Posidonia oceanica, Marine Flowering Plant Endemic to the Mediterranean Sea

Posidonia is derived from the god of seas and oceans Poseidon in Greek mythology. It takes its origin from a terrestrial species resembling our present bulrushes. Posidonia took up residence under the sea about 100 million years ago.

A plant with flowers and fruits

Although they live underwater, they are not marine algae, they have roots, a creeping stem called rhizome and leaves. But more importantly flowers and fruits that allow it to reproduce mainly in asexual manner by cuttings or by growth of the rhizome. The growth of a posidonia occurs a few centimeters a year until it can bloom and germinate in turn. The originality of this flowering, which usually takes place around October, is that it is random: if environmental conditions such as water temperature, the amount of nutrients in the water are not optimal, they do not take place or shift in time. After flowering, the female organs must capture the pollen that forms sticky filaments carried by the currents for fertilization. At this moment, the flower begins to swell at the base to gradually form a fruit that will only be ripe after 6 to 9 months of maturation. This little greenish fruit is known as “the olive of the sea”: and be found in quantity on the beaches in the spring period. Green and located at the end of a peduncle of 10 to 30 cm, hermaphrodite flowers both male and female hide among the long ribboned leaves undulating according to the currents. When it occurs, flowering can be massive and coordinated over a wide geographical area.

Posidonia herbarium threatened

In the Mediterranean, there is only one specie of Posidonia, the “Posidonia Oceanica”. There are, however, 8 other species along the Australian coasts. Increasing external pressures in the Mediterranean threaten herbarium of Posidonia anchorages, pollution, coastal development. That’s why the Posidonia Oceanica has been protected by national decrees since 1988, like all other flowering marine plants.

The other extraordinary characteristics of posidonia

Often considered by divers as monotonous, the posidonia herbarium forms real underwater fields that extend from the surface to about 40 meters deep. Threatened by many factors, posidonia is a mysterious and extraordinary plant.

• – Posidonia is a formidable producer of oxygen. Each day, 1m2 of Posidonia herbarium produces up to 14 liters of oxygen through photosynthesis, which is a yield twice as high as the tropical forest.
• – The Posidonia herbarium has a very high life expectancy. According to some scientists, this species can live several centuries. Some specimens aged over 80,000 years have been found in the Balearic Islands.
• – Although only 1% of the Mediterranean seabed is covered with posidonia, it permanently or temporarily resides in about 25% of the Mediterranean fauna. From the canopy to the roots through the leaves, Posidonia is home to a large number of sedentary and transboundary plant and animal species that shelter, reproduce and feed there. Larvae and juveniles of species living near posidonia are numerous to spend the first years of their life like the castles, the sars or the cuttlefishes.
Credit flickr trebol_a
• – In the fall, when gusts occur, a large number of dead leaves of posidonia accumulate at the beaches in the form of benches that attenuate the strength of the winter swell.