By Mark Kinver
Science and environment reporter, BBC News
At least 235 marine species are living in both polar regions, despite being 12,000km apart, a census has found.
Scientists were surprised to find the same species of "swimming snails" at both poles, raising questions about how they evolved and became so dispersed.
The census, involving 500 researchers from more than 25 nations, was carried out during International Polar Year.
The findings form part of the global Census of Marine Life (CoML) report, which will be published in 2010.
"Some of the more obvious species like birds and whales migrate between the poles on an annual basis," explained Ron O'Dor, CoML's co-senior scientist.
But he added the presence of smaller creatures, such as worms living in mud, sea cucumbers and "swimming snails", at both locations had particularly interested researchers on the project.
One of the swimming snails, or sea butterflies, found in the icy waters of both the Arctic and Antarctic was Cliona limacina.
The creature feeds on Limacina helicina, which is another swimming snail found in the waters of both poles.
Dr O'Dor said that although there was 12,000km separating the two habitats, it did not create a huge barrier for marine wildlife, as a mountain range does for terrestrial species.
"The oceans are a mixing ground," he told BBC News. "There are all kinds of currents that allow things to move around."
He also added that the temperature differences in the oceans did not vary enough to act as a thermal barrier.
"The deep ocean at the poles falls as low as -1C (30F), but the deep ocean at the equator might not get above 4C (39F).
"There is continuity in the ocean as a result of the major current systems, which we call the 'conveyor belt'; a lot of these animals have egg and larvae stages that can get transferred in this water."
'Barcode of life'
Dr O'Dor said that part of the CoML's work included examining organisms' genetic information, which would help the scientists to identify any differences between the seemingly identical species.
"The traditional approach was to describe an organism's physical features, so if these organisms lived in very similar habitats, did very similar jobs and ate similar food, then they often looked very alike even if they came from different origins.
"So we are also working very closely with the Barcode of Life team at the University of Guelph (Canada), and we hope that by 2010 that we will have about 90% of marine species barcoded."
The project aims to develop DNA barcoding as a global standard for identifying species using key genetic markers - much like a shop barcode uniquely identifies a retail product.
"It's a new way to mark or classify things," Dr O'Dor observed.
"Even though organisms look exactly the same and have been identified as being the same type by traditional methods, genetic information can reveal them to be a sub-species or different populations."
COML, which began back in 2000, carried out 17 regional censuses involving more than 2,000 scientists from 82 nations.
Currently, the census teams are collating and examining the data collected by the various surveys, ahead of the publication in October 2010 of the first global Census of Marine Life.