Melayonchis siongkiati Dayrat & Goulding, 2017

Dayrat, Benoît, Goulding, Tricia C., Bourke, Adam J., Khalil, Munawar & Tan, Shau Hwai, 2019, New species and new records of Melayonchis slugs (Gastropoda: Euthyneura: Pulmonata: Onchidiidae), Raffles Bulletin of Zoology 67, pp. 557-585 : 578-581

Melayonchis siongkiati Dayrat & Goulding in Dayrat et al., 2017

( Figs. 17–20 )

Melayonchis siongkiati Dayrat & Goulding in Dayrat et al., 2017: 1871–1878, figs. 9–15.

Type locality. Singapore, Mandai River , 01°26.237ʹN, 103° 45.730ʹE, 2 April 2010, station 6, following the river from the railroad towards the sea, open mangrove forest with tall trees and soft mud, ending on sun-exposed mudflat outside the mangrove with very soft mud .

Type material. Holotype, by original designation, 32/20 [1002] mm ( ZRC.MOL.6501).

Additional material examined. Australia, Northern Territory, upper Darwin Harbour, Elisabeth River, upstream Bennetts Creek , 12°35.26740ʹ, 131°01.31160ʹ, 7 September 2016, 1 specimen 33/22 [5875] mm, station 265, at midtide level, on bank of Rhizophora -dominated tidal creek, deeply buried within moist woody debris and decaying stumps ( NTM P.59483) ; Australia, Northern Territory, upper Darwin Harbour, Elisabeth River, upstream Bennetts Creek , 12°35.26740ʹ, 131°01.31160ʹ, 7 September 2016, 1 specimen 35/20 [5877] mm, station 265, at mid-tide level, on bank of Rhizophora -dominated tidal creek, deeply buried within moist woody debris and decaying stumps ( NTM P.59484) ; Australia, Northern Territory, upper Darwin Harbour, Elisabeth River, upstream Bennetts Creek , 12°35.26740ʹ, 131°01.31160ʹ, 7 September 2016, 1 specimen 28/20 [5878] mm, station 265, at mid-tide level, on bank of Rhizophora - dominated tidal creek, deeply buried within moist woody debris and decaying stumps ( NTM P.59485) .

Distribution ( Fig. 6 ). Australia, Northern Territory (new record). Other records are in Brunei Darussalam, Peninsular Malaysia, Singapore (type locality), and Vietnam (Dayrat et al., 2017: 1872).

Habitat. In the Northern Territory, M. siongkiati was found at one site deeply buried within moist woody debris and decaying stumps, on the bank of a tidal creek, in a Rhizophora -dominated forest. In the rest of its distribution, M. siongkiati usually lives in old forests where it is found on tree trunks and roots as well as logs, though it can occasionally be found on cemented walls of ditches or bridges near mangroves (Dayrat et al., 2017: 1873).

Abundance. Melayonchis siongkiati is extremely rare in the Northern Territory. Only a few specimens at one site were found in years of mangrove exploration by Adam Bourke. In the rest of its distribution, M. siongkiati is abundant only in Brunei and a large population was found at one site in Malaysia (Dayrat et al., 2017: 1873).

Live animals ( Fig. 17 ). The beige background of the notum is irregularly mottled with darker areas, both light and dark brown. The colour of the hyponotum varies between light to dark gray and is marked by a significantly lighter ring at the margin. The foot is pale orange. When the animal crawls undisturbed, the brown ocular tentacles are short and extend for only a few millimeters beyond the notum margin, and the head is small and remains covered by the dorsal notum. The body is elongated, oval, usually not flattened, though animals may seem flattened when crawling. The dorsal notum is not particularly thick. Its surface, when the animal is undisturbed, is not smooth. Dorsal gills and large papillae are absent, but small conical papillae are present. About 15 of those papillae bear a black dorsal eye. A very slightly larger, central papilla bears three black dorsal eyes. In addition, the notum is finely granular. When disturbed, the animal can coil up, and occasionally even form a complete ball. Also, when disturbed, the dorsal notum immediately secretes an abundant oily and shiny mucus. Finally, the dorsal notum of disturbed animals tends to be smooth (instead of finely granular). Live animals are from 28 to 35 mm long.

Digestive system ( Figs. 18 , 19 , Table 4). Examples of radular formulae are presented in Table 4. The length of the rachidian teeth is approximately 30 µm, significantly less than the lateral teeth. The length of the hook of the lateral teeth is approximately 60 µm, excluding the first few (about 5) innermost and outermost lateral teeth which are significantly smaller. The inner lateral aspect of the hook of the lateral teeth is not straight. It is marked by a strong protuberance placed over the preceding adjacent tooth. The tip of the hook is similar across the half row, although it tends to be slightly more pointed in the innermost and slightly more round in the outermost teeth. The intestinal loops are of type III, with a transitional loop oriented at approximately 4 o’clock. A rectal gland is present.

Reproductive system ( Fig. 18B ). The receptaculum seminis (caecum) is spherical ovate. The spermatheca is ovate and connects to the oviduct through a very short duct. The oviduct and the deferent duct are narrow and straight. A vaginal gland is absent.

