How does tripus extirpation affect auditory sensitivity in goldfish?

Otophysine fishes are characterized by Weberian ossicles connecting the swimbladder to the ear acoustically. In order to determine the degree to which these ossicles contribute to auditory sensitivity, the tripus was unilaterally or bilaterally extirpated in goldfish and hearing thresholds determined. The auditory evoked potential (AEP) recording technique was used to measure auditory sensitivity between 100 and 4000 Hz. Bilateral extirpation resulted in a hearing loss at all frequencies ranging from 7 dB at 100 Hz to 33 dB at 2 kHz; no AEPs were detectable at 4 kHz. In contrast to bilateral extirpation, unilateral tripus removal caused no sensitivity change. Pre-exposure to intense white noise caused different threshold shifts in unilaterally versus bilaterally extirpated goldfish. Thresholds increased at all frequencies in unilaterally extirpated goldfish but only at 100 and 200 Hz after bilateral extirpation. The comparison between the hearing generalist Neolamprologus brichardi (family Cichlidae) and the tripus-extirpated otophysine revealed that the latter is still more sensitive than the cichlid. Higher sensitivity in the goldfish after bilateral extirpation as compared to swimbladder elimination indicates that swimbladder oscillations might partly be transmitted to the inner ear independently of the ossicular chain. This suggests that the auditory system in otophysines improves with increasing frequency due to a more efficient connection between the swimbladder and inner ear ensured by the Weberian ossicles.