As It Happens

Ghost crabs make eerie sounds with their stomach teeth to ward off predators

A new study has uncovered that ghost crabs scare away predators by making their insides growl.

X-rays reveal that the crabs make grinding noises with their stomachs when they feel threatened

Researchers used X-rays of ghost crabs to discover the crustaceans make aggressive sounds with their stomachs to ward off predators. (Jennifer Taylor)
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Transcript

Ghost crabs have a reputation for being feisty. The nocturnal crustaceans aren't afraid to tell a predator to back off.

But scientists just learned it's more than the ominous snapping of their pincers that wards off enemies.

After studying the way the crabs communicate when threatened, a team of researchers at the Scripps Institution of Oceanography has discovered the crabs have another noisy line of defence — their stomachs.

"I was pretty blown away," marine biologist Maya deVries told As It Happens host Carol Off. "They are communicating with their stomachs."

The findings were published this week in the Proceedings of the Royal Society B

According to deVries, all crustaceans have what's known as "a gastric mill" in their stomachs.

"It's a set of teeth, basically, in the stomach that they grind together and they can chew up food that way," deVries said. 

"But we learned that they can also make these sounds that sound strikingly similar to the sounds that they make with their claws."

Using vibration-sensing lasers, the team determined the grinding sound was coming from inside the crab's body, likely from the gastric mill.

A new study finds that ghost crabs ward off predators with sounds produced by grinding teeth in their stomachs together. 0:10

But deVries and her colleague Jennifer Taylor suspected the noise wasn't just due to a bit of indigestion. 

They noticed the mysterious noise occurred when the crabs were behaving antagonistically.

They set out to test that theory. But getting a window into the insides of an agitated crab proved difficult. 

"We needed to see the gastric mill actually moving at the same time that the sound was being produced," deVries said. "That's actually really tricky to do!"

Professor Maya deVries was a post-doc fellow at Scripps Institution of Oceanography and the co-author of a new study on crab communication. (Submitted by Maya deVries)

Eventually, the researchers teamed up with medical doctors at UC San Diego Hillcrest Medical Center and they hatched a plan to X-ray the crabs. 

"This is definitely the first time that they had stuck crabs into this machine before," deVries said. "But it worked."

While sporting X-ray protective gear, deVries and her team used remote-controlled Hexbug toys to mimic the crabs' predators. 

Maya deVries says the researchers used robotic remote-controlled Hexbug toys to test how the crabs reacted to a predator. (Dr. Emily L.A. Kelly)

According to deVries, the whole set-up was "a pretty wild scene."

But in the end, the team captured clear video evidence of the crab grinding its gastric mill in sync with the mysterious sound.

"It's really neat because this means that this behaviour, that is so important for them for defending territory and fighting with other crabs, this actually frees up their claws for them to still be able to make the sound with a different part of their body," deVries said.

"It's highly possible that there are a lot of other animals that are doing this and we just haven't discovered it yet."

Written by Kate Cornick and John McGill. Produced by Kate Cornick.

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