Genetic testing has determined a single 4,500-year-old seagrass may have spread over 200 sq km of underwater seafloor – about 20,000 football fields

About 4,500 years ago, a single seed – spawned from two different seagrass species – found itself nestled in a favourable spot somewhere in what is now known as Shark Bay, just off Australia’s west coast.

Left to its own devices and relatively undisturbed by human hands, scientists have discovered that seed has grown to what is now believed to be the biggest plant anywhere on Earth, covering about 200 sq km (77 sq miles, or about 20,000 rugby fields, or just over three times the size of Manhattan island).

The species – a Posidonia australis, also known as fibre-ball weed or ribbon weed – is commonly found along the southern coastlines of Australia.

But when scientists started looking for genetic differences in ribbon weed across the bay, they came across a puzzle. Samples taken from sites that were 180km apart suggested there were not multiple specimens of Posidonia australis, but one single plant.

“We thought ‘what the hell is going on here?’” said Dr Martin Breed, an ecologist at Flinders University. “We were completely stumped.”

Student researcher Jane Edgeloe, of the University of Western Australia (UWA), said about 18,000 genetic markers were examined as they looked for variations in the species that might help them select specimens for use in restoration projects.

But what they found instead was that the same plant had spread using rhizomes in the same way that a lawn can spread from its edges by sending out runners.

“The existing 200 sq km of ribbon weed meadows appear to have expanded from a single, colonising seedling,” she said.

The one plant now spreads out like a meadow, providing habitat for a huge array of marine species including turtles, dolphins, dugongs, crabs and fish.

Ribbon weed rhizomes can grow up to 35cm a year and, using that rate, the authors of the research – published in Proceedings of the Royal Society B – estimate the plant will have needed at least 4,500 years to spread as far as it has.

Dr Elizabeth Sinclair, a co-author of the research at UWA, said they hadn’t given the plant a nickname, and original samples – pulled from the seagrass meadow – originally had 116 different labels with GPS coordinates when they were stored in a deep freeze ready for genetic sampling.

The plant has formed huge, dense meadows that in some areas stretch as far as the eye can see in all directions. The ribbons of the plant are only 10cm long in some places, but up to a metre in others.

Conditions in Shark Bay itself are challenging. The plant has found a way to survive in areas where the salinity is double that elsewhere in the bay, and can thrive in water temperatures as cold as 15C and as hot as 30C.

The seagrass plant’s survival appears to be linked, Sinclair said, to how it had held on to all the chromosomes from its two parents, giving it inbuilt genetic diversity.

“Instead of getting half [of] its genes from mum and half from dad, it’s kept all of them,” she said.

Sinclair and colleagues are still working through the secrets of the giant specimen, but she said it appears to be “largely sterile” and so has to rely on its own ability to grow, rather than disperse seeds.

Breed said the fact the plant “doesn’t have sex” but had survived for so long was a puzzle.

“Plants that don’t have sex tend to also have reduced genetic diversity, which they normally need when dealing with environmental change,” he said.

Breed said they had detected some very subtle mutations in the plant’s genetics across the places it was growing that might also explain its extreme longevity.

The size of the Shark Bay ribbon weed is about 20,000 hectares (49,000 acres) – making it much larger than a stand of quaking Aspen trees in Utah, often referred to as the world’s largest plant, covering 43 hectares.

Associate professor Kathryn McMahon, of Edith Cowan University, was not involved in the Shark Bay research but is an expert on seagrass. She said the method used by the researchers gave her confidence they had identified one single specimen, which she said was “amazing”.

Genetic studies of other seagrass species had estimated the plants could live for between 2,000 and 100,000 years, so McMahon said the estimate that the Shark Bay specimen was 4,500 years old fits into that range.

“They have a versatile growth pattern which contributes to this long life span,” she said. “They can grow towards nutrient-rich patches to access the nutrient they need, or to gaps in the meadow where there is space for them to grow or away from stressful locations.

“All of these characteristics mean that if they are in the right place they can persist over long periods of time.”