Around 4,500 years ago, a single seed – from two different species of seagrass – ended up nestled in a favorable spot somewhere in what is now known as Shark Bay, just offshore the west coast of Australia.
Left to their own devices and relatively undisturbed by human hands, scientists found that the seeds grew into what is now thought to be the largest plant in the world, covering around 200 km2 (77 sq mi, that’s about 20,000 rugby pitches, or just over three times the size of the island of Manhattan).
The species – a Posidonia australisalso known as fiberball grass or ribbon grass – commonly found along the southern coasts of Australia.
But when scientists began looking for genetic differences in ribbons across the bay, they came across a puzzle. Samples taken from sites 180 km apart suggested that there were not multiple specimens of Southern Posidonia, but only one plant.
“We thought ‘what’s going on here?'” said Flinders University ecologist Dr Martin Breed. “We were completely puzzled.”
Student researcher Jane Edgeloe, from the University of Western Australia (UWA), said around 18,000 genetic markers had been examined as they searched for variations in the species that could help them select specimens to use in restoration projects.
But what they found instead was that the same plant had spread using rhizomes in the same way a lawn can spread from its edges by sending out runners.
“The existing 200 km² of ribbon weed grasslands appear to have expanded from a single colonizing seedling,” she said.
The single plant now spreads out like a grassland, providing habitat for a huge range of marine species including turtles, dolphins, dugongs, crabs and fish.
Ribbon weed rhizomes can grow up to 35cm per year and using this rate the authors of the research – published in Proceedings of the Royal Society B – estimate that the plant will have needed at least 4 500 years to spread as far as it did.
Dr Elizabeth Sinclair, co-author of the research at UWA, said they hadn’t given the plant a nickname and that the original samples – taken from the seagrass meadow – had at least originally 116 different tags with GPS coordinates when stored in a deep freeze ready for genetic sampling.
The plant has formed huge dense meadows, which in some areas stretch as far as the eye can see in all directions. The ribbons of the plant are only 10 cm long in some places, but up to a meter in others.
Conditions in Shark Bay itself are harsh. The plant has found a way to survive in areas with twice the salinity of elsewhere in the bay and can thrive in waters as cold as 15°C and as warm as 30°C.
The survival of the seagrass plant appears to be tied, Sinclair said, to how well it retained all the chromosomes from both parents, giving it inherent genetic diversity.
“Instead of getting half [of] his mum and half dad genes, he kept them all,” she said.
Sinclair and her colleagues are still working out the secrets of the giant specimen, but she said it appears to be “largely sterile” and therefore must rely on its own ability to grow rather than disperse seeds.
Breed said the fact that the plant “didn’t have a sex” but survived for so long was puzzling.
“Plants that are genderless also tend to have reduced genetic diversity, which they normally need to cope with environmental change,” he said.
Breed said he detected very subtle mutations in the genetics of the plant across the places it grew, which could also explain its extreme longevity.
Shark Bay’s banded grass size is approximately 20,000 hectares (49,000 acres), making it much larger than a stand of trembling aspens in Utah, often called the world’s tallest 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 beds. She said the method the researchers used gave her confidence that they had identified a single specimen, which she said was “incredible”.
Genetic studies of other seagrass species had estimated that plants could live between 2,000 and 100,000 years. So McMahon said the estimate that the Shark Bay specimen was 4,500 years old fell within that range.
“They have a versatile growth pattern that contributes to that long lifespan,” she said. “They may grow towards nutrient-rich patches to access the nutrients they need, or towards gaps in the meadow where there is room for them to grow or away from stressful places.
“All of these characteristics mean that if they are in the right place, they can persist for long periods of time.”