Muangnua limax Solem, 1966.

(based on type material studied by Solem (1966)). Shell reduced, having only one remnant of coiling, which in living semislug is completely covered by fused shell laps. Mantle lobes form large cephalic shield reaching base of tentacles. Tail long and slender, with hooked caudal horn. Posterior visceral hump rounded, resting in V-shaped body groove. Jaw thin, without median projection or sculpture. Radula possesses tricuspid central, lateral, and marginal teeth. Genitalia, was studied from juvenile specimen, therefore, epiphallus cannot be differentiated. Free oviduct short, vagina rather long, dart apparatus, and other accessory penial organs absent; gametolytic sac (= spermatheca in Solem, 1966) long, finger-like, reaching two-thirds of way to albumen gland; atrium rather short.

The genus Muangnua was originally placed in the family Helicarionidae and tribe Durgelli by Solem (1966), after which Vaught (1989) moved it to the family Ariophantidae (Table 1). Panha (1996), Hemmen and Hemmen (2001), and Nabhitabhata (2009) followed Vaught’s classification. Schileyko (2002) also follows Vaught (1989), but he placed genus Muangnua into the subfamily Ariophantinae and tribe Ariophantini (Table 1). Recently, Bouchet and Rocroi (2005) and Bouchet et al. (2017) moved the tribe Durgellini into the subfamily Durgellinae (Helicarionidae). In this study, we followed Bouchet and Rocroi’s classification. Among the three genera of long elongate, small Southeast Asian semislugs (Parmarion P. Fischer, 1856; Muangnua Solem, 1966; and Laocaia Kuzminykh, 1999), their external morphology is very similar, but they possess many different characters. Parmarion frequently covers their ear-shaped shell with a mantle lobe, whereas Muangnua and Laocaia always cover their finger nail-shaped shell and triangular shaped shell, respectively, with their mantle lobe. The caudal horn overhangs in Muangnua and Laocaia but does not overhang in Parmarion. The postero-dorsal midline keel is present in Parmarion and Muangnua whereas it is present or absent in Laocaia.

Holotype. (Fig. 4) Thailand: Loei Province, Phu Suan Sai sandstone mountain, in small valley of Suan Pa Na Po, type locality covered by evergreen forest with dense undergrowth of bamboo and banana, 17°27'55"N, 100°55'30"E; at 940–960 m above mean sea level, 24–25 October 2011; C. Tumpeesuwan, S. Tumpeesuwan, and member of MSU malacology laboratory leg.; NHMSU-00019. Paratypes. Seven adults and three juveniles, same data as for holotype: NHMSU-00020.

Specific epithet “arborea” derived from Latin word “arboreus” meaning “of trees” referring to the habitat of this new semislug species.

(Table 2) Muangnua arborea Tumpeesuwan & Tumpeesuwan, sp. nov. differs from Muangnua limax Solem, 1966 by its body coloration. Head of M. arborea with three black or pale black strips, for which each lateral side of head possesses lighter stripes from base of lower tentacle back to base of cephalic shield, a darker mid-dorsal line from anterior extremity among tentacles back to one-third of body length under cephalic shield (Figs 3–6), whereas, mid-dorsal line black strips absent in M. limax. There are various sizes of white spots crowded on fringe and keel of living M. arborea, which causes remarkable white “Y” stripe on postero-dorsal side of foot (missing in preserved specimens), whereas, preserved specimen of M. limax possesses three black strips on postero-dorsal side of foot, a darker stripes on mid-dorsal keel, and two lighter stripes on both lateral sides.

Figure 4. 

Muangnua arborea Tumpeesuwan & Tumpeesuwan, sp. nov. dissected holotype specimen NHMSU-00019 A lateral left B lateral right C dorsal D ventral. Photograph by: Benchawan Nahok.

Body : Body is slender, elongated (Figs 2–6). Body length 37.1–45.3 mm and body width 10.2–10.4 mm when slightly retracted. Foot narrow, posterior part of foot laterally depressed and forms steep keel structure (Figs 2B–D, 3–6) tapering posteriorly. Tail long with hooked, caudal horn (Fig. 6B), tail length almost equal to half of length of posterior end of visceral hump to anterior end of head. Posterior lobe of visceral hump rounded, resting in “V” shaped body groove on top of foot (Figs 5, 6). Mantle lobes and shell laps fused and forming large cephalic shield, completely covering shell and visceral hump.

Figure 5. 

Muangnua arborea Tumpeesuwan & Tumpeesuwan, sp. nov. external morphology and coloration of body with visceral hump and V-shaped dorsal groove, A mature snail B immature snail.

Figure 6. 

Muangnua arborea Tumpeesuwan & Tumpeesuwan, sp. nov. A colour pattern variation within population B close-up view of caudal horn (indicated by arrow).

Coloration : Primary color of anterior body is light-ocher-brownish and gradually changes to brown or dark brown at posterior of body. Head possesses two pairs of tentacles (=ommatophores in Dedov et al. 2019). Lower tentacle is short and same color as head. Upper tentacle is black colored at base and gradually changes to light-ocher-brownish at top of tentacle, which contrasts with black eye spot on top of tentacle. Head with three black or pale black strips, for which each side of head possesses lighter stripes from base of lower tentacle back to base of cephalic shield and under its edge, a darker mid-dorsal line from anterior extremity among both tentacles back to one-third of body length under cephalic shield (Figs 3–6). Whole body has pattern of irregularly-dispersed various-sized white spots. Visceral hump with two short vague, pale brown lateral strips (Figs 3A, 5, 6A). Posterior lobe of visceral hump sitting in dark brown to pale black V-shaped depression on postero-dorsal side of foot. Fringe of V-shaped depression connects to steep keel on postero-dorsal side of foot. Various sizes of white spots crowded on fringe and keel, which causes remarkable white “Y” stripe on postero-dorsal side of foot; posterior extremity of this white Y-shaped stripe connects to white caudal horn.

