Melayonchis 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 : 568-569

Melayonchis Dayrat & Goulding in Dayrat et al., 2017

Type species. Melayonchis eloisae Dayrat in Dayrat et al., 2017, by original designation.

Etymology. Combination of Melayu, the Malay word for Malays, and Onchis, one of the old names used for onchidiid slugs (Dayrat et al., 2017: 1860).

Gender. Masculine, gender of onchis ( ICZN, 1999; Article 30.1.1), a word derived from the masculine Greek word ὁ ὂγκος (onchos), which means ʻmassʼ or ʻtumour.ʼ

Diagnosis. Body not flattened. No dorsal gills. Dorsal eyes present on notum. Retractable, central papilla present or absent. Foot wide. Eye tentacles short and narrow. Eyes at tip of ocular tentacles. Elongated, transversal protuberance on each oral lobe. Male opening below and on left of right ocular tentacle (in dorsal view). Pneumostome median. Female opening close to anus. Intestinal loops of types II and III. Rectal gland present. Lateral radular teeth with a strong protuberance on their inner lateral margin. Accessory penial gland and hollow spine present or absent. When present, accessory penial gland with no muscular sac. Penis short (<1 mm), soft papilla with no hooks.

Distinctive features. Externally, Melayonchis slugs could be easily confused with Marmaronchis Dayrat & Goulding in Dayrat et al., 2018, or Wallaconchis Goulding & Dayrat in Goulding et al., 2018b, even though the colour pattern of some Melayonchis species is very distinctive and unmistakable. Internally, however, the short (<1 mm), the soft papilla with no hooks is unique to Melayonchis slugs. In addition, the lateral radular teeth of Melayonchis are characterised by a strong protuberance on their inner lateral margin. A similar protuberance is also present in radular teeth of Platevindex and Marmaronchis slugs. However, the protuberance of Melayonchis radular teeth is much stronger than in Platevindex and Marmaronchis .

Distribution ( Fig. 6 ). Melayonchis is known from the Andaman Islands, both sides of the Strait of Malacca (western Peninsular Malaysia and eastern Sumatra), the South China Sea (southern Vietnam, Brunei Darussalam), and the Northern Territory, Australia. The Strait of Malacca is the core of the geographic distribution of Melayonchis : the Matang mangrove, near Kuala Sepatang, western Peninsular Malaysia, hosts all but one species.

Remarks. Differences among Melayonchis species are summarised in Table 3. Both the diagnosis and the distinctive features above are slightly updated from our previous description (Dayrat et al., 2017: 1860) to incorporate new findings: the retractable central papilla is absent in some species (described as present in the 2017 diagnosis) and, when present, is only slightly larger than the other dorsal papillae; there are no intestinal loops “intermediary between types II and III” and intestinal loops are clearly of type II or of type III ( Fig. 2 ); finally, in 2017, the protuberance on the inner lateral margin of the lateral radular teeth was mentioned as a distinctive feature unique to Melayonchis , but, after dissecting many other onchidiid slugs since then ( Dayrat et al., 2018, 2019a, b; Goulding et al., 2018a, b, c), we know now that a smaller but similar protuberance is found in Platevindex and Marmaronchis , but that the short (<1 mm), soft penial papilla is truly unique to Melayonchis .

Figures

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.

Fig. 2. Onchidiid intestinal type III. Small black arrows indicate the direction of the intestinal transport, which starts in the blue loop. A blue loop turns clockwise. A yellow loop turns counterclockwise. A green loop is transitional between a blue loop and a yellow loop. The orientation of the transitional (green) loop is indicated with a red arrow (e.g., at 3 o’clock in C, at 6 o’clock in I, and at 12 o’clock in J). For details about the Melayonchis and Onchidium individuals illustrated here, see Dayrat et al. (2016, 2017). A, Plate’s (1893: pl. 8, fig. 31a) original figure of a type III, Onchidium nigrum (junior synonym of Onchidium stuxbergi), redrawn (scale unclear), colours indicate Plate’s interpretation; B, same as A, colours indicate a new interpretation based on two parts (in red) drawn by mistake by Plate (see the text); C, O. stuxbergi (from Dayrat et al., 2016: fig. 11B), scale bar = 5 mm; D, O. stuxbergi, (PNM 041200), scale bar = 4 mm; E, O. stuxbergi, [5605], scale bar = 5 mm; F, Melayonchis eloisae, [1011], scale bar = 2 mm; G, M. eloisae, [922], scale bar = 2 mm; H, M. eloisae, [5607], scale bar = 1 mm; I, M. eloisae, [951], scale bar = 2 mm; J, M. aileenae, [5631], scale bar = 2 mm; K, M. aileenae (from Dayrat et al., 2017: fig. 23a), scale bar = 3 mm; L, M. aileenae, [970], scale bar = 2 mm; M, M. siongkiati, [1052], scale bar = 2 mm; N, M. siongkiati, [5608], scale bar = 3 mm; O, M. siongkiati (from Dayrat et al., 2017: fig. 12a), scale bar = 4 mm.