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Tangles of seagrass, algae swirl in the shallow surf

Beached tangles of vegetation from the sea. (Photo by Anita Westervelt)

Published August 5, 2023 in the McAllen Monitor

Story and photos by Anita Westervelt

Nothing says summer like wading carefree in the surf – carefree until a swirling tangle of slimy seaweed wraps around your ankles.

Kicked free, the annoying matter washes ashore, ignored by the casual beach walker. Dedicated beachcombers, though, pay attention to the beached clumps, checking for a rare shell, sea bean, piece of coral, sea critter or perhaps an ancient artifact dredged from the sea. Birds probe the stranded seaweed for a tasty morsel of food.

In its own right, each cluster can be an interesting still life artwork. A more discerning person might note nuances that denote different species left embattled together by the receding tide.

But the colorful tangled debris is not all seaweed. A variety of species often get clumped together as the sea churns beneath the surface. The more common species stranded on local beaches is broad-leaved Gulfweed, Sargassum fluitans. It is not a sea grass but a genus of large brown macroalgae. An important element in dune construction and shoreline stability, it helps to prevent sand erosion from wind and surf. It breaks down into vital nutrients that promote the growth of dune plants, which also aid in dune stabilization.

Broad-leaved Gulfweed, Sargassum fluitans. (Photo by Anita Westervelt)

Two species of sargassum in the Atlantic are unique: S. fluitans and S. natans. They are holopelagic, meaning the alga not only floats around the ocean, but it also reproduces on the high seas. Seaweeds reproduce and begin life on the floor of the ocean. Oxygen-filled pneumatocysts that look like berries add buoyancy to the sargassum.

Sargassum with the oxygen-filled berry-like pneumatocysts that keep it afloat. (Photo by Anita Westervelt)

Where it originates in the Sargasso Sea, the floating masses provide refuge for migratory species and essential habitat for some 120 species of fish and more than 120 species of invertebrates. It is an important nursery habitat that provides shelter and food for endangered sea turtles and commercially important fish species.

Two seagrasses that do frequently get stranded on our beaches are turtle grass, Thalassia testudinum, and manatee grass, Syringodium filiforme. Both are important to the health and success of the Laguna Madre ecosystem. These grasses live completely submerged in marine waters and help stabilize the sea bottom.

Flat blades of Turtle Grass. (Photo by Anita Westervelt)

Turtle grass develops underground rhizomes that can be 10 inches deep; they produce horizontal stems from which broad flat leaf clusters emerge. The leaves grow to 14 inches in length. The grass provides food and shelter for fish and wildlife and is favored by the green sea turtle.

Manatee grass leaf blades are cylindrical with blunt tips; the blades can grow to 20 inches in length. Its root is not as deep as that of turtle grass. Bottlenose dolphins, manatees, urchins and sea turtles eat the blades.

Turtle and Manatee grasses with Sargassum. (Photo by Anita Westervelt)

Rarer to wash ashore is broadleaf sea lettuce, Ulva lactuca, a marine green alga found worldwide in bays and around jetties. Freshly beached, the plant body is bright translucent green, membranous, thin and delicate.

Bright green Broadleaf Sea Lettuce and Sargassum. (Photo by Anita Westervelt)

Another interesting find often mistaken for seaweed is sauerkraut grass or spaghetti bryozoan, Zoobotryon verticillatum. Bryozoans are microscopic aquatic invertebrates that live in colonies of thousands. It is found globally in seagrass beds, drift algae, oyster reefs, dock pilings and debris in bays in warm seas.

Crystal strands of Bryozoan and thin green Manatee and broad Turtle Grasses. (Photo by Anita Westervelt)

Spaghetti bryozoan peppered with large chuncks of tar. (Photo by Anita Westervelt)

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These sources were helpful in writing this article:,,, biology department.



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