Wallaconchis gracile (Stantschinsky, 1907)

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 gracile (Stantschinsky, 1907)

comb. n.

Wallaconchis gracile (Stantschinsky, 1907)  comb. n. Figs 24, 25, 26, 27, 28

Onchidium gracile  Stantschinsky, 1907: 380-383, pl. 12, figs 7-9, pl. 13, fig. 25 (as Oncidium gracile  ).

Type locality.

Mindanao, Philippines. No fresh material was collected from the type locality. However, fresh material was collected from Bohol (Philippines), just north of Mindanao, and from Halmahera (Indonesia), just south of Mindanao.

Type material.

Lectotype, 25/20 mm, designated here (ZMB 103082a). The lectotype was opened previously but all internal organs remain. The other syntype (28/20 mm), which becomes a paralectotype (ZMB 103082b) was dissected by Stantschinsky, but all of the internal organs remain. The paralectotype of W. gracile  is part of W. buetschlii  , which is not an issue because it is no longer a name-bearing type (see the remarks on both W. gracile  and W. buetschlii  ).

Additional material examined.

Indonesia, North Sulawesi, Wori, 01°36.06'N, 124°51.73'E, 4 specimens 20/13 mm [2280], 19/15 mm [2252], 9/5 mm [2276], and 8/5 mm [2277], st 90, old Avicennia  , Sonneratia  , and Rhizophora  mangrove with rocks (UMIZ 00055); Bali, Gilimanuk, 08°10.26'S, 114°26.61'E, 3 specimens 22/22 mm [#1], 19/9 mm [3107] and 17/10 mm [3106], st 155, muddy-sandy beach near a mangrove (UMIZ 00056); North Maluku, Ternate, 00°45.18'N, 127°20.17'E, 2 specimens 15/10 mm [5158] and 15/9 mm [5159], st 220, sandy beach behind a rock wall (UMIZ 00057); Timor, Oesapa, 10°08.69'S, 123°38.21'E, 1 specimen 27/18 mm [5932], st 254, mangrove with fine sand (UMIZ 00074). Philippines, Bohol, Loay, 09°36.23'N, 123°59.72'E, 1 specimen 10/5 mm [3633], st 198, mostly sand, and a few Avicennia  (PNM 041230); Bohol, Loon, 09°49.91'N, 123°48.33'E, 3 specimens 25/10 mm [3653], 25/10 mm [3652] and 21/11 mm [3648], st 201, fine sand in front of Rhizophora  forest (PNM 041231).


Indonesia: Bali, Halmahera, Sulawesi, and Timor. Philippines: Bohol and Mindanao (type locality). All records are new except for the type locality.


(Fig. 24, Table 3). Wallaconchis gracile  is found on muddy sand at the margin of mangroves, by the sea (just a few meters away from it). This type of muddy sand is not as saturated with water as a mudflat, nor as firm as a sandy beach. The slugs are sensitive to changes in the pressure on the muddy sand and usually dig in when approached. Wallaconchis gracile  is specific to its habitat, and slugs are found only where patches of muddy sand are found.


(Table 5). Wallaconchis gracile  cannot be distinguished externally from other Wallaconchis  species. Grey or brown individuals cannot be distinguished from any other Wallaconchis  species. Even patches of color (i.e., red, yellow- orange, or brown-black) do not help distinguish it from similar species ( W. uncinus  , W. nangkauriense  , W. buetschlii  , W. ater  , and W. comendadori  ).

Internally, the attachment of the oviduct to the body wall distinguishes W. gracile  from the species with a similar penial anatomy (i.e., W. sinanui  , W. comendadori  , and W. melanesiensis  ), but is shared with W. buetschlii  . The penial sheath enclosing the penis of W. gracile  distinguishes it from W. buetschlii  . Also, the retractor muscle is thicker in W. gracile  than in W. buetschlii  .

Color and morphology of live animals

(Fig. 25). The dorsal color is usually brown, greyish brown, or dark brown, occasionally with an orange or yellow hue. The color may be homogenous or with brown patterns. Longitudinal stripes or a patch of color (red, yellow-orange, or brown-black) are sometimes present. The hyponotum is light yellow-orange. The foot is light grey.

External morphology.

Between six and eight dorsal papillae bear eyes, with three or four per papilla. 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. 26, Table 4). Examples of radular formulae are presented in Table 4. The length of the rachidian teeth is approximately 15 µm, significantly smaller than that of the lateral teeth. The length of the hook of the lateral teeth gradually increases (from 35 to 45 µm) from the inner teeth to the outer teeth, excluding the innermost and outermost lateral teeth which are significantly smaller. The intestinal loops are of type I.

