Posidonia oceanica, also known as Neptune's Grass, is a seagrass which reproductive characteristics allow it to develop into clonal colony, where each individual of the colony is involved in the identic transmission of the genome, giving rise to other individual plants perduring genetic line. Together they form a super-organism, a single entity, which first "cells" appeared already about 100,000 years ago. This is the estimated age of the oldest living organism on Earth ... in the sea.
These Mediterranean grasses are indeed aquatic plants, whose classical sexual reproduction is complemented by a reproductive ability using propagules: the rhizomatic stems from a parent plant extend the seabed to hang on a suitable support and develop new plants, genetically identical to the previous, forming a herbarium whose size and volume varies significantly. One of these herbaria, recently observed (2006) using DNA comparisons between plants distant over several kilometers, proved to be only a single body of phenomenal size, spanning over 8 km long.
This entity has at the very least 100 million clones plants, fastened together and forming one of the largest living things that we know, that has nothing to envy to other super-organisms such as those formed by Trembling poplars Pando colony in Utah, or the clones mushrooms in Oregon, Armillaria ostoyae. Estimated age of Posidonia oceanica root system is evaluated to nearly 100 000 years, when Pando is estimated to be 80 000 years old (only). This assessment of Posidonia Oceanica gives it title for the oldest living form in the world.