Weedlines: A Sargassum Research Blog

Sargassum Communities

Saw large quantities of weeds today, though none was observed yesterday.

        –Excerpt from journal of Christopher Columbus, Thursday, September 20, 1492.
Sargassum Communities 1

A Sargassum weedline stretching into the horizon in the Gulf of Mexico.

This observation, made over a half century ago by Columbus as he sailed across the Sargasso Sea, still sums up the ephemeral nature of floating Sargassum in the western central Atlantic region, including the Gulf of Mexico. Seemingly here today and gone tomorrow, Sargassum (also known as “gulfweed”), is a brown algae complex composed (primarily) of two species, Sargassum natans and Sargassum fluitans. At sea, Sargassum is buoyed by small gas bladders which allows it to float on the sea surface. Sargassum can be found in several forms as it is transported within and between the central Atlantic region via currents and winds. For example, small clumps of Sargassum can be encountered scattered throughout the ocean. At other times oceanographic fronts converge Sargassum into weedlines that can stretch for miles. And as Ernest Hemingway noted, Sargassum can be found in the form of large, amorphous mats that resemble a “yellow blanket”. Sargassum reproduces vegetatively, however we know relatively little about the oceanic and environmental conditions that result in “good” vs. “bad” Sargassum years. In recent years there have been tremendous Sargassum ‘outbreaks’ in the Caribbean, which plagued the fishing and tourism economies of many islands once much of the seaweed washed ashore. Because Sargassum is constantly on the move, it is very difficult to determine its growth patterns, estimate just how much of it is out there at any given time, and track how it it is transported from region to region.

Sargassum Communities 2

Two species comprise floating Sargassum habitat. Sargassum natans (left) has slender blades and gas bladders with small spines, while Sargassum fluitans (right) has wider blades and smooth gas bladders.

We do know, however, that regardless of its form, where there is Sargassum there is usually a vibrant community of marine life living among the algal fronds and swimming beneath the leafy canopy. At the base of this community are bryozoans, encrusting polychaete worms, barnacles, hydroids and other organisms attach themselves to the algal blades and gas bladders, thus creating a more complex habitat. Many species of mobile invertebrates are found in Sargassum, including some that are uniquely adapted to live in this habitat, such as the aptly named Sargassum Swimming Crab (Portunus sayi), Sargassum Shrimp (Leander tenuicornis), Slender Sargassum Shrimp (Latreutes fucorum), and Sargassum Nudibranch (Scyllaea pelagica).  Over a hundred species of fish are found in association with Sargassum, either during their early life as eggs, larvae or juveniles, or during later life as adults. Some of these fishes are resident species, such as the Planehead Filefish (Stephanolepis hispidus), the Sargassum Pipefish (Syngnathus pelagicus) and the Sargassumfish (Histrio histrio).

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Resident species that have adapted to living in Sargassum include the Sargassumfish (left), Planehead Filefish (center) and Sargassum Nudibranch (right).

Others are more transient, and use the habitat during one or more life stages. Many species of flyingfish, for example, spawn their eggs within Sargassum mats. Larvae and juveniles of Greater Amberjack (Seriola dumerili) and Sailfish (Istiophorus platypterus) have been collected in Sargassum, while Tripletail (Lobotes surinamensis) and Mahi Mahi (Coryphaena hippurus) are commonly encountered as larvae, juveniles and adults. Juvenile Gray Triggerfish (Balistes capriscus) are also a ubiquitous member of the transient fish community. In addition, sea turtles (particularly juveniles) are often found in association with Sargassum, as are many species of foraging seabirds. In an otherwise featureless open ocean, it is believed that floating Sargassum provides foraging areas, refuge from predators, and a point of reference for many marine organisms.

Sargassum Communities 4

Transient species like Mahi Mahi may use Sargassum during multiple life stages, like this juvenile (left) and adult (right).

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