Did Auditory Sensitivity and Vocalization Evolve Independently in Otophysan Fishes?

Abstract

Otophysine fishes have a series of bones, the Weberian ossicles, which acoustically couple the swimbladder to the inner ear. These fishes have evolved a diversity of sound-generating organs and acoustic signals, although some species, such as the goldfish, are not known to be vocal. Utilizing a recently developed auditory brainstem response (ABR)-recording technique, the auditory sensitivities of representatives of seven families from all four otophysine orders were investigated and compared to the spectral content of their vocalizations. All species examined detect tone bursts from 100 Hz to 5 kHz, but ABR-audiograms revealed major differences in auditory sensitivities, especially at higher frequencies (>1 kHz) where thresholds differed by up to 50 dB. These differences showed no apparent correspondence to the ability to produce sounds (vocal versus non-vocal species) or to the spectral content of species-specific sounds. All fishes have maximum sensitivity between 400 Hz and 1,500 Hz, whereas the major portion of the energy of acoustic signals was in the frequency range of 100–400 Hz (swimbladder drumming sounds) and of 1–3 kHz (stridulatory sounds). Species producing stridulatory sounds exhibited better high-frequency hearing sensitivity (pimelodids, doradids), except for callichthyids, which had poorest hearing ability in this range. Furthermore, fishes emitting both low- and high-frequency sounds, such as pimelodid and doradid catfishes, did not possess two corresponding auditory sensitivity maxima. Based on these results it is concluded that selective pressures involved in the evolution of the Weberian apparatus and the design of vocal signals in otophysines were others (primarily predator or prey detection in quiet freshwater habitats) than those serving to optimize acoustical communication.