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Seagrass-grazing dugongs and green sea turtles supercharge the seeds they eat

A dugong (Dugong dugong) feeding on seagrass in Egypt. Image by Julien Willem via Creative Commons (CC BY-SA 2.0)

Banner image: A dugong (Dugong dugong) feeding on seagrass in Egypt. Image by Julien Willem via Creative Commons (CC BY-SA 2.0)

  • By consuming and pooping seagrass seeds, dugongs and green turtles play a vital role in maintaining vital, carbon-sequestering seagrass meadows, a new study reveals.
  • Scientists working in the Great Barrier Reef found that seagrass seeds germinated up to 60% faster after they had passed through the gut of dugongs or green turtles, and also had two to four times greater germination probability.
  • The research highlights the mutual relationship between seagrass and marine mega herbivores, and underscores the shared vulnerability if either party is undermined.
  • Experts say we must do what we can to protect dugongs, turtles and other grazing marine animals if we wish to protect seagrass ecosystems and their many benefits.

For decades, scientists speculated that seagrass seeds served only as a backup to vegetative proliferation; they settled into the sand beneath their parent plants, ready to germinate if anything untoward befell the living meadow. But according to findings published recently in Biotropica, the tiny seeds can disperse over hundreds of miles, and thereafter can germinate rapidly. However, they must first suffer the indignity of being ingested and pooped out by dugongs (Dugong dugong) or green sea turtles (Chelonia mydas).

The new research provides the first evidence that seagrass seeds that have passed through the digestive tracts of these marine mega-herbivores have enhanced germination potential.

Although it’s common knowledge that terrestrial animals boost the germination potential of the fruits and seeds they eat, “for some unknown reason, no one had actually gone out and seen whether that was true for dugongs, turtles and seagrasses,” lead author Samantha Tol, a marine ecologist at James Cook University, Australia, told Mongabay.

“Not only are the seeds germinating, they germinate at a far faster rate than what we’re used to with seagrasses … it confirms that dugongs and turtles are helping to maintain seagrass meadows, keeping them healthy and resilient,” Tol said.

A green sea turtle (Chelonia mydas) feeding seagrass in an intertidal meadow at Lizard Island, Queensland. Image courtesy of TropWATER, James Cook University

Vital ecosystems

While seagrass meadows have a somewhat lower profile than other coastal ecosystems, such as coral reefs or mangroves, they are considered to be one of the most vital ecosystems for mitigating global climate impacts.

During photosynthesis, seagrasses sequester immense amounts of carbon from the water column and bury it in coastal sediments. Seagrass meadows capture carbon up to 35 times faster than tropical rainforests. And even though they cover less than 0.2% of the seafloor, they absorb 10% of the ocean’s carbon.

Furthermore, seagrass meadows are superb nurseries for fish and other marine animals, many of which are commercially important. They also minimize coastal erosion through sediment stabilization, and cycle excess nutrients from the water column, helping to prevent diseases on neighboring coral reefs.

Study lead author Samantha Tol holds a freshly-collected mega-herbivore sample containing seagrass seeds in Cleveland Bay, Queensland. Image courtesy of Samantha Tol

Speedy seeds

Tol and her colleagues had previously confirmed that dugongs and green turtles in the Great Barrier Reef are capable of dispersing more than 500,000 viable seagrass seeds per day across distances of up to 650 kilometers (400 miles). Their next challenge was to ascertain whether the journey through an animal’s digestive tract affects a seed’s germination rate.

To find out, the researchers began the unsavory task of collecting dugong and green turtle feces in Gladstone Harbor and Cleveland Bay in northeast Queensland state.

Although the undertaking “wasn’t pretty,” it was relatively easy: “Luckily for us, literally all they do is eat and poop, so it wasn’t difficult,” Tol said.

The researchers found no shortage of satiated grazers: on one occasion, more than 100 dugongs surrounded their boat, and it was simple to scoop their floating poop out of the water with a net.

