In an effort to clarify the species diversity of onchidiid slugs, the taxonomy of the genus Onchidium Buchannan, 1800 is revised using an integrative approach. New, fresh specimens were collected in a large number of places, including type localities. The genus Onchidium is redefined here as a clade including only three species which are strongly supported by both morphological and molecular data. All three species were already named: the type species O. typhae Buchannan, 1800, O. stuxbergi (Westerlund, 1883), and O. reevesii (J.E. Gray, 1850). With the exception of a re-description of O. typhae published in 1869, all three species are re-described here for the first time. First-hand observations on the color variation of live animals in their natural habitat are provided. The anatomy of each species is described. Important nomenclatural issues are addressed. In particular, Labella Starobogatov, 1976 is regarded as a junior synonym of Onchidium and Labella ajuthiae (Labbé, 1935) and O. nigrum (Plate, 1893) are regarded as junior synonyms of O. stuxbergi. The nomenclatural status of several other species names is discussed as well. Many new records are provided across South-East Asia and precise ranges of geographic distributions are provided for the genus Onchidium and its three species. Distinctive features that help distinguish the genus Onchidium from other onchidiids are provided, as well as an identification key for the three species. In an effort to clarify the species diversity of onchidiid slugs, the taxonomy of the genus Onchidium Buchannan, 1800 is revised using an integrative approach. New, fresh specimens were collected in a large number of places, including type localities. The genus Onchidium is redefined here as a clade including only three species which are strongly supported by both morphological and molecular data. All three species were already named: the type species O. typhae Buchannan, 1800, O. stuxbergi (Westerlund, 1883), and O. reevesii (J.E. Gray, 1850). With the exception of a re-description of O. typhae published in 1869, all three species are re-described here for the first time. First-hand observations on the color variation of live animals in their natural habitat are provided. The anatomy of each species is described. Important nomenclatural issues are addressed. In particular, Labella Starobogatov, 1976 is regarded as a junior synonym of Onchidium and Labella ajuthiae (Labbé, 1935) and O. nigrum (Plate, 1893) are regarded as junior synonyms of O. stuxbergi. The nomenclatural status of several other species names is discussed as well. Many new records are provided across South-East Asia and precise ranges of geographic distributions are provided for the genus Onchidium and its three species. Distinctive features that help distinguish the genus Onchidium from other onchidiids are provided, as well as an identification key for the three species.

Mangroves of the Indo-West Pacific have remained poorly explored, so even the diversity of the onchidiid slugs, which are some of the most abundant animals in mangroves of the Indo-West Pacific, is not well known. Thanks to several years spent exploring mangroves in the Indo-West Pacific (more than 260 stations), especially in South-East Asia, the diversity of mangrove gastropods can now be addressed through revisions following an integrative taxonomy approach (nomenclature, field observations, comparative anatomy and DNA sequences). A new genus of onchidiid slugs is described, Paromoionchis Dayrat & Goulding gen. nov., which includes five species, three of which are new: Paromoionchis boholensis Dayrat & Goulding gen. et sp. nov., P. daemelii (Semper, 1880) com. nov., P. goslineri Dayrat & Goulding gen. et sp. nov., P. penangensis Dayrat & Goulding gen. et sp. nov. and P. tumidus (Semper, 1880) comb. nov. Paromoionchis gen. nov. is distributed from western India to the subtropical waters of Japan (33° N) and southeastern Australia (33° S). The creation of new taxon names is supported by rigorous nomenclature: the types of all existing species names in the family were examined, the original descriptions carefully studied and nomenclatural issues addressed. The diversity and biogeography of this new genus is discussed in a broader context.