Ecoacoustical constraints on hearing in catfishes

Fishes have evolved a wide diversity of hearing sensitivities whose functional significance is unknown. Hearing generalists (e.g., perches, salmonids) are able to detect low frequency sounds up to a few hundred hertz whereas hearing specialists (e.g. catfishes, carps) possess an connection between the inner ear and the swimbladder which results in a broadening of the frequency range up to several kilohertz and an increase in hearing sensitivity. In the preceding FWF-project we found indications that ecoacoustical constraints such as ambient noise might be the driving force in the evolution of the diversity in hearing and less so intraspecific acoustic communication. This eocacoustical constraints hypothesis postulates that low environmental (ambient) noise level results in the selection of enhanced hearing sensitivities in fishes. Catfishes possess a diversity in peripheral hearing structures ranging from large unpaired swimbladders to tiny paired bladders and 1-4 Weberian ossicles transmitting swimbladder vibrations to the inner ears. Reduction in swimbladder size and ossicle number results in a decrease in high frequency hearing abilities. We plan to test the ecoacoustical constraints hypotheses in catfishes. Catfishes are a large order of bony fishes comprising more than 3000 species which live in a number of different habitats. In order to analyze the correlation between hearing, acoustic communication and ambient noise we plan to measure the sound pressure levels and record the noise spectra in different aquatic habitats worldwide and relate those to the species inhabiting these waters. Closely related species living in noisy coral reefs and quiet freshwaters as well as species living in quiet as well as noisy running freshwaters will be compared. Noise levels are therefore expected to be low where catfishes possessing a well developed auditory periphery live. To support our expectation we will measure hearing thresholds of representative catfish species inhabiting these habitats utilizing the auditory evoked potential (AEP) recording technique. The AEP technique has been established in our lab in a preceding project. The hearing curves (audiograms) will be determined under quiet laboratory conditions and during the simultaneous presentation of various habitat noises. This will clarify if and to what extent hearing thresholds shifted (are masked) due to the noise. In order to find out if ambient noise affects acoustic communication, sounds of various catfishes will be recorded and their detectability analyzed using the AEP audiograms. This will show if sound communication is adapted to the environmental noise or if the detection of conspecific sounds is impaired. This will be the first comprehensive investigation eliciting the influence of environmental noise on evolution of hearing and sound communication in closely related fishes. It will in particular demonstrate the importance of ecoacoustical constraints on the selection of accessory hearing structures in catfishes.

The goal of the project was to study the high diversity in peripheral hearing structures (swim bladders, hearing ossicles) and in hearing abilities in fishes as well as the evolution of this variety unknown in other vertebrates. Based on the results of two previous projects the project leader hypothesized that this diversity evolved as an adaptation to different ambient noise conditions. It was intended to test the ecoacoustical constraints hypothesis in catfishes by recording the ambient noise in their habitats worldwide and measure their hearing sensitivities. This plan, however, was much delayed due to unforeseen difficulties in getting the technical equipment necessary and in catching and transporting fishes. Nevertheless, a number of investigations could be carried and published in the course of this project. Investigations in catfishes showed the influence of the development of hearing ossicles, of different swim bladder forms and sizes and of pigmentation (albinism) on hearing abilities. Furthermore, the effect of ambient temperature on sound production and hearing was described in a thorny catfish species. Several investigations were carried out on cichlid fishes in the course of an international cooperation with the Ludwig-Maximilians-Universität in Munich. Most likely, this species-rich family of freshwater fishes reveals the highest diversity of peripheral hearing structures among fishes. Cichlid species which have large swim bladders directly connected to the inner ear hear much better than species lacking such a connection or even possess only a tiny reduced bladder. Noise decreases hearing in species possessing high sensitivities to a higher degree (in accordance with the ecoacoustical constraints hypothesis) than in species having low sensitivities. In addition, we could show that cichlids possess the most complex connection between the swim bladder and the inner ear known in fishes. Furthermore, we found out that the diversity in the periphery is paralleled by a diversity in inner ear and both seem to affect hearing abilities. Finally, a study in toadfishes showed that the auditory system of species lacking peripheral structures for the improvement of hearing is able to detect fine features of vocalizations and differentiate between conspecific (courtship and aggressive sounds) and heterospecific sounds as well as clicks of their predators (bottlenose dolphins). This again indicates that ecoacoustical constraints and less so acoustic communication and orientation were the main selective forces in the development of hearing sensitivities in fishes.