Wallaconchis nangkauriense (Plate, 1893)

Goulding, Tricia C., Khalil, Munawar, Tan, Shau Hwai & Dayrat, Benoit, 2018, Integrative taxonomy of a new and highly-diverse genus of onchidiid slugs from the Coral Triangle (Gastropoda, Pulmonata, Onchidiidae), ZooKeys 763, pp. 1-111: 1

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scientific name

Wallaconchis nangkauriense (Plate, 1893)

comb. n.

Wallaconchis nangkauriense (Plate, 1893)  comb. n. Figs 29, 30, 31, 32, 33

Onchidium nangkauriense  Plate, 1893: 170-171, pl. 12, figs 84, 93-95 (as Oncidium nangkauriense  ).

Paraoncidium nangkauriense  : Labbé 1934: 231.

Type locality.

Nangkauri, eine Insel der Nicobaren [Nancowry, Nicobar Islands, Indian Ocean]. No fresh material was collected from the type locality (access to the Nicobar Islands is extremely restricted). However, fresh material was collected from the Andaman Islands, just north of the Nicobar Islands (see below, additional material).

Type material.

Lectotype, 19/14 mm, designated here (ZMB 45659a). The lectotype was dissected prior to the present study but is well preserved, with all organs remaining except for the penis. The lectotype is designated because it still displays the diagnostic characters of the species (a retractor muscle inserted within the left side of the body wall). The paralectotype (22/15 mm) also was dissected prior to the present study (ZMB 45659b). A piece of the notum is missing, as well as the digestive glands, and a part of the female reproductive system.

Additional material examined.

India, Andaman Islands, South Andaman, Burman Nala, 11°33.23'N, 92°44.00'E, 2 specimens 25/12 mm [1074] and 24/10 mm [1075], st 53, rocky shore with a patch of Rhizophora  , sand and coral rubble, but no mud. (BNHS 50). Indonesia, Sumatra, Lampung, Penegahan, 05°40.40'S, 105°33.76'E, 4 specimens 42/25 mm, 28/15 mm [1787], 23/15 mm [1788], and 23/15 mm [1786], st 78, coral rubble on beach exposed to estuary (UMIZ 00012); North Sulawesi, Bahoi, 01°43.36'N, 125°01.23'E, 2 specimens 25/14 mm [2161] and 18/13 mm [2156], st 85, sand and small rocks outside a mangrove (UMIZ 00013); North Sulawesi, Tamperong, 01°41.51'N, 125°00.80'E, 1 specimen 20/13 mm [2192], st 87, rocks behind mangrove of small Rhizophora  trees (UMIZ 00014); North Sulawesi, Wori, 01°36.06'N, 124°51.73'E, 1 specimen 30/20 mm [2257], st 90, old mangrove forest with Avicennia  , Sonneratia  , and Rhizophora  , with rocks (UMIZ 00016); Ambon, Haruku Island, 03°36.52'S, 128°25.07'E, 1 specimen 22/14 mm [2731], st 127, rocky Sonneratia  mangrove with coral rubble (UMIZ 00017); Lombok, Seriwe Bay, 08°54.55'S, 116°22.22'E, 1 specimen 39/28 mm [2972], st 148, rocks with Avicennia  trees (UMIZ 00018); Bali, Pemuteran, Labuhan Lalang Harbor, 08°08.61'S, 114°32.33'E, 2 specimens 20/12 mm [3136] and 18/12 mm [3129], st 157, coral rubble, rocks and a few Avicennia  (UMIZ 00020). Philippines, Luzon, Batangas, Lian, 13°59.76'N, 120°37.43'E, 1 specimen 28/17 mm [3162], st 181, sandy, open Avicennia  forest (PNM 041204); Luzon, Batangas, Calatagan, 13°55.32'N, 120°37.26'E, 2 specimens 30/17 mm [3208] and 24/14 mm [3217], st 183, rocks in Avicennia  and Rhizophora  mangrove (PNM 041205); Bohol, Maribojoc, 09°44.02'N, 123°47.45'E, 3 specimens 32/20 mm [3269], 32/23 mm [3268], and 28/20 mm [3276], st 191, coral rubble with sand (PNM 041206); Bohol, Loay, 09°36.23'N, 123°59.72'E, 1 specimen 21/14 mm [5763], st 198, mostly sand, and a few Avicennia  (PNM 041229); Bohol, Maribojoc, 09°44.02'N, 123°47.45'E, 2 specimens 28/16 mm [3396] and 18/12 mm [3401], st 200, coral rubble with sand at night (PNM 041207); Bohol, Maribojoc, 09°44.28'N, 123°49.39'E, 1 specimen 28/16 mm [3427], st 202, coral rubble with sand and algae, near Sonneratia  (PNM 041208).


