Wire weed

Sargassum muticum


Photo credit: ©Graça Gaspar

Sargassum muticum
  • The species is known to occur from the intertidal to the subtidal in a range of substrates including hard rock face and Zostera marina (eel grass) beds.

  • This large brown alga has recorded at several locations around the coast of Ireland. S. muticum in its invasive territory differs greatly to specimens in its native Japanese waters. Sargassum can grow up to 16 m in length, forming floating mats on the sea surface. It can grow up to 10 cm per day, and it also has a long life span of 3-4 years. Dense mats of Sargassum can form very quickly. Fronds, if detached, can continue to shed germlings as they drift. The loss of an entire frond is not fatal to the weed, as a new frond can grow from the primary axis that remains.

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Origin and Distribution:
  • Native to waters off Japan and Korea but is now present in continental Europe and Britain. The species is known to have been present at the Isle of Wight in 1973 but may have been present in Europe as early as 1966. It has since spread from the original site of introduction and is known from Norway to Spain. S. muticum is also known to have invaded North America from the 1940’s onwards.

  • The species can occupy hard substrates on sheltered shores where it can from dense monospecific stands excluding other species.
  • Dense S. muticum stands can reduce the available light for understory species, dampen water flow, increase sedimentation rates and reduce ambient nutrient concentrations available for native species.

How did it get here?
  • It is believed that this species arrived with oyster spat introduced for commercial purposes.

Is it found in Northern Ireland?
  • It has been found in Northern Ireland.  More distribution information can be found at NBN Atlas NI.

Prevent Spread:
  • Keep boats and marine equipment free of fouling.
  • Report all sightings.
  • Follow Check Clean Dry protocols.

Current Legislation Position (Northern Ireland only):
  • Sargassum munticum is listed in Schedule 9 of the Wildlife (Northern Ireland) Order 1985 and as such, under Article 15 it is an offence to plant or otherwise cause this species to grow in the wild.
For further queries, you can contact the Invasive Non Native Species (INNS) Team in the Northern Ireland Environment Agency on 028 9056 9558 or Email: invasivespecies@daera-ni.gov.uk

Species Related Files:

Invasive Species Ireland

Invasive Species Ireland