What is sargassum?

December 2003

Sargassum is a seaweed that consists of several species of brown algae, many of which are free-floating masses. Sargassum is also called gulfweed or sea holly, because it has many branched stems and hollow, berry-like floats. The hollow floats aid the plant in staying near the surface, which is necessary for photosynthesis, the process it uses to make food using energy from the sun.

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Pieces of sargassum weed, identifiable by its many stems and hollow, berry-like floats, are often found washed on shore.

There are approximately 150 species of sargassum found throughout the world. However, two main species of free-floating sargassum are found in the Atlantic Ocean; Sargassum natans and S. fluitans. These accumulate in large masses in an area of the Atlantic Ocean known as the Sargasso Sea.

This sea is a huge mass of floating sargassum off the southeastern coast of the United States. It is located near Bermuda, and its elliptical shape covers approximately a two-million-square-mile area, a result of a ring of currents that enclose a large eddy.

Because of the Earth’s rotation, the eddy circulates clockwise. This rotation keeps the sargassum from dispersing into other parts of the ocean. The Sargasso Sea receives little wind or rain, making it ideal for the seaweed to grow.

Without the weed, this area of ocean, currently located in the heart of the Bermuda Triangle, would be a desert. There are few nutrients available here, and sargassum provides a variety of sea life with habitat and food.

One species that uses the sargassum is the loggerhead sea turtle. Loggerheads spend the first few years of life hiding in the sargassum weed. Other animals, such as the sargassum fish (Histrio histrio), actually disguise themselves among the fronds of the weed to ambush prey.

In ancient times, sailors believed the huge masses of seaweed would trap their ships, dooming them to sail forever in the Sargasso Sea. However, there is actually not enough seaweed to entangle even a small boat, much less a large ancient sailing vessel.