Poles apart, but intimately linked. Of the thousands of species that populate Antarctica and the Arctic, it seems hundreds are "bipolar": found at both ends of the 11,000 kilometre span between the poles.

The surveys, part of the international Census of Marine Life, also suggest the Antarctic acts as a cold incubator for species that populate the deep sea around the planet. As ice ages come and go, and the ice shelf advances and retreats, species are isolated, evolve, then released to the global sea floor.

The 235 species that we believe are found at both poles include a great variety of animals, says Julian Gutt of the Alfred Wegner Institute for Polar and Marine Research in Germany. The easiest ones to explain are the large migrating organisms, such as whales and birds. But the list also includes a large number of animals that are thought to live relatively sedentary lives.

Globe-trotting offspring?

For instance, Elpibia glacialis, is a sea cucumber that can be just as comfortable at 80 or 8000 metres beneath the waves - a feat in itself. Even more remarkable, different teams, working at opposite poles, think they have found the same species in both places.

Species of crustaceans, worms and sea butterflies - snail-like animals that swim in the water column - were also found at both poles.

How species that live attached to the seafloor came to live across 11,000 miles (1800 kilometres) of ocean is a bit of a mystery, say the researchers. Gutt's best guess is that their floating larval stage is the key.

For this to be the case, the larvae would have to be regular globetrotters. If only a few individuals had made the pole-to-pole journey just once, their descendants would most likely have evolved into a distinct species.

DNA bar-coding experiments are underway at a facility in Canada to confirm the identity of the sampled species. It is unlikely that all 235 pairs of species are identical matches, says Bodil Bluhm of the University of Alaska, Fairbanks, but "it's also unlikely that they're all different."

Cold storage

Expeditions carried out under the auspices of the Census of Marine Life also revealed that the Antarctic acts as a cold incubator for the rest of the world's seafloor communities.

Every time a new ice age dawns, the polar ice cap thickens and grows, pushing life off the shallow continental shelf, down to around 800 metres. The only place for these animals to go ends up being the very margins of the shelf and the shallow margins of sub-Antarctic islands, where temperature and light conditions are roughly similar to their original habitat.

There, the animals are surrounded by deep water, effectively isolated on underwater ecological islands. Over the course of thousands of years, they evolve into specially adapted species. Then, as the globe warms and the ice retreats, they can once more colonize the continental margins, where they are joined by other species that evolved around other islands.

The Antarctic team at the Census of Marine Life now believe some of these species end up venturing into the deep ocean.

More than 30 million years ago there was not enough oxygen in the deep ocean to support life. But creatures that live there now had to come from somewhere. As some of the conditions in the deep ocean are similar to those on the continental shelf of Antarctica, it is possible that is where the ancestors of deep ocean species came from.

Genetic studies on several species of Antarctic octopus and crustacean have previously confirmed this.

Gutt also thinks the forced isolation of Antarctic species during the last glacial period and subsequent re-colonization of Antarctic waters may be what has led to the great amount of biodiversity that exists around Antarctica (see Antarctic islands surpass Galapagos for biodiversity).