Species description: This is a large, brown seaweed that grows to over 2 m in length, up to 4 m. It is yellowish-brown to dark brown in colour with small spherical air bladders in clusters of 2-6 mm, on stalks and flattened oval blades. It has regular, alternate branching which can become quite bushy. Has a characteristic "washing line" appearance when held out of water. Upper branches are narrow. It is tough and cartilaginous. Dies back in autumn leaving only the small basal holdfast.

Habitat: Epilithic from intertidal rock pools and rock to 10 m deep. Very fast growing and can outcompete native seaweeds.

Distribution: First recorded in Northern Ireland in Strangford Lough in 1995, and now widespread around all coasts. First recorded from the Isle of Wight in 1973, has now spread along the south coast of England and to the Channel Islands. In Ireland it has been confirmed from counties Down, Cork, Kerry and Wexford. Drift material has been found in Co. Sligo. Current data suggests that at least 5 independent events of introduction have occurred. Europe: Mediterranean, Portugal, Spain, France, Netherlands to Scandinavia. Further afield: Japan, introduced to the Pacific coast of North America about 1945 and has since spread from British Columbia to California.

Similar Species: Could potentially be confused with some Cystoseira species but these are small and branch irregularly.

Key Identification Features:

• Small stalked air bladders 2 - 6 mm in size.
• Flattened oval blades.
• Regular alternate branching

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

iNaturalist: Sargassum muticum at iNaturalist World Species Observations database.

WoRMS: Sargassum muticum at World Register of Marine Species. Accepted name: Sargassum muticum (Yendo) Fensholt, 1955. AphiaID: 494791.

Classification: Biota; Chromista; Harosa; Heterokonta; Ochrophyta; Phaeophyceae; Fucophycidae; Fucales; Sargassaceae; Sargassum