According to Swedish newspaper, Helsingin Sanomatf, fresh samples taken in January show that populations were especially high in the depths of the Åland Sea - up to 3,800 individuals per square metre.

Aranda, the research vessel of the Finnish Institute of Marine Research, returned to Helsinki from a one-month monitoring expedition in the Baltic Sea on Friday after taking samples in 78 locations.

In the southern Baltic, off the Danish island of Bornholm, there were 1,460 comb jellies per square metre were found. There were also small numbers of the tiny creatures in other areas. Many were also found in the Gulf of Finland, between Helsinki and the Estonian capital Tallinn.

"However, they were in poor shape - many were dead. The possible reason is the cold temperature and low salinity of the water, or the shortage of nutrition.", said researcher Maiju Lehtiniemi of the Finnish Institute of Marine Research.

The unusually mild winter has nevertheless benefitted the invaders.

No comb jellies were found in the northern part of the Gulf of Bothnia - possibly because of the low salinity.

The comb jellies are similar to jellyfish. They are indigenous to the east coasts of north and South America, appearing in water temperatures ranging from 4 to 32 degrees Celsius, with a salinity of 3-39 parts per mille.

They feed on fish eggs, newly-hatched fish, and zooplankton.

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