Shell (Fig. 7): Reduced to partially calcified cap having no coiling, shell length 7.1–7.4 mm, SW 5.1–5.5 mm., apex not prominent, shell shape similar to human finger nail, white calcified plate covered by transparent pale brown periostracum, normally sloughs off in preserved specimens (Fig. 7). In living semislug, its shell is always completely enclosed by fused shell laps.

Figure 7. 

Shell of Muangnua arborea Tumpeesuwan & Tumpeesuwan, sp. nov. (paratype NHMSU-00020) A external view B internal view C periostracum removed from shell of preserved specimen.

Radula (Fig. 8): According to old specimens preserved since 2011, edge of radula plates macerated and breaks down during radula extraction process. Although, we choose the best specimens from several individuals, nevertheless, most of the marginal teeth were lost or incomplete. Radula plate long ribbon-shape, comprised of at least 123 rows of teeth. Each row composed of more than 44 teeth. Central teeth isosceles triangle, tricuspid, very large broad base, mesocone long lancelate, ectocones prominent but short, not reaching nearly to edge of base plate. First lateral teeth with small entocone appearing on mesocone shaft, ectocone large and plump on outer side of cusps. Laterals have basal plate elongated, increasing as entoconal size increases and ectocone rapidly decreases in size. By 20^th tooth, teeth sub-equally bicuspid, as entocone becomes smaller than mesocone and ectocone reduced to a small side cusp.

Figure 8. 

Radula morphology of Muangnua arborea Tumpeesuwan & Tumpeesuwan, sp. nov. (paratype NHMSU-00020) A radula plate (ribbon-like radula in middle of picture) B central tooth (arrow) and lateral teeth C close-up view of left side of radula D close-up view of middle part of radula E close-up view of left side of radula showing marginal teeth F close-up view of right side of radula.

Genitalia morphology (Fig. 9): Atrium rather short. Penis large and stout, peanut-shape, internally with short cylindrical muscular verge, which attaches to inner surface of penis at proximal end (connected to distal end of epiphallus); inner surface sculpture of penis can be divided into 2 types: upper portion around verge covered with numerous small tubercles, and lower portion covered with transverse folds (Fig. 9B, C); epiphallus length equal to penis, dumbbell-shape; vas deference inserts to epiphallus apically. Vagina very long and slender tube, prostate gland very small and encloses uterus, albumen gland small, hermaphroditic duct convoluted. Dart apparatus (amatorial organ) and other accessory penial organs absent. Gametolytic sac long, finger-like, reaching about half way to albumen gland.

Figure 9. 

Reproductive anatomy of Muangnua arborea Tumpeesuwan & Tumpeesuwan, sp. nov. (paratype NHMSU-00020), A genitalia B inner part of penis C after verge inverted. Abbreviations: hd = hermaphroditic duct, ag = albumen gland, vd = vas deferens, ep = epiphallus, er = epiphallic retractor, p = penis, ve = verge, at = atrium, gs = gametolytic sac, ut = uterus, v = vagina, pr = prostate gland.

According to the most recent information, this species is known only from the type locality. They were found only on plants (Fig. 2), which is normally on the leaves of monocot plants, such as bamboo, banana, etc. (Fig. 3). In the resting stage, this animal normally holds onto the leaves or trunks of plants by inverting their tail and attaching its end to the left side of their head, making their body U-shaped (Fig. 2C). This new species has a similar protective behavior to that described by Dedov et al. (2019) for Laocaia simovi and Wiktor (2002) for Cryptaustenia saltatoria and Cryptaustenia obesa. If they are touched or caught, they will quickly flip, wag, and twist their foot to escape from the predator (Fig. 3B). In addition, for some semislugs in the resting stage, we found them near their fecal matter (Fig. 3A), which might suggest this animal has homing behavior, and they will return to sleep in the same position every day.

According to Schilthuizen and Liew (2008) a semislug is defined as snails with a partially visible shell that, due to their rather small shell, cannot withdraw its body into its shell. Muangnua is a slug-like semislug possessing a reduced shell having only one remnant coil that is always covered by a mantle lobe. There are many species of semislug described and recorded from South and Southeast Asia (Blanford and Godwin-Austen 1908), including, Girasia Gray, 1855 from Himalaya and Assam (India), Cryptogirasia Cockerell, 1898 from Naga Hills (India), from Western Ghat (India) and Ceylon (Sri Lanka), Austenia Nevill, 1878 from the Himalaya, Assam (India) and Burma (Myanmar), Parmarion P. Fischer, 1856 from South China to Java (Indonesia), Minyongia Godwin-Austen, 1916 from Assam (India), and Myotesta Collinge, 1901 from North Vietnam. Of these genera, only Austenia and Parmarion are known in Thailand. Austenia doisutepensis Solem, 1966 has a short body and 1⅔ to 2 whorls; therefore, its mantle lobes and shell laps cannot cover all the shell surface and leave much of the shell exposed (snail-like semislugs). Parmarion martensi Simroth, 1893 has an elongated body, in which the small ear-shape reduced shell is frequently covered by mantle lobes and shell laps (slug-like semislugs). We provide below keys for identifying the genera of slug-like semislugs and species of Muangnua in Southeast Asia.