STOCKHOLM (AFP) - A gluttonous American pseudo-jellyfish, giant Japanese oysters, and an unidentified virus killing seals: strange intrusions are threatening Sweden's seas and fishermen are concerned.

The biggest threat is called mnemiopsis, an animal that measures about 10 centimetres (four inches) and is not technically a jellyfish but has a gelatinous and translucent appearance. It is not harmful to humans.

The species has never before been seen this far north in Europe and could change the region's entire ecosystem.

"Officially it was first seen a year ago... We know they survived the winter. Suddenly at the end of July there were a lot of them," said Lene Friis Moeller, a researcher at Gothenburg University's marine ecology department.