Co-evolution of hearing and sound production in fishes

Hearing in vertebrates in general and in fishes in particular evolved most likely independently from the development of acoustic communication. The detection of a large number of noises generated by prey, predators, feeding animals, or of non-biological origin, was a prerequisite for a successful orientation and adaptation to their environment. At the start of the project I focussed on the question if the diversity of hearing abilities in fishes is in any way related to the variety of sound types generated. In order to answer this question a newly developed technique for measuring the auditory sensitivity of fishes non-invasively was introduced to our laboratory in Vienna. This method is based on the recording of acoustically evoked auditory brainstem responses (ABR) from the skin of the head of fishes. Based on this technique the hearing thresholds of numerous fish species could be measured. Results showed that the hearing sensitivity in 14 species out of 8 families were only weakly correlated to the characteristics of their sounds. In addition, the excellent hearing ability of mute species such as the goldfish are very similar to that of vocalizing catfishes, piranhas and cyprinids. On the other hand fishes possessing poor hearing abilities such as sunfishes and sculpins are nevertheless often communicating acoustically. In the following we wanted to investigate at which developmental stage (age) fishes are able to detect conspecific sounds. For this purpose the auditory sensitivity of croaking gouramis, tropical freshwater fishes, was measured starting shortly after hatching, and compared to the characteristics of sounds produced during fighting. Results showed that hearing develops prior to the ability to produce sounds and that the acoustic signals of juveniles could not be detected in the beginning due to low sound intensities and poor hearing. There exists growing concern that underwater noise polution may affect fishes negatively in addition to higher vertebrates. In a series of experiments where we exposed goldfishes and catfishes to white noise for 12 or 24 hours we could show that fishes suffer a noise-induced hearing loss similar to humans. Contrary to mammals the hearing loss is temporary because lower vertebrates are able to regenerate damaged inner ear hair cells. Vocalizations of fishes resemble croaking, growling, knocking or stridulating sounds, all of which are characterized by a series of pulses. The ABR recording technique was recently expanded so measure the ability of fishes to detect temporal patterns of sounds in general and of conspecific sounds in particular. In all species fishes investigated the temporal pattern of sounds was represented in the auditory brainstem which indicates that they hear small interpuls-intervalls. This may enable fishes to assess the fitness of mates or of opponents as well as to distinguish between conspecific and hererospecific sounds based on differences in temporal features of acoustic signals.