Acoustically induced streaming flows near a model cod otolith and their potential implications for fish hearing

Area of Science:

• * Bioacoustics
• * Fish sensory biology
• * Fluid dynamics

Background:

• * Fish ears, specifically otoliths, are crucial for detecting underwater sound and localizing sound sources.
• * The Atlantic cod (Gadus morhua) ear contains three otoliths and detects sounds from 30 to 500 Hz.
• * The precise mechanisms by which fish ears process acoustic information remain incompletely understood.

Purpose of the Study:

• * To investigate if steady streaming fluid flows around an Atlantic cod otolith model contain acoustically relevant information.
• * To determine if sensory hair cells could detect these fluid flows.
• * To explore the potential role of these flows in fish hearing capabilities.

Main Methods:

• * A 350% scale model of an Atlantic cod otolith was immersed in a viscous fluid.
• * The otolith model was oscillated sinusoidally at frequencies of 8-24 Hz (simulating 280-830 Hz in vivo).
• * Phase-locked particle pathline visualizations were used to analyze fluid velocity, vorticity, and rate of strain.

Main Results:

• * The study identified acoustically relevant information within the steady streaming fluid flows.
• * Calculated displacements from these flows were likely insufficient to account for the hearing thresholds of Atlantic cod.
• * The fluid dynamics observed may offer insights into ultrasound detection mechanisms in certain fish species.

Conclusions:

• * Steady streaming flows near fish otoliths contain acoustic information detectable by sensory hair cells.
• * The magnitude of these flow-induced displacements may not fully explain the hearing sensitivity of Atlantic cod near their hearing threshold.
• * The findings suggest a potential mechanism for ultrasound detection in some fish, warranting further investigation.