Reproductive system

(Fig. 27A). The distal region of the oviduct forms a loop (approximately 1/2 to 3/4 of a circle) attached by fibers of tissue to the inner wall of the visceral cavity. The oviduct is narrow and approximately of the same width as the deferent duct. The surface of the spermatheca is marked by several lobes. A short duct connects the spermatheca to the middle part of the oviduct.

Copulatory apparatus

(Figs 27B, 28). The distal end of the penis lies free within the proximal part of a long, narrow vestibule, and the proximal end of the penis is protected within the penial sheath. The length of the penis varies between individuals, and ranges from approximately 1 - 3 mm. The penis is elongated, narrow, and does not bear hooks (Fig. 28). The narrow penial sheath measures approximately 6 - 8 mm long. Proximally, the vestibule is slightly wider than the penial sheath and gradually widens distally (Fig. 27B). The short retractor muscle is approximately 1.5 - 4 mm long and inserts at the posterior end of the visceral cavity near the rectum. In mature specimens, the deferent duct is convoluted and thicker than the penis.


Onchidium gracile  refers to a Wallaconchis  species due to a unique combination of characters (intestinal loops of type I, no accessory penial gland, and no rectal gland). Stantschinsky’s original description was based on two syntypes which are actually part of two species, hence the necessity of designating a lectotype. The anatomy of one of the former syntypes is in agreement with the species described here, so it is designated as the lectotype to clarify the application of the name W. gracile  . The other former syntype, now a paralectotype without a name-bearing function, is anatomically indistinguishable from W. buetschlii  . This nomenclatural decision helps avoid having to create a new species name for the species described here. Wallaconchis gracile  and W. buetschlii  are similar anatomically (Table 4) and it is understandable that Stantschinsky confused them. Stantschinsky’s original description and illustration (pl. 13, fig. 34) of the male parts of W. gracile  were based on the paralectotype, and therefore correspond to W. buetschlii  .

Stantschinsky (1907: 381, our translation from German) described Onchidium gracile  as "very similar in appearance to O. fungiforme  " and bearing "much resemblance with regard to internal structure." Stantschinsky distinguished O. gracile  from O. fungiforme  externally by the mantle sculpture and the coloration, and internally by the anatomy of the buccal mass and the radular teeth. However, in our experience, all those characters are not informative for distinguishing species in Wallaconchis  (or in any other onchidiid genus for that matter). The holotype (24/19 mm), by monotypy, of O. fungiforme  (SMF 333604/1), was previously dissected by Stantschinsky. Although the digestive system and the female reproductive system are still present, the penial complex is missing. Onchidium fungiforme  needs to be transferred to Wallaconchis  because the holotype shares its unique combination of aforementioned characters given above (see diagnostic features of the genus Wallaconchis  , p. 22). However, the name W. fungiforme  is regarded here as a nomen dubium because its exact application is unclear. The female reproductive parts of the holotype of W. fungiforme  bear the distinctive attachment of the oviduct to the body wall only known in W. gracile  and W. buetschlii  . Of these two species, Stantschinsky’s (1907: 380) description of the penis as "two sharply delineated portions, of which the longer posterior part passes, as a very thin tube, into a gradual thickening" distinguishes it from W. buetschlii  and is only consistent with W. gracile  . However, a species matching the anatomy of W. fungiforme  was not found during extensive sampling in Queensland (29 stations) nor in the MNHN collections from Papua New Guinea. Therefore, it cannot be evaluated whether the male parts of W. fungiforme  differ from those of W. gracile  or not. Given the large distance (more than 2,500 kilometers) between our samples from the Coral Triangle and the type locality of Queensland, it is unclear whether the names W. fungiforme  and W. gracile  could apply to a single widely distributed species, or if W. fungiforme  is distinct and endemic to Australia. Moreover, it is unclear where the holotype of W. fungiforme  was collected exactly since its type locality is simply stated as “Queensland”.

Hoffmann (1928: 82) regarded O. fungiforme  as a synonym of Onchidium ovale  . However, O. ovale  is regarded here as a nomen dubium because its type locality is also unknown. The holotype (22/14 mm) of O. ovale  (ZMB 39045) was dissected by Semper. Its internal organs remain, with the exception of the male parts. Onchidium ovale  clearly refers to a Wallaconchis  species because intestinal loops of type I and a male opening below the right eye tentacle can still be observed in the holotype, and because Semper clearly mentioned the lack of penial accessory gland. The fact that the male parts of the holotype are missing and that they were never illustrated by Semper is yet another reason to regard O. ovale  as a nomen dubium.

Hoffmann (1928: 82) regarded O. gracile  as a synonym of Onchidium palaense  . However, the position of the male opening in Semper’s description of O. palaense  (between the ocular tentacles) indicates that O. palaense  does not refer to a Wallaconchis  species (see remarks on W. buetschlii  ).