(File photo) A picture taken on May 24, 2011,shows one of the three Kamchatka crabs which joined the Oceanopolis, Ocean Discovery Park in Brest, western France. AFP PHOTO / FRED TANNEAU

Paris - Crabs have a sort of inner ear that helps them to hear nearby predators, US scientists have found.

An organ called the statocyst, previously shown to play a role in crustacean balance, is also used for the crab equivalent of hearing, they reported Tuesday in the British journal Proceedings of the Royal Society B.

Biologists at Northeastern University in Massachusetts conducted lab experiments on mud crabs - small crustaceans found in reefs in the Gulf of Mexico - to see how they responded to underwater sounds.

The team placed a thin sensor under the crab's shell to measure electrical activity in the statocyst.

They then placed the crabs in a tank and played recordings of foraging sounds made by three predator fish species - the hardhead catfish, black drum and oyster toadfish.

They found that the crabs abruptly stopped hunting for clams, a behaviour that is a prelude to scurrying for shelter, whenever they heard the sounds from the catfish and toadfish.

But they were far less bothered by the acoustics of the black drum, a fish whose loud foraging noise can be heard from afar and thus may not present an immediate threat.

The team ruled out vibrations or pressure differences as the stimuli to which the crabs reacted. The animals “can detect sound across a range of frequencies,” they concluded. - Sapa-AFP