Cryptosemelus gracilis Collinge, 1902

Pholyotha, Arthit, Sutcharit, Chirasak & Panha, Somsak, 2021, Rediscovering the dancing semislug genus Cryptosemelus Collinge, 1902 (Eupulmonata, Ariophantidae) from Thailand with description of two new species, ZooKeys 1076, pp. 43-65 : 43

Cryptosemelus gracilis Collinge, 1902

Figs 1 , 2C, D , 3A, B , 4 , 5 , 10A

Cryptosemelus gracilis Collinge, 1902: 76, pl. 5, figs 37-39. Type locality: Bukit Besar, State of Nawng Chik [Nong Chik District, Pattani Province, Thailand]. Laidlaw 1933: 221. Benthem Jutting 1949: 71. Zilch 1959: 326. Maassen 2001: 112. Schileyko 2003: 1332.

Type material.

Syntype UMZC I.66448 (one specimen in spirit; Fig. 2D ) from Bukit Besar, Patani [Pattani Province, Thailand], Malay Peninsula.

Other material examined.

Ton Din , Khuan Don District, Satun Province, Thailand (6°43'N, 100°09'E): CUMZ 7954 .

Diagnosis.

Shell globose and pale golden amber. Animal with blue-gray body. Genitalia with large vagina and elongated epiphallus with two small diverticula. Inner sculpture of penis with a small papilla near atrium. Spermatophore with a head filament of several spines and long tail filament with a cluster of small spines at the tip.

Description.

Shell (Fig. 3A, B ). Shell globose, small size (width up to 6.6 mm, height up to 4.2 mm), thin, smooth, polished, and pale golden amber. Whorls 3½ -4, rapidly increasing; body whorl large and well-rounded at periphery. Spire slightly elevated; suture little impressed. Aperture oblique, diagonal, and roundly ovate; peristome simple and thin. Columellar margin simple. Umbilicus imperforate.

Genital organs (Figs 4B-D , 5 ). Atrium (at) short. Penis (p) rather short, cylindrical, and with thin penial sheath covering entire penis. Internal penis surface nearly smooth, with small papilla (protruded tissue) near atrium (yellow arrow in Fig. 4C ). Epiphallus (e1+e2) approximately three times total penis length; e1 cylindrical and gradually smaller in diameter (Fig. 4B ); proximal e2 enlarged with irregularly undulated surface patch; and distal e2 generally smooth surface. Diverticulum (de) having two caeca: one small and one more muscular, thicker, and slightly larger (Fig. 4D ). Penial retractor muscle (prm) thin and attached at junction between e1 and e2. Vas deferens (vd) thin tube connected between distal epiphallus and free oviduct (Fig. 4B ).

Vagina (v) large, cylindrical, and approximately half of penis length. Gametolytic sac (gs) bulbous (Fig. 4B with spermatophore); gametolytic duct (gd) rather short, cylindrical, and somewhat broader at its base. Free oviduct (fo) cylindrical, approximately one and a half times penis length, and encircled with thick tissue in middle (Fig. 4B ).

Spermatophore long (Fig. 5 ). Sperm sac (ss) enlarged and elongate ovalate. Head filament (hf) large and divided into two major branches located opposite: first branch bearing one small bifid spine, and second branch containing several bifid spines (Fig. 5B ). Tail filament (tf) very long tube; terminal part about two-thirds of its length containing a series of tiny spines arranged in spiral rows (Fig. 5D-F ).

Radula (Fig. 10A ). Teeth arranged in a wide U-shape with half row formula: 1-(19-20)-38 teeth. Central tooth square base-plate with symmetrical tricuspid; mesocone large and triangular shape; ectocones small and pointed cusps. Lateral teeth asymmetrical tricuspid, inner teeth square base-plate and then gradually become elongate-shaped at outer teeth. Inner lateral teeth with mesocone large, triangular, and with pointed cusp; ectocone larger than endocone and located near tooth base. Outer lateral teeth: mesocone and ectocone large and pointed tip; endocone very small to nearly absent. Marginal teeth starting at approximately teeth numbers 19 to 20 with obliquely elongate bicuspid; endocone large and pointed tip; ectocone small lanceolate shape with pointed cusp. Outermost marginal teeth shorter and smaller than inner teeth.

