus of the Durgellinae Godwin-Austen, 1888, a subfamily of the enormously diverse limacoid family Helicarionidae Bourguignat, 1877. According to MolluscaBase (2022), the genus currently comprises 21 nominal species. Members of this genus are tree-dwellers that occupy various tree-associated microhabitats such as tree trunks and leaves in tropical forests (Blanford and GodwinAusten 1908). They are distributed from the subtropical forests of eastern India to the tropical or dry forests of Indochina, plus a few records from the Greater Sunda Islands (Blanford and Godwin-Austen 1908; Solem 1966; Schileyko 2002; Vermeulen and Liew 2022). Historically, the genus name Durgella was introduced without a description (Blanford 1863), and so its diagnostic characters are subject to validation. As is the case for several other groups of semislugs, the shell morphology is conservative compared to the other taxonomically informative structures, especially the reproductive organs, mantle edge development, and radula (Blanford and Godwin-Austen 1908; Solem 1966; Pholyotha et al. 2021a b). Godwin-Austen (1881) identified these structures by demarcating the genus based on the revision of its type species, D. levicula (Benson, 1859). Therefore, the genus is characterised by Citation: Pholyotha A, Sutcharit C, Lwin N, Panha S. 2024. Unveiling the diversity of the semislug genus Durgella Blanford, 1863 (Eupulmonata: Helicarionidae) from Thailand and Myanmar, with description of two new species. Zool Stud 63:14. doi:10.6620/ZS.2024.63-14. Zoological Studies 63:14 (2024) doi:10.6620/ZS.2024.63-14 1 © 2024 Academia Sinica, Taiwan a very thin to membranous and vitriniform shell, a welldeveloped mantle, and a pectiniform radula (GodwinAusten 1881; Blanford and Godwin-Austen 1908; Solem 1966). Since then, six species from Thailand and Myanmar and 15 species from other regions have been described or re-located to the genus. However, the genital anatomy of only five species (D. levicula, D. assamica Godwin-Austen, 1881, D. mairangensis Godwin-Austen, 1898, D. rogersi Godwin-Austen, 1907, and D. libas Solem, 1966) has been examined (Godwin-Austen 1881; Blanford and GodwinAusten 1908; Solem 1966). The remaining taxa were provisionally assigned to this genus based solely on their shell traits, which are less informative. Since data on the genital anatomy and radular morphology of Durgella are scarce, the genus has been largely neglected for over a century. Thailand and Myanmar are located in the IndoBurma biodiversity hotspot (Myers et al. 2000), and are home to various endemic limacoid semislug taxa., i.e., Durgella, Cryptosemelus Collinge, 1902, Parmarion Fischer, 1855, Megaustenia Cockerell, 1912, and Muangnua Solem, 1966 (Blanford and GodwinAusten 1908; Solem 1966; Hemmen and Hemmen 2001; Pholyotha et al. 2021a). These semislugs are often encountered in various habitats, especially limestone forests throughout Thailand; numerous ethanol-preserved specimens have been deposited in the Chulalongkorn University Museum of Zoology (CUMZ). Additionally, our 2015–2016 land snail survey in Myanmar, initiated by Fauna & Flora International (FFI), yielded many fresh specimens of durgellinids, which were also deposited in this museum. The newly collected material has prompted the taxonomic revision of all Durgella species recognised in Thailand and Myanmar. This study also provides new information on the mantle morphology, reproductive anatomy, and radulae of these species, together with their distribution. In addition, during our examination of the material, two new Durgella species from Thailand were encountered, and are herein described as new to science. MATERIALS AND METHODS Durgella specimens from Thailand and Myanmar were sampled from several localities including limestone and non-limestone habitats throughout the known distribution range