Agonistic sounds in the clownfish Amphiprion clarkii: implication of the swimbladder in the sound-producing mechanism

Clownfishes are aggressive fishes that use sound production to defend their anemone territory. It has been shown that they produce agonistic sounds using a jaw teeth snapping. At present, this mechanism has highlighted the onset of the sound but has not explained yet which structure is responsible for the sound modulation. Interestingly, some acoustic features such as dominant frequency and pulse duration are directly related to fish size. Such variations are linked to a morphological constraint. Also, the existent relationship between fish size and swimbladder size implies that the swimbladder might be involved in the sound production.
Sound analyses in Amphiprion clarkii showed that the experimental filling of the swimbladder with physiological liquid (NaCl 9‰) significantly modified the acoustic features. The most striking changes were a significant increase in dominant frequency and a significant decrease in pulse duration. These observations highlighted the implication of the swimbladder in sound modulation.
In clownfishes, dominant frequency and pulse duration are morphologically determined signals. The swimbladder appears to modulate these acoustic features by acting as a resonant chamber.