Benoit Dayrat

Introduction. --- Onchidiidae, one of the nine major extant taxa of Pulmonata (Mollusca, Gastropoda), has been understudied since the last onchidiid experts were active more than 70 years ago (e.g., Plate 1893; Hoffmann 1928, 1929; Labbé 1934a, 1934b, 1934c, 1935). In fact, there is no living expert able to reliably identify species of onchidiids, and museum lots are often labeled as “Onchidiidae.” A few large monographs focused on parts of European collections: London, Frankfurt, and Berlin (Plate 1893); Copenhagen and Stockholm (Hoffmann 1928); Paris (Labbé 1934a). However, a global revision based on all collections available has never been attempted: the nomenclatural status of most species names is unknown; as a result, the onchidiid species diversity is in a state of confusion, which in turn makes discovering new species difficult.
Toward a revision. --- In our lab, we have started a taxonomic revision of the Onchidiidae. Our main goals are to: 1) evaluate how many existing species names are valid; 2) redescribe the anatomy of valid species; 3) reconstruct the supra-specific phylogenetic relationships; 4) discuss evolutionary questions, such as the transitions between aquatic and terrestrial habitats -- Onchidiidae is one of the four families of gastropods (out of more than 400 families) to be adapted to marine, freshwater, and terrestrial life.
Natural history. --- Onchidiids are true slugs: they lack an internal shell. Most species are marine and live in the upper intertidal zone, either in rocky, sandy, or muddy habitats, including mangroves. However, two species live in brackish habitats and tolerate fresh water: Onchidium typhae Buchannan, 1800, and Labella ajuthiae (Labbé, 1935). Also, three terrestrial species have been described from high-elevation rainforests: Semperella montana (Plate, 1893), from Sibugan Island, Philippines; Platevindex ponsonbyi (Collinge, 1901), from Borneo; and Platevindex apoikistes Tillier, 1983, from Mindoro, Philippines. The highest elevation record of terrestrial onchidiids found in the literature is the original description of Platevindex ponsonbyi (850–1060 m), although the author has undertaken the study of onchidiids collected up to 1850 m from Mindoro and Panay Islands (Phillipines). Most species tend to be seasonal with a population peak in the summer; when present, onchidiids can be very abundant (e.g., Dey 2006). Onchidiids have a worldwide distribution, with the exception of the Arctic and Antarctic (Hoffmann 1928, 1929).
General morphology. --- Most onchidiids measure from 10 to 50 mm, although a few species are much larger (some specimens of Peronia peronii (Cuvier, 1804) in the collections of the Natural History Museum, NHM, London, measure up to 250 mm). Onchidiids are usually oval and slightly dorso-ventrally flattened. The dorsal notum, larger than the pedal sole, can be smooth or covered with papillae; in some species, some papillae bear photoreceptors sometimes called ‘dorsal eyes’. The ventral surface of the laterally extending dorsal notum, or hyponotum, is smooth. The head bears a pair of retractile cephalic tentacles with apical eyes, as well as a pair of oral lobes. As in other pulmonates, onchidiids possess a lung adapted for breathing air. The lung is located at the posterior end of the body and opens through a contractile pneumostome. In addition, some species possess branched, dorsal papillae, traditionally interpreted as gills (likely to help gas exchange when the animal is submerged). As all pulmonates, onchidiids are hermaphroditic: the female genital opening is posterior; the male copulatory organ is anterior and opens near the right cephalic tentacle.
Monophyly. --- Although the monophyly of onchidiids still needs to be tested with a broad taxon sampling, both in morphological and molecular studies, onchidiid slugs share many unique external and internal features that distinguish them from all other pulmonates (e.g., Barker 2001; Dayrat and Tiller 2002, 2003). In fact, Onchidiidae has never been split and all onchidiids have always been classified within a single taxon, which strongly suggests that they form a natural taxon. Using 18S and 28S rRNA gene sequence data, Vonnemann and co-authors (2005) recently found that Onchidella floridana and Onchidium cf. verruculatum (i.e., Peronia verruculata) formed a highly-supported clade among other pulmonates.
Literature cited. --- See our Catalogue Page.