It grows at the bottom of the strait between Ibiza and Formentera, a vast underwater meadow where posidonia, a type of sea grass, forms a single, ancient organism. Estimates place its age at a minimum of 100,000 years, with a spread of about eight kilometers. In 2006, researchers from CSIC, the University of the Balearic Islands, and collaborators from Portugal, the Caribbean, and the United States described it as the largest living organism on the planet.
More recently, another study called posidonia in this region the world’s largest and oldest plant. A related species found in Australia shares many similarities with the Ibiza-Formentera meadow but belongs to the same genus, Posidonia australis.
Researchers from the University of Western Australia identified a meadow of this marine plant in Shark Bay, on the country’s western coast. The meadow stretches roughly 10 kilometers in length and covers around 180 square kilometers, as reported in the Proceedings of the Royal Society.
Researchers noted that the seagrass beds in Shark Bay are composed of a single clone rather than many distinct plants, a fact revealed while studying genetic diversity across the area.
The lead author, Jane Edgeloe of UWA, explained that the team created a genetic fingerprint using 18,000 markers to map diversity in the meadow. Her team concluded that a single clone occupies 180 square kilometers, making it the largest known plant by area.
To estimate the age, scientists divided the total meadow area by the typical rhizome expansion rate of 15 to 35 centimeters per year. This approach suggested an age of at least 4,500 years.
Despite the Australian finding, the oldest known terrestrial plant remains the giant specimen located beneath the seabed between Ibiza and Formentera. This specimen, while sharing similar proportions with the Australian one, is much taller and has an age of approximately 100,000 years, far exceeding the 4,500 years attributed to Posidonia australis.
Posidonia of Ibiza and Formentera, described as the longest on the planet
How did the Australian meadow reach a surface comparable to Ibiza in far less time? Oceanographer and posidonia expert Carlos Duarte offers an explanation. Duarte notes that Australian angiosperm species grow much faster than Mediterranean varieties, leading to quicker expansion in open water.
In every case, the Ibiza-Formentera meadow remains the longest-lived plant on the planet, according to Duarte, who has extensive experience with these ecosystems.
Duarte emphasizes that marine angiosperms are among the planet’s most significant carbon sinks. He points out that a hectare of Posidonia meadows in the Mediterranean captures as much carbon as fifteen hectares of Amazonian forest, underscoring their role in maintaining climate balance.
The seasonal mooring of yachts on this meadow during summer contributes to tourist pressure that can threaten its conservation. Warming waters and discharges from submarines and ships further jeopardize this important habitat, highlighting a deteriorating situation.
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Beyond conservation concerns, it is essential to recognize the unique vulnerability of Posidonia meadows. Their slow growth, under two centimeters per year, and limited seed production mean that losses take centuries to heal, making protection a priority for years to come.