World’s Largest Plant Is a Seagrass Meadow Bigger Than Brooklyn

The seagrass Posidonia australis
The seagrass Posidonia australis. Rachel Austin

When Australian researchers went to find out how much genetic diversity was contained in the seagrass meadows of Shark Bay, a World Heritage Area in Western Australia, they were in for a surprise. 

The team from The University of Western Australia (UWA) and Flinders University took seagrass shoots covering the range of conditions present in the bay in order to determine how many plants were present and which ones should be used for restoration. 

“The answer blew us away – there was just one!” UWA student researcher and lead author Jane Edgeloe said in a UWA press release. “That’s it, just one plant has expanded over 180km in Shark Bay, making it the largest known plant on earth.”

It turns out that entire meadows in the bay are made up of clones of Poseidon’s ribbon weed (Posidonia australis), the researchers wrote in the study, “Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment,” published in Proceedings of the Royal Society B Wednesday. 

“The existing 200km2 of ribbon weed meadows appear to have expanded from a single, colonising seedling,” Edgeloe said in the press release. 

At more than 112 miles, the plant stretches about the distance between Los Angeles and San Diego, CNN reported. Its area, of around 77 square miles or 49,000 acres, is larger than Brooklyn’s. It’s also larger than the previous largest plant in the world–the cloned Pando quaking Aspen trees in Utah that cover 106 acres.

In addition to its size, the plant is also impressively old. The researchers calculated its age by working backwards from the fact that ribbon weed rhizomes grow as much as 35 centimeters (approximately 14 inches) per year, according to The Guardian. Therefore, the plant would need to be at least 4,500 years old given its size. The meadows now supports marine life like fish, dolphins, crabs, dugongs and turtles. 

The plant is well adapted to the varying conditions in the bay, in part because of a unique feature, according to the press release. It is a polyploid plant, meaning that it received all of the chromosomes from both parent plants instead of just half from each. 

“Polyploid plants often reside in places with extreme environmental conditions, are often sterile, but can continue to grow if left undisturbed, and this giant seagrass has done just that,” study senior author and UWA evolutionary biologist Dr. Elizabeth Sinclair said in the press release. “Even without successful flowering and seed production, it appears to be really resilient, experiencing a wide range of temperatures and salinities plus extreme high light conditions, which together would typically be highly stressful for most plants.”

The scientists are now conducting experiments in the bay to learn more about how the plant can do so in an extreme environment despite its lack of new genetic input over time.

“Plants that don’t have sex tend to also have reduced genetic diversity, which they normally need when dealing with environmental change,” study author and Finders University ecologist Dr. Martin Breed told The Guardian. 

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