Meet the world’s largest plant: a single seagrass clone stretching 180 km in Western Australia’s Shark Bay

Next time you go diving or snorkelling, have a close look at those wondrously long, bright green ribbons, waving with the ebb and flow of water. They are seagrasses – marine plants which produce flowers, fruit, and seedlings annually, like their land-based relatives.

These underwater seagrass meadows grow in two ways: by sexual reproduction, which helps them generate new gene combinations and genetic diversity, and also by extending their rhizomes, the underground stems from which roots and shoots emerge.

To find out how many different individual plants are growing in a seagrass meadow, you have to test their DNA. We did this for meadows of ribbon weed seagrass called Posidonia australis in the shallow sun-drenched waters of the Shark Bay World Heritage Area, in Western Australia.

The result blew us away: it was all one plant. One single plant has expanded over a stretch of 180 km making it the largest known plant on Earth.

We collected shoot samples from ten seagrass meadows from across Shark Bay, in waters where the salt levels range from normal ocean salinity to almost twice as salty. In all samples, we studied 18,000 genetic markers to show that 200 km² of ribbon weed meadows expanded from a single, colonising seedling.