Copulatory apparatus ( Figs. 18C , 20 ). The male anterior organs consist of the penial complex (penial papilla, penial sheath, deferent duct, and retractor muscle) and the accessory penial gland (flagellum and hollow spine). The penial complex and the accessory penial gland share the same vestibule and male opening. The flagellum of the penial gland is coiled. Distally, it ends in a hard, hollow spine. The hollow spine is narrow, elongated, and slightly curved. It measures about 60 μm in diameter at its conical base and narrows down to 15 to 20 μm distally, for a length that varies from 0.6 to 0.7 mm. The hollow spine opens into the proximal region of the vestibule.

The penial sheath protecting the penial papilla is straight and short (approximately 1 mm long) and joins distally the accessory penial gland in the vestibule. The total length from the retractor muscle attachment (proximal end of the penial sheath) to the opening of the vestibule is about 4 mm. The penial papilla consists of a short tube, slightly conical at its base and a slightly enlarged tip. Its length ranges between 0.4 and 0.5 mm. Its diameter is approximately 130 to 150 μm at the base, 150 μm at the tip, and 60 μm in between. There are no penial hooks. The retractor muscle is longer or shorter than the penial sheath and inserts at about half the length of the floor of the visceral cavity. The deferent duct is highly convoluted with many loops.

Remarks. The description of the individuals from the Northern Territory matches perfectly the original description of M. siongkiati from the Strait of Malacca and the South China Sea (Dayrat et al., 2017). Most especially, they share the most distinctive trait of the species, i.e., an abundant secretion of oily mucus when animals are disturbed, which is convenient to identify M. siongkiati in the field. Individuals can also coil up into a ball, but not as systematically and completely as M. eloisae or M. eberlyi . The dorsal colour of live animals, which is quite variable, is harder to use for identification. When crawling animals are observed without being disturbed, they could easily be confused with other species, including species from other genera (e.g., Platevindex , Wallaconchis ). The only differences observed between individuals from the Northern Territory and the rest of the species are minor and can easily be explained by geographic distance and the fact that all three Australian specimens come from the same population ( Table 3): the pedal sole is orange in Australian individuals and grey in the rest of the species; the penial papilla is slightly shorter in Australian individuals (0.4 to 0.5 mm long) compared to the rest of the species (0.5 to 1 mm long).

Figures

Fig. 17. Live animals, Melayonchis siongkiati, Darwin, Northern Territory, Australia, dorsal (A–C), ventral (D, G), and lateral (E, F) views. A, 33 mm long [5875] (NTM P.59483); B, 35 mm long [5877] (NTM P.59484); C, 28 mm long [5878] (NTM P.59485); D, same as C; E, same as A; F, same as B; G, same as A.

Fig. 18. Digestive system, and reproductive system, Melayonchis siongkiati, [5878] (NTM P.59485), Darwin, Northern Territory, Australia. Small arrows indicate the direction of the intestinal transport. Large arrows indicate the orientation of the transitional loop. A, Digestive system (type III, with a transitional loop at 4 o’clock), dorsal view, scale bar = 2 mm; B, Posterior reproductive parts, scale bar = 2 mm; C, Anterior male copulatory parts, scale bar = 2 mm. Abbreviations: ag, accessory penial gland; dd, deferent duct; ddg, dorsal lobe of digestive gland; fgm, female gland mass; hd, hermaphroditic duct; hg, hermaphroditic gland; i, intestine; ov, oviduct; pdg, posterior lobe of the digestive gland; ps, penial sheath; rg, rectal gland; rm, penial retractor muscle; rs, receptaculum seminis; sp, spermatheca; st, stomach; v, vestibule.

Fig. 19. Radula, Melayonchis siongkiati, [5875] (NTM P.59483), Darwin, Northern Territory, Australia. A, Rachidian and innermost lateral teeth, scale bar = 20 μm; B, Right lateral teeth, scale bar = 20 μm. Abbreviations: 1rlt, first, right, lateral tooth; 2rlt, second, right, lateral tooth; hlt, hook of lateral tooth; ilp, inner lateral protuberance; mc, median cusp of rachidian tooth; rt, rachidian tooth.

Fig. 20. Hollow spine of the accessory penial gland (A–D), and penial papilla (E, F), Melayonchis siongkiati, Darwin, Northern Territory, Australia. A, Scale bar = 100 μm, [5878] (NTM P.59485); B, Scale bar = 100 μm, [5877] (NTM P.59484); C, Scale bar = 100 μm, [5875] (NTM P.59483); D, Spine tip, scale bar = 8 μm, same as C; E, Scale bar = 100 μm, same as A; F, Scale bar = 100 μm, same as B.

Fig. 6. Geographic distribution of the six known Melayonchis species. The many blue dots correspond to regions where we collected gastropods since 2010 (with several sites per dot), clearly indicating where we have not found Melayonchis (e.g., Philippines, Sulawesi, Queensland). So far, Melayonchis was only found in the Andaman Islands, on both sides of the Strait of Malacca (western Peninsular Malaysia and eastern Sumatra), Singapore, southern Vietnam, Brunei Darussalam, and Northern territory. The colours used for each species are the same as those used in Figs. 3–5.