Phylum: Ochrophyta   Family: Sargassaceae

Sargassum muticum (Yendo) Fensholt 1955

Wireweed; Japan Weed

An invasive species, originating from Japanese waters, it first appeared on the Isle of Wight in 1973. It's self fertile, fast growing and relatively long lived. It was first observed in the Outer Hebrides in 2013

References: 
Fish & Fish (2011) - A Student's Guide to the Seashore;
Bunker, et al - Seaweeds of Britain and Ireland