Posidonia oceanica, named after the ruler of the sea, Poseidon, and commonly called Neptune grass, is a seagrass which plays an important role in the Mediterranean ecosystem.

120 million years ago Posidonia oceanica covered the coastal plains of an ocean that straddled the equator. Drifting over millennia with the tectonic plates, today Posidonia occurs only in the Mediterranean and around the southern coasts of Australia.

 

Posidonia oceanica lives in a narrow coastal strip, normally on beds of soft sediment, at depths of between 5 and 40 to 50 metres, depending on water clarity. It is a true plant, in that it can bear flowers and disperse seeds. However, flowering is quite rare with less than one flower found per 10 square metres per year.

It plays an important role in oxygenating and clarifying coastal waters, provides a habitat for a rich diversity of plants and animals, acts as a safe breeding area for many species, and protects beaches from erosion. Sea grasses also play a role in protecting our planet from the increasing build up of carbon dioxide. They act as a "carbon sink", like land-based plants, absorbing carbon dioxide from the atmosphere and thereby helping to slow down the effects of global warming.

 

Posidonia is a large, long-living but very slow-growing seagrass. Its shoots, which are able to live for at least 30 years, are produced at a slow rate from rhizomes which grow horizontally by only 1 to 6 centimetres each year. Over centuries the rhizomes form mats which rise up into reefs that help to trap sediment and mediate the motion of waves, thus clarifying the water and protecting beaches from erosion. Dead leaves are shed in the autumn and can be seen washed up on beaches.