Until now, the warm tropics have been seen as a barrier keeping polar bears in the Arctic separate from penguins in the Antarctic. Only a few creatures have been known to live in both polar regions, such as long-migrating grey whales or Arctic terns.
'At least 235 species live in both polar seas despite an 11,000-km (6,835 miles) distance in between,' a decade-long international project to map the world's oceans , Census of Marine Life, has found.
A shell-less pteropod or swimming snail, clione limacina, found in both Arctic and Antarctic watersSpecies living at both poles include cold-water worms, crustaceans, sea cucumbers and snail-like pteropods.
They make up two percent of the 7,500 Antarctic and 5,500 Arctic animals known to date, out of a global total estimated at up to 250,000.
'The Arctic and Antarctic are much more alike than we thought,' senior scientist of the census Ron O'Dor said. Genetic studies were being carried out to confirm that the 235 species were identical.
A bean-sized swimming snail, limacina helicina, which has crossed the tropics to live in both seas. The creature spins a mucus-net off its paddle-like foot-wings to trap algae and other small particles on which it feedsThe findings, along with a discovery that the frigid seas teem with life, raise questions about where common polar species 'originated and how they wound up at both ends of the earth,' the census said in a statement.
Among theories were that larvae of some species could be swept northwards from Antarctica by chill currents along the deep floor of the Atlantic Ocean -- away from warm surface waters in the tropics that would kill them.
'Animals can be dispersed over such long distances at the deep sea floor,' senior member of the census Julian Gutt, of the Alfred Wegener Institute in Germany, said.
A sand-flea hyperoche capucinus, a common predator swimming in polar waters'The most likely direction is from the Antarctic.'
But he said scientists were yet to discover cold-loving species in the depths near the equator that would back up the theory.
Ice Ages may have helped species disperse.
The nemertean pelagonemertes rollestoni has a yellow stomach that reaches out to feed all parts of the bodyDuring Ice Ages, Antarctica's ice smothered surrounding seas and caused new northbound currents that could have carried species such as sea spiders or crustaceans known as isopods.
Genetic studies have traced many types of octopus to an Antarctic ancestor.
Among other findings, researchers said smaller marine species of copepods, a sort of crustacean, were replacing larger ones in some Arctic waters, perhaps because of shifts linked to global warming.
One of the Antarctic's ice fish, which can withstand temperatures that freeze the blood of all other types of fish
'A change in these few species might impact the whole food system,' Rolf Gradinger of the University of Alaska said. The larger copepods were key food for creatures such as whales and seabirds.
Among bizarre creatures, one of the Antarctic ice fish known as Chionodraco hamatus can withstand temperatures that would freeze the blood of other fish.
The census, the results of which are due next year, is seeking to lay down a benchmark for judging long-term shifts in the oceans. The U.N. General Assembly has asked for regular assessments of the oceans to gauge the impact of pollution, over-fishing and climate change.