The clownfish clacks its jaws together to create its unique 'chirps' and 'pops' during territorial or courtship displays

Credit: Eric Parmentier

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SYDNEY: The star of Finding Nemo spoke English, but real clownfish communicate in an even more surprising way, research has revealed.

Eric Parmentier from the University of Liege, Belgium and colleagues found that clownfish, Amphiprion clarkii, clack their jaws together to create the 'chirps' and 'pops' they emit during territorial disputes or courtship displays. The researchers believe their findings, published today in the U.S. journal Science, could apply to the noise making machinery of a host of related fish species.

Noisy Nemo

Scientists have known for nearly 80 years that clownfish produce a swift succession of unusual clacking noises when they spot an intruder or a potential mate. "Clownfish are prolific 'singers,'" said Parmentier. "Although clownfish sounds were recorded as early as 1930, the mechanism of sound production has remained obscure."

To decode A. clarkii's utterances, Parmentier's team came up with a three-pronged research strategy. They used a combination of high-speed video imaging, X-ray technology and sound recording to analyse the physical basis of sounds made by one clownfish approached by another in a classic Nemo scene: milling around a sea anemone.

Sonic chew

The researchers found that the clownfish's sounds were accompanied by a series of rapid movements, including elevation of the head and closing of the lower jaws. "Sound results from the collisions of the jaw teeth, transferring energy to the jaws that are presumably the sound radiator," they wrote.

When they dissected the clownfish, Parmentier and colleagues discovered an unusual 'sonic ligament' that acts like a drawbridge, pulling on the lower jaw and closing the mouth. They confirmed the ligament's role by cutting it, which resulted in the loss of sound production.

According to the researchers, the head movements observed during sound production resemble the fish's feeding motions, but are of greater magnitude. This led them to suspect that the mechanism could be an 'exaptation', or a novel adaptation, of clownfish feeding behaviour. The most famous example of an exaptation is probably the mammalian ear, which evolved from reptilian jaw bones.

A window to other species

Parmentier and colleagues believe this sound production mechanism might be shared by other types of fish. Apart from the other 26 Amphiprion species, "the sonic ligament is present in other members of the damselfish family, many of whom produce communication sounds," they wrote.

Andrew Bass, who studies the neural basis of sound-producing fish at Cornell University in the U.S., believes this may be understating the case. "Considering that half of all living vertebrates are fishes, there is the potential that this mechanism is especially widespread."

"This new mechanism of sound production for fish is a wonderful discovery that once again highlights the remarkable diversity exhibited by fish," he said.