Abstract :
[en] In Teleostei, the evolution of the ability to produce sounds has provided a great diversity
of sonic mechanisms, even at the intrafamilial level. However, the constraints modulating the
evolution of sound-producing mechanisms and hearing structures are very little supported by
comparative anatomy studies and it is not determined in fish if the evolutionary constraints are
related to intraspecific communication or environmental characteristics.
Serrasalmidae (Characiformes) are neotropical freshwater fishes inhabiting most of the
major river systems in South America. This family is currently divided into three clades: the
« pacu », the « Myleus » and the « piranha » clades.
The ability to produce sounds has been reported in some species belonging to the
« piranha » clade, within the genera Pygocentrus and Serrasalmus. The ability to detect sounds
has been reported only in Pygocentrus nattereri.
This thesis aims to improve our knowledge on acoustic communication in the family
Serrasalmidae. The main goal was to determine, in a phylogenetic context, whether soundproducing
and -detecting apparatuses coevolved within this family. The hypothesis is that the
species having developped the most complex sound-producing mechanisms also improved their
ability to discriminate different types of sounds. An alternative hypothesis is that auditory
ablities are more related to the biology of fishes: carnivorous species would have better hearing
than herbivorous species to locate their living prey. The ability to produce and detect sounds
was therefore investigated using different techniques in a large panel of serrasalmid species.
The ability to produce sounds is uneven in the family Serrasalmidae. In the « pacu »
clade, Piaractus brachypomus produces single low frequency pulses by contracting its hypaxial
musculature. In the « Myleus » clade, several species belonging to the genera Myleus and
Myloplus produced no sound, whatever the experimental conditions. In the « piranha » clade,
Pygopristis denticulata and Catoprion mento produce sounds composed of a varying number
of pulses. They possess a sonic muscle extending from the second rib to the first rib (P.
denticulata) or to the swimbladder (C. mento). Pygocentrus spp. and Serrasalmus spp., as well
as Catoprion mento, are able to utter drumming sounds (tonal vocalizations characterized by
the presence of harmonics in the power spectrum) when hand-held. Sonic muscles of the genera
Pygocentrus and Serrasalmus originate at the base of the second rib and attach to a tendon
surrounding the swimbladder ventrally.
Investigation of the auditory system morphology in various serrasalmid species showed
that the hearing structures (otoliths and Weberian ossicles) are similar in overall shape and
position in all the species investigated. Moreover, hearing range and hearing sensitivity are
quite comparable among the species, despite differences in life-history traits (vocal vs. nonvocal
and herbivorous vs. carnivorous species). Therefore, hearing capacities are not related to
the ability to produce sounds or to the diet.
Acoustic communication appears to be of importance within carnivorous species since
the most sensitive hearing range covers the frequency spectrum of acoustic signals. However,
hearing ability is not related to the capacity to emit sounds. The auditory system seems to be
highly conserved within this family, suggesting that it was shaped by other constraints than
acoustic communication. Sonic organs developed secondarily in the most derived species and
could be related to their radiation.
