Sargassum is eaten by a wide range of animals, from tiny crustaceans and sea urchins to sea turtles, herbivorous fish, and even specialized deep-sea organisms. What consumes it depends largely on where the sargassum is: floating in open ocean, attached to coastal rocks, or sinking to the seafloor.
Small Grazers in the Water
The most consistent sargassum eaters are small invertebrates. Amphipods (tiny shrimp-like crustaceans), snails, and sea urchins all graze directly on sargassum tissue. In Northern Ireland, the amphipod Dexamine spinosa feeds heavily on sargassum, and in the Pacific Northwest, the small snail Lacuna vincta is a primary grazer. One amphipod species, Ampithoe marcuzii, shows a strong preference for sargassum even when other algae are available. In lab tests with several types of seaweed offered simultaneously, it ate sargassum almost exclusively.
Floating sargassum mats in the open ocean also support a shrimp called Hippolyte coerulescens, a dedicated herbivore that lives and feeds among the algae. These small grazers are the base of the sargassum food web, and their constant nibbling helps shape how the algae grows and spreads.
Sea Turtles, Especially Young Ones
Young green sea turtles are among the most notable sargassum consumers. In the Gulf of Mexico, sargassum appeared in the stomachs of over 93% of oceanic-stage green turtles studied, making it the dominant item in their diet by a wide margin. Even slightly older turtles that had recently settled into coastal waters still had sargassum in 63% of their stomachs, particularly during spring months when floating mats are abundant.
The relationship is more complicated than it looks, though. Chemical analysis of turtle tissue suggests many of these turtles aren’t actually absorbing much nutrition from the sargassum itself. Their body chemistry more closely reflects the small animals living on the algae: tiny crabs, shrimp, snails, and other creatures that cling to floating mats. So while juvenile green turtles consume enormous quantities of sargassum, they may be eating it partly as a byproduct of hunting the animals attached to it.
Herbivorous Fish
Sea chubs (Kyphosus species) are the most well-documented fish grazers of sargassum. These reef fish have guts specifically adapted for digesting algae, including microbial fermentation similar to what happens in a cow’s stomach. On subtropical reefs off the Brazilian coast, where rocks are heavily covered in sargassum, researchers found that most sub-adult and adult sea chubs fed mainly on sargassum. Juveniles tended to eat other types of algae and small crustaceans, then shifted toward a sargassum-heavy diet as they grew larger.
This pattern, moving from a mixed diet to a primarily herbivorous one, is common in reef fish that graze on tough brown algae. Larger fish have the gut length and microbial communities needed to extract nutrition from sargassum’s complex carbohydrates.
The Community Living on Floating Mats
Floating sargassum mats in the Atlantic function as drifting ecosystems, and many of the animals living on them feed not on the sargassum itself but on the organisms growing on its surface. Nudibranchs (sea slugs) like Scyllaea pelagica and Fiona pinnata eat the tiny hydroids, bryozoans, and other colonial animals that encrust sargassum fronds. Small crabs such as Portunus sayi and Planes minutus hunt mobile prey within the mat, while the slender shrimp Latreutes fucorum does the same.
This distinction matters. The sargassum mat supports two overlapping food webs: one built on animals that eat the algae directly, and another built on predators that eat the grazers and other small creatures using the mat as habitat.
What Eats Sargassum on the Seafloor
When sargassum sinks, it doesn’t go to waste. A 2024 study published by the Royal Society described a deep-sea isopod (a relative of woodlice and pill bugs) specifically adapted to consume sargassum that drifts down from the surface. This creature has serrated, grinding mouthparts built for breaking apart tough algal tissue, and its gut contains specialized bacteria that can break down the complex sugars in seaweed and even fix nitrogen to supplement its diet. It also uses a distinctive swimming stroke to reach sinking sargassum before it hits the bottom.
This discovery suggests that sargassum plays a larger role in deep-sea food webs than previously understood, delivering energy from sunlit surface waters to organisms thousands of meters below.
Sargassum as Livestock Feed
Humans have also explored feeding sargassum to animals on land. Research on using brown algae in cattle and goat diets shows promise: at low concentrations (1 to 10% of the diet), sargassum supplementation improved digestive efficiency and actually reduced methane and carbon dioxide emissions during fermentation in the gut. The fiber and mineral content make it a potentially useful feed additive for ruminants.
The major obstacle is arsenic. Pelagic sargassum collected along the Mexican Caribbean coast contained arsenic levels between 24 and 172 parts per million (dry weight). Under European regulations, seaweed used as animal feed must contain less than 40 ppm. A full 86% of the samples exceeded that limit. The arsenic concentration varies depending on where the sargassum drifted before washing ashore, since mats that pass through polluted waters absorb more contaminants. Salinity is another concern: the seaweed needs to be thoroughly washed before use to avoid disrupting gut microbes in livestock, though research suggests moderate salt intake doesn’t significantly harm rumen function.
So while sargassum can technically be fed to cattle, the heavy metal content of most wild-harvested sargassum, particularly the massive blooms now arriving on Caribbean and Gulf coastlines, makes it unsuitable without careful testing and processing.