Acoustic communication and the evolution of hearing in fishes

Fishes have evolved a diversity of sound-generating organs and acoustic sig nals of various temporal and spectral content. Additionally, representative s of many teleost families such as otophysines, anabantoids, mormyrids and holocentrids possess accessory structures that enhance hearing abilities by acoustically coupling air-filled cavities to the inner cat. Contrary to the accessory hearing structures sudl as Weberian ossicles in o tophysines and suprabranchial chambers in anabantoids, sonic organs do not occur in all members of these lasa. Comparison of audiograms among nine rep resentatives of seven otophysan families from four orders revealed major di fferences in auditory. sensitivity, 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- local species) or to the spectral content of species-specific sounds. In an abantoids, the lowest auditor thresholds were found in the blue gourami Tri chogaster trichopterus, a species not thought to be vocal. Dominant frequen cies of sounds corresponded with optimal hearing bandwidth in two out of th ree vocalizing species. Based on these results, it is concluded that the selective pressures involv ed in the evolution of accessory hearing structures and in the design of vo cal signals were other than those serving to optimize acoustic communicatio n.