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Forget blue whales, redwood trees and aspen groves. There’s a new biggest organism in town. It’s a huge patch of seagrass, the size of about 20,000 rugby fields. And it’s been hiding in plain sight in Shark Bay, at the western tip of Western Australia!

For the team who discovered this massive plant, the discovery came as a bit of a surprise. They were looking for lots of different plants. “We often get asked how many different plants are growing in seagrass meadows and this time we used genetic tools to answer it,” says Dr Elizabeth Sinclair.

“The answer blew us away – there was just one!” says researcher Jane Edgeloe. “That’s it, just one plant has expanded over 180 kilometres in Shark Bay, making it the largest known plant on Earth. The existing 200 square kilometres of ribbon weed meadows appear to have expanded from a single, colonising seedling.”

The team collected samples of the seagrass Posidonia australis from different meadows across Shark Bay. Back in the lab, they generated a genetic ‘fingerprint’, that compared 18,000 different genetic markers. Samples from 9 out of 10 meadows came back remarkably similar. They were parts of a single clone!

That wasn’t the only genetic surprise in store. Posidonia australis is a flowering plant, and a seed usually gets half of each parent’s genes to create a full set. In this case, the super spreading seagrass got an entire copy from both parents, giving it twice as many genes as usual.

The team reckon the extra genes might make this seagrass especially suited to the challenging conditions. In some places, Shark Bay can be twice as salty as regular sea water. It has a huge range of average temperatures, and extremely bright sunlight.

It also looks like the extra genes mean this plant can’t flower or produce seeds of its own. That means the grass probably spread very slowly, maybe 15 to 35 centimetres per year. The team estimate that, given how big and widespread the grass is, it could be 4,500 years old!

What are seagrasses?

Seagrasses are the only flowering plants that live in the ocean. As well as growing from seeds, they also spread by putting out sideways stems called runners, which grow their own roots and continue the meadow.

The seagrasses at Shark Bay support a vibrant ecosystem, including dugongs, dolphins, turtles and sharks. These plants are also good at fighting climate change by storing carbon.

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