The APG system (1998) and APG II system (2003) accept this genus as constituting the sole genus in the family Posidoniaceae, which it places in the order Alismatales, in the clade monocots. The AP-Website[4] concludes that the three families Cymodoceaceae, Posidoniaceae and Ruppiaceae form a monophyletic group. Earlier systems classified this genus in the family Potamogetonaceae or in the family Posidoniaceae but belonging to order Zosterales.
Posidonia oceanica has nitrogen fixation capabilities via symbiosis and other species may as well.[5]
The species described by Linnaeus, Posidonia oceanica, is found in the Mediterranean; the rest are located around the southern coast of Australia. Some species are endemic seagrasses of Western Australia, all the Australian species are found in that region's diverse habitats.
This arrangement was divided into two complexes: the Posidonia australis and Posidonia ostenfeldii groups.[10] Some species descriptions may only be regional characteristics, and may need further revision.[6][11]
In 2006 a clonal colonyofP. oceanica was discovered south of the island of Ibiza which measured 8 km across and is possibly up to 100,000 years old.[12]
^Cambridge, M.L. and Kuo, J. (1979) Two new species of seagrass from Australia, Posidonia sinuosa and Posidonia angustifolia (Posidoniaceae). Aquat. Bot. 6, 307–328 f. Keulen
^Kuo, J. and Cambridge, M.L. (1984) A taxonomic study of the Posidonia ostenfeldii complex (Posidoniaceae) with descriptions of four new Australian seagrasses. Aquat. Bot. 20, 267–95. f. Keulen
^den Hartog, C. (1970) Seagrasses of the World. Verh. Kon. Ned. Akad. Wetens. Afd. Naturk. Ser. 2 59:139 f. Keulen
^Kuo, J. and McComb, A.J. (1989) In: "Biology of Seagrasses. A treatise on the biology of seagrasses with special reference to the Australian region." (Eds. A.W.D. Larkum, A.J. McComb, S.A. Shepherd) (Aquatic Plant Studies 2) (Elsevier, Amsteredam). p. 6-73 f. Keulen
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