Marine Biologist Dr. Paul Arena in the Sargasso Sea dives among sargassum. Sargassum muticum is a large brown seaweed of the class Phaeophyceae. Some sargassum is visible off the Florida coast. Large, pelagic mats of sargassum in the Sargasso Sea act as one of the only habitats available for ecosystem development; this is because the Sargasso Sea lacks any land boundaries. The sargassum patches act as a refuge for many species in different parts of their development, but also as a permanent residence for endemic species that can only be found living on and within the sargassum.