Scientists have known that piranhas, the sharp-toothed meat-eaters which Theodore Roosevelt called "the most ferocious fish in the world," produced sound, but they never knew what those sounds meant -- until now.
The researchers found that the fish make three different sounds depending on their behavior. The University of Liège team published their results in the current issue of The Journal of Experimental Biology.
According to the journal, Millot and Parmentier used a hydrophone -- an underwater microphone -- to record the sounds piranhas made while in a tank. They also videotaped the fish so they'd know what the fish were doing while barking.
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.
Wired reports that vibrations from the piranhas' swim bladders are responsible for the barking sound. The "percussive beat" emitted when the fish chase each other is produced when they grind their teeth, according to a video the researchers made available to National Geographic (see below).
Twenty-five species of piranhas exist in the wild today, but only "two or three" species pose a threat to humans, Parmentier said.
In particular, the red-bellied piranha's voracious appetite for fresh meat is a big reason many scientists have shied away from studying any in-water vocalizations, he added.
That said, the researchers didn't come away unscathed. "We both visited the hospital because we were bitten and Sandie's finger was nearly cut in half," Parmentier told The Journal of Evolutionary Biology's Kathryn Knight, referring to Sandie Millot, one of the article's co-authors.
Red-bellied piranas are native to South America, but one was found this summer in a Texas lake, according to the Houston Chronicle. Prior to the 2011 discovery, the last piranha found in Texas was in 1982.
Last month, piranhas attacked the heels and toes of about 100 visitors at a beach in Brazil.
WATCH & LISTEN: Barking Red-Bellied Piranhas