India: Nicobar Islands (type locality) and Andaman Islands. Indonesia: Ambon, Bali, Lombok, Sulawesi, and southeastern Sumatra. Philippines: Bohol and Luzon. All records are new except for the type locality.


(Fig. 29, Table 3). In the Andaman Islands, W. nangkauriense  is found in rocky mangroves, on the roots of Rhizophora  covered with algae (Fig. 29B). Throughout the rest of its range, W. nangkauriense  is found predominantly on coral rubble or small stones, which may be mixed with mangrove trees (Fig. 29C, -D), adjacent to a mangrove (Fig. 29A) or a few isolated mangrove trees (Fig. 29E), or completely without mangrove trees (Fig. 29F). Wallaconchis nangkauriense  frequently occurs with W. ater  .


(Table 5). Wallaconchis nangkauriense  cannot be distinguished from other Wallaconchis  species based on external features. Brightly-colored individuals (e.g., red, yellow, and orange) cannot be distinguished from W. ater  and W. graniferum  . Grey or brown specimens cannot be distinguished from any other Wallaconchis  species. Internally, however, the extremely long oviduct is a diagnostic feature of W. nangkauriense  . The long retractor muscle of the penis inside the posterior body wall is also distinctive (although it is not present in all individuals).

Color and morphology of live animals

(Fig. 30). In sandy habitats, sand grains may be stuck to the dorsal notum. The dorsal coloration is highly variable, often a mottling of two or three colors. The most common colors are grey, brown, black, and yellow; combinations of red, green and orange also occur. The colors of the hyponotum and foot are variable. The hyponotum is usually light grey or cream, occasionally dark grey or white. The foot is either cream, light grey or dark grey. The color of the ocular tentacles varies between yellow-orange, light brown and reddish brown.

External morphology.

The number of dorsal papillae with eyes (three or four per papilla) is between five and 15, but, exceptionally, 25 papillae were observed in one specimen. There is a retractable papilla with eyes in the center of the dorsal notum, which may be slightly raised above the dorsal surface.

Digestive system

(Fig. 31, Table 4). Examples of radular formulae are presented in Table 4. The length of the rachidian teeth is approximately 20 µm, significantly smaller than that of the lateral teeth. The length of the hook of the lateral teeth gradually increases, from 40 to 65 µm, along the half row from the inner teeth to outer teeth (excluding the innermost and outermost lateral teeth, which are significantly smaller). The intestinal loops are of type I.

Reproductive system

(Fig. 32A). Posteriorly, the oviduct is extremely long, convoluted, and slightly narrow. The spermatheca is spherical and joins the distal region of the oviduct through a thin, short duct.

