Melayonchis tillieri Dayrat, 2019

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 : 573-578

Melayonchis tillieri Dayrat , new species

( Figs. 12–16 )

Type locality ( Fig. 12E ). Malaysia, Peninsular Malaysia, Matang , off Kuala Sepatang , Crocodile River, Sungai Babi Manpus, 04°49.097ʹN, 100°37.370ʹE, 19 July 2011, station 28, old and open Rhizophora forest with tall trees, hard mud, creeks, and many dead logs .

Type material. Holotype, 50/20 [973] mm, designated here ( USMMC 00070 ).

Additional material examined. Malaysia, Peninsular, Malaysia, Matang , facing the fishermen’s village on the other side of the river, 04°50.154ʹN, 100°36.368ʹE, 20 July 2011, 1 specimen 33/17 [972] mm, station 29 (same site as station 256 in 2016), oldest and open Rhizophora forest of tall and beautiful trees, with hard mud, many creeks and logs ( USMMC 00071 ) ; Peninsular Malaysia, Kuala Sepatang , 04°50.217′N, 100°36.826′E, 26 July 2016, 1 specimen 36/18 [5957] mm, station 256 (same site as station 29 in 2011), oldest and open Rhizophora forest of tall and beautiful trees, with hard mud, many creeks and logs ( USMMC 00072 ) ; Peninsular Malaysia, Kuala Sepatang , 04°50.605′N, 100°38.133′E, 27 July 2016, 1 specimen 36/14 [5970] mm, station 257, old forest with tall Rhizophora trees, high in the tidal zone (ferns), in the educational mangrove preserve ( USMMC 00073 ) ; Peninsular Malaysia, Kuala Sepatang , 04°50.605′N, 100°38.133′E, 27 July 2016, 1 specimen 52/20 [5972] mm, station 257, old forest with tall Rhizophora trees, high in the tidal zone (ferns), in the educational mangrove preserve ( USMMC 00074 ) .

Distribution ( Fig. 6 ). Malaysia, Peninsular Malaysia, Malacca Strait, Kuala Sepatang (known from only three sites very close to each other).

Etymology. Melayonchis tillieri is dedicated to Simon Tillier, former PhD advisor of the first author at the Muséum national d’Histoire naturelle (Paris, France), for all the support and freedom he provided during those important years.

Habitat ( Fig. 12 ). Melayonchis tillieri lives in high intertidal, well-preserved, old mangrove forests. It crawls on tree trunks and roots as well as logs and even young stems. It is not found directly on the mud. It is the only onchidiid species that we ever found crawling on a green stem, high above ground (4 feet).

Abundance. Melayonchis tillieri is a rare species. We found only five individuals at three different sites in the Matang mangrove, near the fishermen’s village of Kuala Sepatang, western Peninsular Malaysia. Two sites are facing the village on the other side of the large river and are only accessible by boat: the site of station 28 (2011) and the site of stations 29 (2011) and 256 (2016). The third site is the mangrove educational preserve accessible by the road (station 257, in 2016). Only one individual was found at station 28, one at station 29, one at station 256, and two at station 257. The site of station 257 was visited four other times (station 27 in 2011 and stations 258, 262, and 263 in 2016) but M. tillieri was not found there.

Live animals ( Fig. 13 ). Live animals are not covered dorsally with a thin layer of mud. The notum is homogenously beige.

The hyponotum is pure white. The foot varies from pure white to beige. When the animal crawls undisturbed, the ocular tentacles extend normally for only a few millimeters beyond the notum margin but exceptionally for up to one centimeter, and the head is small and remains covered by the dorsal notum.

The body is elongated, not flattened. The dorsal notum is moderately thick and flexible. Its surface, when the animal is undisturbed, is finely granular, but not completely smooth. Dorsal gills and papillae are absent. There are between 15 and 25 single black dorsal eyes and a central group of three dorsal eyes. There is no retractable central papilla. When the animal is disturbed, it does not become a ball, and its dorsal notum does not become completely smooth. Live animals are from 33 to 52 mm long.

External morphology ( Fig. 14A ). The colour of preserved specimens is similar to live animals (homogenously beige dorsally and whitish ventrally). The width of the pedal sole is approximately a third of the total width. The pneumostome is median. Its position on the hyponotum relative to the notum margin and the pedal sole varies among individuals (from very close to the pedal sole to about half way between the pedal sole and the notum margin). The male aperture (opening of the copulatory complex) is located below the right ocular tentacle, slightly to its left side in dorsal view. In one individual, the deferent duct was observed (by transparency) following the peripodial groove from its anterior end all the way to its posterior end.

