Melayonchis siongkiati Dayrat and Goulding, 2017

Dayrat, Benoît, Goulding, Tricia C., Apte, Deepak, Bhave, Vishal & Ngô Xuân, Qu ảng, 2017, A new genus and four new species of onchidiid slugs from South-East Asia (Mollusca: Gastropoda: Pulmonata: Onchidiidae), Journal of Natural History (J. Nat. Hist.) 51 (31 - 32), pp. 1851-1897 : 1871-1877

Melayonchis siongkiati Dayrat and Goulding sp. nov.

( Figures 9–15 )

Type locality

Singapore, Mandai River , 01°26.237 N, 103° 45.730 E, 2 April 2010 [station 6, following the river from the railroad towards 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, designated here: one specimen 32/20 [DNA 1002] mm, in formalin and 70% alcohol, with a piece in 95% alcohol, leg. B. Dayrat and S.K. Tan ( ZRC.MOL.6501).

Additional material examined

Malaysia, Peninsular Malaysia , Matang, Crocodile River, 04°49.521 ʹ N, 100°37.630 ʹ E, 11 July 2011, 5 specimens (28/17 to 17/ 12 mm), leg . B . Dayrat and T . Goulding [station 16, tall Rhizophora trees with soft and hard mud] ( USMMC 00012 ); Malaysia , Peninsular Malaysia, Merbok, 05°39.035 ʹ N, 100°25.782 ʹ E, 12 July 2011, 1 specimen (32/ 15 mm) ,

leg. B. Dayrat and T. Goulding [station 21, deep Rhizophora forest with old, tall trees, hard mud, many small creeks and dead logs] ( USMMC 00013); Malaysia, Peninsular Malaysia, Merbok, 05°40.143 ʹ N, 100°26.178 ʹ E, 12 July 2011, 1 specimen (8/ 6 mm), leg. B. Dayrat and T. Goulding [station 22, mostly Rhizophora , soft mud and some very soft mud near creek] ( USMMC 00014); Malaysia, Peninsular Malaysia, Kuala Sepatang, 04° 50.434 N, 100°38.176 E, 18 July 2011, 2 specimens (28/15 and 25/ 16 mm), leg. B. Dayrat and T. Goulding [station 27, old forest with tall, old Rhizophora trees, high in the tidal zone (ferns), following boardwalk in educational preserve, reached a creek lower in the tidal zone, with mud] ( USMMC 00015); Malaysia, Peninsular Malaysia, Matang, off Kuala Sepatang, Crocodile River, Sungai Babi Manpus, 04°49.097 ʹ N, 100°37.370 ʹ E, 19 July 2011, 19 specimens (40/25 [#2] to 11/ 9 mm; 34/20 [#1], 26/20 [DNA 920] and 17/11 [DNA 947] mm), leg. B. Dayrat and T. Goulding [station 28, old and open Rhizophora forest with tall trees, hard mud, creeks, and many dead logs] ( USMMC 00016); Malaysia, Peninsular Malaysia, Matang, close to the jetty, facing the fishermen’ s village on the other side of the river, 04°50.154 ʹ N, 100°36.368 ʹ E, 20 July 2011, 4 specimens (25/16 to 16/ 12 mm), leg. B. Dayrat and T. Goulding [station 29, oldest and open Rhizophora forest of tallest and beautiful trees, with hard mud, many creeks, and many dead logs] ( USMMC 00017); Brunei Darussalam, Pulau Siaru, Temburang, 04°49.066 ʹ N, 115°02.250 ʹ E, 26 July 2011, 38 specimens (35/21 [#1] to 10/6 [#2] mm; 12/9 [#3]), leg. B. Dayrat, T. Goulding and S. Calloway [station 30, mostly Nypa palms, by the river] ( BDMNH); Brunei Darussalam, Sungai Brunei, 04°53.756 ʹ N, 114°59.496 ʹ E, 26 July 2011, 3 specimens (27/14 to 10/ 10 mm), leg. B. Dayrat, T. Goulding and S. Calloway [station 31, very large, tall Rhizophora trees, soft mud] ( BDMNH); Brunei Darussalam, Pulau Pyatan, Teluk Brunei, 04°55.246 ʹ N, 115°02.764 ʹ E, 27 July 2011, 46 specimens (35/22 [DNA 1052] to 10/ 6 mm), leg. B. Dayrat, T. Goulding and S. Calloway [station 32, open mangrove with a few sparse old trees, and large old logs, by the river] ( BDMNH); Brunei Darussalam, Pulau Kaingara, 04°57.020 ʹ N, 115°01.785 ʹ E, 28 July 2011, 23 specimens (33/17 to 12/ 8 mm), leg. B. Dayrat, T. Goulding and S. Calloway [station 33, open mangrove with medium Rhizophora trees and logs, by the river] ( BDMNH); Brunei Darussalam, Mentiri, Jalan Batu Marang, 04°59.131 ʹ N, 115°01.820 ʹ E, 29 July 2011, 12 specimens (28/17 to 17/ 10 mm), leg. B. Dayrat, T. Goulding and S. Calloway [station 34, old mangrove with tall Rhizophora trees with high roots and Thalassina mounds] ( BDMNH); Vietnam, Can Gio, 10°27.803 ʹ N, 106°53.288 ʹ E, 16 July 2015, 2 specimens (31/18 and 30/20 [DNA 5608] mm), leg. T. and J. Goulding [station 228, high intertidal, open forest of tall Rhizophora trees, hard mud, near a small river] ( ITBZC IM 00010); Vietnam, Can Gio, 10°27.620 ʹ N, 106°53.316 ʹ E, 17 July 2015, 1 specimen (37/ 25 mm), leg. T. and J. Goulding [station 231, open mangrove with large Avicennia trees, soft mud, some dead logs] ( ITBZC IM 00011).

