It's big, it's old and it lives under the sea -- and now an international research collaboration with The University of Western Australia's Ocean's Institute has confirmed that an ancient seagrass holds the secrets of the oldest living organism on Earth.

Ancient giant Posidonia oceanica reproduces asexually, generating clones of itself. A single organism -- which has been found to span up to 15 kilometres in width and reach more than 6,000 metric tonnes in mass -- may well be more than 100,000 years old.

"Clonal organisms have an extraordinary capacity to transmit only 'highly competent' genomes, through generations, with potentially no end," said Director of UWA's Oceans' Institute Winthrop Professor Carlos Duarte.

Researchers analysed 40 meadows across 3,500 kilometres of the Mediterranean sea. Computer models helped demonstrate that the clonal spread mode of Posidonia oceanica, which as all other seagrasses can reproduce both sexually and asexually, allows them to spread and maintain highly competent clones over millennia, whereas even the most competent genotypes of organisms that can only reproduce sexually are lost at every generation.

"Understanding why those particular genomes have been so adaptable to a broad range of environmental conditions for so long is the key to some interesting future research," Professor Duarte said.

Seagrasses are the foundation of key coastal ecosystems but have waned globally for the past 20 years. Posidonia oceanica meadows are now declining at an estimated rate of five per cent annually.

"The concern is that while Posidonia oceanica meadows have thrived for millennia their current decline suggests they may no longer be able to adapt to the unprecedented rate of global climate change."

The genus Posidonia occurs only in the Mediterranean and Australian waters.

The findings have been published in the online journal PLoS ONE.