of the genus (Fig. 1). The specimens from Myanmar were collected during 2015 to 2016 under an MoU between the Forest Department, the Ministry of Natural Resources and Environmental Conservation and Forestry, Myanmar, and FFI. The semislugs were obtained by direct visual searching and hand collecting in favourable microsites, such as under leaves, trunks or branches. Prior to preservation, images of living specimens were taken using a Nikon camera (DSLR D850) with a Nikon 105 Macro lens (AFS VR Micro-Nikkor 105 mm f/2.8G IF-ED). Living specimens were euthanised following the standard two-step protocol (American Veterinary Medical Association 2020), and then fixed in 95% (v/v) ethanol for further work. Identification of Durgella species was based on the original literature and several related publications (Godwin-Austen 1881 1883 1898 1907 1910; Blanford and Godwin-Austen 1908; Solem 1966), and comparisons with available type specimens and the reference collection. For shell morphology, adult specimens were measured for size, and whorls were counted using a stereoscopic light microscope. Dissections were performed on three to ten ethanolpreserved specimens of each species to study their external morphology and genital anatomy. Photographs of representative shells and genitalia were taken using the Nikon DSLR D850 with 105 mm macro lens, while the inner anatomy of the genitalia was imaged using a stereo microscope with the Cell’D Imaging Software. Radulae were prepared as previously described (Pholyotha et al. 2021a) and then observed and imaged using scanning electron microscopy (SEM; JEOL, JSM-6610 LV). All voucher specimens of Durgella spp. examined in this study have been deposited in the Chulalongkorn University Museum of Zoology (CUMZ), Bangkok, Thailand. Morphological and anatomical abbreviations In this study, the term ‘proximal’ refers to the region closest to the genital opening, while ‘distal’ refers to the region furthest away from the genital opening. Abbreviations used in images: ant-ldl (anterior left dorsal lobe), at (atrium), cf (caudal foss), ch (caudal horn), da (dart apparatus), e1 (portion of epiphallus nearer to penis), e2 (portion of epiphallus nearer to retractor muscle), ec (epiphallic caecum), fo (free oviduct), gd (gametolytic duct), gs (gametolytic sac), lsl (left shell lobe), p (penis), post-ldl (posterior left dorsal lobe), prm (penial retractor muscle), rdl (right dorsal lobe), rsl (right shell lobe), v (vagina), vd (vas deferens). Institutional abbreviations: CUMZ, Chulalongkorn University, Museum of Zoology (Bangkok, Thailand); NHMUK, when citing lots deposited in The Natural History Museum (NHM, London, United Kingdom); SMF, Senckenberg Forschungsinstitut und Naturmuseum (Frankfurt am Main, Germany); ZMB, Museum für Naturkunde (Berlin, Germany). page 2 of 29 Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan RESULTS TAXONOMY Superfamily Helicarionoidea Bourguignat, 1877 Family Helicarionidae Bourguignat, 1877 Subfamily Durgellinae Godwin-Austen, 1888 Genus Durgella Blanford, 1863 Nanina (Durgella) Blanford, 1863: 76. Macrochlamys (Durgella) – Stoliczka 1871: 248. Durgella – Godwin-Austen 1881: 291. Godwin-Austen 1883: 142. Godwin-Austen 1898: 60. Godwin-Austen 1907: 205. Blanford and Godwin-Austen 1908: 213. Thiele 1931: 1007. Zilch 1959: 327. Solem 1966: 49. Vaught 1989: 98. Schileyko 2002: 1297. Type species: Helix levicula Benson, 1859, subsequent designation by Stoliczka (1871: 250). Description: Shell subglobose to globose, imperforate to perforate, small to medium sized, thin to fairly solid, polished and pale to dark yellowish. Whorls 3–4½ rapidly increasing, smooth or with microscopic sculpture on protoconch and teleoconch. Aperture large and oblique. Fig. 1. Map of Thailand and Myanmar showing the geographic distribution of eight species of Durgella based on