Undaria pinnatifida (Harvey) Suringar

Common names: Wakame, Japanese Kelp.

Description: Thallus laminate, yellowish to dark brown, usually 1-2 m, occasionally 3 m or more in length; holdfast spreading, dichotomously branched and claw-like, giving rise to a flattened oar-like stipe with a "fried-egg" like margin with small proliferations and basally with beautifully lobed sporophylls that coil around it when mature; stipe continuing into the fond as a flattened midrib that bears broadly lobed laciniate fronds with a roughly pyramidal shape.
Habitat: Lower intertidal and very shallow subtidal (no more than a few m), particularly in sheltered locations, growing particularly on marinas, buoys, and similar floating structures in harbours. Often occurring on boat-hulls.
Distribution: In Ireland, Carrickfergus, Co. Antrim (2014); Kilmore Quay, Co. Wexford (2016), Dun Laoghaire, Co. Dublin (2017); Greystones, Co. Wicklow (2020). Now widespread on the east coast in harbours and bays. Elsewhere in Europe: S England, NW France, NW Spain, Mediterranean. Native to Pacific Russia, Japan, China and Korea.
Key characteristics: Frilly sporophylls coiling around the base of the flattened stipe at the base. A similar flattened midrib is not found in any other kelp in the Atlantic. Alaria esculenta has a midrib which is not flattened and the frond of Alaria is not lobed, although it may be similar laciniate.
Uses: Cultivated and collected in the wild in Japan, China and Korea for human consumption as "Wakame" (Japan) and cultivation on ropes was successfully carried out in Brittany for a time. Fucuiodan from Wakame may be effective in the prevention of certain cancers.
Notes: This is an introduced kelp from the W Pacific, now found in the NE Atlantic and the W Mediterranean. Elsewhere similar adventive populations have been found in New Zealand, Tasmania, and Argentina. It is thought to have been introduced into the southern hemisphere by fishing fleets from SE Asia. The plants shown here are from Tasmania. Undaria is a particular pest in Venice (see bottom two pictures) where plants foul the step leading to St Mark's Square (right) and cover the anchor chains of the vaporetto and traghetto stops (left).


Kraan, S. (2017). Undaria marching on; late arrival in the Republic of Ireland. Journal of Applied Phycology 29(2): 1107-1114.

Minchin, D. & Nunn, J. (2014). The invasive brown alga Undaria pinnatifida (Harvey) Suringar, 1873 (Laminariales: Alariaceae), spreads northwards in Europe. BioInvasions Records 3(2): 57-63.

Identification guide for selected marine non-native species (National Museum Wales). (Download free PDF, about 3.5 Mb).

Photographs © Dan Minchin (top and middle from N. Ireland); M.D. Guiry (bottom two from Venice).