The seagrass in Shark Bay, Australia, covers an area the size of about 20,000 football fields. It is the largest known plant on Earth – roughly three times the size of Manhattan.

     Using genetic testing, scientists have determined that the large underwater meadow in Western Australia is, in fact, one plant.

     It is believed to have spread from a single seed over at least 4,500 years. The seagrass covers about 200 square km (77 square miles), according to researchers from the University of Western Australia.

     The team stumbled upon the discovery by accident at Shark Bay, about 800km north of Perth.

They had set out to understand the genetic diversity of the species – also known as ribbon weed – which is commonly found along parts of Australia’s coast.

     Researchers collected shoots from across the bay and examined 18,000 genetic markers to create a “fingerprint” from each sample. They had aimed to discover how many plants made up the meadow.

     “The answer blew us away – there was just one!” said Jane Edgeloe, the study’s lead author.

“That’s it, just one plant has expanded over 180km in Shark Bay, making it the largest known plant on Earth.”

     The plant is also remarkable for its hardiness, having grown in locations across the bay with wildly variable conditions.

     “It appears to be really resilient, experiencing a wide range of temperatures and salinities plus extreme high light conditions, which together would typically be highly stressful for most plants,” said Dr. Elizabeth Sinclair, one of the researchers.

     The species generally grows like a lawn at a rate of up to 35cm a year. This is how researchers estimated it has taken 4,500 years to sprawl to its current size.

     Seagrasses are the only flowering plants which grow in marine environments.

     There are about 60 species of fully marine seagrasses. They evolved from terrestrial plants which recolonised the ocean 70 to 100 million years ago.

     The name seagrass stems from the many species with long and narrow leaves, which grow by rhizome extension, and often spread across large “meadows” resembling grassland; many species superficially resemble terrestrial grasses.

     Like all plants, seagrasses photosynthesize, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. Most species undergo submarine pollination and complete their life cycle underwater. While it was previously believed this pollination was carried out without pollinators, and purely by sea current drift, this has been shown to be false for at least one species, Thalassia testudinum.

     Crustaceans, such as crabs and worm larvae have both been found with pollen grains. The seagrass produces nutritious mucigenous clumps of pollen, which attract, and then stick to the crustaceans just like the nectar in terrestrial flowers do to bees and other pollinators.

     Seagrasses form dense underwater seagrass meadows which are among the most productive ecosystems in the world. They function as important carbon sinks and provide habitats and food for a diversity of marine life comparable to that of coral reefs.

     They are ubiquitous throughout the world. So the chances are that the one plant in Shark Bay may not be the largest plant in the world. Others, out there, are almost certainly bigger.

     It reminds me of a discovery made in the Rocky Mountains that documented that all Aspen Trees on the same location are, in fact, related and interconnected: One Aspen tree, just like one seagrass. Amazing.

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