Most Unwanted: Wire weed
Sargassum muticum
Habitat: Marine
Threat: Loss of protected species
Status: Established
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. In its home range it is relatively small innocuous seaweed. 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
What is it?
This is a species of alga that belongs to the division Phaeophyta. 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. It reproduces both sexually and via floating fragments. Fertilisation usually takes place on the surfaces of the reproductive bodies (receptacles) and thousands of fertilised "germlings" are released. These settle rapidly, usually near the parent alga. 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.
Impact
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.
Where is it from?
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.
Where are they in Ireland?
Species is now widespread around the coast of Ireland with definite records in Counties Down, Louth, Wexford, Cork, Kerry, Galway and Sligo. It is likely that the species has a much wider distribution and will spread to new areas to colonise all coastal areas.
How did it get here?
It is believed that this species arrived with oyster spat introduced for commercial purposes.
How are they spreading in Ireland
Natural dispersal.
Contaminant of oyster spat.
Fouling on boats and other marine equipment.
What you can do
Keep you boat and marine equipment free of fouling.
Report all sightings.
