Auditory chain reaction: Effects of sound pressure and particle motion on auditory structures in fishes

Abstract

Despite the diversity in fish auditory structures, it remains elusive how otolith morphology and swim bladder-inner ear (= otophysic) connections affect otolith motion and inner ear stimulation. A recent study visualized sound-induced otolith motion; but tank acoustics revealed a complex mixture of sound pressure and particle motion. To separate sound pressure and sound-induced particle motion, we constructed a transparent standing wave tubelike tank equipped with an inertial shaker at each end while using X-ray phase contrast imaging. Driving the shakers in phase resulted in maximised sound pressure at the tank centre, whereas particle motion was maximised when shakers were driven out of phase (180˚). We studied the effects of two types of otophysic connections-i.e. the Weberian apparatus (Carassius auratus) and anterior swim bladder extensions contacting the inner ears (Etroplus canarensis)-on otolith motion when fish were subjected to a 200 Hz stimulus. Saccular otolith motion was more pronounced when the swim bladder walls oscillated under the maximised sound pressure condition. The otolith motion patterns mainly matched the orientation patterns of ciliary bundles on the sensory epithelia. Our setup enabled the characterization of the interplay between the auditory structures and provided first experimental evidence of how different types of otophysic connections affect otolith motion.

Revealing sound-induced motion patterns in fish hearing structures in 4D: a standing wave tube-like setup designed for high-resolution time-resolved tomography

“…We focused on the otophysan saccular otoliths for two reasons. First, this structure showed a rotational movement induced by sound pressure in a previous study which could be easily identified even in radiographs (Schulz-Mirbach et al, 2020) and was also in line with the motion pattern hypothesized by experts in the last century (von Frisch, 1938, de Burlet, 1929, Wohlfahrt, 1932. Second, the 0° in-phase and 180° out-of-phase conditions in the previous study clearly differed from each other by the evoked motion pattern of this otolith with the rotational motion only identifiable under the 0° in-phase condition.…”

“…In the case of the two-shaker setup, the distinct rotational motion of the glass catfish sagittae is suggestive of a quite effective separation of sound pressure and sound-induced particle motion in the tube at the tested frequencies (350 Hz,450 Hz). This rotational motion of the sagittae was expected based on our previous study in goldfish at the ESRF using a standing wave tube-like tank (Schulz-Mirbach et al, 2020) and according to former theoretical considerations (von Frisch, 1938, Freiin von Boutteville, 1935, de Burlet, 1929. In that study on goldfish, the 0° in-phase condition and thus the sound pressure maximum in the center of the tank induced the oscillation of the swim bladder walls which, in turn, resulted in the motion of the coupled Weberian ossicles and finally in a rotation of the saccular otoliths.…”

“…1 µPa) might influence the motion patterns of the auditory structures. A previous study using 2D radiography tested a series of different SPLs while subjecting fish to a 200 Hz stimulus ( Schulz-Mirbach et al, 2020 ). The amplitudes of the moving saccular otoliths and Weberian ossicles indicated a decrease in motion with decreasing SPL values while the motion patterns themselves were the same regardless of the SPL tested.…”

“…In previous studies (Schulz-Mirbach et al, 2018, 2020, a stimulus frequency of 200 Hz resulted in a successful observation of the 2D in-situ motion of otoliths applying hard X-ray phase contrast imaging. Accordingly, we tested stimuli of 100 and 200 Hz (one-shaker setup).…”

“…Nowadays, synchrotron radiation-based techniques provide powerful approaches to perform imaging of internal structures at high spatio-temporal resolution non-invasively ( Mokso et al, 2015 ; Rack et al, 2010 ; Walker et al, 2014 ). Recent studies using hard X-ray phase contrast imaging at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) provided first insights into the sound-induced motion of fish auditory structures ( Schulz-Mirbach et al, 2018 , 2020 ). In these radiographic experiments using a standing wave tube-like tank, the motion of the swim bladder and the otoliths in the inner ears of goldfish ( Carassius auratus , Otophysa) and the cichlid Etroplus canarensis was successfully visualized and motion patterns could be described qualitatively.…”