A large brown seaweed (with a frond often over 1m long), the stem has regularly alternating branches with flattened oval blades and spherical gas bladders. It is highly distinctive and olive-brown in colour. Wireweed is an invasive species from the Pacific that appeared on the Isle of Wight in 1973, having spread to Britain from France. It competes with native species such as seagrasses and is considered a nuisance in harbours, beaches and shallow waters.Sargassum muticum is also known as 'Japweed' and 'Japanese weed'.