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Baltic Sea jellyfish keeps researchers in Sweden guessing

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Baltic Sea jellyfish keeps researchers in Sweden guessing
07:14 CET+01:00
Marine biologists in Sweden remain puzzled by a species of jellyfish newly discovered in the Baltic Sea which they first feared was a harmful invasive species.

"It has previously been feared that the invasive jellyfish that disrupted marine ecosystems in the Black Sea had arrived in the Baltic Sea," Stockholm University said in a statement issued Thursday.

But researchers at the university studying the spread of the American pseudo-jellyfish "have found that their tests showed it was a completely different species never before seen in the waters of the Baltic."

Researchers raised the alarm in 2007 when they reported the appearance of Mnemiopsis, an animal that measures about 10 centimetres and is not technically a jellyfish but has a gelatinous and translucent appearance.

It is not harmful to humans.

The species had never before been seen as far north in Europe and it was feared it could change the region's entire ecosystem.

"We were surprised, to say the least, when genetic tests showed that all samples contained Mertensia, a completely different species," Elena Gorokhova at the university's department of system ecology said.

"The Mertensia jellyfish has never before been observed in the Baltic Sea, but since it is a very well-spread Arctic species, it has probably been here for a while," she said.

She said the morphology of the two species may have confused researchers.

The main fear concerning the Mnemiopsis species was that it devours the plankton that fish eat, and could therefore affect the whole food chain in the Baltic.

The species also has the capacity to reproduce quickly -- it is hermaphroditic and can self-fertilize -- and is also very resistant.

Researchers initially believed that the Mnemiopsis had spread north on board ships that travelled to Sweden from the Netherlands.

"The good news is that it seems as though the American pseudo-jellyfish has not travelled as far as the Baltic Sea, as previously feared," Gorokhova said.

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