THC Science

Posidonia oceanica
Scientific classification
Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Liliopsida
Order:	Alismatales
Family:	Posidoniaceae
Genus:	Posidonia
Species:	P. oceanica
Binomial name
	Posidonia oceanica; (L.) Delile
	Posidonia oceanica range

Posidonia oceanica is a species of seagrass that is endemic to the Mediterranean Sea. The marine plant forms large meadows that are considered to be of high importance to the environmental conservation of the region. The fruit is free floating and known as the olive of the sea (olive di mare). Balls of decomposing detritus from the foliage of the plant, known as egagropili, are found along nearby shore-lines.

Description

File:P11007181.JPG

Egagropili - Posidonia spheroids

A flowering plant occurring in dense meadows, or along channels, in sands of the Mediterranean. It is found at depths from 1 metre, and depending on the clarity of the water, up to 35 metres. Subsurface rhizomes and roots provide stability in the sands it occupies, erect rhizomes and leaves reduce the accumulation of silt.

The leaves are ribbon-like, appearing in tufts of 6 or 7, and up to 1.5 metre long. The average width is around 10 mm. They are bright green, perhaps becoming brown with age, have 13 to 17 parallel veins. The terminus of the leaf is rounded or absent through damage. They arranged in groups with older leaves on the outside, longer and differing in form from the younger leaves they surround.

The rhizome type stems are in two forms; growing beneath the sand up to 0.15 m and rising above the sand to give its tufted appearnace. All stems are approximately 10 mm thick and upright in habit. This arrangement of the rhizomes eventually forms a matte; the surface contains the active parts of the plant, whereas the centre is a dense network of roots and decomposing stems.

The flowering plant is known by the common name Neptune grass.^[1] In 2006 a huge clonal colony of Posidonia oceanica was discovered south of the island of Ibiza. At 8 km across and possibly up to 100,000 years of age, it may be one of the largest and oldest clonal colonies on Earth.^{[citation needed]}

Distribution

This species is found only in the Mediterranean Sea; occupying an area about 3% of the basin (corresponding to a surface area of about 38,000 km 2), representing a kind of key marine coastal ecosystem. A sign of a nearby occurence of Posidonia is the presence of masses of decomposing leaves on beaches. Plant material has been used for composting, but this is currently prohibited by italian laws prohibiting the use of marine algae and plants for this purpose.

Taxonomy

The binomial for this species, Posidonia , is given for the god of the seas Poseidon, and oceanica refers to the once wider distribution. Carl Linnaeus gave the first botanical description of this species in Systema Naturae, although the genus was named as Zostera. The APG system (1998) and APG II system (2003) accept the genus as constituting the sole genus in the family Posidoniaceae, which it places in the order Alismatales, in the clade monocots. The AP-Website concludes that the three families Cymodoceaceae, Posidoniaceae and Ruppiaceae form a monophyletic group.^[2] Earlier systems classified this genus in the family Potamogetonaceae or in the family Posidoniaceae but belonging to order Zosterales.