Atlantic ghost crabs have taken the notion of a grumbling stomach to a whole new level, scientists have discovered, and they’re more than just hangry. Ghost crabs have teeth in their stomachs they use to growl a warning at aggressors, like small crabby dogs.

Ghost crabs, like many other crustaceans, make up for a lack of teeth in their mouths by having a set in their gut – a gastric mill – to grind up and process their food. However, researchers have discovered they also use these teeth to growl during aggressive encounters when their pincers are busy. It’s the first evidence of an animal using stomach sounds to communicate, they say.

While studying Ocypode quadrata, lead author Jennifer Taylor of the Scripps Institution of Oceanography and colleagues noticed that as well as the typical aggressive sounds the crabs made by rubbing their pincers together when provoked, they also seemed to be making other curious sounds.

"We could hear the crabs 'growling' at us, but we could not see how they were producing the sound," Taylor told Newsweek. "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, Taylor and co. reveal how they discovered this unique secret weapon coming from the gastric mill, the only internal part of the crab that could make a sound.  

The team discovered that the crabs – both males and females – only produce the sound under hostile conditions, by approaching them with essentially dead crabs on sticks. Most crabs readily waved their angry little pincers at the imposters, making stridulations – the warning sound created by rubbing their claws, which are lined with bristles, together.

To work out where the second “growling” was coming from, they first attempted an endoscope through the mouth. Unfortunately, a crab crushed it, so they used laser doppler vibrometry, a non-invasive instrument that measures vibrations without contact by bouncing lasers off a surface. The strongest vibrations came from the crabs' gastric region. Suspecting the sound was coming from the gastric mill, they X-rayed the crabs while they wrestled with their deceased foes, and managed to capture the gastric mill in action.

The crabs' gastric mill consists of three main "teeth", calcified plates, that rub against each other to grind up their food. The researchers concluded this was the only internal apparatus that could make a sound, which surprised them because the crabs already have claws specially evolved to communicate acoustically, so why would they need a second method?

One possible answer is that the sound the claws make is to warn off others at a distance, whereas the stomach growling is used to continue the threat when an opponent is so close the claws are engaged in fisticuffs. Either way, the evolution of this dual communication method is unusual.

"Animals have co-opted a variety of structures for sound production," the researchers said in a press release, "but this is the first documented example of one using gastric sounds for communication."

This new discovery has thrown up more questions than answers, like how do live foes react to this sound in a non-lab setting, which the researchers intend to explore.