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How does it look like?
Posidonia oceanica is a higher aquatic plant with roots, trunk, leaves, flowers and fruits. It is normally found in the form of meadows that cover important areas of the seabed. The trunks are underground and can reach a thickness of 10 to 20 centimetres, covering the seabed vertically and horizontally. From the trunk emerge bundles of green, ribbon-like leaves, about one centimetre wide. The length of the leaves can be up to 120cm. The leaves are also characterised by 13 to 17 parallel nerves, which run from the beginning to the end of the leaf.
Where does it live?
Posidonia is endemic to the Mediterranean and is always found in well-lit areas, in extreme cases up to 60-80 metres deep. In fact, it is considered that the upper limit where we find this plant marks the level at which we pass from the infralittoral (the illuminated zone) to the circalittoral (the shaded zone) where photosynthesis is no longer possible. It normally takes root on sandy bottoms, although it can also take root on muddy or rocky bottoms. The trunks (or rhizomes) fix the sand and for this reason Posidonia is important in the creation of beaches. As for the upper limit, posidonia can colonise very shallow areas, reaching the surface, but only when there are no waves, which is what determines this upper limit. Posidonia meadows are currently in clear regression. Factors affecting them are water pollution, trawling, the indiscriminate use of anchors for anchoring boats and the dredging of the seabed.
How does it reproduce?
The reproduction of Posidonia is similar to that of other plants. It can reproduce asexually, by fragmentation or by regeneration of the trunks or rhizoids. This is the way in which Posidonia gains ground and enlarges the surface that covers the seabed. This process is very slow. It is estimated that in this way it can advance one metre every 10 to 20 years. The other system of reproduction is through flowers and fruits. Flowering is not regular and the reasons that trigger it can be diverse, although there are theories that relate the blooms to rises in water temperature. The flowers are spike-shaped and produce a very dense pollen that is carried by the water to other plants. The fruit is shaped like a green olive and floats as long as the pulp does not decompose. Transported by the waves and sea currents, the fruit will eventually fall to the bottom where, if conditions are optimal, it can generate a new plant. But the latter is smaller and its leaves are much thinner.
Curiosities
· It has been counted in a meadow of Posidonia up to 400 species of algae and several thousand species of animals
· Posidionia does not withstand changes in salinity or dirty water. It is also very sensitive to water pollution. It is therefore considered a species that serves as an indicator of environmental quality
· In Catalonia it is also known as the glassmakers' seaweed. Historically, the dried leaves of Posidonia were collected from the beaches and one of the uses they were given was as wrapping and protection for glass pieces during transport
· The dried leaves have also been used to fill mattresses and cushions, with the particularity that they repel bedbugs
· Posidonia has also sometimes replaced straw, both as food for animals and in the manufacture of sand bricks.
Taxonomy
Kingdom: Plantae, Subkingdom: Viridaeplantae, Phylum: Tracheophyta, Class: Magnoliopsida, Order: Alismatales, Family: Posidoniaceae, Genus: Posidonia |
