Sargassum muticum | Wireweed



Conservation status

Not Assessed

Legal status

Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted). Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

Native status


First reported in the wild



Invasive species - risk of High Impact

Irish status


Introduction pathways - 1

Transport Stowaway

Introduction pathways subclass - 1

Hitchhikers on ship/boat

Introduction pathways - 2


Introduction pathways subclass - 2

Natural dispersal across borders of invasive aliens

Invasive score


NAPRA Ireland risk assessed


Species Biology


'Wiry' appearance on rocks, consists of different strands with conspicuous lumps. Large brown seaweed, frond (stem) can be over 1m long, alternating branching of flattened oval blades with gas bladders.


Significant differences between invertebrate communities underneath and at control areas in Strangford Lough, with a higher abundance of smaller, opportunistic, r- selected (generally short lived, early coloniser) species inside the canopy (Strong et al., 2006). By out competing native species Sargassum muticum can significantly reduced primary productive in Lough Hyne (Salvaterra et al., 2013) which could affect ecosystem functioning. These changes in Lough Hyne lead to reductions in species richness and diversity in the faunal community (Salvaterra et al.,2013).




Monoecious species can spread by fragmentation. Highly variable in reproductive development between sites with different wave exposure on the Irish west coast (Baer & Stengel, 2010). Germlings develop on receptacles and disperse within 30m (generally ~3m) of the parent plant (Deysher & Norton, 1982) facilitating rapid within bay dispersion, with long range dispersion mainly by detached fertile branches, which are also capable of vegetative regrowth (Deysher & Norton, 1982; Kraan, 2008).

Pathway and vector description

First recorded in Europe in the Isle of Wight in 1973 (Farnham et al., 1973), it was likely introduced to Europe on oysters (Farnham et al.1973) either from its native Japan or from British Columbia where it was introduced previously (Kraan, 2008). It was likely introduced to Ireland attached to boats as the first record in Ireland comes from Strangford Lough (Boaden, 1995), though it may have dispersed to Ireland naturally from Britain. Once established in Ireland it can rapidly colonise new areas, with estimates of spread of 2-3km per year within bays and 54km per year around the Irish coast dispersal rates estimated (Kraan, 2008).

Mechanism of impact


Broad environment


Habitat description

Attached to rocky substrates, tolerant to a wide range of temperatures and salinities (Kraan, 2008).

Species group


Native region

Temperate Asia


World distribution(GBIF)

Irish distribution

Established - Widespread. Established though Baer & Stengel report a "patchy distribution" (2010). Likely under recorded in areas.

Native distribution

Native to Japan, now widely introduced, present in Europe and North America (Kraan, 2008).

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2021

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How can you help

Report any sightings to the National Biodiversity Data Centre.



Boaden, P. J. S. (1995). The adventive seaweed Sargassum muticum (Yendo) Fensholt in Strangford Lough, Northern Ireland. The Irish Naturalists' Journal, 111-113.

Baer, J., & Stengel, D. B. (2010). Variability in growth, development and reproduction of the non-native seaweed Sargassum muticum (Phaeophyceae) on the Irish west coast. Estuarine, Coastal and Shelf Science, 90(4):185-194.

Deysher, L. & Norton, T.A. (1982) Dispersal and colonisation in Sargassum muticum (Yendo) Fensholt. Journal of Experimental Marine Biology and Ecology 56: 179-195

Farnham W.F., Fletcher R.L. & Irvine L.M. (1973) Attached Sargassum found in Britain. Nature 243:231–232.

Kraan, S. (2008). Sargassum muticum (Yendo) Fensholt in Ireland: an invasive species on the move. Journal of Applied Phycology, 20(5):825-832.

Strong, J. A., Dring, M. J., & Maggs, C. A. (2006). Colonisation and modification of soft substratum habitats by the invasive macroalga Sargassum muticum. Marine Ecology Progress Series, 321:87-97.

Salvaterra, T., Green, D. S., Crowe, T. P., & O’Gorman, E. J. (2013). Impacts of the invasive alga Sargassum muticum on ecosystem functioning and food web structure. Biological invasions, 15(11), 2563-2576.

Global Invasive Species Database

CABI Datasheet