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Enormous, ancient seagrass plant discovered in Shark Bay, WA

Donna Coutts, June 5, 2022 2:30PM Kids News

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Australian researchers have discovered that the seagrass Posidonia australis in the shallows of WA’s Shark Bay is the largest plant on Earth. Picture: University of Western Australia media_cameraAustralian researchers have discovered that the seagrass Posidonia australis in the shallows of WA’s Shark Bay is the largest plant on Earth. Picture: University of Western Australia


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Australian scientists have accidentally found the largest known plant on Earth.

The ancient and enormous seagrass plant is in World Heritage-listed* Shark Bay, off the Western Australian coast.

It stretches 180km across, covers about 200sq km of the shallow seabed* and is thought to be at least 4500 years old.

The team of scientists from the University of Western Australia (UWA) and Flinders University were studying Shark Bay Posidonia australis seagrass to learn about the genetic* diversity* across that plant population, or how different or similar individual plants were to each other.

Knowing this would help experts decide which plants would be good to collect and transplant* in other places to help repair damaged seagrass meadows*.

“We often get asked how many different plants are growing in seagrass meadows and this time we used genetic tools to answer it,” said UWA evolutionary* biologist* Dr Elizabeth Sinclair, who was part of the research team.

Western Australia map showing giant seagrass - cropped media_cameraThis map of Western Australia shows the location and spread of the seagrass. Picture: Abi Fraser

UWA marine* biologist and seagrass expert Jane Edgeloe said the team took samples from seagrass shoots across Shark Bay with the goal of building a complex* genetic picture or map of each plant, like a fingerprint.

But to the scientists’ surprise, the tests revealed the seagrass was actually one gigantic plant.

“The answer blew us away – there was just one!” Ms Edgeloe said.

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

Seagrass in Shark Bay has to withstand* big variations in conditions. Water temperature can vary from 17C to 30C, some areas are dark while others are in bright sunlight and salinity – how salty the water is – varies from normal to double that saltiness.

media_cameraAn aerial view of the south passage of Shark Bay, near the town of Denham in Western Australia. Picture: Australia’s Coral Coast

Flinders University ecologist* Dr Martin Breed said these conditions would typically* be highly stressful for plants.

Scientists expected that the genetic diversity of different plants was what allowed the seagrass to cope with the varying conditions. Instead, Dr Breed said, the one plant may have slight genetic differences across its habitat*.

The seagrass plant is actually a special type of clone* called a polyploid.

In most species of animals and plants, including other seagrass plants, each parent contributes half its genetic material to its offspring*. A polyploid clone has multiple sets of the same genetic material.

The fact it has so much genetic material may explain its success coping with the varying conditions.

There are now experiments set up across Shark Bay to understand more about this giant plant.

The research results are published in the journal Proceedings of the Royal Society B.

Shark Bay is a World Heritage Area, which means the United Nations Education, Scientific and Cultural Organisation (UNESCO) has decided it has “outstanding universal value”.

Shark Bay is important for its seagrass, for the dugong that graze on the seagrass and for the ancient algae colonies that form domes called stromalites. It is also home to five endangered mammal species.

media_cameraDugongs graze on the seagrass in Shark Bay, Western Australia.

The seagrass in Shark Bay has been able to grow so large because it has been undisturbed for thousands of years.

Scientists at Edith Cowan University are mapping Australia’s seagrass meadows to help understand the risks for their survival and how best to protect them. The record-breaking rains on the east coast of Australia this year, for example, could be very damaging for seagrass, which needs light to survive. Mud washed into the sea blocks out sunlight, slowing seagrass growth or killing the plants.


  • World Heritage-listed: United Nations Education, Scientific and Cultural Organisation (UNESCO) has decided it has “outstanding universal value”
  • seabed: floor of the sea or ocean
  • genetic: describing the genes of a living thing
  • diversity: variety
  • transplant: move from one place to another
  • meadows: grasslands
  • evolutionary: describing how living things change over time
  • biologist: scientist of living things
  • complex: complicated; with lots of parts
  • withstand: resist
  • typically: normally, ordinarily
  • habitat: the place and conditions where something lives
  • clone: copy
  • offspring: children; next generation


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  1. Where is the giant seagrass plant?
  2. How old is this seagrass?
  3. What is Jane Edgeloe an expert on?
  4. What is the temperature range of the water in Shark Bay?
  5. Why can flooding rains be dangerous for seagrass?


1. Below the surface
Draw a picture showing what you would see under the surface of the water at Shark Bay. Include as much detail as possible based on the information given in the news story. Add labels and captions to provide key details.

Time: allow 20 minutes to complete this activity
Curriculum Links: English; Science; Geography; The Arts

2. Extension
Use a physical or online map to find your current location. Then, find another location that you are familiar with (for example, a family holiday spot or where a relative lives) that is around 180km away, to give yourself a better idea of just how far the seagrass plant has spread.

Time: allow 5-10 minutes to complete this activity
Curriculum Links: English; Science; Geography

To sum it up
After reading the article, use your comprehension skills to summarise in a maximum of three sentences what the article is about.

Think about:

  • What is the main topic or idea?
  • What is an important or interesting fact?
  • Who was involved (people or places)?

Use your VCOP skills to re-read your summary to make sure it is clear, specific and well punctuated.

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