Sargassum (brown algae) washing up on shore

Sargassum

Sargassum on the beach.

Courtesy of Tom Baird
Published: Thursday, July 10, 2014 at 11:55 AM.

By Tom Baird

 

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.



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