the specimens examined herein. Only D. rhaphiellus is not shown on the map, as its type locality of “Siam” [Thailand] is too imprecise. N page 3 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Mantle extensions partially covering shell when fully extended; right shell lobe (rsl) broad and triangular; left shell lobe (lsl) triangular in shape, varying in width from narrow to broad, and reflected over the shell near the respiratory orifice (Figs. 2–4). Right dorsal lobe (rdl) broad, large, crescent shaped, and reflecting to partially cover the shell. Left dorsal lobe (ldl) long, large, crescent shaped, and undivided (Fig. 2A, B) or divided (Fig. 2C, D) into anterior left dorsal lobe (ant-ldl) and posterior left dorsal lobe (postldl). Fig. 2. Synoptic illustration of mantle extensions of the genus Durgella. A, B: four mantle extensions with left dorsal lobe undivided. C, D: five mantle extensions with left dorsal lobe divided into anterior and posterior left dorsal lobes. White arrow indicates pneumostome or breathing pore. page 4 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Genitalia with thin penial sheath, penial retractor muscle attached to epiphallic caecum, very short vagina, and short to long gametolytic duct. Flagellum absent. Dart apparatus well-developed or absent. Radular teeth arranged in a nearly straight line and gradually developing a curved edge. Central tooth generally reduced with small unicuspid, bicuspid or asymmetrical tricuspid. Lateral and marginal teeth undifferentiated, excessively numerous, elongated bicuspid with several tiny accessory ectoconal cusps on the outer edge (or pectiniform). In life, anterior body varying in colour from pale yellowish-brown to blackish; posterior body generally yellowish-brown to reddish-brown, and with a black gradient that is darkest at the dorsal and lightest at the foot margin. Mantle extensions thin, transparent, and varying in colour from cream to yellowish, reddishbrown, or dark grey. Sole tripartite and lateral foot margin present. Caudal foss (cf) indented. Caudal horn (ch) long, large, and greatly overhung (Figs. 2A, C, 3, 4). Habitat and behavior observations: After rainfall, these semislugs actively crawl on the leaves of plants, trunks or branches of bamboo, banana leaves, shrubs, or small trees. They also perform a tail flicking action when touched or disturbed; then, they often drop to the ground or are suspended in air by a long strand of sticky mucus from the posterior body to leaves or branches. This is probably an anti-predator behaviour that has also been observed in D. mairangensis Godwin-Austen, 1898 and D. salius (Benson, 1859), a species endemic to India (Godwin-Austen 1898; Blanford and GodwinAusten 1908). In the resting stage or during aestivation, the semislugs normally attach themselves to the leaves or stems of plants by entirely withdrawing their body into the protective shell (Fig. 3C). Many copulating pairs of D. libas, D. pentata sp. nov., and D. nulla sp. nov. were observed (Fig. 4D) in the humid and moist conditions of the rainy season, and they clearly exhibited reciprocal copulation. No direct observations of egg-laying were made in our survey, but many individuals produced clutches of 20–40 eggs within our mesh collecting bags; the eggs were translucent, whitish, soft, and oval to spherical shaped (Fig. 4A–C). Remarks: The name Durgella was first proposed by W.T. Blanford (1863: 76) as a subgenus of Nanina Gray, 1834 without description or designation of a type species. Instead he only mentioned that it contained three nominal species, namely Helix levicula Benson, 1859, Helix mucosa Blanford & Blanford, 1861, and Helix seposita Benson, 1859. Later, F. Stoliczka (1871) re-described Helix honesta Gould, 1846 from Myanmar, and he also recognised Durgella as a subgenus of Macrochlamys Gray, 1847, and