Once the team had enough samples, they extracted the seeds of a common Australian seagrass, Zostera muelleri, and set up laboratory experiments to compare the germination of excreted seeds with fresh seeds dropped naturally from seagrass plants.

They found that excreted seeds germinated 10 to 35 days (up to 60%) faster than those dropped directly from seagrass plants. Seeds that passed through the animals also had two to four times greater germination probability.

Tol said she was surprised by how fast the excreted seeds germinated. “We had near-complete germination in less than 30 days. It was so quick! To the point where you double-check your data because everybody knows that usually seagrass seeds have a really slow and low germination rate,” she said.

The clear-cut results emphasize the importance of marine mega-herbivores to seagrass, according to the researchers. By transporting seeds to new locations and enhancing germination, dugongs and green turtles improve the connectivity and genetic diversity of seagrass meadows, thereby helping them recover from disturbances, such as cyclone damage.

A germinating seagrass seed of Zostera muelleri in one of the JCU experiments, this particular specimen grew a leaf. Image courtesy of Samantha Tol

A mutualistic relationship

“This study helps fill in some knowledge gaps on seagrass seed dispersal and seed germination,” study co-author Jessie Jarvis, from the University of North Carolina Wilmington, told Mongabay in an email, adding that such information can be applied to the conservation, management and restoration of these important ecosystems.

The new study also adds to the growing number of publications that highlight the key roles that grazers play in seagrass ecology.

“We knew from [our] previous work that mega-herbivores were transporting seeds, but this research really highlighted the mutualistic connection that seagrasses have with their grazers,” Jarvis said. It indicates the “two-way nature of the relationship, mega-herbivores utilizing seagrass for food and seagrasses adapting for greater seed dispersal and germination via marine mega-herbivores.”

The researchers say they now want to test this approach in U.S. waters to see if grazers like manatees and other turtles perform a similar function. They say they also want to confirm whether these animals are directly targeting seagrass flowering shoots or if they are ingesting them incidentally.

Tol said she plans to investigate whether seagrass seeds are gaining other benefits from their passage through grazing animals’ bodies. During her experiments, she found that some seeds were infected with either a bacterial or fungal pathogen that inhibits germination. She said she was intrigued to find the incidence of infection was dramatically lower in excreted seeds. “So it also looks like passing through the gut could have a potential sterilization effect,” Tol said.

While the new research identifies the benefits of the relationship between seagrasses and marine mega-herbivores, it also highlights the shared vulnerability if either party is undermined.

“We need to make sure that we are doing whatever we can to protect dugong and green sea turtles, because clearly these two animals are important to the health and resilience of seagrass,” Tol said. Removing grazers from the marine ecosystem would not only affect seagrasses, it would have knock-on consequences for mangroves, coral reefs and other marine habitats.

“As seagrass becomes more stressed due to impacts of global climate change, understanding the interactions between mega-herbivores and seagrasses may be key to successful management and conservation of both,” Jarvis said.

A seagrass meadow at Green Island, Queensland. Many of the intertidal meadows in Queensland are frequented by saltwater crocodiles, adding an element of jeopardy to fieldwork. Image courtesy of Samantha Tol

Banner image: A dugong (Dugong dugong) feeding on seagrass in Egypt. Image by Julien Willem via Creative Commons (CC BY-SA 2.0)

Citations:

Tol, S. J., Jarvis, J. C., York, P. H., Congdon, B. C., & Coles, R. G. (2021). Mutualistic relationships in marine angiosperms: Enhanced germination of seeds by mega‐herbivores. Biotropica. doi:10.1111/btp.13001

Tol, S. J., Jarvis, J. C., York, P. H., Grech, A., Congdon, B. C., & Coles, R. G. (2017). Long distance biotic dispersal of tropical seagrass seeds by marine mega-herbivores. Scientific Reports, 7(1). doi:10.1038/s41598-017-04421-1