Posidonia oceanica is an endemic algae of the Mediterranean Sea, and is probably one of the most important algae species in the whole Mediterranean. It is paradoxical that the specific name of this species is ‘oceanica’ being an endemic species of the Mediterranean and that we will not find anywhere else. There have been unsuccessful attempts to change the name to Posidonea mediterranea, which would be more appropriate. In any case, Posidonia oceanica is an algae that forms large underwater prairies, which are a true ecosystem teeming with life in which many species, both plant and animal, live.
The rhizome of the Posidonia oceanica is more or less buried in the sediment, forming downwards little roots and upwards narrow, taped leaves with parallel veins, which are grouped in fascicles of 4 to 8 leaves. The leaves are usually 1 cm wide and can reach up to one meter in length. At the base of the leaves a fascicle is formed from which all the leaves emerge. The outermost leaves are the oldest and longest, while as we move towards the interior of the fascicle, the leaves are younger and smaller. In the fall months, the larger leaves darken and shed, leaving behind part of the tissue structure in the fascicle, giving the base of the fascicle a scaly, fibrous, brown appearance. The detached leaves, dragged by the currents, are deposited in depth or on the shore, forming benches characteristic of beaches. These structures act as a protective barrier against wave erosion. In the autumn months it is also possible to observe the blooming of Posidonia oceanica. Its flowers do not stand out because of their color or their structure, they are light green flowers grouped in groups of 4-10 inflorescences on a peduncle of about 10-30 cm long, which is born from the base of the foliar fascicle. The fertilized flowers, form the fruit, known as sea olive, which after 6 months, mature, freeing themselves from the specimen and disseminating thanks to the currents. During its journey through the currents, the olive pod disintegrates, and the seed falls to the bottom, where it will germinate and give rise to a seedling.
As we have previously mentioned, Posidonia oceanica is a species of vital importance to the coastal system, since they act as structures that limit coastal erosion. However, the role of Posidonia oceanica goes much further, since they form the basis of one of the most diverse and productive ecosystems of the Mediterranean. At the level of its leaves, we can find epiphytic algae, that is, algae that grow on the leaves of Posidonia oceanica. Some examples of these epiphytic algae are Myirionema strangulans, Fosliella farinosa, Melobesia lejolisii, Giraudya sphacelarioides, Castagnea irregularis, Castagnea cylindrica and Ascocyclus orbicularis. We can also find epiphytic invertebrates, which usually account for up to 95% of the species that are attached to the leaves of Posidonia. A group constituted by only 5 species: Monotheca posidoniae, Sertularia perpusilla, Campanularia asymetrica, Electra posidoniae and Microporella johannae. Other species that can be found in the leaves, moving through them, and sometimes feeding on them, are to give some examples the gastropods Petalifera petalifera, Glossodorin gracilis, Rissoa variabilis, Rissoa ventricosa, Rissoa decorata, Alvania montagui, Bittium retriculatum, Rissoina bruguieri, Cantharidus striatus and Trapania fusca; the bivalve Propeamussium hyalium; the echinoderm Asterina pancerii; the antomedusa Eleutheria dichotoma; or the crustaceans Porcellidium viride, Astacilla mediterranea, Idotea hectica, Cestopagurus timidus, Pargus anachoretus, Achaeus cranchii, Pisa nodipides and Pisa muscosa. Another group of species that we can find in the Posidonia oceanica meadows are swimming species that rest on the leaves, such as the arachnid Pontharachna punctulum, the crustaceans Siriella clausi, Hippolyte prideauxiana and Thoralus cranchi, the jellyfish Cadibena radiatum, the quetognato Spadella cephaloptera, the cephalopod Sepiola rondeletti, or the fishes Apleton microcephalus, Hippocampus hippocampus, Hippocampus guttulatus. In summary, the presence of Posidonia oceanica meadows is one of the best indicators of ecosystem health and quality of coastal marine waters.
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