consecutively designated Helix levicula as the type. However, the taxonomic treatment in Stoliczka (1871) was not accepted and has never been used. Later, GodwinAusten (1881: 291) re-described D. levicula and discovered several unique characters from both Nanina and Macrochlamys; therefore, he raised Durgella as a distinct genus. This interpretation of the genus has been widely accepted (e.g., Blanford and Godwin-Austen 1908; Thiele 1931; Zilch 1959; Solem 1966; Schileyko 2002; Preece et al. 2022). Concerning the type species designation, G. Nevill (1878: 24) erroneously designated Helix honesta Gould, 1846 as the type species. This designation is invalid because it does not involve an originally included nominal species (Blanford 1863; ICZN 1999: Art. 69). Durgella birmanica (Pfeiffer, 1847) (Figs. 1, 3A, 5A–C, 6A, 9A, B, 11A, B) Vitrina birmanica Pfeiffer, 1847: 65. Type locality: Mergui imperii Birmanorum [Myeik District, Tanintharyi Region, Myanmar]. Pfeiffer 1848: 498. Reeve 1862: Vitrina pl. 8, fig. 59. Hanley and Theobald 1876: 61, pl. 152, fig. 7. Helicarion birmanicus [sic] – Tryon 1885: 177, pl. 41, fig. 42. Cryptosoma ? birmanicum – Blanford and Godwin-Austen 1908: 212. Megaustenia birmanicum – Solem 1966: 78. Material examined: The type specimen could not be located. MYANMAR: Phra (Buddha) Cave, Tanintharyi Township, Tanintharyi Region (11°13'46.2"N, 99°10'34.3"E): CUMZ 14233 (4 preserved specimens). Diagnosis: Shell globose, fragile, polished, and dark yellowish; animal with five mantle extensions; genitalia with very short vagina, short epiphallic caecum, moderate gametolytic duct, and small dart apparatus. Description: Shell (Figs. 5A–C, 9A, B). Shell globose, small to slightly medium sized (width up to 11.0 mm, height up to 5.4 mm), transparent, very thin, fragile, glassy, and dark yellowish or yellowishcorneous. Surface of protoconch smooth (Fig. 9A, B); surface of body whorl smooth. Spire slightly elevated; suture slightly impressed. Whorls 3¼–4, convex and rapidly increasing; last whorl enlarged and wellrounded. Aperture oblique, diagonal, roundly lunate, width greater than height; peristome simple. Columellar margin thin and little expanded near umbilicus. Umbilicus minute. Genital organs (Fig. 6A). Atrium (at) enlarged and very short. Penis (p) long, cylindrical, and with thin penial sheath encircling about half of penis length. Epiphallus (e1 + e2) shorter than penis; e1 more slender and narrower than penis; e2 bulbous in shape and shorter than e1. Epiphallic caecum (ec) short and small; page 5 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 3. Living snails of seven Durgella species. A: D. birmanica CUMZ 14233. B, C: D. levicula; B: CUMZ 14248; C: CUMZ 14250. D: D. erratica CUMZ 14234. E: D. siamensis CUMZ 14232. F: D. libas CUMZ 14237. G: D. pentata sp. nov. paratype CUMZ 14240. H: D. nulla sp. nov. paratype CUMZ 14228. page 6 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan penial retractor muscle (prm) thin and attached at tip of epiphallic caecum. Vas deferens (vd) long and slender tube. Vagina (v) very short or inconspicuous. Dart apparatus (da) small, long cylindrical, and connected to atrium chamber. Gametolytic duct (gd) slightly shorter than penis, and enlarged at base with conical shape; gametolytic sac (gs) bulbous in shape. Free oviduct (fo) slightly shorter than penis; proximal part enlarged and bulbous; distal part slender and long cylindrical. Oviduct enlarged lobules; prostate gland running alongside oviduct. Radula (Fig. 11A, B). Teeth numerous and pectiniform, arranged in wide-angled V-shaped rows, half row ca. 420 teeth. Central tooth reduced with two claw-shaped cusps. Lateromarginal teeth elongated bicuspid, outer edge serrated with three to six prominent cusps, diminishing gradually in size outwards. Outermost teeth slightly shorter than inner