OSLO At least 235 types of cold-loving creatures thrive in both Arctic and Antarctic seas, puzzling scientists about how they got to both ends of the earth, a study showed on Sunday.
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 from 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," according to the Census of Marine Life, a decade-long international project to map the world's oceans with results due in October 2010.
Species 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," Ron O'Dor, senior scientist of the census, told Reuters. Genetic studies were being carried out to confirm that the 235 species were identical.
The 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," Julian Gutt of the Alfred Wegener Institute in Germany, a senior member of the census, told Reuters. "The most likely direction is from the Antarctic."
He said, however, that he knew of no finds of cold-loving species in the depths near the equator to back up the theory.
Ice Ages may have helped species disperse.
During 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.
"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 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.
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(Editing by Janet Lawrence)