Distribution

Singapore (type locality), Peninsular Malaysia, Brunei Darussalam, Vietnam .

Etymology

M. siongkiati is dedicated to Siong Kiat Tan, from the Lee Kong Chian Natural History Museum (National University of Singapore), who kindly spent some time with the first author in the mangroves of Singapore and generously shared his excellent field knowledge of the local fauna, especially snails and slugs.

Habitat ( Figure 9 )

Melayonchis siongkiati is mostly found in the high intertidal, and it especially favours old and open Rhizophora forests with dead logs. It lives on trunks and roots of mangrove trees, often not muddy but covered with algae instead. It can also be found on dead logs. Occasionally, when abundant, it can also be found on muddy rocks and cemented walls at the margin of a mangrove. It is not found directly on mud.

Abundance

Melayonchis siongkiati was abundant only in Brunei, where we found many specimens at almost every site that we visited. In Malaysia, we found a few individuals in most sites that seemed to be the right habitat, and a large population was found only at one site. It is uncommon in Singapore and Vietnam.

Colour and morphology of live animals( Figures 10 , 11 )

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. That thin layer of mucus makes them very cryptic. It may also help prevent desiccation. The colour of the dorsal notum appears after the thin muddy layer is removed. The background of the notum, light brown or greyish, is irregularly mottled by dark brown areas. In addition, the background can occasionally be mottled with whitish, irregular areas also. The colour of the hyponotum varies from light grey to dark brown or dark blue but is always marked by a significantly lighter ring at its margin. The foot is grey (light to dark). The brown ocular tentacles are short and extend for only a few millimetres beyond the notum margin when the animal crawls undisturbed. The head is small and remains covered by the dorsal notum as the animal crawls.

