A standing wave tube-like setup designed for tomographic imaging of the sound-induced motion patterns in fish hearing structures.

Abstract

Background: Modern bony fishes exhibit a considerable variation in the morphology of their hearing structures, and the morphological composition of these has been studied for centuries. However, the precise interaction and contribution of individual structures to hearing remains unclear in many species. Measurements of their motion in situ are challenging and pose the risk of damage or altering results through invasive intervention. Recent developments in time-resolved synchrotron-radiation-based tomography have opened up possibilities for non-destructive quantification of the micron-level motion patterns of the auditory system. However, the strict requirements for miniaturised acoustic environments compatible with tomographic imaging hinder the production of ideal and well-characterised sound fields. To address this issue, we present the design of a miniature standing wave tube-like setup equipped with the necessary sensors to tune and monitor the sound field in situ, thereby generating and recording the desired acoustic conditions during experiments.

Results: By incorporating hydrophones into the tube of the standing-wave setup, we achieved a precise adjustment of the acoustic field within the tube at various frequencies. We generated and measured frequencies up to 2 kHz that fall within the relevant hearing spectrum of otophysan fish. The setup allows for the determination and adjustment of sound pressure levels during tomographic measurements, and phases can be regulated to achieve distinct differences between maximum (0° phase shift) and minimum (180° phase shift) sound pressure at the centre of the test tube.

Conclusions: We are able to visualise the motion of the peripheral auditory structures from the swim bladder to the Weberian ossicles and the otoliths (sagittae) in terms of maximum and minimum (sound-induced particle motion) sound pressure, respectively. This methodology has been successfully applied to various otophysan fish species and is demonstrated in the example of a glass catfish (Kryptopterus vitreolus). Our setup not only enhances our understanding of basic principles in fish bioacoustics but also sets a new standard for non-invasive, high-resolution imaging techniques in the field of aquatic sensory biology.