Seagrasses are flowering plants that grow in shallow coastal waters around the world. If you’re wondering what animals eat these underwater prairies, you’re not alone. Seagrasses play a vital role in marine ecosystems by stabilizing sediment, improving water quality, and providing food and shelter for many animals.
But which animals actually eat seagrass? Let’s take a closer look.
If you’re short on time, here’s a quick answer to your question: Fish, sea turtles, waterfowl, manatees, and dugongs all consume seagrass to some degree. Green sea turtles and dugongs are the largest seagrass grazers, with seagrass making up the majority of their diet.
Green Sea Turtles Depend on Seagrass Meadows
Seagrasses Are the Mainstay of Their Diet
Seagrasses form an integral part of the diet for green sea turtles, which are classified as endangered on the IUCN Red List. These marine reptiles spend their time grazing on seagrass beds stretching across shallow coastal waters.
Their favorite varieties include turtle grass and shoal grass that thrive in abundance along the coasts of Florida and the Caribbean islands (NOAA).
An adult green turtle needs to consume around 2.2-9 pounds of seagrass each day to meet its nutritional requirements, making these underwater meadows the mainstay of its diet (Christianen et al., 2012).
Their grazing helps the turtles obtain sufficient nutrients and minerals like nitrogen, phosphorus, calcium, and potassium that aid growth and maintenance.
Grazing Helps Maintain Healthy Seagrass Beds
The green turtles also facilitate seagrass growth through their grazing activities. As these megaherbivores trim the leaves, they boost light penetration and simulate nutrient recycling in marine ecosystems.
Their grazing prevents excessive nutrient accumulation in plant tissues, keeping the seagrasses in a state of perpetual growth (Christianen et al., 2014). So the turtles’ grazing and trampling actually foster plant productivity.
However, both seagrasses and green sea turtles exist in a delicate balance. Excess removal of seagrass blades through overgrazing poses survival issues for the turtles. Similarly, inadequate grazing impedes seagrass growth and hampers light penetration.
But within threshold limits, the green turtles help maintain healthy and resilient seagrass meadows through their feeding activities.
Dugongs Are Specialized Seagrass Grazers
Dugongs Can Eat Over 50 Pounds Daily
Dugongs, also known as sea cows, are gentle marine mammals that inhabit shallow coastal waters throughout the Indo-Pacific region. These large herbivores are specialized grazers on seagrasses and can consume astonishing amounts each day.
According to research, adult dugongs may eat anywhere from 33-99 pounds (15-45 kg) of seagrass per day, with an average daily intake around 77 pounds (35 kg)!
The dugong’s diet consists almost exclusively of seagrasses. Their large, muscular snout combined with specialized lips and mouth allow them to uproot entire seagrass plants and mow down large meadows of these underwater plants.
Dugongs will swallow the plants whole, then digest the leaves and stems with their hindgut fermentation system. This allows them to maximize nutrient absorption from the fibrous seagrass.
To sustain their massive bulk of up to 1100 pounds (500 kg), dugongs need to spend most of the day, around 5-8 hours, grazing on seagrass meadows. Their grazing paths often appear as “feeding trails” through seagrass beds. Dugongs may travel several miles each day between prime feeding areas.
This constant grazing provides an important ecological role in trimming fast-growing seagrass meadows.
Vulnerable to Seagrass Loss and Habitat Degradation
As seagrass specialists, dugongs are highly vulnerable to declines in seagrass abundance and distribution. Loss of seagrass beds due to coastal development, water pollution, trawling activities, and climate change all threaten dugong survival.
Their slow reproductive rates also make it difficult for populations to recover from these losses. They are listed as Vulnerable on the IUCN Red List of Threatened Species.
Protecting remaining seagrass habitats will be crucial for dugong conservation. Organizations like Seagrass Watch monitor seagrass health and educate communities about the importance of seagrass ecosystems. Public awareness and habitat restoration projects can also aid dugong recovery.
With dedicated conservation efforts, we can ensure these unique seagrass grazers continue grazing for generations to come.
Parrotfish and Surgeonfish Occasionally Nibble on Seagrass
Some Species Target Seagrass When Other Food is Scarce
Although parrotfish and surgeonfish are not primarily herbivores, some species will occasionally graze on seagrass, especially when other food sources are limited. For example, the magnificent parrotfish is known to feed on seagrass in parts of the Caribbean when algae or coral is scarce.
According to a study on parrotfish feeding habits in the Florida Keys, five parrotfish species grazed on the seagrass Thalassia testudinum, with the redband parrotfish consuming the most. The grazing was likely opportunistic when their preferred food like algae was unavailable.
Surgeonfish may also eat small amounts of seagrass, but they tend to prefer algae. For example, a study on the diet of the blue tang surgeonfish in the Caribbean found that only 3% of their diet consisted of seagrass, while the rest was mostly algae.