teeth. External appearance (Fig. 3A). Animal with blackish body and light yellow near foot margin. Five mantle extensions well-developed and fleshy grey. Caudal horn raised, large, and blackish. Distribution: This species is known from a single locality only at Phra (Buddha) Cave, Tanintharyi Region, Myanmar (Fig. 1). Remarks: Concerning the authorship of this nominal name, Pfeiffer (1847) stated ‘Vitrina birmanica Phil.’, and the specimen was from the R.A. Philippi collection. However, there is no clear evidence that Philippi provided him with the species name and description. Therefore, Pfeiffer (1847) is the sole author of this nominal name (ICZN 1999: Arts 50.1, 50.1.1). Previous works placed this species in either the semislug genus Helicarion Férussac, 1821 or in Megaustenia, which is a replacement name of Cryptosoma Theobald, 1857 (Tryon 1885; Blanford and Godwin-Austen 1908; Solem 1966). However, Helicarion is mainly distributed in Australia (Schileyko 2002), while Megaustenia is native to mainland Southeast Asia (Schileyko 2002). Based on the shell characters, D. birmanica is distinct from all Megaustenia species because it has a small shell (9 mm) with 4 whorls, whereas Megaustenia spp. have much larger shells (18–25 mm) with fewer than 4 whorls Fig. 4. Eggs and mating behavior of Durgella species recorded in the field. A: eggs of D. libas from Wachirathan Waterfall, Chiang Mai Province. B: eggs of D. pentata sp. nov. from Tham Chiang Dao, Chiang Mai Province. C: eggs of D. nulla sp. nov. from Phu Pha Lom, Loei Province. D: mating pairs of D. nulla sp. nov. from Phu Pha Lom, Loei Province. page 7 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan (Table 1; Blanford and Godwin-Austen 1908; Solem 1966). Several semislug specimens collected from the Tanintharyi Region (approximate type locality) in this study conform to the shell characteristics of D. birmanica. Our morphological study of shell sculpture, mantle morphology, genital anatomy, and radular morphology suggests that this species belongs to the genus Durgella. Durgella levicula (Benson, 1859) (Figs. 1, 3B, C, 5D–F, 6B, C, 9C, D, 11C, D) Helix levicula Benson, 1859: 391. Type locality: Phie Than, raro occurrens [Payathonzu or Phaya Thone Zu, Kayin State, Myanmar]. Pfeiffer 1868: 48. Stoliczka 1871: 250. Hanley and Theobald 1876: 37, pl. 90, figs. 1, 4. Nanina (Durgella) levicula: Blanford 1863: 81. Tryon 1886: 111, pl. 37, figs. 8, 9. Nanina levicula – Nevill 1878: 26. Durgella levicula – Godwin-Austen 1881: 291, pl. 20, figs. 1, 2 (shell), pl. 21, figs. 1, 3, 3a, 5 (anatomy). Godwin-Austen 1898: 61, pl. 76, fig. 1, 1a, 1b. Blanford and Godwin-Austen 1908: 213, fig. 75. Solem 1966: 49. Preece et al. 2022: 167, fig. 75a. Material examined: Holotype NHMUK 1888.12.4.506 from Tenasserim (figured in Preece et al. 2022). MYANMAR: Mule-ik Range, Tenasserim [Mulayit Taung (Hill), Kayin State, Myanmar]: NHMUK 1903.7.1.785 (18 shells). THAILAND: Khao Wang Khamen, Sai Yok District, Kanchanaburi Province (14°23'18.4"N, 98°55'30.5"E): CUMZ 14245 (10 preserved specimens). Sai Yok waterfall, Sai Yok District, Kanchanaburi Province (14°26'01.8"N, 98°50'53.0"E): CUMZ 14246 (3 preserved specimens). Wat Sunantha Wanaram, Sai Yok District, Kanchanaburi Province (14°32'05.1"N, 98°49'43.4"E): CUMZ 14247 (3 preserved specimens). Tham Than Lot, Si Sawat District, Kanchanaburi Province (14°39'44.8"N, 99°18'20.2"E): CUMZ 14248 (11 preserved specimens). Wat Tham Ong Chu School, Si Sawat District, Kanchanaburi Province (14°48'05.3"N, 99°05'03.6"E): CUMZ 14249 (1 preserved specimen). Sahakon Nikhom, Thong Pha Phum District, Kanchanaburi Province (14°44'35.7"N, 98°49'06.2"E): CUMZ 14250 (1 preserved specimen). Wat Tha Khanun, Thong Pha Phum District, Kanchanaburi Province (14°44'36.4"N, 98°38'15.7"E): CUMZ 14251 (7 preserved specimens). Wat Pa Tham Pha Daeng, Thong Pha