Posidonia is not an algae, but a marine phanerogam, specifically an angiosperm, which is a plant with roots, stems, leaves, flowers and fruits. The stems are called rhizomes, since they are subterranean, and grow in two directions: horizontally, from which the roots arise and serve to extend out; and vertically, from which bundles of 4 to 8 long leaves emerge. The leaves are of an intense green colour, one centimeter wide and up to 1,5 meters long, absorb nutrients and photosynthesize. Posidonia grows approximately 1 cm/year and forms meadows up to 8 km long, in which 1 m² can have up to 10.000 leaves.


As a species, the oldest posidonia fossils belong to the Cretaceous, about 120 million years ago. But as an organism, it is one of the most long-lived, having been found in the Balearic Islands one specimen with an age of about 100.000 years.


It is located at depths from 0 to 50 meters in warm, crystalline and uncontaminated waters, with an optimum temperature between 15ºC and 20ºC, on hard (rocky) and soft (sandy) substrates.


It is endemic to the Mediterranean Sea, which means that it is only found naturally in this part of the world. The total known area is 12.247 km².


More than 400 plant species and 1000 animals inhabit the posidonia meadows, which provide them with a habitat in which they can rest, feed and reproduce.


Sexual reproduction

It is given thanks to the production of inflorescences that contain between 3 and 10 hermaphrodite flowers, these give rise to fruits, called olives, that disperse.

Asexual reproduction

It is given by stolons, lateral buds that emerge from the stems (rhizomes) and grow horizontally, which is why the same plant can reach many kilometers of total length.


The leaves present an annual cycle in which they develop, grow and are shed. They are born at the end of winter, and there is more quantity at the beginning of summer, when they are bigger and greener. Higher temperatures cause many organisms to colonize the meadows, which in exchange does not allow light to arrive properly and the leaves can not carry out photosynthesis, they acquire a yellowish coloration, then brown and finally die. The leaves remain attached to the stem until autumn, when they are torn off by the strong waves, a process that ends between January and March.

The fiber balls are remains of dead leaves and rhizomes mixed with sand, they are seen on the beaches in winter and the beginning of spring.

The flowering occurs between September and November and not every year, but it is necessary that the state of the meadow and environmental conditions are adequate.

The olive (fruit) appears between March and June.


Classified by the IUCN

Least concern

Classified by CITES

Not listed

Spanish Catalogue of Threatened Species

Not listed but included in the List of Wild Species in Regime of Special Protection in the Mediterranean

Habitats Directive

Priority habitat to be conserved within the territory of the European Union (Habitat 1120)

Fisheries legislation

It is prohibited to use fishing gear such as trawling, purse seine or dredges on seabed that harbor seagrass communities (EC nº1967/2006)