Posidonia: The Ancient Seagrass Meadow That Keeps the Mediterranean Alive

Snorkel out from almost any Mediterranean beach, swim past the sand into the deeper water, and you will eventually float over a meadow. Long green leaves wave below you in slow currents. Small fish hover above them. Light filters down in shifting bars. This is Posidonia oceanica — an endemic Mediterranean seagrass that is, by some measures, the oldest living thing on earth.

A single Posidonia meadow off Formentera, in the Spanish Balearics, has been carbon-dated to roughly 100,000 years old. It is a single clonal organism, spreading by underground rhizomes, that has been quietly growing since before modern humans left Africa. It outlived the last ice age. It is still there.

Posidonia matters far beyond its age. The meadows cover something like 12,000 square kilometers of Mediterranean seabed and produce more oxygen per square meter than the Amazon rainforest. They are nurseries for hundreds of fish species — including most of the commercial ones — and they trap sediment, keeping the water clear and the coastline stable. When a Posidonia meadow disappears, the beach behind it usually disappears within a few decades, eroded by waves the meadow used to break.