Seagrasses are flowering plants just like those growing on the land, except that they have become adapted to life permanently under water. Unlike algae, sea plants have roots, a stem, leaves, flowers and fruit. These plants are often called phanerogams, from the Greek “phaneros” (visible) and “gamos” (marry), literally “visible marriage”, thus stressing the presence of the flower, the key component in sexual reproduction. Seagrass meadows are some of the most endangered marine habitats. Occurring in coastal areas, they are particularly vulnerable to disturbance by human activity, such as pollution and the physical destruction of its habitat. Posidonia oceanica is the largest species of marine plant present in the Mediterranean and is endemic to this sea. The species is characterized by long, ribbon-like leaves, averaging 50 cm in length (although they can exceed 100 cm) and 1 cm in width. The leaves have 13-17 veins arranged longitudinally. Posidonia oceanica used to form huge meadows in a vast, discontinuous belt stretching from Punta Sdobba to Punta Tagliamento. By the late 1960s it had virtually completely disappeared from the entire gulf. This withdrawal is probably due to changes in the climate, the species’ vulnerability to various pollutants and to shellfishing (warty venus, Venus verrucosa, and smooth clam, Callista chione) with suction pumps and rakes. Today’s remnants lie off the coast of Grado, in two areas known as “Pietre di S. Agata” and S. Gottardo”. These two expanses are important from an ecological point of view as they represent the northernmost presence of P. ocenica in the Mediterranean. Moreover, this is a monoclonal population, with all the plants sharing the same genetic origin (genet). This characteristic makes the meadow especially vulnerable to changes in its environment.