Clownfish are prolific “singers” that produce a wide variety of sounds in both reproductive and agonistic behavioural contexts.
Amphiprion sounds are produced in trains of pulses. Sounds are typically accompanied by rapid movements that include an elevation of the head, lowering of the hyoid bar and anterior part of the branchial basket, a backward movement of the pectoral girdle, and finally a closing of the lower jaws. Synchronization of sound pulses with the video images indicates that sound is produced when the hyoid apparatus is rapidly lowered and the mouth simultaneously closed. Manipulations of specimens show that low amplitude elevations of the skull lower the jaws and branchial basket, a phenomenon well-known in fish feeding. Rather than accentuating this movement, a higher amplitude elevation of the head actually forces the mouth to close by a previously unknown mechanism. Dissections revealed an unusual sonic ligament was responsible for the rapid lower jaw elevation. The ligament joins the hyoid bar (ceratohyal) to the internal part of the mandible (coronoid process of the articulo-angular) and can be compared to a drawbridge. The ligament acting as a cord forces the mandible to turn around its articulation during the lowering of the hyoid bar forcing the mouth to close. Sound results from the collisions of the jaw teeth, transferring energy to the jaws that are presumably the sound radiator.
Species-specific sounds are produced by all 27 Amphiprion species and appear to be supported at least by interspecific variation in teeth shape. The sonic ligament is present in other members of the damselfish family, many of whom produce communication sounds. The homologous-ligament mechanism is likely involved in sound production throughout this large family and represents a novel skeletal adaptation for a new behavioural function. This functional movement seems to be an exaptation of the feeding mechanism.
Synchronous underwater audio–video recordings and a passive acoustic detector were used to study the behaviour of Dascyllus flavicaudus. These damselfish produced sounds during six different behaviours.
(1) The sound associated with the signal jump consisted of a male rising in the water column and then rapidly swimming down while making a pulsed sound. This behaviour was initiated when female(s) entered the male territory or in response to signal jumps made by other males. The male territory spread from 50 cm to about 1 m around nests usually situated at the base of a coral colony.
(2) The visiting sound is produced by males when females visit the nests. In this case, males and females perform a pseudo-spawning behaviour during visiting, with both passing over the nesting surface.
(3) The conspecific chase consisted of a simple pursuit and was made when males chased other D. flavicaudus away from their territory area.
(4) The heterospecific chase corresponded to the pursuit of fishes of another species. This was
performed by territorial males.
(5) During conspecific fighting, fish were face-to-face and made coming and going movements. Simultaneously, both fish rotated around a common axis. During this rotation, fish always tried to keep their head facing their adversary. During these fights, submission postures were sometimes observed: the fish stopped swimming and showed its flank. It was not easy to determine which of the fish produced sounds because both specimens showed an identical behaviour when fighting.
(6) The behaviour of heterospecific fighting was comparable to that displayed during conspecifics fighting. It was observed only in some Acanthuridae eating algae on the coral colonies next to the nests. No submission behaviour was observed, and no sound was detected in the Acanthuridae.