From Scientific American:
Two years ago, several research vessels shipped out to the North and the South poles to assemble a census of creatures living under the ice. One of the most surprising results was a discovery that 235 identical species lived on opposite sides of the world but were undocumented anywhere else. It's easy to understand how massive humpbacks can swim from Arctic to Antarctic waters, but most of the miniature worms, snails and crustaceans on the researchers' list are no bigger than grains of rice. How could tiny creatures adapted for the frigid waters travel 9,500 kilometers through warmer climes to reach the opposite pole?
Under the microscope, these invertebrates sometimes look like shredded plastic bags or shrimp with bullhorns. It's unclear how they could cross a swimming pool, let alone the globe. So, their “bipolarity” poses a 160-year mystery of the ocean—one that has only grown with time. “If bipolar species are as common as our initial list suggests, it really means we don't appreciate the mechanisms that are important for connectivity in the ocean as well as we thought,” says Russ Hopcroft, project leader of the Arctic portion of the Consortium for Ocean Leadership's Census of Marine Life.