Scientists observe crabs eating juvenile crown-of-thorns starfish
By Holly Richardson
ABC Far North
Topic:Great Barrier Reef
9m ago9 minutes agoThu 13 Mar 2025 at 3:35am
A large green spiky coral on top of a large coral system.
Crown-of-thorns starfish feed on coral, sometimes causing significant damage. (Supplied: Australian Institute of Marine Science)
In short:
Scientists have observed several crab species eating juvenile crown-of-thorns starfish (CoTS) on the Great Barrier Reef.
They have also found areas with more CoTS also had more starfish-eating crabs.
What's next?
Researchers say the crabs could be key to controlling the coral-damaging starfish.
Small crabs could be the invisible force controlling crown-of-thorns starfish, according to a new study from multiple Australian research institutes.
The paper, published in scientific journal PNAS this week, is one of the first to consider what animals eat the notorious starfish in their juvenile stages.
Study co-author Sven Uthicke said the Great Barrier Reef had been experiencing outbreaks of crown-of-thorns starfish (CoTS) since the 1960s.
Dr Uthicke, who is also the principal research scientist at the Australian Institute of Marine Science, said the coral-eating animals could cause significant damage to the already vulnerable reef, especially during their periodic population booms.
a crab eating a small starfish
Swimming crabs were observed eating juvenile crown-of-thorns starfish in aquarium experiments. (Supplied: Kennedy Wolfe)
Dr Uthicke said the booms happened every 15-17 years, although scientists were not sure why.
From tiny beginnings
After spawning, the crown-of-thorns larvae float in the water column.
They are about 1 millimetre in size before growing to almost 1 centimetre by the time they are a few weeks old.
Dr Uthicke said they then settled on areas of coral rubble.
"At that stage, they're very vulnerable to predation, and that's a part of the life cycle which hasn't been well studied yet,"
he said.
Building on previous research which looked at what crab species would eat juvenile crown-of-thorns starfish in aquariums, scientists went out on the reef and surveyed the crabs and CoTS present.
a small, many-armed starfish
Crown of thorns starfish are more vulnerable when they are juveniles. (Supplied: AIMS/ Florita Flores)
They also analysed DNA inside the guts of crabs to discover what they were eating.
Dr Uthicke said they knew of at least seven crab species that ate CoTS juveniles in the wild.
"Schizophrys aspera, is a decorator crab, and that seems to be really the star of the CoTS-eating crabs there," he said.
"They have been shown to eat up to 20 crown-of-thorns previously per day."
Dr Sven Uthicke holds up a netting containing a crown-of-thorns starfish.
Sven Uthicke nets a crown-of-thorns starfish. (Supplied: Australian Institute of Marine Science)
Dr Uthicke said they also noticed areas that had more crabs had fewer CoTS.
"To us, it looks like where you have a lot of crabs, you get much less crown-of-thorn outbreaks, which is why we're suggesting that they're really playing an important role in reducing the outbreaks,"
he said.
Dr Uthicke said the key thing researchers did not yet understand, was what caused the differences in crab densities.
divers looking at coral
Scientists search for CoTS predator crabs among coral rubble. (Supplied: University of Queensland/ Morane le Nohaic)
He said fishing zones could play a part by possibly changing the populations of crab-eating fish.
"We could think about … we could enhance the crab populations; we could artificially breed crabs and release them to the reef," he said.
But he said scientists would need to have a better understanding of what impact that could have on the reef.
Hidden army
Professor of Marine Biology at the University of Sydney, Maria Byrne, said research on the "hidden army" of tiny starfish could be a crucial missing link.
many small, many-armed starfish in a container
Juvenile CoTS are hard to find in the wild. (Supplied: AIMS/ Maria Gomez Cabrera)
"They're hidden in the rubble, so it's one of these parts of the jigsaw puzzle of the CoTS that was probably kind of almost not considered or put in the too hard baskets," she said.
Professor Byrne said to understand a species, scientists should always ask "where do the babies come from".
She said more data still needed to be gathered on different reefs, types of rubble and exactly how many starfish different crabs were eating, especially before any kind of bio-control could be looked at.
"It's certainly worth considering," she said.
"It would take a lot of research to get there and first of all, you've got to figure out how to culture these little crabs.
Some thick, spiky red crown-of-thorns starfish are seen crawling around branches of white coral.
An outbreak of CoTS can damage coral on the Great Barrier Reef. (Supplied: AIMS)
"You'd have to generate tens of thousands of them, really, to make a difference."
She said it was an important area to keep researching, with rubble areas increasing due to climate change.
"We need to know what are the other aspects, the other missing links in the jigsaw puzzle," she said.
Posted9m ago9 minutes agoThu 13 Mar 2025 at 3:35am
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