information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Wireweed (Sargassum muticum)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.
Only coastal and marine records shown



A large brown seaweed (with a frond often over 1m long), the stem has regularly alternating branches with flattened oval blades and spherical gas bladders. It is highly distinctive and olive-brown in colour. Wireweed is an invasive species from the Pacific that appeared on the Isle of Wight in 1973, having spread to Britain from France. It competes with native species such as seagrasses and is considered a nuisance in harbours, beaches and shallow waters.

Recorded distribution in Britain and Ireland

Sargassum muticum has spread along the south coast to the Isles of Scilly and along the north Cornish coast to Lundy. Populations have also been recorded in Strangford Lough, Northern Ireland and Loch Ryan and the Firth of Clyde, Scotland.

Global distribution



Grows on hard substrata in shallow waters and can also tolerate estuarine conditions. It can out-compete local species because it is fast growing, can reproduce within the first year of life and being monoecious can fertilize itself.

Depth range


Identifying features

  • Fronds often more than 1 m in length.
  • Olive-brown in colour.
  • Regular alternating branches along stem.
  • Flattened oval blades and spherical gas bladders along branches.

Additional information

Sargassum muticum is also known as 'Japweed' and 'Japanese weed'.

Listed by

- none -

Further information sources

Search on:


  1. Hardy, F.G. & Guiry, M.D., 2003. A check-list and atlas of the seaweeds of Britain and Ireland. London: British Phycological Society

  2. Hayward, P., Nelson-Smith, T. & Shields, C. 1996. Collins pocket guide. Sea shore of Britain and northern Europe. London: HarperCollins.

  3. Howson, C.M. & Picton, B.E., 1997. The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]

  4. JNCC (Joint Nature Conservation Committee), 1999. Marine Environment Resource Mapping And Information Database (MERMAID): Marine Nature Conservation Review Survey Database. [on-line]

  5. National Biodiversity Network (NBN) Atlas website. Available from:  Accessed 01 April 2017

  6. Norton, T.A. (ed.), 1985. Provisional Atlas of the Marine Algae of Britain and Ireland. Huntingdon: Biological Records Centre, Institute of Terrestrial Ecology.


This review can be cited as:

Pizzolla, P.F 2008. Sargassum muticum Wireweed. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19-09-2018]. Available from:

Last Updated: 29/05/2008