Scientists recorded the sounds that red-bellied piranhas use to communicate with one another and identified three types of vocalizations. The findings are reported in the Journal of Experimental Biology and described in Inside JEB.Authors Sandie Millot, Pierre Vandewalle, and Eric Parmentier from the University of Liege, Belgium, knew that the fish produced some sounds, but did not know why or in what contexts. The researchers recorded the sounds of captive piranhas with a hydrophone (an underwater microphone) while also filming the fish  as they interacted. The fish were generally silent and peaceful, cruising around their tank. Vocalizations were only produced during rare confrontations, usually over food.Three distinct types of sounds were recorded. The first was a bark, produced by piranha displaying aggressively to one another but not fighting. This is the same sound that piranhas produce when picked up by the scientists, noted Parmentier. The other two sounds were a short drum-like beat, produced when chasing each other and fighting for food, and a softer croak sound produced by their jaws when biting and snapping at each other.The research team also investigated the mechanisms by which piranhas produce these sounds.  Like many sound-producing fish, piranhas were known to make noises using muscles attached to their swim bladders, gas-filled organs that help maintain buoyancy.  Parmentier and colleagues stimulated these muscles and found that they contract and relax 150 times per second to vibrate the swim bladder. The muscles themselves were driving the vibration of the swim bladder and determining the frequency of the bark and drum sounds.Next, Parmentier wants to know if piranhas are also vocal during mating, a question that will likely take him and his team into the Amazon. It turns out that piranhas are not as easily persuaded to mate in captivity as they are to fight.Click here to listen to a recording of the three types of vocalizations (barks, drum beats, and croaks) recorded from piranhas.