Melayonchis eberlyi Dayrat & Goulding, 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 : 569-573

Melayonchis eberlyi Dayrat & Goulding , new species

( Figs. 7–11 )

Type locality ( Fig. 7 ). Malaysia, Peninsular Malaysia, Kuala Sepatang , 04°50.434′N, 100°38.176′E, 18 July 2011, station 27, old forest with tall Rhizophora trees, high in the tidal zone (ferns), in educational mangrove preserve .

Type material. Holotype, 40/ 20 mm [950], designated here (USMMC 00066).

Additional material examined. Malaysia, Peninsular Malaysia, Kuala Sepatang , 04°50.605′N, 100°38.133′E, 2 August 2016, 1 specimen 30/17 [6042], station 262, old forest with tall Rhizophora trees, high in the tidal zone (ferns), in educational mangrove preserve ( USMMC 00067 ) ; Peninsular Malaysia, Kuala Sepatang , 04°50.605′N, 100°38.133′E, 3 August 2016, 1 specimen 33/15 [6044], station 263, old forest with tall Rhizophora trees, high in the tidal zone (ferns), in educational mangrove preserve ( USMMC 00068 ) ; Peninsular Malaysia, Kuala Sepatang , 04°50.605′N, 100°38.133′E, 3 August 2016, 1 specimen 25/14 [6045], station 263, old forest with tall Rhizophora trees, high in the tidal zone (ferns), in educational mangrove preserve ( USMMC 00069 ) .

Distribution ( Fig. 6 ). Malaysia, Peninsular Malaysia, Malacca Strait, Kuala Sepatang (known from only one site).

Etymology. Melayonchis eberlyi is dedicated to the Eberly College of Science at the Pennsylvania State University for being understanding and supportive of our biodiversity exploration.

Habitat ( Fig. 7 ). Melayonchis eberlyi is found in the high intertidal. It lives on mangrove tree trunks and roots. It is not found directly on mud.

Abundance. Melayonchis eberlyi is known from one only site where it is hard to find. We found only one individual in 2011 (station 27). We went back four times to that site in 2016 (stations 257, 258, 262, and 263) and we found only three additional individuals, even though we were specifically looking for that species. At that site, other onchidiid species are common, including M. eloisae , the sister species of M. eberlyi .

Live animals ( Fig. 8 ). Even though they are not found directly on mud, live animals are covered dorsally with a thin layer of muddy mucus and the colour of their dorsal notum can hardly be seen. The background of the notum is usually dark (black or brown), but it can also be light brown. The background is mottled with white areas, some of which may form two irregular longitudinal lines on either side of the medial axis. The colour of the hyponotum varies between light grayish and black but is always marked by a white ring at the margin. The colour of the foot varies from yellow to bright orange. When the animal crawls undisturbed, the 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 oval, not flattened. The dorsal notum is thick. Its surface, when the animal is undisturbed, is not smooth. Dorsal gills and large papillae are absent, but small conical papillae are present. Between 15 and 20 of those papillae bear a single black dorsal eye. A slightly larger, central papilla bears three black dorsal eyes. In addition, the notum is finely granular. When the animal is disturbed, however, it forms an almost perfect ball, and its dorsal notum becomes completely smooth. Animals preserved without first being relaxed remain coiled up into a ball. Live animals are from 25 to 40 mm long.

External morphology. Preserved specimens tend to lose their distinct, live colour traits. The ventral colour of preserved specimens, in particular, is homogenously whitish. The width of the pedal sole ranges from approximately a third to half of the total width. The anus is posterior, median, close to the edge of the pedal sole. On the right side (to the left in ventral view), a peripodial groove is present at the junction between the pedal sole and the hyponotum, running longitudinally from the buccal area to the posterior end, a few millimeters from the anus and the pneumostome. The pneumostome is median. Its position on the hyponotum relative to the notum margin and the pedal sole varies among individuals but averages in the middle. The position of the female pore (at the posterior end of the peripodial groove) does not vary much among individuals. In the anterior region, the left and right ocular tentacles are superior to the mouth. Eyes are at the tip of the ocular tentacles. Inferior to the ocular tentacles, superior to the mouth, the head bears a pair of oral lobes. On each oral lobe, there is an elongated, transversal protuberance, likely with sensitive receptors. The male aperture (opening of the copulatory apparatus) is located below the right ocular tentacle, slightly to its left side, in dorsal view.