Phum District, Kanchanaburi Province (14°38'44.7"N, 98°39'33.8"E): CUMZ 14252 (8 preserved specimens). Thee Lor Su, Umphang District, Tak Province (16°02'47.1"N, 98°51'09.2"E): CUMZ 14253 (22 preserved specimens). Wat Tham Khao Kriap, Lang Suan District, Chumphon Table 1. Comparison of the morpho-anatomical characteristics of Durgella Blanford, 1863 and morphologically similar genera of helicarionoidean semislugs from mainland Southeast Asia. The genitalia characters depicted are in respect to type species, or otherwise stated. References: 1 = Blanford and Godwin-Austen (1908), 2 = Solem (1966), 3 = Schileyko (2002), 4 = Schileyko (2003), and 5 = Pholyotha et al. (2021a) Genus (type species) Shell Genitalia Radular teeth Caudal horn Ref. Shell size (width) Umbilicus Whorls Dart apparatus Epiphallic caecum Flagellum Durgella Blanford, 1863 (Helix levicula Benson, 1859)* small to medium minute to narrow 3–4½ present1present absent central unicuspid or asymmetrical bior tricuspid, lateromarginal bicuspid with several tiny cusps on outer side present 1, 2, 3 this study Cryptaustenia Cockerell, 1891 (Vitrina planospira Benson, 1859 (= Vitrina succina Reeve, 1862))* small to medium minute 3–4½ present2absent absent central tricuspid, lateral bior tricuspid, marginal bicuspid present 1, 2, 4 Cryptosemelus Collinge, 1902 (Cryptosemelus gracilis Collinge, 1902)* small to medium absent 3½–4 absent absent absent central tricuspid, lateral tricuspid, marginal bicuspid absent 5 Megaustenia Cockerell, 1912 (Vitrina praestans Gould, 1843)* medium to large absent 3–3½ present present present central tricuspid, lateral tricuspid, marginal bicuspid present 1, 2, 3 *Genitalia of the type species were examined. 1Durgella assamica Godwin-Austen, 1881, D. rogersi Godwin-Austen, 1907 and D. nulla sp. nov. have no dart apparatus (Blanford and Godwin-Austen 1908; this study). 2Cryptaustenia gadinodromica Solem, 1966 have no dart apparatus (Solem 1966). page 8 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 5. Shells of Durgella species. A: D. birmanica modified from Hanley and Theobald (1876). B, C: D. birmanica CUMZ 14233. D–F: D. levicula; D: NHMUK 1903.7.1.785; E: CUMZ 14251; F: CUMZ 14245. page 9 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 9. Microsculpture of Durgella shells: protoconch and early teleoconch (first column), and close-up view of protoconch (second column). A, B: D. birmanica CUMZ 14233. C, D: D. levicula CUMZ 14251. E, F: D. erratica CUMZ 14234. G, H: D. siamensis CUMZ 14232. page 16 of 29 Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 10. Genitalia of Durgella species. A: D. erratica CUMZ 14234. B, C: D. siamensis CUMZ 14232; B: general view of the genital system; C: internal structure of the penis and epiphallus. White arrow indicates the end of the penis. page 17 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 11. Representative SEM images of the radula. A, B: D. birmanica CUMZ 14233. C, D: D. levicula CUMZ 14253. E, F: D. erratica CUMZ 14234. G, H: D. siamensis CUMZ 14232. Yellow arrow indicates the central tooth. page 18 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 12. Shells of Durgella species. A, B: D. libas CUMZ 14236. C, D: D. pentata sp. nov.; C: holotype CUMZ 14239; D: paratype CUMZ 14240. E, F: D. nulla sp. nov.; E: holotype CUMZ 14227; F: paratype CUMZ 14228. page 19 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 13. Microsculpture of Durgella shells: protoconch and early teleoconch (first column), and close-up view of protoconch (second column). A, B: D. libas CUMZ 14236. C, D: D. pentata sp. nov. paratype CUMZ 14240. E, F: D. nulla sp. nov. paratype CUMZ 14228. expanded near umbilicus. Umbilicus minute. Genital organs (Fig. 14). Atrium (at) enlarged and very short. Penis (p) slightly long, enlarged with irregular shape, and with thin penial sheath