External appearance (Figs 1 , 2C, D , 4A ). Living animal with reticulated skin, blue-gray to blackish body marked by conspicuous oblique grooves running downwards. Four mantle extensions well-developed and same color as body. Shell lobes enlarged to cover entire shell; left shell lobe (lsl) smaller than right shell lobe (rsl); left dorsal lobe (ldl) larger than right dorsal lobe (rdl). Sole divided into three parts longitudinally. Caudal horn absent.

Distribution, habitat, and behavior observations.

Cryptosemelus gracilis can be found in Satun, Yala, Songkhla, and Pattani Provinces in southern Thailand (Fig. 1 ). We searched after rain and found the semislug populations normally hiding under the slope of rocks or the tree trunks, and sometimes climbing on the rocks or low branches of plants. When the semislug are disturbed, they escape by quickly flipping and wagging their tail, and then falling on the floor. Information on its natural predators and parasites remains scarce, but the carnivorous slug genus Atopos and streptaxid snails were found sympatrically with this semislug.

Remarks.

A specimen of C. gracilis was first discovered from 'Bukit Besar’, the Malay Language, which means 'Big Mountains’ in Thai Language ( Skeat 1901; Annandale and Robinson 1906). However, this type locality is now referred to as the Namtok Sai Khao National Park area that is situated on the boundary of Pattani, Yala, and Songkhla Provinces in southern Thailand.

In this study, we examined specimens from Satun Province, which are identical to the syntype in having a blue-gray body with prominent oblique lines running downwards on the posterior body, large right shell lobes that covered the apex of the shell, and no caudal horn. Benthem Jutting (1949) provisionally attributed three semislug specimens from Telom Valley, Gunong Siku, Pahang State (1,000 m altitude) as C. gracilis s.l., but this was without any description or illustration. Based on our observation, all recognized Cryptosemelus species generally have a restricted distribution, and tend to occur at low altitudes near the mean sea level. Therefore, we consider that those semislug specimens from Pahang State probably belong to a distinct taxon from C. gracilis s.s. However, this semislug population needs to be re-examined to confirm their taxonomic status.

Figures

Figure 1. Geographic distribution and living animals of Cryptosemelus gracilis, C. betarmon sp. nov., and C. tigrinus sp. nov. based on the specimens examined herein. All not to scale. Black symbols indicate type locality and white symbols indicate other localities.

Figure 2. Comparison of shell and mantle lobes among the three monotypic semislugs described by Collinge (1902) A, B Apoparmarion partridgii Collinge, 1902 A original illustration and B syntype UMZC I. 66414 C, D Cryptosemelus gracilis Collinge, 1902 C original illustration and D syntype UMZC I. 66448 E, F Paraparmarion elongatus Collinge, 1902 E original illustration F syntype UMZC I. 66522. Credits: A, C, E after Collinge (1902) and B, D, F online catalogues of the UMZC, Cambridge.

Figure 3. Shells A, B Cryptosemelus gracilis specimen CUMZ 7954 C, D C. betarmon sp. nov. C holotype CUMZ 7959, and D paratype CUMZ 7960 E, F C. tigrinus sp. nov. E holotype CUMZ 7955, and F paratype CUMZ 7956.

Figure 4. Cryptosemelus gracilis specimen CUMZ 7954 A both sides of animal showing four lobes of mantle and B-D genitalia: B general view of the genital system C internal structure of the penis, and D external structure of epiphallus (e 2). White arrow indicates the end of the penis. Yellow arrow indicates the protruded tissue inside the penis near the atrium.

Figure 5. Spermatophore of Cryptosemelus gracilis specimen CUMZ 7954 A general view of the spermatophore B-F SEM images B head filament showing branching spines C region of tail filament near sperm sac showing no spine D alternate-spined region of tail filament near the end of spines from the tip E opposite-spined region of tail filament near the tip region, and F tail filament showing branching spines on the tip region. Yellow arrow indicates the end of spines from the tip.

Figure 10. Representative SEM images of the radula A Cryptosemelus gracilis specimen CUMZ 7954 B C. betarmon sp. nov. paratype CUMZ 7960, and C C. tigrinus sp. nov. paratype CUMZ 7956. Central tooth indicated by ' C'; blue color indicates central tooth row; yellow color indicates the transition of outer lateral to marginal teeth.