Birth and Evolution of Acoustic Communication in Piranhas (Serrasalmidae)

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.