encircling from atrium to almost middle of penis. Inner sculpture of penis uniform throughout with small conical penial pilasters (pp). Epiphallus (e1 + e2) same as penis length but its diameter much narrower; e1 rather long and slender; e2 shorter and bulbous. Inner sculpture: e1 with small longitudinal dash-like pilasters, and e2 with one large longitudinal pilaster. Epiphallic caecum (ec) short, small, and oblong; penial retractor muscle page 20 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan (prm) thick and attached at tip of epiphallic caecum. Vas deferens (vd) long and thin tube. Vagina (v) very short or nearly absent. Dart apparatus (da) rather short, enlarged cylindrical, and connected to atrium chamber. Gametolytic duct (gd) proximally enlarged and tapering to smaller tube distally; gametolytic sac (gs) enlarged and bulbous. Free oviduct (fo) slightly shorter than penis; proximally enlarged and bulbous; distal part cylindrical. Oviduct enlarged lobules; prostate gland running alongside oviduct. Fig. 14. Genitalia of Durgella libas CUMZ 14236. A: general view of the genital system and B: internal structure of the penis and epiphallus. White arrow indicates the end of the penis. page 21 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Radula (Fig. 17A, B). Teeth pectiniform, arrangement and shape similar to those of D. levicula, and with half row ca. 430 teeth. Central tooth very reduced to tiny unicuspid. Lateromarginal teeth elongated bicuspid and outer edge serrated with several tiny cusps. Outermost teeth shorter than inner teeth. External appearance (Figs. 3F, 4A). Posterior body with pale reddish-brown to light yellow-brown on dorsal and light yellow near foot margin; anterior body light yellow or creamy. Four mantle extensions welldeveloped and creamy. Caudal horn raised, large, and reddish-brown. Distribution: The species is known from Chiang Mai Province, northern Thailand (Fig. 1) and the newly collected samples suggest that this species prefers habitats in high mountainous areas. Remarks: Solem (1966) listed the specimens from Ban Kao and Sai Yok [both from Kanchanaburi Province, western Thailand] as this species. However, we have collected many Durgella specimens from these two localities and found that the soft body colour, shells, and genital morphology are as those of D. levicula instead. Therefore, we conclude that D. libas is probably restricted to northern Thailand. Durgella pentata Pholyotha & Panha, sp. nov. (Figs. 1, 3G, 4B, 12C, D, 13C, D, 15, 17C, D) urn:lsid:zoobank.org:act:398E44AA-406A-4C4C-9BE84BC191484BBF Type material: Holotype CUMZ 14239 (Fig. 12C; width 11.5 mm, height 6.7 mm). Paratypes same locality as holotype: CUMZ 14240 (52 preserved specimens; Fig. 12D; width 11.0 mm, height 6.5 mm), 14241 (24 preserved specimens), NHMUK (2 shells). Other material examined: THAILAND: Wat Tham Klaeb (Wat Tham Nguang Chang), Chiang Dao District, Chiang Mai Province (19°33'32.3"N, 99°03'46.2"E): CUMZ 14242 (7 preserved specimens). Pha Daeng Cave-Luang Cave, Chiang Dao District, Chiang Mai Province (19°34'40.7"N, 99°03'43.4"E): CUMZ 14243 (1 preserved specimen). Ban Pang Mayao, Chiang Dao District, Chiang Mai Province (19°26'10.5"N, 99°04'09.2"E): CUMZ 14244 (20 preserved specimens). Type locality: Tham Chiang Dao, Chiang Dao District, Chiang Mai Province, Thailand (19°23'37.5"N, 98°55'39.6"E). Diagnosis: Shell depressedly subglobose, thin, polished, dark yellow with a creamy tinge, and aperture roundly lunate, broader than high; animal with five mantle extensions; genitalia with very short vagina, small epiphallic caecum, short gametolytic duct, and very large dart apparatus. Description: Shell (Figs. 12C, D, 13C, D). Shell depressedly subglobose, small to medium (width up to 11.7 mm, height up to 7.2 mm), transparent, polished, smooth, and yellowish-corneous or dark yellow