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Aim Our aim was to test for transoceanic dispersal in direct-developing species of Onchidella (Mollusca: Gastropoda). As these slugs are ecologically associated with buoyant bull-kelp ( Durvillaea), and are known to raft, we predicted that they would show evidence of recent genetic connectivity among isolated landmasses. Location Cool-temperate and subantarctic coasts of the Southern Hemisphere (New Zealand and its subantarctic islands, southern Chile, and the Falkland Islands). Methods We employed two commonly used genetic marker types - mitochondrial DNA (mt DNA) sequences and amplified fragment length polymorphisms ( AFLPs) - to resolve phylogeographical and population genetic structuring of Onchidella populations on a transoceanic scale. Results Contrary to previous taxonomic views that classify these Southern Ocean Onchidella as at least three separate species, our genetic data suggest that the studied populations belong to a single taxon, whose distribution is widespread. The analyses reveal substantial mt DNA differentiation between several mainland New Zealand populations (e.g. Kaikoura, Purakaunui Bay), apparently consistent with low rates of dispersal. However, subantarctic samples show widely shared mitochondrial haplotypes and AFLP similarity between distant populations. Main conclusions Our study suggests recent common ancestry for geographically separated New Zealand subantarctic and South American populations of Onchidella. Their transoceanic similarities are likely to have been driven by passive rafting mediated by circumpolar ocean currents, as previously documented for several epifaunal species strongly associated with buoyant Durvillaea. [ABSTRACT FROM AUTHOR]