Sargassum

Sargassum is a genus of brown macroalgae in the order Fucales, comprising more than 300 species that grow mainly across tropical and subtropical oceans. Its fronds carry spherical, gas-filled bladders called pneumatocysts that keep the algae afloat near the surface for photosynthesis, and some species reach several meters in length with brown to dark olive-green coloring. Most Sargassum grows attached in shallow coastal waters and coral reefs, but two species, Sargassum natans and Sargassum fluitans, are holopelagic, reproducing only by fragmentation and drifting freely with ocean currents; the Sargasso Sea in the open Atlantic takes its name from the large mats of Sargassum that collect there. These floating mats form a habitat of their own, sheltering more than 100 species across eleven phyla, including fish, crabs, sea turtles and migratory birds.