What happens? When the fish 'throws’ its head backwards, inertia brings about the opening of the mouth and, simultaneously, a pulling down of the tongue. The latter is connected to the lower jaw by two ligaments, and their being stretched beyond a certain point leads to a very rapid closing of the mouth (it takes less than 10 milliseconds), and causes the jaws to snap together. The subsequent clashing of teeth is the source of sound production.

To achieve their goals, the University of Liège’s researchers filmed Clark’s anemonefish producing sounds in the defence of their territory. A high speed camera (500 images per second) was used. They then slowed down the images thus gathered and analysed the animal’s movements, when they came to understand that they needed to focus their research on the head.

Nonetheless, how this sound is amplified still remains to be determined. According to Eric Parmentier, either the movement of the jaws generates vibrations within the ligaments, or, another possibility, the amplification is carried out in the gas bladder. This will be the focus of the next research project carried out by the Functional and Evolutive Morphology Lab.

The next step was to insert small balls of lead in several bony parts of the skull. The fish were then filmed again with a high speed camera which was this time linked to an X-ray machine of the same sort as those used in hospitals to observe the skeleton. In this way it became possible to follow the relative movements of the bony structures, the lead balls blocking the X-rays and thus appearing white on the film. The analysis of the data gathered in this way revealed the mechanism by which the clownfish's sound is produced.

Nevertheless, the results obtained needed to be confirmed. Two complementary experiments were thus carried out. Through these it was demonstrated that a fish in which the ligaments connecting the lower jaw to the tongue had been cut made no sound. And if an individual fish’s teeth were cut it went on to produce another type of sound. The necessary operations were obviously carried out under anaesthetic.

No-one can any longer have doubts: Nemo speaks!