Southern Hemisphere coasts are biologically connected by frequent, long-distance rafting events

Globally, species distributions are shifting in response to environmental change, and those that cannot disperse risk extinction. Many taxa, including marine species, are showing poleward range shifts as the climate warms. In the Southern Hemisphere, however, circumpolar oceanic fronts can present formidable barriers to dispersal4. Although passive, southward movement of species across this barrier has been considered unlikely, the recent discovery of buoyant kelp rafts on beaches in Antarctica demonstrates that such journeys are possible. Rafting is a key process by which diverse taxa – including terrestrial and coastal marine species – can cross oceans. Kelp rafts can carry passengers, and thus can act as vectors for long-distance dispersal of coastal organisms. The small numbers of kelp rafts previously found in Antarctica do not, however, shed much light on the frequency of such dispersal events. We here use a combination of high-resolution phylogenomic analyses (>220,000 SNPs) and oceanographic modelling to show that long-distance biological dispersal events in the Southern Ocean are not rare. We document tens of kelp (Durvillaea antarctica) rafting events of thousands of kilometres each, over several decades (1950 – 2019), with many kelp rafts apparently still reproductively viable. Modelling of dispersal trajectories from genomically-inferred source locations shows that some distant landmasses are particularly well connected, for example South Georgia and New Zealand, and the Kerguelen Islands and Tasmania. Our findings illustrate the power of genomic approaches to track, and modelling to show frequencies, of long-distance dispersal events.