Researchers knew picking up red-bellied piranhas—among the few piranhas dangerous to humans—could prompt croaks from the fish. Even so, no one had studied their sounds in water or provided good evidence for the barks' evolutionary role.

Now a fish tank, an underwater water microphone, and a video camera have helped uncover three different piranha calls—all tied to a variety of grumpy behaviors.

"We knew piranhas were able to make sounds but were not satisfied with the explanation for how they do it," said biologist Eric Parmentier of the Université de Liège in Belgium. "We wanted to know how they do this and what these sounds might mean to other fish."

(Piranha pictures: See the fossil "megapiranha" species.)

Twenty-five species of piranhas exist