The effect of vaterite deposition on sound reception, otolith morphology, and inner ear sensory epithelia in hatchery-reared Chinook salmon (Oncorhynchus tshawytscha)

The inner ear of teleost fishes contains three calcareous structures (otoliths) that are part of the organs for hearing and balance. The largest of these structures, the sagitta, is usually composed of calcium carbonate crystals in the form of aragonite, but the calcium carbonate also occurs less frequently in a clear crystallized form called vaterite. We investigated the functional consequences of otolith crystal structure on hearing in juvenile Chinook salmon (Oncorhynchus tshawytscha) using the auditory brainstem response technique. A significant loss of sensitivity (2.5-6.5 dB) occurred within the primary hearing range (100-300 Hz) among salmon that had at least one vateritic sagitta. Auditory thresholds were not significantly different in fish with one vs. two vaterite sagittae. Crystallized sagittae were significantly larger and less dense than their aragonite counterparts. Saccular epithelium shape and hair bundle orientation patterns did not differ between saccules with different crystal types. There was, however, a propensity for the saccular epithelia from vateritic sagittae to have fewer sensory hair bundles. We conclude that significant hearing loss was associated with the occurrence of vateritic sagittae and suggest that hearing loss is caused by the lower density of the vaterite otoliths.