Keep an eye out for Sargassum Weed

Know Your Islands

The next time you walk along the shore, or if you are floating on the surface after a great dive to Cayman’s reefs and you see a floating mass of brown algae, carefully examine it and you may discover a unique community of plants and animals!

Sargassum Weed

Observing these algae, called sargassum seaweed, can be a great source of education (and entertainment!) for people of all ages.

The main source of the sargassum found floating throughout the Gulf of Mexico, Atlantic Ocean and Caribbean comes from the Sargasso Sea. This area is a two-million-square-mile portion of the Atlantic Ocean extending from the West Indies to the Azores. Four different currents rotate around the sea creating a wall of cool water that contains the Sargasso Sea. In its interior lies a vast calm where huge patches of sargassum collect. The weed is drawn to the centre of the sea by two different methods: rotation and evaporation.

The rotation of the earth spins the currents to the centre of the sea, carrying the weed and since this central area is relatively stable, it evaporates quickly which in turn creates small surface currents replacing the water that was lost to evaporation. Even though these currents work to keep the sargassum centrally located in the Sargasso Sea, prevailing winds, storms and spiralling currents help disperse the weed throughout the world’s oceans.

Sargassum gets its name from the Portuguese word for grapes which resemble the small gas-filled bladders that help keep surface varieties afloat and anchored species standing upright. The golden-green leaves are saw-toothed and feel sturdy and resilient. There are at least six known species of sargassum, but Sargassum natans and Sargassum fluitans are two species that are frequently found together.

Sargassum plays an important role in the life cycles of hundreds if not thousands of marine animals. These floating mats serve as a source of food or home to a wide variety of sea life and it is something like a floating hotel to a numerous organisms. Organisms like small anemones, flatworms, hydroids, pipefish, small crabs, shrimp and Sargassum may be discovered in this weed. Most of these animals have evolved into forms that resemble the shape of the algae and are also similar in color to the gold-green leaves.

As a marine habitat, sargassum is one of the most dynamic and important features in the ocean used by pelagic organisms, particularly juvenile fish. Young fish become easy prey for predators, and sargassum provides a source of refuge from this predation. As more creatures cling to the fronds, the weight begins to overcome the buoyancy provided by the gas bladders and the alga begins to sink towards the bottom of the ocean. As the weed sinks deeper in the water column, the resulting increase in pressure will completely collapse the bladders and eventually this leads to death where it decomposes and serves as food for other organisms.

Protect Cayman Wildlife! For more information, to share your knowledge or if you would like to get involved with the many activities in the National Trust’s Know Your Islands Program, please visit, or contact [email protected] or 949-0121.

The weekly column from the National Trust is submitted by Marnie Laing, Education Programs Manager at the Trust. The Trust can be contacted at 949-0121 or via email at [email protected]

Can you find the Sargassum fish? Photo: Submitted

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