Digestive system ( Figs. 14 , 15 , Table 4). Examples of radular formulae are presented in Table 4. The length of the rachidian teeth is approximately 15 to 25 µm, significantly less than the lateral teeth. The length of the hook of the lateral teeth is approximately 40 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 of the lateral teeth is mostly tapered (square). The intestinal loops are of type III, with a transitional loop oriented between 12 and 3 o’clock. A rectal gland is present.

Reproductive system ( Fig. 14D ). The receptaculum seminis (caecum) is spherical to 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. 14E , 16 ). The male anterior organs consist of the penial papilla, the penial sheath, the vestibule, the deferent duct, and the retractor muscle. There is no accessory penial gland. The penial sheath is short (approximately 1 mm long) and straight, followed by a longer vestibule (approximately 3 mm long). Given that there is no accessory penial gland, the penial sheath and the vestibule are continuous. Both could be described as a penial sheath of approximately 4 mm long. Proximally, the penial sheath protects a penial papilla which consists of a short tube, slightly conical at its base. Its tip is marked by a distinct pattern of five longitudinal folds separated by deep grooves. Its length ranges between 0.6 and 0.9 mm. Its diameter is approximately 250 to 300 μm at the base and 200 μm at the tip. There are no penial hooks. The retractor muscle is longer than the penial sheath and inserts at about the anterior third of the floor of the visceral cavity. The deferent duct is highly convoluted with many loops.

Remarks. Melayonchis tillieri is rare but it is one of the few unmistakable onchidiid species. In the field, we immediately recognised it as a distinct species and we called it the “white, smooth species” because of its pure white hyponotum and very finely granular dorsum. Internally, M. tillieri also differs from other Melayonchis species , especially by its distinctive penial papilla ( Table 3).

Figures

Fig. 12. Habitats, Melayonchis tillieri, at the only three sites where M. tillieri is known, near Kuala Sepatang, Peninsular Malaysia. A, From the boat, view of station 256 (in 2016, same site as station 29 in 2011), old and open Rhizophora forest of tall and beautiful trees, with hard mud, many creeks and logs; B. Landing place to access station 256; C, Station 256, inside the forest; D, same as C; E, Station 28 (in 2011, type locality), old and open Rhizophora forest with tall trees, hard mud, creeks, and many logs; F, Station 257 (in 2016), old forest with tall Rhizophora trees, high in the tidal zone (ferns), in the educational mangrove preserve.

Fig. 13. Live animals, Melayonchis tillieri, dorsal (A–D) and ventral (E–G) views. A, Holotype, 50 mm long [973] (USMMC 00070); B, 36 mm long [5957] (USMMC 00072); C, 36 mm long [5970] (USMMC 00073); D, 52 mm long [5972] (USMMC 00074); E, same as A; F, same as B; G, same as D.

Fig. 14. External morphology, digestive system, and reproductive system, Melayonchis tillieri. Small arrows indicate the direction of the intestinal transport. Large arrows indicate the orientation of the transitional loop. A, Ventral, anterior view, scale bar = 5 mm, [5972] (USMMC 00074); B, Digestive system (type III, with a transitional loop at 12 o’clock), dorsal view, scale bar = 5 mm, holotype [973] (USMMC 00070); C, Digestive system (type III, with a transitional loop at 1 o’clock), dorsal view, scale bar = 2 mm, same as A; D, Posterior reproductive parts, scale bar = 4 mm, same as B; E, Anterior male copulatory parts, scale bar = 3 mm, same as B. Abbreviations: dd, deferent duct; ddg, dorsal lobe of digestive gland; ddt, deferent duct (by transparency); f, foot (pedal sole); fgm, female gland mass; h, hyponotum; hd, hermaphroditic duct; hg, hermaphroditic gland; i, intestine; mo, male opening; ol, oral lobe; ot, ocular tentacle; ov, oviduct; pdg, posterior lobe of the digestive gland; ppg, peripodial groove; ps, penial sheath; rg, rectal gland; rm, penial retractor muscle; rs, receptaculum seminis; sp, spermatheca; st, stomach; tp, transversal protuberance (on oral lobe); v, vestibule.

Fig. 15. Radula, Melayonchis tillieri. A, Rachidian and innermost lateral teeth, scale bar = 10 μm, [973] holotype (USMMC 00070); B, Right lateral teeth, scale bar = 20 μm, [5972] (USMMC 00074). 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. 16. Penial papilla, Melayonchis tillieri. A, Scale bar = 100 μm, [973] holotype (USMMC 00070); B, Scale bar = 100 μm, [5972] (USMMC 00074); C, Scale bar = 100 μm, [972] (USMMC 00071); D, Tip of the papilla, scale bar = 40 μm, same as C.

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.