Ribbon weed meadow in Shark Bay, Western Australia
Ribbon weed meadow in Shark Bay, Western Australia

World’s Largest Organism Discovered Underwater

Off the western Australian coast, in Shark Bay, a field of seagrass big enough to cover Washington, DC, has flourished for more than four millennia, a new study finds.

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Andy Carstens is an intern at The Scientist. He has a bachelor's degree in chemical engineering from the Georgia Institute of Technology and a master's in science writing from Johns Hopkins University.

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ABOVE: Ribbon weed (Posidonia australis) meadow in Shark Bay, Western Australia Rachel Austin, University of Western Australia

A bed of seagrass off the coast of Australia has spent roughly 4,500 years expanding across the ocean floor and is the largest plant discovered so far, a study published yesterday (June 1) in Proceedings of the Royal Society B finds. The seagrass, which appears to be a hybrid of Poseidon’s ribbon weed (Posidonia australis) and an unknown species, has apparently thrived using clonal growth instead of sexual reproduction, Science reports.

The researchers who discovered the gargantuan bed of seagrass originally set out to conduct a genetic survey, hoping to find specimens to replant in restoration projects, The Guardian reports. After analyzing nearly 150 samples from 10 seagrass meadows between 2012 and 2019, they didn’t see the genetic variation they expected, according to the study. Instead, they found that the shoots they sampled from nine of the meadows all came from the same plant.

“The existing 200 square kilometers of ribbon weed meadows appear to have expanded from a single, colonizing seedling,” study coauthor Jane Edgeloe, a marine biologist at the University of Western Australia, tells The Guardian.

Normally, ribbon weed has 20 chromosomes, but this behemoth blanket of seagrass has 40, suggesting it’s actually a cross between the ribbon species and a yet-unidentified species. “Instead of getting half [of] its genes from mum and half from dad, it’s kept all of them,” coauthor Elizabeth Sinclair, an evolutionary biologist at the University of Western Australia, tells The Guardian.

Furthermore, rather than dispersing seeds via sexual reproduction, this seagrass appears to clone itself as its rhizomes—underground plant stems—slither and grow outward, expanding its root structure across the seafloor, according to the study. Using previous estimates of rhizome growth rates, the researchers were able to approximate the giant clone’s age. 

Kathryn McMahon, a coastal biologist at Edith Cowan University who was not involved in the work, tells The Guardian that she agrees the seagrass meadow originates from a single specimen and that its estimated age fits within predicted possible ranges.

The researchers found it puzzling that a clone had survived for such a long period of time, according to The Guardian. That’s because the genetic diversity that comes with sexual reproduction normally imparts adaptations that enable it to survive a range of environmental conditions. “Plants that don’t have sex tend to also have reduced genetic diversity, which they normally need when dealing with environmental change,” coauthor Martin Breed, an ecologist at Flinders University, tells The Guardian.

Though more research is needed understand why it has thrived for so long, the researchers detected genetic mutations across the meadow’s geographical range that could have contributed to its millennia-long lifespan, reports The Guardian.