Current studies on the ontogenetic development of acoustic communication in the labyrinth fish Trichopsis vittata and the toadfish Halobatrachus didactylus show that the ability to detect conspecific sounds develops during growth. In order to investigate at which stage sound detection starts in an otophysine fish, we investigated the ontogenetic development of auditory sensitivity and vocalizations in the mochokid catfish Synodontis schoutedeni. Mochokids are also called squeakers because they produce broadband stridulation sounds during abduction and adduction of pectoral fin spines. Fish from five different size groups – from 22 mm standard length to 126 mm – were studied. Hearing thresholds were measured between 50 Hz and 6 kHz using the auditory evoked potentials (AEP) recording technique; stridulation sounds were recorded and their sound pressure levels determined. The smallest juveniles showed the poorest hearing abilities of all size groups between 50 Hz and 1000 Hz. At higher frequencies (5 and 6 kHz), hearing sensitivity was lowest in the largest groups. Duration of abduction and adduction sounds, sound pressure level and pulse period increased, while the dominant frequency of sounds decreased with size. Comparisons between audiograms and sound spectra revealed that the most sensitive frequencies correlate with the dominant frequencies of stridulation sounds in all Synodontis size groups and that all specimens are able to detect stridulation sounds. This study demonstrates that the hearing ability in the siluriform S. schoutedeni changes during ontogeny, whereas no changes were observed in two cypriniform species investigated earlier. Furthermore, S. schoutedeni is able to communicate acoustically at all stages of development, which contrasts to prior findings in teleosts.
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