Abstract
Regarding the basics of ear structure-function relationships in fish, the actual motion of the solid otolith relative to the underlying sensory epithelium has rarely been investigated. Otolith motion has been characterized based on a few experimental studies and on approaches using mathematical modeling, which have yielded partially conflicting results. Those studies either predicted a simple back-and-forth motion of the otolith or a shape-dependent, more complex motion. Our study was designed to develop and test a new set-up to generate experimental data on fish otolith motion in-situ. Investigating the basic parameters of otolith motion requires an approach with high spatial and temporal resolution. We therefore used hard X-ray phase contrast imaging (XPCI). We compared two anatomically well-studied cichlid species, Steatocranus tinanti and Etroplus maculatus, which, among other features, differ in the 3D shape of their otoliths. In a water-filled tank, we presented a pure tone of 200 Hz to 1) isolated otoliths embedded in agarose serving as a simple model or 2) to a fish (otoliths in-situ). Our new set-up successfully visualized the motion of otoliths in-situ and therefore paves the way for future studies evaluating the principles of otolith motion.
Highlights
In teleost fish, otoliths are solid calcium carbonate biominerals overlying the respective sensory epithelium in the inner ear[1]
Otoliths vary greatly in mass and shape among teleost species[2,8]. This raises the question of how these characters affect the relative motion between otolith and sensory epithelium and the stimulation of sensory hair cells[9,10]
The otolith–sensory epithelium system acts as an accelerometer because particle acceleration is the relevant stimulus for the otolith organs in the fish ear[6,17]
Summary
Otoliths are solid calcium carbonate biominerals overlying the respective sensory epithelium in the inner ear[1]. Otoliths, especially those of the saccule, show a species-specific shape[2] and are about three times denser If the fish moves in a sound field, the denser otolith lags behind relative to the movement of the sensory epithelium[1]. First experimental studies[18] indicated a simple forward and backward motion between the otolith and sensory epithelium. This was adopted for initial mathematical modelling[19,20]. Recent studies on the biting and chewing mouthparts of living cockroach Periplaneta americana[27] or flying blowfly[28] using synchrotron radiation imaging techniques stimulated our idea that movement of otoliths provoked by a sound stimulus could be visualized in-situ as well
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