The body is usually not flattened, but some animals may seem flattened, especially when crawling. The dorsal notum is elongated, oval. The dorsal notum is not particularly thick. Its surface, when the animal is undisturbed, is not smooth. Dorsal gills are absent. Large papillae are absent but small conical papillae are present. About 15 of those papillae bear a black ‘dorsal eye’ (occasionally two or three). A larger, central papilla bears three black ‘dorsal eyes’. In addition, the notum is finely granular. When the animal is disturbed (typically, if one touches its dorsum), it can coil up but does not form a complete sphere ( Figure 10b, d ). Also, its dorsal notum immediately secretes a distinct oily mucus that makes the dorsum shiny and oily. In fact, in the field, we called this ‘the oily species’ ( Figures 10a , 11c ). Also, the dorsum of disturbed animals tends to be smooth (instead of finely granular), but the small conical papillae inflate into slightly larger bumps. Animals that are preserved without first being relaxed remain coiled up into a sphere. Crawling individuals can measure up to 40 mm, but most of them measure about 20 to 25 mm on average. Preserved specimens no longer display the distinctive colour traits of live animals. The ventral colour, in particular, is homogeneously whitish or creamish, occasionally dark ( Figure 11b, d, f ).

Digestive system ( Figures 12a , 13 )

Examples of radular formulae are ( Figure 13 ): 80 × (165–1–165) in BDMNH station 30 #1 (35 mm long), 40 × (80–1–80) in BDMNH station 30 #2 (10 mm long), and 85 × (160–1– 160) in USMMC 00016 #2 (40 mm long). The length of the rachidian teeth is about 30 µm, significantly less than that of the lateral teeth. Along a half row, all the lateral teeth do not have exactly the same shape. Their length (about 60 µm) is similar, although there seems to be a slight, gradual increase from innermost to outermost teeth (excluding the first few innermost and outermost lateral teeth which are significantly smaller than the rest of the lateral teeth). Finally, the tip of the hook is fairly 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 pattern of its loops is of type III ( Figure 12a ).

Reproductive system ( Figures 12b , 14 , 15 )

There is a small and ovate receptaculum seminalis (caecum) along the hermaphroditic duct. The spermatheca (for the storage of exosperm) is large and elongated, and connects to the oviduct through a very short and narrow duct ( Figure 12b ).

The male anterior organs ( Figure 14 ) consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle) and penial accessory gland (flagellum, duct and hollow spine). The penial complex and the penial accessory gland share distally the same vestibule and same anterior male opening ( Figure 14 ). The length of the flagellum of the penial gland varies among individuals. However, in all specimens dissected, the flagellum of the penial gland is coiled. The gland is a tube with a dead end proximally: it is characterised by two portions of distinct diameter: the proximal portion, or flagellum, is slightly wider than the distal portion or gland duct. That distinction in diameter, however, does vary among individuals. In immature specimens, the flagellum and the duct may not be distinguishable. Distally, the duct of the gland ends in a hard, hollow spine ( Figure 15a, b ), which conveys the secretion from the flagellum to the outside (and the partner). The hollow spine is narrow and elongated, slightly curved. It measures about 100 μm in diameter at its conical base and narrows down to 20 μm distally, for a length from 0.8 to 0.9 mm. The hollow spine opens into the proximal region of the vestibule.

The penial sheath is short (less than 4 mm in length) and straight. The penial sheath protects a penis which consists of a short, straight papilla, from 100 to 130 μm in diameter and from 0.5 to 1 mm in length ( Figure 15c, d ). There are no penial hooks. The insertion of the retractor muscle marks the separation between the penial sheath and the deferent duct ( Figure 14 ). The retractor muscle is longer or shorter than the penial sheath but inserts at about half the length of the visceral cavity. The deferent duct is convoluted with many loops (in immature specimens, the deferent duct is significantly less convoluted).

