Yesterday, we posted about Poseidon’s ribbon weed, Posidonia australis, a clonal seagrass. Researchers recently discovered large meadows of the self-cloning sea grass growing in Shark Bay, off the westernmost tip of Australia. DNA testing of the seagrass, covering over 180km, an area three times the size of Manhattan, established that the meadows were made up of a single plant, making the self-cloning seagrass the largest single plant in the world.

The post brought to mind a similar story about clonal sea grass from a decade ago involving a related species, Posidonia oceanica, known as Neptune grass, that is believed to be the oldest living thing on the planet.

The seagrass reproduces by asexually generating clones of itself, so meadows spanning vast areas of the seabed are genetically identical and counted as one organism.

In 2012, researchers from the University of Western Australia’s Ocean’s Institute analyzed the DNA of the seagrass at 40 sites across 3,500 kilometers of the Mediterranean Sea, from Spain to Cyprus. By calculating the plant’s annual growth rate, the team determined that the meadows are between 80,000 and 200,000 years old, making the Neptune grass the oldest known living thing on earth.

What makes Neptune grass so interesting, beyond its apparent longevity, is that it has a very high carbon absorption capacity, being able to soak up 15 times more carbon dioxide every year than a similar-sized area of the Amazon rainforest.

The bad news is that the research, which appears in the journal PLos ONE, cautions that rising water temperatures and coastal construction projects have slowed the seagrass’ growth and it’s dying out at a rate faster than it’s growing. The study warns that if trends continue “the outlook for this species is very bad.” 

Posidonia oceanica meadows are now declining at an estimated rate of 5% annually.