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.

A new genus of onchidiid slugs, Wallaconchis Goulding & Dayrat, gen. n., is described, including ten species. Five species were previously described but known only from the type material: Wallaconchis ater (Lesson, 1830), W. graniferum (Semper, 1880), W. nangkauriense (Plate, 1893), W. buetschlii (Stantschinsky, 1907), and W. gracile (Stantschinsky, 1907), all of which were originally classified in Onchidium Buchannan, 1800. Many new records are provided for these five species, which greatly expand their known geographic distributions. Five species are new: Wallaconchis achleitneri Goulding, sp. n., W. comendadori Goulding & Dayrat, sp. n., W. melanesiensis Goulding & Dayrat, sp. n., W. sinanui Goulding & Dayrat, sp. n., and W. uncinus Goulding & Dayrat, sp. n. Nine of the ten Wallaconchis species are found in the Coral Triangle (eastern Indonesia and the Philippines). Sympatry is high, with up to six species found on the island of Bohol (Philippines) and eight species overlapping in northern Sulawesi (Indonesia). Wallaconchis is distinguished from other onchidiids by its bright dorsal colors (red, yellow, orange) but those are extremely variable and not useful for specific identification. Internally, the reproductive system can be used to identify all Wallaconchis species. The copulatory organs of Wallaconchis species are especially diverse compared to other onchidiid genera, and the possible role of reproductive incompatibility in species diversification is discussed. All specimens examined were freshly collected for the purpose of a worldwide revision of the Onchidiidae Rafinesque, 1815. The species are well delineated using DNA sequences and comparative anatomy. Mitochondrial DNA analysis yields thirteen molecular units separated by a large barcode gap, while nuclear DNA yields nine units. By integrating nuclear DNA and mitochondrial DNA with morphology, ten species are recognized. The natural history of each species (e.g., the microhabitat where they are found) is also documented. Nomenclature is addressed thoroughly (the types of all onchidiid species were examined, lectotypes were designated when needed, nomina dubia are discussed). Morphological characters, transitions to new microhabitats, and diversification processes are discussed in the context of a robust molecular phylogeny.

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.

Aim Little is known about phylogeography and cryptic diversity of parasites in the marine environment. The acanthocephalan Profilicollis altmani parasitizes intermediate hosts that are broadly distributed around the Americas and final hosts that are highly motile. We investigated the spatial genetic structure of this acanthocephalan found in three species of Emerita crabs: (1) to test whether land masses serve as biogeographic barriers promoting ocean basin divergence among parasite lineages or species; and (2) to test whether the distribution of parasite species matches the distribution of different crab host species.Location The Pacific, Atlantic and Gulf coasts of the USA, and the Pacific coast of Panama and Chile.Methods Sequences of cytochrome c oxidase subunit I (COI) and ribosomal internal transcribed spacers (ITS) were obtained from 204 acanthocephalans. Parasites were sampled from crabs in 15 sampling localities. These sequences were analysed with coalescent-based methods and other population genetic analyses to infer phylogeographic patterns.Results Haplotype diversity for COI sequences was high (0.96) among parasites sampled, but nucleotide diversity was low (0.071) and there was no distinct geographic pattern between regions. Pairwise genetic distances were generally low, although there was a degree of population structure between oceans. Sequence comparisons showing an excess of low divergence alleles and a bimodal mismatch distribution provide evidence of either past selective events or demographic expansions. No variation was observed in the ITS sequences.Main conclusions The lack of geographic patterning in haplotype diversity of this parasite indicates that gene flow is probably occurring between ocean basins. In addition, the low genetic diversity suggests that the acanthocephalan parasitizing E. analoga in Chile is conspecific to the species found parasitizing several Emerita species along the coasts of North America, and is thus a cosmopolitan parasite that is most likely dispersed long distances by marine birds that serve as definitive hosts.