Posidonia oceanica

Referred to as Neptune grass or even Mediterranean tapeweed.

Posidonia oceanica is a marine sea grass species found only in the Mediterranean Sea. Like any other form of sea grass, it grows as large underwater meadows which can be found in the submerged photic zones of sheltered coastal waters. These meadows are of utmost importance to the ecosystem and is a protected species, both in Gozo and Malta.  Balls of large fibrous material from its foliage can be found along seashores, this is known as egagropili


The Posidonia oceanica lives in dense meadows or along sandy channels in the Mediterranean basin. It is a flowering plant; the fruit is free floating and is called by Italians “the olive of the sea” (l’oliva di mare).

Depending on water clarity, the plant can be found between 1 – 35 metres depth.  It is stabilized by subsurface roots and rhizomes.  Erect leaves and rhizomes are used to reduce the accumulation of silt around the plant.

The leaves appear ribbon shaped in tufts of 6 or 7, with an average width of around 10 millimetres. The leaves are bright green in colour and can reach up to 1.5 metres in length. They have 13 to 17 parallel veins along the leaves, which turn brown with age. The rhizome stems are approximately 10 millimetres thick and of upright nature. They form over time a thick, dense mat, while the surface contains active plant parts above the sand, the center is a network of roots and old decomposing stems.


Found only in the Mediterranean Sea, but sadly in decline. The Posidonia oceanica now occupies only around one third of the basin of the Mediterranean. It grows only in clean waters, so it is a well-known marker for non-polluted water. Posidonia covers an area of about 38,000 square kilometres and can be detected by masses of decomposing leaves piled high on the beaches.

Taxonomy of Posidonia oceanica

The genus Posidonia is named after the famous Greek god of the Seas, Poseidon. Oceanica is the name given to its former wide distribution. Posidonia was first listed in Systema Naturae under the name of Zostera. In 1998 the AGP system and APG system ll, renamed it into a sole genus in the family Posidiniaceae, which falls under the clade monocots.


Photo Janet Bulmer @ www.divesmartgozo.com

Divesite: Xwejni Bay