Alien Species Alert: Undaria Pinnatifida (Wakame or Japanese Kelp)

Since the early 2000s, the Japanese kelp, Undaria pinnatifida, native to the northwest Pacific,

occurs on all continents except – so far – Africa and Antarctica, and it has become one of the

main target species for biosecurity. In an analysis ranking species traits of 113 introduced

seaweeds in Europe, it was the third most invasive seaweed. There are several reasons for its

success as an invader, especially its great ability to colonize artificial substrates and disturbed

areas rapidly, as well as shells of oysters and mussels, and it can grow very fast, reaching

lengths of up to 2–3 metres. Other reasons are its high tolerance for adverse conditions, such

as high turbidity and eutrophication, and the nearly invisible gametophytes’ ability to survive

being out of water for more than a month and act as a “seed bank”. The reproductive output is

large, and zoospores may be released all year-round, which contributes to its colonization

potential. Further, Undaria often develops into a fouling problem. This not only affects ships

and boats, but also structures used in aquaculture and molluscs growing on the seabed. On the

other hand, it has economic value as a source of food (“wakame”), which has been the

motivation for intentional introductions to some areas for farming.