You'd think that the world's largest plant would stand out like a sore thumb.
But researchers have today revealed there's a plant about 4,500 years old and measuring 180 kilometres across living right under our noses in Western Australia.
Genetic testing has revealed that what was once thought to be part of a giant seagrass meadow in the shallow waters of Shark Bay, near Carnarvon, was actually a single massive clone of Posidonia australis seagrass.
According to the researchers, who published their findings in Proceedings of the Royal Society B today, that makes it the single largest plant known to exist on Earth.
"Certainly by land area, to the best of our knowledge, it is [the largest plant in the world]," said evolutionary biologist and study co-author Elizabeth Sinclair from the University of Western Australia.
The researchers had initially set out to study how genetically diverse the seagrass meadows were.
But they had an inkling that something was slightly off about the seagrass in the area, according to study co-author, ecologist and ecosystem health researcher Martin Breed from Flinders University.
"We were a bit suspicious because the plants around there don't act like normal seagrass," Dr Breed said.
"They don't flower as much, don't seed as much, so these signs of reproductive activity were a little bit unremarkable."
But when they took samples from 10 meadows throughout the Shark Bay area, they never expected nine of them to return a genetic match.
Instead, they were planning to use their research to inform which plants to use for restoration of the meadows, to help with their resilience against threats like bleaching.
"Our PhD student Jane Edgleloe did the DNA work and we soon realised that it was far more complicated than we were expecting. It blew us away," Dr Breed said.
Being a polyploid mutant has its advantages
Being a clone probably helps to explain why this single plant has been so successful.
It is what is referred to as a polyploid, meaning it possesses multiple sets of the same chromosomes, according to Dr Sinclair.
"It's got double the number of chromosomes as Posidonia on the coast," she said.
"That one has 20 chromosomes, this one has 40 chromosomes."
Most plant species, as well as humans and most animals, possess single pairs of chromosomes — known as diploidy.
Polyploidy in this case has occurred because at some stage, a Posidonia plant has hybridised with another related species.
The disadvantage of this is that it is no longer able to reproduce sexually, so doesn't get any genetic variation across generations that might allow it to adapt to changing conditions over time.
But the plus side is it possesses double the amount of genetic variability to begin with.
"What that does is double the amount of genetic diversity you have in that plant and probably broadens its capacity to deal with the challenges that come from its environment," Dr Sinclair said.
While polyploidy can be a disadvantage, the conditions in Shark Bay are extremely variable and having double the chromosomes appears to be paying off for the Posidonia, according to Dr Breed.
"It can be 17 degrees in the water one day and 30 degrees the next and the salinity [varies widely]. The seagrass is able to hold the fort even when you've got massively fluctuating conditions," Dr Breed said.
"There was a big heatwave in 2010 -2011 — about a third of seagrasses were knocked off. But [it] bounced back very well."
4,500 years old
During the last Ice Age, about 20,000 years ago, Shark Bay was exposed on the continental shelf.
But as the ice caps thawed and the oceans rose, it filled in around 8,000 years ago.
The researchers hypothesised that there were likely many seagrass plants that colonised the region soon after that.
Posidonia australis hybridised and then edged out its competitors, Dr Breed said.
"Historically, there were probably a bunch of these diploid plants and it just so happens with time and survival of the fittest, this one emerged as the dominant plant."
Given the time needed for the plant to clone itself across such a huge expanse, the researchers conservatively estimate it to be at least 4,500 years old.
"The plant has spread over about 200 square kilometres and plants that have the same genetic fingerprint are spread apart by about 180 km," Dr Sinclair said.
"We know how fast the oceanic species grow — you take that into account and the fact that the plants don't actually grow in a straight line but they branch, you can get an estimate of about 4,500 years.
