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This family has one genus, Posidonia, with one species found around the coast of the Mediterranean Sea, and two species found in still, shallow, intertidal waters in southern Australia from Shark Bay, WA, to near Newcastle, NSW.

Characteristic features of the family Posidoniaceae in Australia include:

  • sea-grasses with creeping rhizomes and linear, often spotted and streaked leaves that sheath the stem edgewise and have a small flap (ligule) at the base of the blade
  • flowers small, lacking a perianth, borne in short spikes or racemes at the apex of a leafy shoot
  • stamens 3, producing thread-like pollen
  • ovary superior, comprising a single carpel with one ovule, developing into a fleshy, floating fruit


Perennial marine aquatic herbs rooted in the substrate, with their leaves all or mostly submerged. Perennating by rhizomes. Vegetative reproduction by rhizomes. Internal secretions not obvious. Plants glabrous. Leaves distichous, cauline, sessile. Stipule-like lobes present or apparently absent. Lamina simple, symmetric, filiform, acicular, subulate or linear; base lobed or auriculate; margins entire, ±flat; venation parallel with the midrib, conspicuous, and the tertiary venation reticulate; surfaces not punctate; herbaceous or leathery. Leaf ligule present. All the flowers bisexual. Inflorescences terminal or axillary, consisting of spikes or racemes. Bracts and bracteoles present. Pollination by water. Flowers odourless, sessile. Floral disc absent; nectaries absent. Perianth absent or vestigial. Fertile stamens 3 or 4, at least partly fused with the ovary or style, distinct from each other, all ±equal. Staminal filaments absent. Anthers dorsifixed, not versatile, opening outwards by longitudinal slits, 2-celled; appendages apical. Ovary superior and sessile. Carpel 1; ovary with 1 locule. Style single and unbranched, or absent and the stigma ±sessile. Ovule 1, sessile; placentation basal. Fruit a dry or fleshy, indehiscent or tardily dehiscent drupelet (camara). Disseminule micro-surface ±smooth, white or green, glossy. Seeds 1 per fruit. Aril absent. Cotyledons 1. Embryo straight.
(Note: this description has been generated from the coded data compiled for the key. Any errors in the key data will be reflected in the descriptions.)

A treatment of the family Posidoniaceae has not yet been published in the Flora of Australia. It will appear in Volume 39.

Australian genera of Posidoniaceae (as recognised for the Flora of Australia)


Posidonia australis (fruiting plant)
Photo: S.Jacobs S.Jacobs 

Posidonia australis (habit)
Photo: M.Fagg ANBG 

Posidonia australis (habit)
Photo: S.Jacobs S.Jacobs