Distinctive field diagnostic features

A table at the end of the introduction summarises the most important features that can help distinguish and identify Melayonchis species ( Table 3). In the field, individuals of M. siongkiati can easily be identified because their dorsal notum secretes a distinctive oily mucus as soon as they are disturbed, which makes their dorsum shiny and oily. They can also coil up into a sphere, but not as systematically and completely as M. eloisae . The dorsal colour of live animals, which is quite variable, is harder to use for identification. When animals are observed without being disturbed while crawling, then they are much harder to identify and could easily be confused with other species, including species from other genera (such as Platevindex ).

Figures

Figure 9. Habitats for Melayonchis siongkiati. (a) Type locality, Singapore, Mandai River, open mangrove forest with tall trees and soft mud (station 6). (b) Malaysia, Peninsular Malaysia, Matang, old and open Rhizophora forest with tall trees, hard mud, creeks and many dead logs (station 28). (c) Brunei Darussalam, Pulau Pyatan, open mangrove with a few sparse old trees, and large old logs, by the river (station 32). (d) Vietnam, Can Gio, open mangrove with large Avicennia trees, soft mud, some dead logs (station 231).

Figure 10. Live specimens, Melayonchis siongkiati. (a) Holotype, dorsal view (with oily secretion), 32 mm long [DNA 1002], Singapore (ZRC.MOL.6501). (b) Same as (a). (c) Dorsal view, 30 mm long, Brunei Darussalam, station 32 (BDMNH). (d) Same as (c). (e) Dorsal view, 30 mm long [DNA 5608], Vietnam (ITBZC IM 00010). (f) Dorsal view, 37 mm long, Vietnam (ITBZC IM 00011).

Figure 11. Live specimens,Melayonchis siongkiati. (a) Dorsal view, 28 mm long, Malaysia (USMMC 00015). (b) Ventral view, same as (a). (c) Dorsal view,25 mm long, Malaysia (USMMC 00016).(d) Ventral view,same as (c). (e) Dorsal view, 25 mm long, Brunei Darussalam, station 34 (BDMNH). (f) Ventral view, same as (e).

Figure 12. Melayonchis siongkiati, Malaysia (USMMC 00016, #1). (a) Digestive system. (b) Posterior hermaphroditic (female) reproductive system. Abbreviations: dd, deferent duct; ddg, dorsal lobe of digestive gland; fgm, female gland mass; hd, hermaphroditic duct; hg, hermaphroditic gland; i, intestine; ldg, lateral lobe of the digestive gland; ov, oviduct; pdg, posterior lobe of the digestive gland; rg, rectal gland; rs, receptaculum seminis; sp, spermatheca; spv, spermoviduct; st, stomach. Scale bar: a = 4 mm; b = 3 mm.

Figure 13. Radula, Melayonchis siongkiati. (a) Rachidian and innermost lateral teeth, Malaysia (USMMC 00016,#2). (b) Rachidian and innermost lateral teeth, same as (a). (c) Outermost lateral teeth,same as (a). (d) Lateral teeth, same as (a). (e) Rachidian and innermost lateral teeth, Brunei Darussalam, station 30 (BDMNH, #1). (f) Lateral teeth, inferior view, same as (e). The arrows indicate the lateral protuberance which is characteristic of the teeth in Melayonchis. Scale bars: a, d = 40 μm; b, c, e = 20 μm; f = 4 μm.

Figure 14. Anterior, male apparatus, Melayonchis siongkiati. (a) Malaysia (USMMC 00016, #1). (b) Brunei Darussalam, station 30 (BDMNH, #3). Abbreviations: ag, accessory gland; agf, accessory gland flagellum; dd, deferent duct; ps, penial sheath; rm, retractor muscle; v, vestibule. Scale bar: a = 2 mm; b = 0.5 mm.

Figure 15. Anterior, male apparatus, Melayonchis siongkiati. (a) Hollow spine of penial accessory gland, Brunei Darussalam, station 30 (BDMNH, #1). (b) Hollow spine of penial accessory gland, Malaysia (USMMC 00016, #1). (c) Penis, Malaysia (USMMC 00016, #2). (d) Penis, same as (b). Scale bars = 100 μm.