In the shallow waters of Western Australia’s Shark Bay lies a beautiful seagrass meadow. Often known as Poseidon's ribbon weed (Posidonia australis) – after Poseidon, the Greek god of the sea – strapweed or fireball weed, its green fronds waft gently in the shallow waters of the bay.
But this seagrass meadow had been hiding a secret for thousands of years.
Researchers were astounded when they realised this extensive 77-square-mile meadow – larger than the Caribbean country of Aruba – was made up of one individual that had cloned itself to reach this epic size over around 4,500 years.
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“We often get asked how many different plants are growing in seagrass meadows and this time we used genetic tools to answer it,” says evolutionary biologist Dr Elizabeth Sinclair, from UWA’s School of Biological Sciences and the UWA Oceans Institute in a statement.
So, they used genetic tools to try and find out the answer.
“The answer blew us away – there was just one!” adds UWA student researcher Jane Edgeloe. “One plant has expanded over 180km in Shark Bay, making it the largest known plant on earth.
The discovery was described in the journal Proceedings of the Royal Society B: Biological Sciences in June 2022.
“Spawned from a single seed, based on this species' growth rate […] it is estimated to be around 4,500 years old,” writes Guinness World Records in its entry confirming Shark Bay’s Posidonia australis seagrass is the world’s largest plant by area. “It has spread over the millennia via underground clonal shoots known as rhizomes meaning that the entire plant is connected and shares the same DNA, though the authors of the study note that certain patches (ramets) in a clonal plant can become separated over time so gaps can emerge.”
Aside from being a giant clone of itself (the seagrass meadow is also the world’s largest clone, as it happens), there’s something else weird about this gigantic plant: it has twice as many chromosomes as it should.
Diploids – including humans – have one complete set of chromosomes from each parent (two sets in total) while polyploids have more than two complete sets. The experts believe this happened because the plant is a hybrid of two diploid ‘parent’ plants.
“The new seedling contains 100 per cent of the genome from each parent, rather than sharing the usual 50 per cent,” explains Sinclair. “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.”
Seagrass is the only flowering plant that spends its entire life fully immersed in saltwater. Like other seagrass meadows, this habitat is an important carbon store and supports many different animals including fishes, turtles and dugongs.
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Despite having survived for thousands of years, the future of Shark Bay’s Posidonia australis isn’t certain. A marine heatwave in 2010–2011 damaged around one third of the enormous meadow and could have released vast amounts of carbon dioxide into the atmosphere. Protecting seagrass meadows is vital, write the authors of a 2018 study describing the losses caused by this event: “With heatwaves predicted to increase with further climate
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