Meeting Abstract
Within piranhas, sound production is known in carnivorous species whereas herbivorous species were thought to be mute. As these carnivorous sonic species have a complex sonic apparatus, we hypothesise that transitional forms could be found in some serrasalmid species. We investigate sound production in different species covering all the Serrasalmidae phylogenetic tree to understand the evolution of the sonic mechanism in this family. The results highlight the evolutionary transition from a simple sound-producing mechanism without specialised sonic structures in the herbivorous species (Piaractus and Myloplus) to a sonic mechanism involving large, fast-contracting sonic muscles vibrating the swimbladder in the genera Pygocentrus and Serrasalmus. Hypaxial muscles in herbivores primarily serve locomotion, but some bundles caused sound production during swimming accelerations, meaning these muscles have gained a dual function. Sound production therefore seems to have been acquired through an exaptation event, i.e. the development of a new function (sound production) in existing structures initially shaped for a different purpose (locomotion). In further evolutionary stages (Catoprion and Pygopristis), some bundles are distinguishable from other hypaxial muscles and insert directly on the swimbladder. At this stage, the primary function (locomotion) is lost in favour of the secondary function (sound production). In the last stage (Pygocentrus and Serrasalmus), the muscles and insertion sites are larger and the innervation involves more spinal nerves, improving calling abilities. The comparison of sounds and sonic mechanisms shows the evolution of acoustic communication corresponds to a trajectory where the initial exaptation event is then subject to adaptations.