Their seagrass consumption occurred when algal food was limited.
Their Grazing Provides Benefits to the Seagrass
Although parrotfish and surgeonfish do not rely heavily on seagrass, their occasional grazing provides benefits to the seagrass bed. For instance, their grazing helps thin dense seagrass shoots, allowing sunlight to penetrate to lower leaves.
One Florida study found parrotfish grazing led to higher seagrass productivity.
Additionally, parrotfish and surgeonfish excrete nitrogen and phosphorus in their waste that can fertilize and stimulate seagrass growth. One estimate suggests parrotfish can supply over 60% of soluble phosphorus to coral reef areas.
By promoting seagrass growth and nutrient cycling, these fish support productive and vibrant seagrass ecosystems.
Waterfowl Consume Seagrass Shoots and Seeds
Brant Geese Rely on Seagrasses in Winter
Brant geese (Branta bernicla) are small goose species that breed in the Arctic during the summer. When winter arrives, many brant travel south to coastal seagrass beds to feed on the nutritious shoots and roots of eelgrass (Zostera marina) and widgeon grass (Ruppia maritima).
Studies show over 90% of their winter diet consists of seagrasses.
Some major wintering sites for brant geese with abundant seagrass include Izembek Lagoon in Alaska, Baja California in Mexico, and Boundary Bay in Canada. Up to 30,000 brant have been counted grazing on the seagrasses in Boundary Bay estuary alone.
The health and productivity of these seagrass meadows is critical for supporting the brant goose populations.
Wigeons Also Forage on Seagrasses
The Eurasian wigeon (Mareca penelope) and American wigeon (Mareca americana) are two closely related species of dabbling ducks that also feed extensively on seagrasses. These birds utilize coastal wetlands and estuaries during the fall and winter months, where they can dive to reach seagrass leaves, shoots, and seeds up to 20 inches deep.
Species | Main Seagrass Food Source |
---|---|
Eurasian wigeon | Ruppia maritima, Zostera marina |
American wigeon | Ruppia maritima |
A study in Mexico found seagrasses comprised 62% of Eurasian wigeon diets and 93% for American wigeons. Their herbivorous feeding frees up invertebrates to be food for other species. So despite consuming lots of seagrasses themselves, wigeons help support biodiversity within these habitats.
Manatees Will Opportunistically Graze on Seagrass
Seagrasses Provide Fallback Food Source for Manatees
Seagrass meadows serve as a vital food source for the endangered West Indian manatee (Trichechus manatus), providing up to 60% of their diet along the Florida coast alone. When they’re not feasting on seagrasses (Thalassia, Syringodium, etc.
), manatees will eat an assortment of aquatic plants and other vegetation. But seagrass remains an integral pillar for sustaining manatee nutrition year-round.
According to the research group Save the Manatee, manatees often opportunistically graze on a variety of seagrasses – they’re not overly picky. In fact, whereas some marine herbivores carefully target certain species over others based on nutrient content, manatees demonstrate far less selectivity.
Their hefty bodies require incredible caloric intake, which compels them to eat a lot and continuously.
An adult manatee may chow down on a whopping 30-40 kilograms of seagrasses and other vegetation every single day! This unique feeding behavior and their sensitivity to cold water temperatures (they need to stay in warm waters above 20°C to survive) generally keeps the mammals close to the shoreline where seagrass is abundant.
Seagrass Loss Threatens Florida Manatees
Sadly, in recent years escalating seagrass decline along Florida’s coasts has become an existential threat to local manatee populations. Conservation groups estimate that 58% of the state’s seagrass ecosystems have been lost over the past century.
There are numerous factors behind this dilemma: nitrogen pollution that accumulates in seagrass habitats, rising ocean temperatures and acidification, invasive species destroying seagrass beds, increased boating activity damaging seagrass, etc.
2010 | At least 766 Florida manatees died |
2013 | A staggering 830+ manatee fatalities |
2021 | Over 1,100 manatee deaths, breaking annual records |
What we’re witnessing is a devastating decline for a beloved creature already classified as endangered under the ESA. If seagrasses keep disappearing at this rate, manatee conservation in Florida appears extremely bleak.
State and federal wildlife agencies are investigating the deaths and developing emergency response plans. But saving the manatees will also require extensive seagrass restoration projects across Florida’s ecosystem.
Conclusion
In the underwater seagrass meadows that carpet shallow coastal areas, a diverse array of animals nibble and graze on these productive plants. Green sea turtles and dugongs are the most dependent on seagrasses, relying on them for the bulk of their diet.
But many fish, waterfowl, and marine mammals will also opportunistically consume seagrass shoots and seeds when other food sources are scarce. Understanding the interconnectivity between seagrasses and the animals that eat them highlights the need for continued seagrass conservation and restoration.