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 (110 miles) making it the largest known plant on Earth.