Posidonia oceanica

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Posidonia oceanica

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. This marine plant forms large underwater meadows that are considered to be of high importance to the environmental conservation of the region. The fruit is free floating and known in Italy as 'the olive of the sea' (l'oliva di mare^[1]). Balls of fibrous material from the foliage of the plant, known as egagropili, are washed up to nearby shore-lines.

1 Description
2 Distribution
3 Taxonomy
4 References
5 External links

[edit] Description

Egagropili - Posidonia spheroids

P. oceanica is a flowering plant that occurs 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, and have 13 to 17 parallel veins. The terminus of the leaf is rounded or sometimes absent because of damage. They are 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 found in two forms: one growing beneath the sand up to 0.15 m and the other 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.^[2] 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.^[3]

[edit] 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 km²), representing a kind of key marine coastal ecosystem.

A sign of a nearby occurrence of Posidonia is the presence of masses of decomposing leaves on beaches. Such plant material has been used for composting, but this is currently prohibited by Italian laws that restrict the use of marine algae and plants for this purpose.

Illustration of P. oceanica habitat

[edit] Taxonomy

The genus epithet, Posidonia , is named for the Greek god of the seas Poseidon, and the species epithet, oceanica, refers to the once wider distribution. Carl Linnaeus gave the first botanical description of this species in Systema Naturae, although the genus was then named 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 Angiosperm Phylogeny Website concludes that the three families Cymodoceaceae, Posidoniaceae and Ruppiaceae form a monophyletic group.^[4] Earlier systems classified this genus in the family Potamogetonaceae or in the family Posidoniaceae but belonging to order Zosterales.

[edit] References

mattes along a channel

^ Posidonia oceanica (Italian). “Dopo essere fecondato, in estate fa crescere e maturare il suo frutto, l’oliva di mare (si chiama così perché ha una forma arrotondata).”
^ Posidonia oceanis.
^ Ibiza Spotlight (28 May 2006). Ibiza's Monster Marine Plant. Retrieved on 2007-05-09.
^ AP-Website