Sargassum muticum, commonly known as wire weed, strangle weed or ‘Jap’ weed, is a perennial brown seaweed that grows just below the low water mark in the shallow sublittoral or in areas of standing water on the lower shore.

Sargassum muticum is a member of the fucoid family,  but is distinctly different to the wracks found around the UK coastline.  Normally brown, but colour varies to orange and light yellow with high light intensity and high surface water temperatures.  S. muticum originates from the western Pacific, where within its natural range (south-east Asia to southern Russia), it is one of the smaller Sargassum species, growing to around 1m long.

Sargassum muticum is a non-native species that has been accidentally introduced to other parts of the world and once removed from the constraints of its native environment, it becomes an
aggressive coloniser and grows larger.  There are concerns that Sargassum muticum competes and displaces indigenous species of brown seaweeds, and on Lundy steps are being taken to remove it from the areas where it has colonised in the Landing Bay and Devil's Kitchen.  

There are a number of other implications relating to recreational and commercial uses of coastal areas where dense growth occurs - floating mats of drifting Sargassum muticum can potentially affect water sports, sailing, It can foul fishing lines and nets and the steering gear of small boats as well as trapping marine debris and being cast ashore.

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