Sargassum (brown algae) washing up on shore

A lot of seaweed is washing up on the beach right now.  Often called Gulfweed, the clumps are all one or more species of Sargassum, which is a brown algae.  The brown algaes are more often associated with colder, more northern coastlines…  Think of the giant kelps off the Pacific coast.  One notable exception is Sargassum, which includes several species of algae of the open ocean.  Changes in weather patterns and ocean currents will sporadically bring rafts of Sargassum onto our shores.

When at the beach, pick up a clump of the Gulfweed and inspect it.  There are little, round, grapelike balls among what appears to be leaves. The little balls are small air bladders that buoy the algae mass up at the surface so that it can get sunlight for photosynthesis.  The air bladders are an adaptation to living in the open ocean. Otherwise, the algae would rapidly sink into darkness and die.

What few people realize is that these floating rafts of seaweed contain a whole community of highly specialized organisms that spend their lives drifting in the open ocean.  Some of these creatures are found nowhere else. The Sargassum community is like something found in science fiction, floating islands of strange and unusual creatures.

Many of these animals have shapes and colors that mimic and blend in with the sargassum.  Some are sessile, that is they attach to the algae, while most crawl about freely on the drifting seaweed.  The young of some oceanic fish use the sargassum as a refuge from larger, predaceous fish and hang about in or under the floating islands.

Some of the types of creatures that have become adapted to the rafts of sargassum are flatworms, polychaete worms, shrimps, crabs, an anemone, and various hydroids.  The Sargassumfish (Histrio histrio) has an irregular outline and mottled coloration that mimics its leafy environment.  The little fish can swim rapidly, but spends most of its time crawling about the Sargassum using its pectoral fins like arms.  It blends with the algae so well that it easily ambushes any prey, including fish that swim nearby.  The sargassum crabs, Callinectes marginatus and Portunus sayi, have mottled brown coloration enabling them to blend with the algae. There is a little sargassum shrimp, Latreutes fucorum, that crawls about the floating clumps of algae and may be hard to spot until it moves.  There is even a carnivorous nudibranch that has large leaf-like lobes along its back that make it almost impossible to distinguish from its floating algae host.

All of the different species that inhabit the floating sargassum are closely related to inshore fauna and probably derived from those species.  Nevertheless, the sargassum fauna is a dwarf fauna. They are much smaller than their close relatives. It is also a warm-water fauna and is most prolific where water temperature is greater than 64 degrees Fahrenheit.  The surface water temperature of the Gulf of Mexico in July is around 86 degrees F. and the rafts of Sargassum abound in associated life forms.

When the Sargassum is washed into shore, most of these animals abandon the bundles of seaweed or die and are washed out of the seaweed in the surf.  Good ways to get an impression of the life forms in the sargassum is to collect some fresh sargassum as it is still in the surf and shake it out in a bucket of seawater, or collect it at sea from a boat using a dip net or bucket.  Once in a bucket of water you can examine it in more detail and see some of the myriad creatures living there.

There are several species of Sargassum.  Some species are anchored to the bottom using holdfasts.  Meadows of these species of Sargassum can be found in clear, shallow water from the West Indies to the northern Gulf of Mexico.  These species reproduce sexually and therefore introduce genetic variation.  Pieces of these algae break off and float in the currents.  However, there are some species of Sargassum that exist only as free-floating islands in the open seas. They reproduce vegetatively.  Amazingly, some of the associated life forms that live on the Sargassum are restricted to only one of the Sargassum species and are found nowhere else.

While the rafts of Gulfweed are sometimes a nuisance to swimmers, the decaying algae fertilizes the beach and makes it more productive for burrowing beach creatures.  The tides will return the nutrients back to the sea.  The next time you are wandering the shoreline, closely inspect some of the clumps of Sargassum that have washed ashore.  You’ll be holding a fascinating world of life that only lives drifting on the open ocean.

Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.


Photo caption: Sargassum on the beach, courtesy of Tom Baird

Tom Baird, starfl

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