Wallaconchis Goulding & Dayrat

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 Goulding & Dayrat

gen. n.

Genus Wallaconchis Goulding & Dayrat View in CoL gen. n.

Type species.

Wallaconchis sinanui , designated here.


Combination of Wallace, for Alfred Russel Wallace, one of the first naturalists to widely study the fauna of the Indo-Malay Archipelago, and Onchis , one of the names used to refer to onchidiid slugs. The core of the geographic distribution of this genus in Indonesia (Bali, Lombok, Sulawesi, and eastern Indonesia) corresponds to a region sometimes referred to as Wallacea, also after A. R. Wallace, which makes it especially fitting here.


Masculine, gender of Onchis .


Body not flattened. Dorsal gills absent. Dorsal eyes present on notum, mostly in groups of three or four eyes. Central dorsal papilla bearing eyes present and retractable, but usually not raised above the dorsal surface. Eyes at the tip of short ocular tentacles. Male opening below the right ocular tentacle. Oral lobes typically grey or dark brown. Pneumostome median. Foot wide. Visceral cavity pigmented (in black), which varies in intensity between individuals. Intestinal loops of type I. Rectal gland absent. Accessory penial gland absent. Penial morphology highly diverse, from short and straight to long and coiled. Penial hooks absent or present.


Every available genus name was carefully evaluated when searching for a name for this genus, which included examination of the type specimens of the type species of all existing genera, as well as the analysis of all the original descriptions. Nine existing species names are transferred to Wallaconchis , five of which are valid, one of which is a synonym, and three of which are nomina dubia. Historically, these five valid species names have been placed in three different genera: Onchidium Buchannan, 1800, Paraoncidium Labbé, 1934, and Scaphis Labbé, 1934. However, all five species were originally described in Onchidium (sometimes with the unjustified emendation Oncidium ), traditionally used by default for many unrelated onchidiids ( Dayrat 2009): Onchidium ater Lesson, 1830; Onchidium graniferum Semper, 1880; Oncidium nangkauriense Plate, 1893; Oncidium buetschlii Stantschinsky, 1907; and Oncidium gracile Stantschinsky, 1907. The one species name which is a synonym, Oncidium keiense Hoffmann, 1926, and the three species names which are nomina dubia ( Onchidium ovale Semper, 1880; Oncidium simrothi Plate, 1893; Oncidium fungiforme Stantschinsky, 1907), were also originally created in the genus Onchidium . From this point forward, the unjustified emendation Oncidium is not repeated throughout the paper and is systematically replaced by the correct spelling Onchidium . The genus Onchidium actually refers to a different taxon including only three species and characterized by features (e.g., large dorsal conical papillae, rectal gland, accessory penial gland) that are absent in the species described here (see Dayrat et al. 2016). Hoffmann (1926) decided not to create a new genus for Onchidium keiense , despite the distinctive morphology of the copulatory parts: "The peculiarity of the penis could almost suggest the idea that we are dealing here with a whole new genus. In the genus Onchidium , however, there are already species which differ quite considerably from each other, so that I have no reason to create a new genus for my species." (1926: 24, translated from German).

Hoffmann (1928) briefly mentioned the morphological similarity of six Onchidium species ( Onchidium papuanum Semper, 1880, Onchidium palaense Semper, 1880, Onchidium ovale , O. buetschlii , O. nangkauriense , and Onchidium chameleon Brazier, 1886) which all lack an accessory penial gland, and five of which also lack a rectal gland: "The following six species are quite close to one another, and there are usually very few characteristic features which can be used for differentiation." (1928: 81) Hoffmann (1928) also mentioned that some of those six names would likely become synonyms as new material became available, but none of these names has been proposed as synonymous. Three of these names are transferred to Wallaconchis here ( O. ovale , O. buetschlii , and O. nangkauriense ). Onchidium chameleon belongs to the genus Onchidina Semper, 1882 ( Dayrat and Goulding 2017). Onchidium palaense and O. papuanum are poorly known because their type material is lost. However, Semper’s description of the position of the male aperture as "between the two very small eye tentacles" in O. palaense (1882: 276) and "exactly in the center line between the tentacles and mouth" in O. papuanum (1882: 277) indicates that they belong to another genus. A few years later, Labbé (1934) created the genus name Paraoncidium for eleven Onchidium species with no accessory penial gland, including the six aforementioned species considered morphologically similar by Hoffmann (1928). However, P. chameleon , the type species of Paraoncidium , belongs to the genus Onchidina and Paraoncidium is a junior synonym of Onchidina ( Dayrat and Goulding 2017). The genus Onchidina , which so far is monotypic, differs greatly from Wallaconchis (e.g., dorsal eyes absent, male aperture on right lateral side of right ocular tentacle, rectal gland present).

