In the case of seagrass, you might have a meadow containing genetically different members of the same species that will breed and share traits, with the strongest members surviving environmental changes, such as in temperature or salinity.

But Shark Bay’s Poseidon’s ribbon weed is different, as it is a hybridisation of two similar species thousands of years ago and cannot reproduce sexually.

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University of Western Australia evolutionary biologist Elizabeth Sinclair, who was a senior author on the paper, said the plant was still genetically tough, with twice the chromosomes of other Posidonia, as it had 100 per cent of the genomes from both its parents rather than the usual 50 per cent.

“So it has a lot of genetic diversity in it, which is why it has been able to persist as long as it has,” she said.

“It’s blown a lot of people away, including people in our research team.”

There are seagrass clones off the east coast of the US, which run about 50 kilometres long, making the Shark Bay find three times as large.

The Shark Bay ribbon weed is also bigger than the clonal colony of quaking aspen in Utah, known as Pando, which is made up of 47,000 trees and previously held the title of the world’s biggest plant.

Saving the seagrass

Seagrass is believed to have started colonising Shark Bay about 8000 years ago as the ocean rose.

The biggest threat now facing the seagrass, which is great at storing carbon, is climate change.

The last major heatwave event, in 2010-11, killed or damaged hectares upon hectares of seagrass in Shark Bay and other marine plants along the west coast.

A lot of the long blades of the ribbon weed also died off but, six years on, shoot density has been recovering in parts of the bay.

The more-prevalent wire weed, favoured by dugongs, has not been as resilient and is more of a priority for scientists.

Seagrass in the bay filters the water, making it look very clear, and keeps the bay shallow thanks to the accumulation of sediment and dead plants.

There is also little freshwater input and evaporation makes it a lot saltier than other places in the world.

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Sinclair said the seagrass kept building up banks and helped maintain the environment where the 3.5 billion-year-old stromatolites – living fossils that grow 0.3 millimetres a year – occur in Hamelin Pool.

“Those two organisms are tightly linked, what happens to the seagrass happens to the stromatolites,” she said.

“It’s a very delicate balance up there ... luckily for us in WA it’s all protected so that does limit the damage that can occur ... but climate change we can’t escape.”

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