In this study, we describe a new dancing semislug from a limestone hill area in northeastern Thailand. Cryptosemelus niger sp. 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.

Genital system, karst topography, limestone hill, mantle extensions, protective behavior, shell lobes

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, 2019 in 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, 2021 and C. tigrinus Pholyotha, 2021 in 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.

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.

Figure 1. 

Map of type locality of Cryptosemelus niger S. Tumpeesuwan & C. Tumpeesuwan, sp. nov.

Figure 2. 

Cryptosemelus niger S. Tumpeesuwan & C. Tumpeesuwan, sp. nov. Photograph by Kitti Tanmuangpak A shell morphology of holotype NHMSU-00054 B living snail.

Figure 3. 

Cryptosemelus niger S. Tumpeesuwan & C. Tumpeesuwan, sp. nov. Living snail in natural habitats. Photograph by Chanidaporn Tumpeesuwan 12 December 2018 A C. niger crawling on soil in limestone crevice (white circle) B close-up view in living position C, D semislug was moved to green leaf for taking photo C left side represents left shell lobe (lsl) D right side represents right shell lobe (rsl), left shell lobes (lsl), and left dorsal lobe (ldl).

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).

The abbreviations used were as defined by Blanford and Godwin-Austen (1908), Solem (1966), Pholyotha et al. (2020) and Pholyotha et al. (2021a, b): ag, albumen gland; at, atrium; e1, portion of epiphallus nearer to penis; e2, portion of epiphallus nearer to retractor muscle; fo, free oviduct; gd, gametolytic duct; gs, gametolytic sac; hd, hermaphroditic duct; ldl, left dorsal lobe; lsl, left shell lobe; ovt, ovotestes; p, penis; prm, penial retractor muscle; pro, prostate gland; rsl, right shell lobe; v, vagina; vd, vas deferens; ut, uterus.

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.

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 2002, 2003; Pholyotha 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.

This research project was financially supported by Thailand Science Research and Innovation (TSRI). We would like to thank Mr. Sophon Thip-Apakul and all the staff from Phu Pha Lom Forest Park, Loei Province, for their help when conducting our fieldwork in 2012–2013. We are deeply indebted to Nual-anong Wichaikul when she worked at the SEM laboratory, Faculty of Science, Mahasarakham University, for their help with sample processing and SEM work. We are very grateful to everybody who assisted with the land snail collection. Special thanks to Jolyon Dodgson for kindly 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.