Posidonia oceanica L. Del., a Mediterranean endemic species, is one of the best-known as well as most endangered plants in the Mediterranean Sea. It is not an alga but a fiowering plant, i.e. a vascular plant with all the characteristic Uody parts - rhizome and roots, leaves, flowers and fruits. At first sight it reminds us of grasses; this is the reason why we usually refer to it as to a grass, which can be also said of other flowering plants living in our sea - Cymodocea nodosa, Zostera marina and Zostera nana.
Posidonia oceanica is the largest "seagrass' in the Adriatic Sea. Its name - given after Poseidon, the chief god of the sea - seems very appropriate indeed, for its extensive underwater meadows that spread from the shore to the depth of 40 metres represent one of the key ecosystems of the Mediterranean Sea. Most often they are found on silty and sandy floors, less often on hard ground. Its rhizomes, which can grow horizontally or vertically, are densely intertwined and constitute a kind of a secondary Bottom. This may be several decimeters thic and can at places create actual reefs. At the tip of each rhizome there is a shoot of a centimetre wide and even more than a metre long leaves. Their numhers vary during the year, ranging Between 5 and 8. The older leaves situated on the outer side of the shoot fall off but are soon replaced by new ones growing in the shoot's interior. 
The meadows of Posidonia oceanica are of great importance for the marine ecosystem, not only in view of producing oxygen and organic substances (approx. 20 tons/ha/year) but also as a biotone for an infinite number of marine organisms dependent on such meadows in terms of their diet, habitat, shelter, etc. There are also many sessile organisms, which live attached to the surface of the leaves and rhizomes. There is of course more than enough place for them, considering that the actual surface area of all leaves in a single square metre of a meadow ranges from 20 to 50 m2. Posidonia meadows are at the same time a very important factor in the diminishing of erosion. With their undulation they slow down the wave motion a great deal and thus the impact of the sea exerted on the shore. In spite of all these positive characteristic, less and less natural habitats of this plant have been observed in the Mediterranean particularly due to various construction works, pollution, drag netting, anchoring, etc. 
Posidonia oceanicau has been declared endangered in our country as well. Its only natural habitat is in no more than 50 metres wide and a kilometre long belt between Koper and Izola (or, to be more specific, between Zusterna and Rex). This habitat is also the only one in the Gulf of Trieste and along the entire western coast of Istra. The meadow, which starts immediately along the shore, is not a homo-geneous one, but consists of a number of larger and smaller islets, in between which Cymodea nodosa is found on sandy fioor and different algal species on hard ground. In this meadow numerous marine organisms have been noted, most perceivable amongst them being Pina nobiliss, various sponges, crabs (Maia verrucosa, Pagurus sp.), gastropods (Murex sp.) and bivalve molluscs (Chlamis varius, Aca noe). The habitat represents an ecosystem that plays an important part in the exchange of substances in the sea and, which is of particular significance, provides a biotope, hiding place and a refuge for numerous marine organisms of the Gulf of Trieste and thus contributes towards the diversity of the underwater world as well as to the conservation of its ecological process.