Finally, a new combination was proposed by Labbé (1934) for Onchidium ater as Scaphis atra . However, Scaphis Starobogatov, 1976 is a synonym of Peronia Fleming, 1822 which refers to a different group of onchidiids characterized by dorsal gills, which are lacking in all the species described here.

Overall, none of the three genus names historically used in binomials for the species described here ( Onchidium , Paraoncidium , Scaphis ) could apply to our new genus.

Geographic distribution

(Fig. 6). Wallaconchis species are distributed from the Andaman Islands (India) in the Bay of Bengal to Vanuatu in the western Pacific. Nine out of ten species are sympatric over at least part of their distribution. Eight out of ten species are found widely within the Coral Triangle. Two species are known from only one station each, which suggests that they are endemic to a small region and specialized to a particular habitat. All species are found in tropical waters.


(Table 3). Wallaconchis slugs are found in various intertidal habitats. Two new species ( W. comendadori and W. melanesiensis ) live predominantly in the rocky intertidal, with large rocks covered in thin layers of algae, which may or may not be adjacent to mangrove trees. Two species ( W. nangkauriense and W. ater ) are commonly found in coral rubble habitats, where Avicennia trees often grow. Three species ( W. buetschlii , W. uncinus , and W. sinanui ) are found on firm mud (i.e., mud which is not deep and not saturated in water). Two species ( W. gracile and W. buetschlii ) are found on sandy mud, two species ( W. achleitneri and W. buetschlii ) on coarse sand, and one species ( W. graniferum ) on fine sand. Many Wallaconchis species can be found together on diverse substrates at stations with various habitats. For instance, at our station 198, on the southern coast of Bohol, Philippines, six species were present without any other onchidiids on just a few square meters of fine sand mixed with coral rubble and muddy sand.

Distinctive diagnostic features.

A unique combination of characters can be used to determine whether a specimen is part of a Wallaconchis species: no dorsal gills, intestinal loops of type I, no rectal gland, and no accessory penial gland. This combination is not found in any other genus and these anatomical features can be easily and quickly observed through dissection. It is more challenging to identify Wallaconchis slugs using external characters, but it is not impossible. Wallaconchis slugs exhibit the highest degree of color variation in the Onchidiidae . In some species, the dorsal notum can be red, yellow, green, orange, brown, grey, black, etc., when most onchidiids are usually just brown, marbled with light to dark brown. Such bright dorsal colors are also found in Peronia , but Peronia is easily distinguished from Wallaconchis (the dorsal notum of Peronia bears gills that are absent in Wallaconchis ). However, the three Wallaconchis species which are found on mud inside and around mangroves only occasionally exhibit patches of bright colors, and thus, can be easily mistaken for other genera found in mangroves. The male opening can also help identify Wallaconchis slugs at the generic level. In other genera, the male opening may be to the left of the right ocular tentacle (e.g., Melayonchis , Onchidium ), or to the right of the right tentacle (e.g., Peronina , Onchidina ), while in Wallaconchis it is directly below the right ocular tentacle ( Dayrat et al. 2016, 2017, Goulding et al. in press). Another character which may be observed in Wallaconchis by naturalists in the field, although difficult to appreciate, is that live animals produce a sticky mucus from the ventral surface, which is most noticeable with large animals. Other onchidiids also produce some mucus, but that mucus is usually wet, while the mucus of Wallaconchis can be likened to drying glue.