fibre-ball weed

Posidonia australis is a flowering plant occurring in dense meadows, or along channels, in white sand. It is found at depths from 1–15 m (3–49 ft). 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 and 11–20 mm (0.43–0.79 in) wide. They are bright green, perhaps becoming browned with age. The terminus of the leaf is rounded or absent through damage. They are arranged in groups with older leaves on the outside, longer and different in form from the younger leaves they surround. The species is monoecious. The flowers appear on small spikes on leafless stems, two bracts on each spike. The plant pollinates by hydrophily, by dispersing in the water. Posidonia australis reproduction usually occurs through sexual or asexual methods but, under extreme conditions, by pseudovivipary. A 2013 study showed that P. australis can sequester carbon 35 times more efficiently than rainforests. In 2022, a study by the School of Biological Sciences and Oceans Institute at The University of Western Australia showed that a single plant of this species can grow vegetatively by using rhizomes to cover an extensive area, similar to buffalo grass. …

This species is found in waters around the southern coast of Australia. In Western Australia it occurs in the Shark Bay region, around islands of the Houtman Abrolhos, and southward along the coast of the Swan Coastal Plain. The species is recorded at the edge of the Esperance Plains, the Archipelago of the Recherche, at the southern coast of the southwest region. The range extends to the east to coastal areas of New South Wales, South Australia, Tasmania, and Victoria. A sign of a nearby occurrence of Posidonia is the presence of masses of decomposing leaves on beaches, forming fibrous balls.