What is the world’s largest seagrass species?

Posidonia australis is the world's largest seagrass species. It grows in coastal waters around southern Australia and forms extensive underwater meadows.

In 2022, genetic research identified a single clone of Posidonia australis in Shark Bay, Western Australia, covering about 180 square kilometres. The plant spread mainly through repeated vegetative growth, producing genetically identical shoots connected through its rhizomes.

Seagrasses are flowering plants, not seaweeds. Their roots stabilize sediment, while their leaves provide habitat and nursery areas for fish and invertebrates. They also store carbon in their sediments. The Shark Bay clone's enormous size reflects its long-term clonal expansion; it does not mean one individual has a single leaf or stem across the entire area.