The seagrass forms large underwater meadows that form an important part of the ocean's ecosystem. Presence of the plant is a sign that the waters around it are clean and pollution free.
Posidonia can only be found in the Mediterranean Sea where it is slowly disappearing, occupying an area of only about 3% of the basin. This corresponds to a surface area of about 38,000 square kilometres (15,000 sq mi).
Posidonia Oceanica thrives in clean waters, and its presence is a marker for lack of pollution. The fruit is free floating and known in Italy as 'the olive of the sea' (l'oliva di mare). Balls of fibrous material from its foliage, known as egagropili, wash up to nearby shorelines. It is a flowering plant which lives in dense meadows or along channels in the sands of the Mediterranean.
It is found at depths from 1 to 35 meters (3.3 to 115 feet), depending on the water clarity. Subsurface rhizomes (stems) and roots stabilize the plant while standing and erect rhizomes and its leaves reduce silt accumulation.
Video: Posidonia Oceanica
Recently, Sophie Arnaud-Haond of Ifremer, in Plouzané, France, and the Universidade do Algarve and colleagues sampled and sequenced DNA from Posidonia oceanica – a seagrass – at 40 sites along 3500 km of sea floor from Spain to Cyprus. This plant, like other seagrasses, reproduces by cloning, so it grows as one big organism comprised of genetically identical cells.
Based on the known (and slow) growth rate of this grass, the researchers estimate that the patch of genetically uniform plants along 15 km of coastline of the island of Formentera must be between 80,000 and 200,000 years old, making it the oldest living thing known on Earth.
In 2006 a huge clonal colony of the seagrass was discovered south of the island of Ibiza. At 8 kilometers (5 miles) across, this colony is estimated to be around 100,000 years old.
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