﻿New species of the dancing semislug Cryptosemelus Collinge, 1902 (Eupulmonata, Ariophantidae) from Loei Province, northeastern Thailand with a key to genera of mainland Southeast Asian semislugs and a key to species of the genus

﻿Abstract In this study, we describe a new dancing semislug from a limestone hill area in northeastern Thailand. Cryptosemelusnigersp. nov. differs from the three recognized congener species from western and southern Thailand, due to differences in their body and shell lobes coloration, appearance of penial caecum, shape and surface texture of penis and epiphallus, and radula formula and morphology.


Introduction
Cryptosemelus Collinge, 1902 is a member of family Ariophantidae. Its common characters comprise a shell with reduced number of whorls, body with well-developed mantle extensions, tail without caudal horn and genitalia without flagellum and amatorial organ. This genus differs from other long-tail semislug genera described from the Malay Peninsula. It differs from Apoparmarion due to more shell whorls and the absence of a caudal horn, flagellum and dart apparatus; and, it differs from Paraparmarion by the presence of both right and left shell lobes, whereas, the latter genus presents only a right shell lobe (Collinge 1902;Pholyotha et al. 2021a) (Table 1).
Cryptosemelus has been referred to as a 'dancing semislug' because of its dance-like movement that it makes when it is disturbed or attacked (Collinge 1902). This protective behavior has also been reported in other species, including Laocaia simori Dedov & Schneppat, 2019in Dedov et al. (2019, Cryptaustenia altatorial Wiktor, 2002 and Cryptaustenia mirabilis Wiktor, 2002, Muangnua arborea Tumpeesuwan & Tumpeesuwan, 2019; Cryptosemelus gracilis Collinge, 1902, C. betarmon Pholyotha, 2021and C. tigrinus Pholyotha, 2021in Pholyotha et al. (2021a, Ibycus spp. and Helicarion spp. (Junn Kitt Foon, pers. comm.) According to a recent study by Pholyotha et al. (2021a), the type species of the genus, Cryptosemelus gracilis, and the recently named species C. betarmon and C. tigrinus were studied and described from their genital anatomy and radula morphology. These three species are characterized by differences in the anatomical details of their penis, epiphallus and spermatophore. We discovered and examined this lovely semislug from a limestone hill in Loei Province, Thailand, and it possesses distinct characters of external shell morphology, mantle lobes coloration, radula and genital organs. Thus, we describe it here as a new species of Cryptosemelus.

