Picture of
Our largest alga

Perennial primary shoots


Summer and winter appearance


Origin and distribution

Japweeds journey to Sweden

Establishment in Sweden and Denmark

Establishment and development in the Koster area

Development along the Swedish west coast

Origin and distribution 

Japweed has its origins in Japan where it grows together with 27 other closely related specie in coastal waters that are effected by warm oceanic currents. The Sargassum-family has spread worldwide and most of the related specie have never been in contact with the Sargasso Sea, where the masses of alga comprise mainly of two specie, Sargassum natans and Sargassum fluitans. It is not unique to find Sargassum-specie in Europe. 3-6 specie are found growing naturally in the Mediterranean.
   During the 1940´s, japweed was found along the west coast of North America, probably as a result of the earlier imports of the "giant japanese oyster". It is believed that japweed originated from spores or small plants on the shells of the oysters. At present, the alga has spread from Alaska to Mexico.
   After adult "giant japanese oysters" were introduced to France, drifting examples of japweed were first discovered in 1971 outside Portsmouth in England. Two years later, an attached population was found off the Isle of Wight. Since then, japweed has spread along the coasts of Europe and to the Mediterranean. In the Limfjorden (Denmark), japweed was discovered in 1984, for the first time in Scandinavian waters. Here, dense populations up to 2m in length were present from the surface and down to about 2m. These populations were best developed amongst the bladder and toothed wracks at depths of between 0,5-1 m. Even along the west coast of Jutland several finds have been made. During 1992, the first attached examples were found in the Danish area of the Kattegatt. Along the south coast of Norway, the first drifting examples were found in 1984 and it was not until 1988 that the first attached examples were noted. Japweed is now found along the whole of the Norwegian south coast.




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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson