Unravelling the sound-induced motion of fish auditory structures: A non-invasive synchrotron radiation-based approach

Modern bony fishes display a high morphological diversity in their auditory structures. Yet, unraveling the sound-induced in situ interaction of the auditory structures has been challenging. Synchrotron radiation-based imaging techniques with high spatial and temporal resolutions now provide a powerful tool to study the functional morphology of such structures in a non-invasive way. We investigated two species representing two types of otophysic connections, namely, goldfish (Weberian apparatus) and Etroplus canarensis (swimbladder extensions contacting the inner ears) asking how different ancillary auditory structures affect otolith motion. Fishes were subjected to a 200 Hz pure tone in a 2L-standing wave tube-like setup while performing 2D radiography based on hard x-ray phase-contrast imaging. The shakers at each end of the tube were either driven 0° in-phase or 180° out-of-phase resulting in maximized sound pressure or sound-induced particle motion in the tube center, respectively. In both species, saccular otolith motion was more pronounced when the swimbladder walls oscillated under 0° in-phase condition and the motion patterns mainly matched the respective orientation patterns of ciliary bundles on the sensory epithelia. In future, we will quantify the motion patterns of fish auditory structures by applying “sound tomography” to cover the three-dimensional aspect of the moving structures.