Destructive organism found ‘thriving’ in surprising place at iconic Aussie destination

The spawn of starfish that kill the Great Barrier Reef have been discovered feasting on toxic blue-green algae bacteria.

Michael Dahlstrom
·Environment Editor
Updated ·2-min read
Two crown-of-thorns starfish larvae at two weeks old, viewed under a fluorescence microscope. They are blue, yellow and pink in colour.
Crown-of-thorns starfish larvae at two weeks old, viewed under a fluorescence microscope. Source: Corinne Lawson

One of Australia’s most iconic tourist destinations, the Great Barrier Reef, has been under attack from invasive crown-of-thorns starfish since the 1960s. Now researchers have discovered its larvae “thriving” in a surprising place.

Visible only under a microscope, the starfish’s tiny offspring have been found feasting almost exclusively on Trichodesmium. This blue-green algae bacteria clusters on the surface of the ocean like an oil slick and it’s referred to colloquially as “sea sawdust”. In large amounts, it's known to close beaches because it can cause skin irritations.

What’s stunned scientists from the University of Queensland and Southern Cross University about their discovery is that Trichodesmium is highly toxic and was also thought to have little nutritional value. It had been thought its main purpose was to create nitrogen from gases in the atmosphere which it makes available to other sea life.

Related: Warning to beachgoers as Aussie shoreline turns 'toxic' red

A group of people at the Whitsundays, walking along sand to the Great Barrier Reef.
Queensland's iconic Great Barrier Reef has been plagued by outbreaks of crown-of-thorns starfish since the 1960s. Source: Michael Dahlstrom

“Until now, not much has been known about sea sawdust as a food source, so we were certainly surprised to say the least,” UQ’s Dr Benjamin Mos said.

“Blue-green algae blooms can extend hundreds to thousands of kilometres across the ocean and often float on the surface in large rafts like sawdust — hence the name.”

While it’s well known the Great Barrier Reef now experiences regular coral bleaching and climate change is a major cause of the problem. What’s been less understood is why major outbreaks of crown-of-thorns starfish occur.

Information on what Crown of thorns starfish are.
Facts about crown of thorns starfish. Images: Getty

But the new discovery about this surprising food source has led scientists to hypothesise that human activity may also be to blame for surges in starfish numbers. That’s because the use of fertiliser by farmers, along with sewage and stormwater runoff likely all fuel increased blooms of Trichodesmium.

Now researchers hope they may be able to target the larvae where it is feeding.

“If we can figure out how to reduce the impact of [crown of thorns starfish], we might give coral reefs a little more time,” Southern Cross University’s Professor Dworjanyn said.

“We don’t yet know if sea sawdust blooms result in more adult [starfish] on coral reefs, so this research needs more work. However, our findings could be an important part of cracking that puzzle.”

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