Hearing sensitivity and the anatomy of the auditory system in the clown knifefish, Notopterus chitala, were studied using behavioural techniques and scanning electron microscopy. While many structural features of the inner ear were similar to those found in other teleost species, the saccular endorgan in Notopterus was unusual in having three discrete epithelial regions. These regions could be distinguished from one another by regional differences in portions of the otolith overlaying them, by hair-cell orientation patterns, and by their position relative to the swimbladder. Notopterus was found to be able to detect sounds from 100 to 1000 Hz and best sensitivity occurred at 500 Hz, where the mean threshold was - 30 dB (re: 1 microbar). For frequencies from 300 to 700 Hz there was substantial variation in threshold values and indications of a bimodal distribution of thresholds. One hypothesis that may tie the ultrastructural and behavioural results together is that the ear in Notopterus is anatomically and functionally differentiated to mediate independent detection of two sound properties: changes in pressure and particle motion.

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