SUMMARYOtophysine 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.

BackgroundSurveys of ontogenetic development of hearing and sound production in fish are scarce, and the ontogenetic development of acoustic communication has been investigated in only two fish species so far. Studies on the labyrinth fish Trichopsis vittata and the toadfish Halobatrachus didactylus show that the ability to detect conspecific sounds develops during growth. In otophysine fish, which are characterized by Weberian ossicles and improved hearing sensitivities, the ontogenetic development of sound communication has never been investigated. We analysed the ontogeny of the auditory sensitivity and vocalizations in the mochokid catfish Synodontis schoutedeni. Mochokid catfishes of the genus Synodontis are commonly called squeakers because they produce broadband stridulation sounds during abduction and adduction of pectoral fin spines. Fish from six different size groups - from 22 mm standard length to 126 mm - were studied. Hearing thresholds were measured between 50 Hz and 6 kHz using the auditory evoked potentials recording technique; stridulation sounds were recorded and their sound pressure levels determined. Finally, absolute sound power spectra were compared to auditory sensitivity curves within each size group.ResultsThe smallest juveniles showed the poorest hearing abilities of all size groups between 50 and 1,000 Hz and highest hearing sensitivity at 5 and 6 kHz. The duration of abduction and adduction sounds and the pulse period increased and sound pressure level (in animals smaller than 58 mm) increased, while the dominant frequency of sounds decreased with size in animals larger than 37 mm. Comparisons between audiograms and sound spectra revealed that the most sensitive frequencies correlate with the dominant frequencies of stridulation sounds in all S. schoutedeni size groups and that all specimens are able to detect sounds of all size groups.ConclusionsThis study on the squeaker catfish S. schoutedeni is the first to demonstrate that absolute hearing sensitivity changes during ontogeny in an otophysine fish. This contrasts with prior studies on two cypriniform fish species in which no such change could be observed. Furthermore, S. schoutedeni can detect conspecific sounds at all stages of development, again contrasting with prior findings in fishes.

BackgroundThe Weberian apparatus of otophysine fishes facilitates sound transmission from the swimbladder to the inner ear to increase hearing sensitivity. It has been of great interest to biologists since the 19th century. No studies, however, are available on the development of the Weberian ossicles and its effect on the development of hearing in catfishes.Methodology/Principal FindingsWe investigated the development of the Weberian apparatus and auditory sensitivity in the catfish Lophiobagrus cyclurus. Specimens from 11.3 mm to 85.5 mm in standard length were studied. Morphology was assessed using sectioning, histology, and X-ray computed tomography, along with 3D reconstruction. Hearing thresholds were measured utilizing the auditory evoked potentials recording technique. Weberian ossicles and interossicular ligaments were fully developed in all stages investigated except in the smallest size group. In the smallest catfish, the intercalarium and the interossicular ligaments were still missing and the tripus was not yet fully developed. Smallest juveniles revealed lowest auditory sensitivity and were unable to detect frequencies higher than 2 or 3 kHz; sensitivity increased in larger specimens by up to 40 dB, and frequency detection up to 6 kHz. In the size groups capable of perceiving frequencies up to 6 kHz, larger individuals had better hearing abilities at low frequencies (0.05–2 kHz), whereas smaller individuals showed better hearing at the highest frequencies (4–6 kHz).Conclusions/SignificanceOur data indicate that the ability of otophysine fish to detect sounds at low levels and high frequencies largely depends on the development of the Weberian apparatus. A significant increase in auditory sensitivity was observed as soon as all Weberian ossicles and interossicular ligaments are present and the chain for transmitting sounds from the swimbladder to the inner ear is complete. This contrasts with findings in another otophysine, the zebrafish, where no threshold changes have been observed.

Pigmentation disorders such as albinism are occasionally associated with hearing impairments in mammals. Therefore, we wanted to investigate whether such a phenomenon also exists in non-mammalian vertebrates. We measured the hearing abilities of normally pigmented and albinotic specimens of two catfish species, the European wels Silurus glanis (Siluridae) and the South American bronze catfish Corydoras aeneus (Callichthyidae). The non-invasive auditory evoked potential (AEP) recording technique was utilized to determine hearing thresholds at 10 frequencies from 0.05 to 5 kHz. Neither auditory sensitivity nor shape of AEP waveforms differed between normally pigmented and albinotic specimens at any frequency tested in both species. Silurus glanis and C. aeneus showed the best hearing between 0.3 and 1 kHz; the lowest thresholds were 78.4 dB at 0.5 kHz in S. glanis (pigmented), 75 dB at 1 kHz in S. glanis (albinotic), 77.6 dB at 0.5 kHz in C. aeneus (pigmented) and 76.9 dB at 1 kHz in C. aeneus (albinotic). This study indicates no association between albinism and hearing ability. Perhaps because of the lack of melanin in the fish inner ear, hearing in fishes is less likely to be affected by albinism than in mammals.

Hypostomus pantherinus was described by Kner in 1854 based on a single young specimen collected by Natterer with a type-locality just asserted as Guaporé. No robust scientific paper focusing the distribution, taxonomy and ecology of this species has been published since the description. Here, based on the analysis of the holotype, recently collected material, and on an extensive searching on ichthyological fish collections, H. pantherinus Kner 1854 is redescribed and its geographical distribution is established to be in the rio Iténez-Guaporé and the mouth of its Bolivian and Brazilian tributaries. Hypostomus pantherinus is distinguished from congeners mainly by villiform teeth, conspicuous black, sharp and widely spaced spots on head, and dark blotches on trunk and fins, keels along lateral series of plates, sharp crests on supraoccipital and on compound pterotic-supracleithrum, less than 25 spots on compound pterotic-supracleithrum, one blotch per interradial membrane along each row on dorsal fin, and by usually attaining about 160 mm SL in size.

Catfish possess a high diversity in accessory hearing structures, hearing sensitivities, and sound-generating mechanisms. Nevertheless, the ontogeny of their hearing and sound communication remains unknown. We investigated the development of Weberian ossicles and hearing sensitivity in the African bullhead catfish from postlarval stages up to adults and also examined the ontogenetic development of hearing and sound production in the yellow marbled squeaker catfish. In the smallest bullhead catfishes, the Weberian ossicles and interossicular ligaments are not fully developed. They are unable to detect sounds at low levels and high frequencies. In later stages of both species tested, hearing sensitivity increases with size at low frequencies and decreases at high frequencies. In the squeakers, the duration of stridulation sounds, sound pressure level, and pulse period increase, whereas the dominant frequency decreases with size. The most sensitive frequencies correlate with the dominant frequencies of stridulation sounds in all size groups, enabling all stages to detect communication sounds. Our studies show two different trends of changes in hearing sensitivity in catfishes: one prior to and the other after full development of the Weberian ossicles. Squeakers of all sizes are able to communicate acoustically. These results contrast with prior findings in teleosts. [Work supported by FWF.]

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