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.