Ghost Crabs Have Stomach Teeth That They Use to Growl at Each Other

Ghost crabs have teeth in their stomachs that they use to growl at one another with, scientists have discovered. This "unusual" communication system, researchers say, is used during aggressive interactions—similar to the way dogs growl during hostile encounters.

Crabs, as with other species like lobsters and crayfish, are known to have teeth in their stomachs. In the absence of teeth in their mouths, this feature, called a gastric mill, allows them to process food.

"There are three main teeth—a medial tooth and two lateral teeth—that are essentially elongated, hard (calcified) structures," Jennifer Taylor, from the University of California, San Diego, told Newsweek. "They are part of the gastric mill apparatus in the stomach, where they rub against each other to grind up food."

Taylor and colleagues noticed that Ocypode quadrata—the Atlantic ghost crab—produced a strange noise. "We could hear the crabs 'growling' at us, but we could not see how they were producing the sound," she said. "After finding no external structures and movements that could produce the distinctive sound, we started to look internally."

In their study, published in the Proceedings of the Royal Society B: Biological Sciences, the team show how this strange growling noise was coming from the gastric mill. This, they found, is the only internal structure that would be able to produce the sound.

The team looked at the behavior associated with the sound, and found it was produced by both males and females during aggressive encounters. "We only heard crabs make this sound during aggressive encounters," Taylor said. "It was most common when the crabs were attacking and striking with their claws. Most crabs readily attacked any object that came too close."

These crabs have also evolved special structures on their claws that allow them to make warning sounds—meaning they have evolved two ways of acoustic communication. Taylor said their claws appear to be helpful for communicating to other crabs at a distance: "Perhaps to warn off competitors or other threats before they get too close. If a potential threat gets too close, then using the stomach apparatus could be advantageous because it allows the crabs to use their claws for fighting while continuing to communicate with sound."

She said they were extremely surprised to find ghost crabs had co-opted their stomach teeth for the purpose of communication, and they now hope to find out more about how the sound signals. "There are many more questions to answer now, including, for example, how these sound signals are acted upon by other crabs and potential predators, how crabs use these sounds in their natural environment, as opposed to a laboratory setting, and how prevalent is this form of communication across crab species," Taylor said.

Correction 9/12, 6:48 a.m. The original article said Jennifer Taylor was from the University of California, Davis. This has been corrected to say the University of California, San Diego.

atlantic ghost crab
An Atlantic ghost crab. Scientists have discovered these crabs use their stomach teeth to make a growling sound during aggressive encounters. iStock