Sargassum or sargasso weed are common names for brown marine algae in the genus Sargassum. Only two species, Sargassum natans and Sargassum fluitans, live in high seas, with a strictly pelagic cycle. They float on the surface, forming large and dispersed clumps called rafts. They are fully functioning ecosystems which are home to many living species, ranging from bacteria to fish.
There are also 350 other benthic Sargassum species, which live attached to the seabed in coastal areas. They are distributed in several regions of the world.