The world's largest seagrass meadow, a single clone covering 200 square kilometers. It's 4,500 years old and produces more oxygen per hectare than rainforest.
Daderot, CC0, via Wikimedia Commons
One Plant to Rule Them All
In the shallow, sunlit waters of Shark Bay, a UNESCO World Heritage site, is a vast underwater meadow. For years, it was assumed to be a sprawling collection of individual seagrasses. A genetic study by researchers from The University of Western Australia and Flinders University revealed a startling truth: the entire 200-square-kilometer meadow is a single plant. Using 18,000 genetic markers, scientists created a "fingerprint" of samples taken from across the bay, some separated by 180 kilometers of water. The results showed they were all genetically identical.
This colossal organism is a clone of Posidonia australis, also known as ribbon-weed. It is estimated to be at least 4,500 years old, having sprouted from a single seed when the area was flooded after the last ice age. Its immense size makes it the largest known plant on Earth. The plant has spread not by flowering and seeding, but by extending rhizomes—underground stems—outward, similar to how a lawn grass spreads. This slow and steady expansion, at a rate of up to 35 centimeters per year, allowed it to colonize an area equivalent to three times the size of Manhattan.
Its longevity and size lies in its unusual genetics. The plant is a polyploid, meaning it contains twice the number of chromosomes of its relatives. It likely originated as a hybrid of two different seagrass species, and instead of taking half the chromosomes from each parent, it retained 100% of the genome from both. This genetic quirk has given it resilience, allowing it to thrive in Shark Bay's challenging environment, which features wide-ranging temperatures from 15°C to 30°C and salinity levels that can be double that of normal seawater.
An Ecological Powerhouse
The Shark Bay seagrass meadows are a cornerstone of the local marine ecosystem. The dense underwater fields provide essential food and shelter for a diverse array of animals, including crabs, fish, and sea turtles. The bay hosts one of the world's most significant populations of dugongs, with around 11,000 individuals that feed almost exclusively on the seagrass. Migrating humpback whales also use the bay as a staging post.
Besides providing habitat, the seagrass performs important ecological functions. Through photosynthesis, these meadows release vast amounts of oxygen. They are also incredibly efficient at carbon sequestration, is a "blue carbon" sink. Seagrass ecosystems can capture and store carbon in their sediments at a rate much faster than terrestrial forests. The extensive root system of Posidonia australis helps to stabilize the seabed, preventing erosion and maintaining water clarity.
The sheer scale of this single plant shows the role of seagrass meadows. However, they are under threat. A severe marine heatwave in 2010-11 destroyed a significant portion of the meadow, releasing an estimated nine million metric tons of stored carbon dioxide back into the atmosphere. While showing signs of recovery, the event shows the vulnerability of even this ancient and resilient organism to the impacts of climate change.
💡Fun Facts
This single plant is a polyploid, possessing a complete double set of chromosomes from two parent species.
The plant has survived and grown in water with salinity levels up to twice as high as the open ocean.
The seagrass meadow provides the primary food source for a population of over 10,000 dugongs.
It expanded from a single seedling over 4,500 years by sending out underground runners called rhizomes.
The Shark Bay World Heritage Area is a natural landscape, accessible at all times. Specific visitor centers or parks within the area may have set hours.
Admission
Access to the general Shark Bay area is free. National parks within the region, such as Francois Peron National Park, and the Monkey Mia Reserve may require entry fees.
Accessibility
The seagrass meadows are underwater and best experienced by boat, with opportunities for snorkeling in calmer areas. Much of the surrounding terrestrial area, including Francois Peron National Park, requires a 4WD vehicle. The main town of Denham and Monkey Mia are accessible by 2WD.
