Integrative taxonomy of the genus Onchidium Buchannan, 1800 (Mollusca, Gastropoda, Pulmonata, Onchidiidae)

Abstract

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 Onchidium typhae Buchannan, 1800, Onchidium stuxbergi (Westerlund, 1883), and Onchidium reevesii (J.E. Gray, 1850). With the exception of a re-description of Onchidium 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 Onchidium nigrum (Plate, 1893) are regarded as junior synonyms of Onchidium 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.

Integrative taxonomy of a new and highly-diverse genus of onchidiid slugs from the Coral Triangle (Gastropoda, Pulmonata, Onchidiidae)

“…All analyses were run several times and yielded the same result. Ten other onchidiid species and their corresponding sequences were selected from previous studies from our lab as out-groups (Dayrat et al 2011, 2016, 2017; Dayrat and Goulding 2017): Onchidella floridana (Dall, 1885), Onchidella celtica (Cuvier in Audouin and Milne-Edwards, 1832), Onchidella patelloide (Quoy & Gaimard, 1832), Onchidina australis (Semper, 1880), Platevindex luteum (Semper, 1880), Peronina tenera (Stoliczka, 1869), Peronia sp. (Okinawa), Peronia sp.…”

“…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.…”

“…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.…”

“…4D) illustrated the penial anatomy of an onchidiid species from Hong Kong which matches the penial anatomy of the species described here perfectly. Britton misidentified that species as Paraoncidium reevesii (J. E. Gray, 1850) because the valid name Onchidium reevesii J. E. Gray, 1850 actually applies to a species with both an accessory penial gland and a rectal gland, both of which Britton noted were absent from his Hong Kong specimens (Dayrat et al 2016). Britton’s description (1984: 184) of the intestinal loops of type II is unlikely given the distinctive penial anatomy he illustrated (known only in this onchidiid species).…”

“…An important component of integrative taxonomy is sound nomenclature; this revision has required the examination of the type material of all onchidiid species, the detailed analysis of all species descriptions, and the re-evaluation of the nomenclatural status of every genus- and species-group name. First, monophyletic genera had to be delineated, and now each genus is being revised separately (Dayrat et al 2016, 2017; Dayrat and Goulding 2017; Goulding et al in press). …”