Species description: A large dark brown to golden brown kelp that grows 1-3 m in length. It has a broad, flat frond that is divided into finger-like projections, although young specimens will not yet have these. The flat, broad midrib, around 3 cm wide, is diagnostic. The holdfast is claw-like and in older specimens the flattened stipe has wavy curled reproductive frills.

Habitat: Usually found on artificial substrata on marinas and harbours, but will grow on any hard surface. Grows from low intertidal to around 18 m. In some areas in the world it is abundant in the sublittoral mainly in estuaries. Capable of tolerating low salinity.

Distribution: First reported in Northern Ireland in 2015 from Carrickfergus marina. Now known to be present in Carrickfergus, Bangor, Glenarm and Ardglass marinas and Mahee Island in Strangford Lough, as well as a few other scattered locations. Recorded from a number of locations in southern Britain, rarer in the north and elsewhere in Ireland there are a few scattered records.

Similar Species: Alaria esculenta is not so broad and does have a midrib, but it is superficially similar to young Undaria pinnatifida that have no yet developed the finger-like projections.

Key Identification Features:

• Broad, flat frond with midrib.
• Older specimens have finger-like projections on frond.
• Wavy, curled margin to flattened stipe on older specimens.

Distribution Map from NBN: Undaria pinnatifida at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Undaria pinnatifida at iNaturalist World Species Observations database.

WoRMS: Undaria pinnatifida at World Register of Marine Species. Accepted name: Undaria pinnatifida (Harvey) Suringar, 1873. AphiaID: 145721.

Classification: Biota; Chromista; Harosa; Heterokonta; Ochrophyta; Phaeophyceae; Fucophycidae; Laminariales; Alariaceae; Undaria