with a creamy tinge. Protoconch surface sculptured with faint spiral furrows and regular strong malleation (Fig. 13C, D). Spire slightly elevated; suture impressed. Whorls 4–4½, convex, rapidly increasing; last whorl enlarged and well-rounded. Aperture oblique, diagonal, roundly lunate, width greater than height; peristome simple. Columellar edge thin and little expanded near umbilicus. Umbilicus minute. Genital organs (Fig. 15). Atrium (at) enlarged and very short. Penis (p) long, cylindrical, and with thick penial sheath encircling about half of penis length. Inner sculpture of penis with longitudinal penial ridges from proximal to near middle and then transforming to small conical penial pilasters (pp). Epiphallus (e1 + e2) slightly shorter than penis; e1 cylindrical and narrower than penis; e2 short and bulbous shape. Inner sculpture: e1 with one thickened longitudinal fold running from proximal to near middle; e2 distally with dense papillae, and one thickened longitudinal pilaster. Epiphallic caecum (ec) short and small; penial retractor muscle (prm) thick and attached at tip of epiphallic caecum. Vas deferens (vd) long and slender tube. Vagina (v) very short or inconspicuous. Dart apparatus (da) very large, long cylindrical, and connected to atrium chamber. Gametolytic duct (gd) short, about half of penis length, proximally enlarged and then tapering to smaller and slender tube; gametolytic sac (gs) enlarged and bulbous. Free oviduct (fo) shorter than penis; proximally enlarged and bulbous; distal part shorter and cylindrical. Oviduct enlarged lobules; prostate gland running alongside oviduct. Radula (Fig. 17C, D). Teeth pectiniform and numerous with half row ca. 380 teeth. Central tooth reduced to bicuspid. Lateromarginal teeth elongated bicuspid, unclear separation between endocone and ectocone and outer edge serrated with inconspicuous cusps. Outermost marginal teeth shorter, smaller, and more conspicuous serrated cusps than inner teeth. External appearance (Figs. 3G, 4B). Dorsal of posterior body yellow to dark brown and then transforming to light yellow below foot margin; anterior body light yellowish. Five mantle extensions welldeveloped and creamy. Caudal horn raised, large, and yellowish-brown to dark brown. Etymology: The specific name “pentata” is from the Greek word meaning five, referring to the presence of five mantle extensions. Distribution: The new species is known from Tham Chiang Dao and the vicinity of Chiang Dao District, Chiang Mai Province, northern Thailand page 22 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 15. Genitalia of Durgella pentata sp. nov. paratype CUMZ 14240. A: general view of the genital system and B: internal structure of the penis and epiphallus. White arrow indicates the end of the penis. page 23 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan Fig. 16. Genitalia of Durgella nulla sp. nov. paratype CUMZ 14228. A: general view of the genital system and B: internal structure of the penis and epiphallus. White arrow indicates the end of the penis. page 24 of 29Zoological Studies 63:14 (2024) © 2024 Academia Sinica, Taiwan (Fig. 1). Remarks: Durgella pentata sp. nov. differs from D. levicula, D. erratica, D. siamensis, and D. libas in the presence of five mantle extensions, while the other species have four mantle extensions. This new species also differs from the others by having a protoconch sculptured with strong malleation, while D. levicula, D. erratica, and D. siamensis have rather smooth or faint spiral furrows without malleation, and D. libas has spiral furrows with faint malleation. Fig. 17. Representative SEM images of the radula. A, B: D. libas CUMZ 14238. C, D: D. pentata sp. nov. paratype CUMZ 14240. E, F: D. nulla sp. nov. paratype CUMZ 14228. Yellow arrow indicates the central tooth. page 25 of 29Zoological Studies 63:14 (2024)