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.

Twenty-seven dead seals found on the beach in Kattegat.

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.

A wild giant Japanese Oyster.

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.

A Mnemiopsis Leiydi.

The mnemiopsis belongs to the ctenophore family and is now firmly established in the waters off Sweden's east and west coasts.

Originally from the United States, it has already shown just how much damage it can do in the Black Sea where it spread at the end of the 1980s, likely dumped there by ships emptying the water in their holds while in port.

"We have seen from the Black Sea that they can have a big impact on the ecosystem. The main thing is that they eat the plankton that the fish eat ... and can thereby change the whole food chain," Friis Moeller said.

Numerous fish species have already disappeared from Swedish waters, though pollution and overfishing have done their bit too. The mnemiopsis probably spread north on board ships that travelled to Sweden from the Netherlands.

Fishermen are worried that the alien species will make the situation even worse.

"We are very concerned about that. We initiated a program with Gothenburg University to get a better understanding of the existence of jellyfish," said Axel Wenblad, the director of the Swedish Board of Fisheries, a government agency.

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

"If we find out that there is a significant risk, what can we do about it? Frankly speaking, we can't do very much," Wenblad admitted.

Sweden has more than 2,000 professional fishermen, who each year catch around 330,000 tonnes of fish and crustaceans, an industry worth around one billion kronor (108 million euros, 154 million dollars).

In August, alarm bells also rang over the discovery of giant Japanese wild oysters in Swedish waters.

"We can't be sure where they are coming from," said Anna-Lisa Wrange, a marine biologist at the Tjaernoe laboratory in western Sweden.

One explanation could be that the oyster larvae may have been transported to Sweden by the sea current and established themselves during optimal conditions, given the relatively warm temperatures this summer.

In the 1980s, Swedish oyster growers tried to breed the species, known as Cassostrea Gigas and which is common in European oyster parks, but failed because the water was too cold for them to reproduce.

"We have no idea if they will survive the Swedish winter," Wrange said.

If they do, the giant oysters could disturb the balance in the ecosystem since they tend to settle in blue mussel beds and could lead to a decline in that population if the two species are found to be rivals.

This could "change the entire fauna. For example, sea birds eat blue mussels as food. They can't really eat those oysters, they're very spiky and large to digest, plus they are very hard to remove," she said.

There is no scientific information on why these two invading species are now found so far north but climate change appears to have played a role.

"There seems to be a link to climate changes that have occurred in the past few years," said Wrange, stressing however that more scientific studies were needed.

Climate change could also be responsible for a mysterious virus that killed about 250 seals in Danish and Swedish waters this summer, said Tero Haeroenen, a scientist at the Swedish Museum of Natural History.

The outbreak started on the small island of Anholt, in the Kattegatt waters between Denmark and Sweden.

The as-yet unidentified virus, which attacks the seals' respiratory systems, has also been detected in the Baltic Sea.

There is no help yet available for the stricken seals.

"We have to identify the virus first, which is very difficult," Haeroenen said.

The infected seals die a horrible death.

"It is a very painful death because in the end they suffocate. Their lungs are clogged by mucus," he said, adding that the virus does not spread to humans.