A species of Posidonia, submerged flowering plants found in mediterraean climates. A perennial rhizomatous herb that appears as stands in marine habitat. This species is found at depths from 0.5 to 20 metres on white sands, in coastal waters that may be sheltered or subject to intense wave action. The habit of Posidonia robertsoniae is as a clumping stand, it is rhizomatous plant that puts out shoots with one or two leaves. The leaf blades are 2.5 to 4 millimetres wide, and 1.5 metres long. They are slightly leathery and uneven on the upper surfaces. Between 6 and 9 veins appear on each leaf. The flowering period is primarily during the months of August and September.
This species is contained by the Posidoniaceae family, one of eight occurring in southern Australia. The ninth member, Posidonia oceanica, is found in the Mediterranean sea. The species was first described in Aquatic Botany in 1984, based on a holotype collected at King George Sound in 1981. Several related species within Posidonia are described as the Posidonia ostenfeldii complex, a group that may be in need of revision. The species Posidonia coriacea, which occurs in a wider distribution range, and this one may not be distinct, both lack evidence of genetic variation. The differing morphology of these species may be subject to environmental influence and their characters can overlap.
- Kuo, J. & Cambridge, M.L. (1984) Aquatic Botany: 281, figs 3, 8a, 14, 22, 28, 32, 34a, 35b, 37c, 38c. Type: "HOLOTYPE: Western Australia, King George Sound, Albany, 8 m deep, 22 Dec., 1981, M.L. Cambridge & H.Kirkman
- Campey, M.L.; Waycott M.; Kendrick G.A. (January 2000). "Re-evaluating species boundaries among members of the Posidonia ostenfeldii species complex (Posidoniaceae) - morphological and genetic variation" ([dead link]). Aquatic Botany 66 (1): 41–56(16) Research article. doi:10.1016/S0304-3770(99)00015-7. http://www.ingentaconnect.com/els/03043770/2000/00000066/00000001/art00015.
- Rippey, Elizabeth; Rowland (Reinette), Barbara (2004) . Coastal plants: Perth and the south-west region (2nd ed.). Perth: University of Western Australia Press. pp. 258. ISBN 1-920694-05-6.
- "Posidonia robertsoniae". FloraBase. Department of Environment and Conservation, Government of Western Australia. http://florabase.dec.wa.gov.au/browse/profile/108.
- "Posidonia robertsoniae". Australian Plant Name Index (APNI), IBIS database. Centre for Plant Biodiversity Research, Australian Government. http://www.anbg.gov.au/cgi-bin/apni?taxon_id=2449.