Sometimes called ‘Strap-weed’ or ‘Ribbon-weed’, Posidonia australis is a large species of seagrass that has long, blade-like leaves and can grow up to 1m in length, but typically around 30-60 cm.
Posidonia australis forms a dense canopy that is important for sheltering many marine animals, particularly juvenile snapper, King George whiting and blue swimmer crabs.
Posidonia australis also plays host to a diverse range of small organisms called epiphytes (plants) and epifauna (animals) living in microhabitats and grazing on the leaves, stems and root systems. These organisms are in themselves important contributors to the overall productivity of seagrass meadows and, due to their rapid growth, can be useful indicators of the nutrient loading in the water column. Even in a decomposing state, seagrass leaf litter (now known as detritus) comprises the main diet for many marine species.
It can form large, dense stands (called meadows), and is also often found mixed with other species of seagrass such as Amphiblous (Wireweed), Zostera (eelgrass) and Halophila (paddleweed). The expansion of meadows of Posidonia australis occurs primarily by the lateral (sideways) growth of the rhizomes. Sexual reproduction is via the production of flowers (male and female reproductive organs on the same plant) that are pollinated underwater. Posidonia australis fruits in November and December in WA and the floating fruits are distributed by currents before splitting open to expose the seed.
Researchers from our project partners at UWA recently proved that an individual Posidonia australis plant growing in Shark Bay is the world’s largest organism. This 4500-year-old plant covers 200 sq km and grew from a single seed. Imagine the difference 1 million seeds can make!