Sound production mechanism in the clownfish Amphiprion clarkii (Amphiprioninae, Pomacentridae)

Clownfishes live in social group within sea anemones. They are prolific “singers” that produce a wide variety of sounds, described as “pops” and “chirps”, involved in both reproductive and agonistic interactions. Although clownfish sounds were recorded since 1930, the mechanism of sound production has remained unresolved.
<br />The sounds used to describe the sonic mechanism were directed towards hetero- and conspecifics that approach their sea anemone host. Sound recordings were synchronized using a high speed video (500 fps) coupled or not with an X-ray system. These systems allowed to quantify the movements of external and internal bones during sound production. Sounds were typically accompanied by rapid (< 30 ms) head movements such as elevation of the skull, lowering of the hyoid bar and the anterior part of the branchial basket, retraction of the pectoral girdle, and finally closing of the mouth. Synchronization of sound pulses with X-ray images indicates that sound is produced when the hyoid apparatus is completely lowered and the mouth closed by a previously unknown mechanism.
<br />Dissections of freshly dead specimens reveal an unusual ligament responsible for the rapid mouth closing. This ligament joins the hyoid bar to the internal part of the mandible. Acting as a cord, it forces the mandible to turn around its articulation during the lowering of the anterior part of the branchial basket, forcing the mouth to close. Sounds result from the collision of the jaw teeth, transferring energy to the jaws that are presumably the sound radiator.