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It has been known for a long time that crabs have teeth inside their stomachs to grind up food, but scientists have just found out that ghost crabs also use these teeth for communication. In particular, growling (although to me it sounds like more of a click than a growl - see for yourself below).
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Crabs are well-known to make clicking sounds with their claws to deter intruders by rubbing the claw's sharp ridges together. But when the intruder comes too close, the crabs will raise their claws in a come-and-get-me style, which prevents them from making sounds with them. This was when scientist Jennifer Taylor noticed that one ghost crab continued to make sound - but not with it's claws.
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Taylor X-rayed a box of ghost crabs that felt threatened by various objects, including a toy crab and a tiny robot. The X-ray fluoroscopy videos showed that the growling sound coincided with the movement of the teeth in their stomach (or gastric mills), and they were not eating at the time.
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So, if you stumble across a ghost crab on the beach and you hear it growling, you'd better back off. The little guy is letting you know that you've crossed the line and you don't want to be in the bad books of a crab.