Size matters: diversity in swimbladders and Weberian ossicles affects hearing in catfishes

Otophysine fish possess Weberian ossicles, which connect the swimbladder to the inner ear and improve hearing ability. There is a high diversity in the morphology of the swimbladder and Weberian apparatus in catfishes, which might affect hearing. We have examined these structures in representatives of six families with large, single bladders (Ariidae, Auchenipteridae, Heptapteridae, Malapteruridae, Mochokidae, Pseudopimelodidae) and five subfamilies from two families (Callichthyidae, Loricariidae) having small, paired, encapsulated bladders. We tested their hearing abilities utilizing the non-invasive auditory evoked potential recording technique. Species with single, non-encapsulated, free airbladders possess one, three or four ossicles, whereas species with encapsulated bladders possess one or two. The relative sizes of the bladders and ossicles were significantly smaller in the latter group. All species were able to detect sound stimuli between 50 Hz and 5 kHz. Interspecific differences in hearing sensitivity varied at most by 24 dB below 1 kHz, whilst this variation increased to more than 50 dB at higher frequencies. Catfishes with free bladders had lower thresholds above 1 kHz than those having encapsulated ones. The relative lengths of swimbladders and of ossicular chains were correlated with hearing sensitivity above 1 and 2 kHz, respectively. The number of ossicles affected hearing at 4 and 5 kHz. These results indicate that larger bladders and ossicles as well as higher ossicle numbers improve hearing ability at higher frequencies in catfishes. We furthermore assume that the tiny bladders have minimized their hydrostatic function but were not completely lost because of their auditory function.