Piranhas communicate with sound

Scientists have discovered that the fearsome fish use sounds to communicate - often intimidating their rivals rather than attacking. With underwater microphones, scientists recorded the sounds the fish made when they confronted one another. They reported in the Journal of Experimental Biology that each of these three sounds appeared to contain a different "message". Lead researcher Eric Parmentier, from the University of Liege, Belgium, has studied sound production and communication in a wide variety of fish species, including the charismatic clownfish and the spectacularly ugly toad fish. He already knew that piranhas made sounds, but wanted to understand why. Many fish use noises to attract a mate, so the sounds are an important indicator that the fish are reproducing. "Eventually, if we understand the behaviour that's associated with the sounds, we might be able to listen to the sea and explain to fishermen: 'Now's not the best time to start fishing'." Dr Parmentier and his colleagues put a hydrophone - an underwater microphone - into a tank of piranhas in their lab and filmed the fish as they interacted. They recorded three distinct sounds. The first was a bark that the fish produced when they "displayed" to each other - confronting one another face to face but not fighting. The other two were a drum-like percussive beat, which piranhas produced when they chased one another, and a softer croak they made when biting each other. These physical fights were usually over food. Most of the time, though, the fish swam around peacefully, making no noise and engaging in none of these underwater conflicts. It was only through hours of painstaking observation that the researchers managed to capture the behaviour. "For animals, it's less expensive [in terms of energy] to make a lot of noise and impress the other guys, rather than fight," explained Dr Parmentier. Good vibrations Piranhas, like many other "noisy" fish, produce sounds by vibrating their swim bladders - gas-filled organs in their bodies that help regulate their buoyancy. The team also studied the very high-speed muscles that drove this vibration. "This muscle contracts and relaxes 150 times every second to vibrate the swim bladder," Dr Parmentier explained to BBC Nature. He and his team hope to go on to study the fish in their Amazon river home to find out more about their acoustic repertoire.