The Gulf weed (Sargassum fulvellum) belongs to the family Sargassaceae. It is an annual brown algae. Sargassum is a seaweed that is widely distributed in the temperate and tropical regions, and about 60 species are known around Japan. It grows in the reefs along the coasts of Honshu, northern Kyushu and northern Shikoku, excluding the Sanriku coast, and is about 2 m high. The stem is flexible, has many branches, and is tightly twisted with trilobes or prisms. The leaves are lanceolate and are characterized by notches. It has elliptic and obovate bubbles and gains buoyancy in seawater to become flowing algae. It has been used as food, fertilizer, and decoration since ancient times.