Posidonia oceanica is found throughout the Mediterranean Sea. It forms massive clonal colonies, in which genetically identical specimens form one giant interconnected super-organism that can last for hundreds of thousands of years. Those colonies are older than human history.
Image by Alberto Romeo.
Posidonia oceancia, popularly known as Neptune grass, is one of a few contenders for the title of longest-lived organisms on Earth. While the individual clones are small and short-lived, the underlying root system binds them together as one organism that is essentially a vast continuation of the original ancestor. Technically speaking, the seagrass is only partially clonal, meaning it can use both asexual and sexual reproduction techniques to propagate itself.
It might be that mix of reproductive techniques - the former crucial to preserving the integrity of the original genetic material, the latter essential in adapting to changing environmental conditions - that has allowed these seagrass colonies to endure for tens, if not hundreds of thousands of years. Speaking to the BBC, French researcher Sophie Arnaud-Haond explains her team's new findings on the ultra-ultra-longevity of the seagrass:
"Most of [these genetic transcriptions errors] are expected to have a negative impact; through generations [the organism] will degenerate and eventually disappear. The age of clonal organisms should therefore be limited as well. We were able, through modelling, to demonstrate that Posidonia oceanica has a mode of clonal spread, common to other seagrasses, that allows the plant to avoid the accumulation of deleterious mutations and explains how it escapes this theoretical rule. The estimated age of Posidonia oceanica clones imply these have been surviving under a broad range of environmental conditions, including much lower sea-water temperatures than those recorded nowadays, showing therefore extraordinary adaptive capacities."
These Neptune grass colonies have hit upon a winning formula to survive in the face of changing environmental conditions for the last hundred thousand years, although Arnaud-Haond says the current climate change may present too great a hurdle for at least some of the colonies to survive:
"These findings show that Posidonia oceanica's clones have been able to survive fluctuating environmental conditions, and meadows we observe nowadays are the outcome of millenia of evolution. On the other hand, changes are occurring nowadays at an unprecedented rate and the sharp decline of seagrass meadows in general - and Posidonia oceanica in particular - are raising concerns as to the ability of this slow growing species and its old, patiently selected clones, to face these new and rapidly occurring environmental challenges."