Material and methods
Sixty specimens were collected from November to December 2012, June to September 2013, and 12 th October 2018 in the Phu Pha Lom limestone area (17°33'62"N, 101°52'31"E), elevation about 380-390 m above mean sea level, in the Mueang Loei District, Loei Province, northeastern Thailand (Fig. 1). The specimens were composed of fifty empty shells and 10 living specimens, which were collected from both leaf litter and the ground surface. The living specimens were photographed in their natural habitat (Figs 2B,3), and then euthanized and preserved in 70 (v/v) ethanol for morphological and anatomical studies. Specimens were classified and identified from the literature, such as Collinge (1902), Blanford and Godwin-Austen (1908), Solem (1966), Schileyko (2003) and Pholyotha et al. (2021a). For the descriptive study, adult shells were measured for size using a vernier caliper and the number of whorls were counted. Photomicrographs were taken using a scanning electron microscope (JEOL, JSM-6460 LV) housed at the Central Laboratory, Faculty of Science, Mahasarakham University, Thailand. Eight specimens were dissected and examined under a stereoscopic light microscope. Descriptions of the new species herein are attributed to the first and the third authors, as indicated below. Type specimens and other voucher specimens were deposited in the Natural History Museum of Mahasakham University, Maha Sarakham, Thailand (NHMSU).
For the description of the genital system; 'proximal' refers to the region closest to the genital opening and 'distal' refers to the region outermost from the genital opening. Type species. Cryptosemelus gracilis Collinge, 1902. Cryptosemelus has a reduced shell of three to four whorls, well-developed mantle extensions with two dorsal lobes and right shell lobe covering the apex and larger than the left shell lobe. Caudal horn absent. Genital system without flagellum and dart apparatus.  Diagnosis. Animal with blackish body, shell lobes with blackish reticulated skin (Figs 2, 3). Genitalia with very short cylindrical vagina, smooth elongated cylindrical epiphallus, without penial caecum (Fig. 4). Radula with bicuspid lateral teeth (Fig. 5B, C).
Genital system (N = 3) (Fig. 4). Atrium (at) short. Penis rather long and cylindrical, with thin penial sheath covering entire penis. Penial retractor muscle (prm) present, short, thin and attached at junction of e1 and e2. Epiphallus (e1+e2) length is slightly equal to penis length, surface smooth, e1 cylindrical and gradually smaller in diameter, e2 cylindrical and larger than e1. Flagellum absent. Vas deferens long. Vagina is shorter than penis, cylindrical. Gametolytic duct (gd) thickened at base, gametolytic sac (gs) swollen gland at distal end. Free oviduct (fo) is shorter than vagina. Uterus and prostate gland long and stout.
External appearance (Figs 2B, 3). Living semislug with reticulated skin, blackish to dark body marked by conspicuous grooves running downward. Four mantle extensions well developed and same color body. Shell lobes do not cover entire shell. Caudal horn absent.
Etymology. The species name "niger" is derived from Latin word, meaning "black" referring to blackish body and mantle extensions.
Distribution, habitat and behavior observations. Cryptosemelus niger sp. nov. is a species restricted to the dry evergreen forest that covers the Phu Pha Lom limestone area, Loei Province, Thailand. We searched after rain and found the specimens normally hiding on the ground and in the leaf litter. When the snails were disturbed, they escaped by quickly flipping and waging their tail. Information on its natural predators is unknown, but the carnivorous land snails, Discartemon sp.     Remarks. Cryptosemelus niger sp. nov. differ from other Cryptosemelus species by its black body and mantle lobes. The shell seems to have a more rapidly descending whorl than in C. gracilis and C. betarmon, but is similar to C. tigrinus. Cryptosemelus gracilis also lack a penial caecum but differs in the undulated surface patch on the proximal part of e2. The epiphallus and penis are cylindrical without a caecum, diverticulum, and granulate surface. The radula of the new species differs from all Cryptosemelus in having bicuspid lateral teeth, whereas other species have tricuspid lateral teeth (Table 2, Figs 2A,B,4,.

Discussion
The absence of a caudal horn is the unique character shared between Cryptosemelus and Paraparmarion (Collinge 1902;Blanford and Godwin-Austen 1908;Solem 1966;Schileyko 2002Schileyko , 2003Pholyotha et al. 2021a). Cryptosemelus differs from Paraparmarion in that the left shell lobe is well developed, whereas it is missing in Paraparmarion (Collinge 1902;Pholyotha et al. 2021a).
All previously described species of Cryptosemelus show no epiphallic caecum, flagellum, and dart apparatus. Shell lobes coloration, appearance of the penial caecum, shape and surface sculpture of the epiphallus, and radula morphology are considered as taxonomically informative and these can be used to distinguish the new species from all recognized Cryptosemelus species.
We have improved the key to the genera of mainland Southeast Asian slug-like semislugs provided by Tumpeesuwan and Tumpeesuwan (2019b) for identifying slug-like semislugs in mainland Southeast Asia and provide a key to species of genus Cryptosemelus below.
Key to genera of mainland Southeast Asian slug-like semislugs Since 2007, an intensive survey on land snail diversity in limestone and non-limestone hills in northeastern Thailand has been continuously conducted and published (Tumpeesuwan 2007;Tumpeesuwan 2010a, 2010b;Srihata et al. 2010;Tanmuangpak et al. 2012;Jumlong et al. 2013Tanmuangpak 2016;Sasang 2019;Nahok 2020). In total, 16 species have been described as new to science, comprising 11 species from limestone hills, four species from sandstone hills, and one species from volcanic hills , 2017, 2019a, 2019bTanmuangpak et al. 2015Tanmuangpak et al. , 2017Nahok et al. 2020Nahok et al. , 2021aNahok et al. , 2021bDeeprom et al. 2022;Tanmuangpak and Dumrongrojwattana 2022;Tongkerd et al. 2023). Future studies on the malacofauna of the northeastern part of Thailand require more surveys in overlooked and isolated natural areas. checking the manuscript. We thank Edmund Gittenberger, Junn Kitt Foon, and Menno Schilthuizen for their valuable comments and suggestions. Animal Care and Use Protocol Review No. IACUC-MSU-17/2021.