Next time you go diving or snorkeling, 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.
The underwater seagrass meadows grow in two ways: by 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 Poseidon Australia 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 (112 miles) 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 (77 miles²) of ribbon weed meadows expanded from a single, colonizing seedling.
How did it evolve?
What makes this seagrass plant unique from others, other than its enormous size, is that it has twice as many chromosomes as its relatives. This makes it what scientists call a “polyploid.”
Most of the time, a seagrass seedling will inherit half the genome of each of its parents. Polyploids, however, carry the entire genome of each of their parents.
There are many polyploid plant species, such as potatoes, canola, and bananas. In nature, they often reside in places with extreme environmental conditions.
Polyploids are often sterile, but can continue to grow indefinitely if left undisturbed. This seagrass has done just that.
How old is this plant?
The sandy dunes of Shark Bay flooded some 8,500 years ago, when the sea level rose after the last ice age. Over the following millennia, the expanding seagrass meadows made shallow coastal banks and sills through creating and capturing sediment, which made the water there is also a lot of light in the waters of Shark Bay, as well as low levels of nutrients and large temperature fluctuations. Despite this hostile environment, the plant has been able to thrive and adapt to saltier.
It is challenging to determine the exact age of a seagrass meadow, but we estimate the Shark Bay plant is around 4,500 years old, based on its size and growth rate.
Other huge plants have been reported in both marine and land systems, such as a 6,000-tonne quaking aspen in Utah, but this seagrass appears to be the largest to date. Other huge seagrass plants have also been found, including a closely related Mediterranean seagrass called Psionic oceanic, which covers more than 15 km (9 miles) and may be around 100,000 years old.
Even without successful flowering and seed production, the giant plant appears to be very resilient. It experiences a wide range of water temperatures (from 17ºC/63ºF to 30ºC/86ºF in some years) and salt levels.
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