Sargassum seaweed, also called gulfweed, a tropical pelagic brown algae kept afloat by its berry-like, gas-filled bladders. Sargassum natans, also known as Sargassum bacciferum and Fucus natans. Namesake of the Sargasso Sea, where vast floating mats of this seaweed can be found. This seaweed plays vital role in the life cycles of a multitude of marine animals. Silver Bank Humpback Whale Sanctuary, Dominican Republic.