Scientists discover ‘biggest plant on Earth’ off Western Australian coast

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.