There is much concern about the potential for invasive species to enter the only marine region left with no known exotics - the Southern Ocean. Attention has focused on planktonic larval travel, shipping (ballast water and hull fouling) and marine debris as transport mechanisms. There is, however, another source of transport for biota across the Polar Frontal Zone - hitchhiking on megafauna, such as seals. In this study we report the frequency and burden of barnacles Lepaa australis attached to Antarctic fur seals Arctocephalus gazella coming ashore to breed at Bird Island, South Georgia. In the austral summers of 2001/2002, 2002/2003 and 2003/2004, female fur seals with barnacles attached arrived at Bird Island in late November/early December and peaked in mid-December. About 4 % of female fur seals carried barnacles, with the mean burden being - 10 barnacles. Pedunculate barnacles seem, therefore, to be entering the Southern Ocean in large numbers every year in the South Georgia region and probably elsewhere. We also found adult barnacles attached to a macaroni penguin and (perhaps the furthest and fastest travelling marine larvae) young stages (cyprids) on the leg ring of a wandering albatross. Barnacle plates provide a hard substratum to which other fouling marine organisms can attach, and thus travel as secondary hitch-hikers. We found polychaete worms (Spirorbidae) and a bryozoan colony (Ceneporella antarctica) encrusting a stalked barnacle attached to a pycnogonan (Collosendeis scotti). We suggest that fouling hitch-hiking barnacles (on migrating megafauna) offer a considerable natural mechanism for potential colonisers of Antarctic waters, This is of particular importance at the current time given the context of strong regional warming in the Scotia Arc-Antarctic Peninsula region
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