Sound production in piranhas and relatives: comparisons between species

Acoustic communication plays an important role in the life of many teleost species where it is mainly involved in agonistic and/or courtship behavior. Despite the large number of Serrasalmidae species (92), sound production has been described only in the genera Serrasalmus and Pygocentrus. The aim of this study was first to investigate the sound producing abilities of different Serrasalmidae species and then to describe and understand the corresponding mechanisms.
One herbivorous species, Piaractus brachypomus, produces sounds composed of a single pulse. The mechanism would involve the vibration of the bladder due to the hypaxial musculature contraction. In contrast, the calls emitted by Serrasalmus rhombeus, Serrasalmus compressus, Serrasalmus marginatus, Serrasalmus elongatus, Pygocentrus nattereri and Pristobrycon eigenmanni are all harmonic sounds composed of several pulses without inter-pulse interval. They all show the same kind of mechanism: the sound results from the forced vibration of the swimbladder following the contraction of sonic muscles that are attached to a perpendicular tendon surrounding ventrally the bladder. A last species, Pygopristis denticulata, is able to produce another type of sounds. It consists of several pulses with irregular pulse period and is likely produced by a sonic muscle inserting on the skull and on the rostral part of the swimbladder.
The relatively high diversity of sound types and mechanisms in Serrasalmidae will be used in the future to understand the evolutionary development of this particular behavior. Do the mechanisms evolve separately or is there a continuity between them?