Digestive system ( Figs. 9A, B , 10 , Table 4). Examples of radular formulae are presented in Table 4. The rachidian teeth are unicuspid: the median cusp is always present; there are no distinct lateral cusps on the lateral sides of the base of the rachidian tooth. The length of the rachidian teeth is approximately 25 to 35 µm, significantly less than the lateral teeth. The lateral aspect of the base of the rachidian teeth is straight, or very slightly concave. The length of the hook of the lateral teeth gradually and slightly increases (from innermost to outermost) from approximately 50 µm to 65 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 varies from (slightly) pointed to round. The intestinal loops are of type III, with a transitional loop oriented between 6 and 8 o’clock. A rectal gland is present.

Reproductive system ( Fig. 9C ). The receptaculum seminis (caecum) is spherical to ovate. The spermatheca is nearly spherical 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. 9D , 11 ). 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 distally by a longer vestibule (approximately 4 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 5 mm long. Proximally, the penial sheath protects a penial papilla which consists of a short tube, conical at its base and club-shaped at its tip. Its tip is marked by a distinct pattern of five protuberances alternating with five grooves. Its length is approximately 0.8 mm. Its diameter is approximately 200 μm at the base and at the tip, and 70 μm in between. There are no penial hooks. The retractor muscle is longer 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. Even though we found only one individual in 2011, we immediately recognised that Melayonchis eberlyi was a distinct species. We also immediately realised that it was closely related to M. eloisae because individuals of both species share exactly the same colour pattern and coil up into a ball when disturbed. However, individuals of M. eberlyi are significantly larger ( Table 3) and so we originally called this species the “big, black and white ball” species in the field. Internally, M. eberlyi also differs from M. eloisae ( Table 3): for instance, the penial papilla of M. eloisae is narrow and elongated and its tip is marked by a rim with approximately 9 teeth, while the penial papilla of M. eberlyi is not narrow and marked by a distinctive, club-shaped tip. That being said, the largest individuals of M. eloisae (~ 25 mm long) could easily be confused with the small individuals of M. eberlyi (~ 25 mm long).

Figures

Fig. 7. Habitats, Melayonchis eberlyi, unique site (type locality) where Melayonchis eberlyi was found and which corresponds to the stations 27 (in 2011), 262 (in 2016), and 263 (in 2016) (A, B). Old forest with tall Rhizophora trees, high in the tidal zone (ferns), in the educational mangrove preserve near Kuala Sepatang, Peninsular Malaysia.

Fig. 8. Live animals, Melayonchis eberlyi, dorsal (A–C) and ventral (D–F) views. A, Holotype, 40 mm long [950] (USMMC 00066); B, 30 mm long [6042] (USMMC 00067); C, 33 mm long [6044] (USMMC 00068); D, same as A; E, 25 mm long [6045] (USMMC 00069); F, same as B.

Fig. 9. Digestive system and reproductive system, Melayonchis eberlyi. 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 8 o’clock), dorsal view, scale bar = 4 mm, [950] holotype (USMMC 00066); B, Digestive system (type III, with a transitional loop at 6 o’clock), dorsal view, scale bar = 2 mm, [6045] (USMMC 00069); C, Posterior reproductive parts, scale bar = 3 mm, same as A; D, Anterior male copulatory parts, scale bar = 2 mm, same as A. Abbreviations: 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. 10. Radula, Melayonchis eberlyi, [6042] (USMMC 00067). A, Rachidian and innermost lateral teeth, scale bar = 20 μm; B, Right lateral teeth, scale bar = 20 μm. Abbreviations: 1llt, first, left, lateral tooth; 1rlt, first, right, lateral tooth; 2llt, second, left, 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. 11. Penial papilla, Melayonchis eberlyi. A, Scale bar = 100 μm, [950] holotype (USMMC 00066); B, Scale bar = 100 μm, [6045] (USMMC 00069); C, Scale bar = 100 μm, [6042] (USMMC 00067); D, Tip of the papilla, scale bar = 30 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.