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.

The discovery of bipolar species dates back to the 1840s expeditions of Victorian explorer James Clark Ross and his two heavy-duty battle cruisers, the HMS Erebus and Terror. During missions to map the North and South poles, he collected samples of marine flora and fauna that looked remarkably similar. He theorized that somehow these tiny species had been able to survive not only the icy waters that would eventually sink his ships, but also a journey halfway around the planet.