Copulatory apparatus

(Figs 32B, C, 33). The distal end of the penis lies free within a long, narrow vestibule, and its proximal end is protected within the penial sheath (in Fig. 32C, distal part of penial sheath removed). The length of the penis varies between individuals and geographic localities and ranges from approximately 9-22 mm. In specimens from the Philippines, the penis tends to be longer and frequently folds back upon itself within the vestibule. The penis is elongated, narrow, and smooth with no hooks (Fig. 33). The length of the penial sheath varies from a few millimeters (in small individuals) to longer than the body length. In addition, the proximal end of the penial sheath may be partially hidden within the left posterior body wall (Fig. 32B). The deferent duct is highly convoluted with many loops; however, in immature specimens, the deferent duct is significantly less convoluted. The retractor muscle enters the body wall at the posterior end of the visceral cavity near the rectum and lies (partially or completely) hidden inside the body wall. The insertion of the retractor muscle varies. In large specimens, the retractor muscle may extend anteriorly inside the left side of the body wall and insert up to half way up the body; in smaller specimens, it inserts within the posterior region of the left body wall. Exceptionally, in large specimens from southeastern Sumatra and the Andaman Islands, the retractor muscle inserts on the posterior wall of the visceral cavity, near the rectum, and not within the body wall. Finally, in small specimens from the Andaman Islands, the retractor muscle inserts on the body wall, near the heart, on the right side of the visceral cavity.


Onchidium nangkauriense  belongs to Wallaconchis  because it shares its unique combination of characters (intestinal loops of type I, no rectal gland, no dorsal gills, no penial accessory gland). Also, there is strong evidence that the name W. nangkauriense  applies to the species being described here. Indeed, Plate (1893: pl. 12, fig. 84) illustrated an extremely long oviduct, a character which matches exactly the species described here and has not been found in any other onchidiid species, as well as a long retractor muscle entering the body wall at the posterior end of the body cavity and extending anteriorly inside the left side of body wall, a peculiar character only known in this onchidiid species.

In the same publication, Plate also described Onchidium simrothi  from the same type locality as O. nangkauriense  (Nangkauri, Nicobar Islands, India). Onchidium simrothi  is transferred here to Wallaconchis  but is regarded as a nomen dubium because important characters are not mentioned in the original description and could not be observed in the type material. The two syntypes of O. simrothi  are small (10/6 and 10/5 mm) and in very poor condition (they previously dried, and most of the organs are missing, with the exception of parts of the female reproductive system and of the digestive system). Plate does not indicate whether the rectal gland is present or absent, but the intestine of type I and the absence of an accessory penial gland indicate that the two syntypes of O. simrothi  belong to a Wallaconchis  species or a Platevindex  species. However, the male opening of O. simrothi  (below the right tentacle) and its central dorsal tubercle with multiple eyes are inconsistent with Platevindex  . We therefore consider that O. simrothi  belongs to Wallaconchis  . Given that both W. simrothi  and W. nangkauriense  were described from the Nicobar Islands, that the Nicobar Islands are within the geographic range of only one Wallaconchis  species ( W. nangkauriense  ) and are very far from the limits of distribution of all other Wallaconchis  species, it is most likely that both names are synonyms. According to Plate, the difference between O. simrothi  and O. nangkauriense  is the length of their penis (4 mm long and 55 mm, respectively). However, our observations show that the small specimens (< 20 mm long) of W. nangkauriense  from the Andaman Islands (which are very close to the Nicobar Islands) have a smaller penis with a retractor muscle inserted halfway down the body, which could explain the shorter penis described by Plate in the tiny (10 mm long) syntypes of O. simrothi  . However, strictly speaking, neither Plate’s original description of O. simrothi  nor the dried remains of its type material can be used to apply this name to any species. Therefore, O. simrothi  is transferred to Wallaconchis  but is regarded as a nomen dubium.

Onchidium nangkauriense  was transferred by Labbé (1934) to his genus Paraoncidium  (because of the lack of penial accessory gland in O. nangkauriense  ), but he did not comment on the species itself. Labbé (1934) re-described O. simrothi  based on new material from New Guinea, which is far from the type locality (Nangkauri, Nicobar Islands). However, Labbé’s specimens were not part of a Wallaconchis  species (and thus could not be part of O. simrothi  ) because their median male opening is incompatible with Wallaconchis  . Considering the lack of information about the reproductive anatomy in Plate’s original description of O. simrothi  , Labbé likely applied O. simrothi  arbitrarily to his specimens from New Guinea, based mostly on the preserved color and the width of the foot in relation to the hyponotum, aspects which do not suffice to distinguish species. Hoffmann (1928) included both O. nangkauriense  and O. simrothi  in his revision but did not examine any new material or add further comment.