Paromoionchis Dayrat & Goulding, 2019

Dayrat, Benoît, Goulding, Tricia C., Khalil, Munawar, Apte, Deepak, Bourke, Adam J., Comendador, Joseph & Tan, Shau Hwai, 2019, A new genus and three new species of mangrove slugs from the Indo-West Pacific (Mollusca: Gastropoda: Euthyneura: Onchidiidae), European Journal of Taxonomy 500, pp. 1-77: 19-20

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Paromoionchis Dayrat & Goulding

gen. nov.

Paromoionchis Dayrat & Goulding   gen. nov.

Type species

Onchidium tumidum Semper, 1880   , designated here.


Body not flattened. No dorsal gills. Dorsal eyes present on notum. Retractable, central papilla (usually with four dorsal eyes) present, often raised above dorsal surface. Eyes at tip of short ocular tentacles. Male opening below right ocular tentacle and to its left. Foot wide. Pneumostome median, on ventral hyponotum. Intestinal loops of type II. Rectal gland absent. Accessory penial gland present or absent. When present, accessory penial gland with muscular sac. Penis with or without hooks.

Differential diagnosis

No external diagnostic feature unambiguously distinguishes Paromoionchis   gen. nov. from all other genera (which is not surprising because many onchidiid species from different genera are very similar externally). However, Paromoionchis   gen. nov. is characterized by a unique combination of internal and external characters: no dorsal gills, male opening below and to the left of the right eye tentacle, no rectal gland and intestinal loops of type II (see Labbé 1934a: 177, fig. 3, for a comparison of digestive types). According to our data, any onchidiid slug with this combination of characters must belong to a species of Paromoionchis   gen. nov.


The name Paromoionchis   is a combination of parómoios (παρóμoιoς), which means ʻsimilarʼ in Greek (because members look very similar externally) and onchis, a word derived from the Greek onchos (ὁ ὂγκος) and one of the early names used to refer to onchidiid slugs.


Masculine, the gender of onchis (ICZN Art. 30.1.1), a word derived from the masculine Greek word ὁ ὂγκος (onchos), which means ʻmassʼ or ʻtumor.ʼ As a result (ICZN Art. 31.2), the ending of the specific name tumidum   (a Latin adjective) must be changed from neuter (because Onchidium   is a name of neuter gender) to masculine ( tumidus   ).


The new genus described here is distributed from the western coast of India in the west, all the way to the subtropical waters of Japan (~33° N), Papua New Guinea and the subtropical waters of southeastern Australia (~32° S) in the east ( Fig. 6 View Fig ). We did not find Paromoionchis   gen. nov. in South Africa, Madagascar or Mauritius, but it is possible that it is present in areas east of Papua New Guinea, such as Fiji and New Caledonia, where we did not collect.


The five known species of Paromoionchis   gen. nov. primarily live on mud, in or next to mangroves, which explains why three species have just been discovered now, because the mangroves of South- East Asia have been very poorly explored. Occasionally, these slugs can also be found in or on muddy logs, coral rubble, sandy mud or even sand with very little mud in it. Paromoionchis tumidus   , which is widespread and very common, can be found in nearly all these habitats, even though the mud surface remains where it is most commonly found, like all other species of the genus. Because members of Paromoionchis   gen. nov. prefer the mud surface, live animals are often covered with mud.


A new generic name is needed because no existing name applies to the clade described here. Our remarks are based on the examination of all the type specimens available, especially those of the type species of all genera, the careful analyses of all the original descriptions (especially when no type specimens were available), and our ongoing taxonomic revision of each genus of the family. Three existing generic names are junior synonyms of Onchidella J.E. Gray, 1850   , which is not found in the tropical Indo-West Pacific and is characterized by a completely different anatomy ( Dayrat 2009; Dayrat et al. 2011b). Seven generic names apply to the clade including all the onchidiid slugs with dorsal gills, i.e., Peronia Fleming, 1822   ( Dayrat 2009). Labella Starobogatov, 1976 is a junior synonym of Onchidium Buchannan, 1800   , which applies to a distinct clade including three species ( Dayrat et al. 2016). Paraoncidium Labbé, 1934   is a junior synonym of Onchidina Semper, 1882   , which applies to a distinct monotypic genus from southeastern Australia (Dayrat & Goulding 2017). Peronina Plate, 1893   applies to a clade including slugs characterized by a pneumostome located at the margin between the dorsal notum and the ventral hyponotum. Platevindex Baker, 1938   applies to a clade including species with a distinctly flattened body and a narrow foot. Semperoncis Starobogatov, 1976   applies to species characterized by a very different anatomy and which are adapted to terrestrial life in the Philippines ( Dayrat 2010). And, finally, Melayonchis Dayrat & Goulding, 2017   applies to a distinct clade including slugs with a different anatomy (Dayrat et al. 2017).