Sound production in Carapus boraborensis results from the action of different sonic muscles terminating in complex tendons, which have hooks that fit over a tubercle on the swimbladder wall. The primary sonic muscles (PSM) draw progressively the forepart of the swimbladder until the hook releases the tubercle. This allows the swimbladder to snap back to its resting position, which initiates the onset of the sound. In the present study, the morphology of the C. boraborensis sound-producing apparatus and the resulting sounds were compared with Encheliophis gracilis and Carapus homei. The main difference concerns the direct insertion of the PSM on the swimbladder in C. homei and E. gracilis and, concurrent sonic characteristics. These morphological features also allow both fish to produce an additional kind of sound with more compact pulses, each being clearly composed of two parts and each having a different frequency. The sound-producing system could be compared with a guitarist who makes a sound in releasing a guitar string and modulates it by moving his/her finger along the string. However, E. gracilis possess more filtered sounds than C. boraborensis and C. homei, probably because of the unusual shape of its swimbladder. This study highlights the diversity and plasticity of sonic mechanisms and their implication in the development of sonic repertoire in evolving species.
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