The Role of Gas-Holding Structures in Fish Hearing: An Acoustically Evoked Potentials Approach

Abstract

Fish varies greatly in their sensitivity to underwater acoustic signals. Over the past century, scientists have designed many methods to investigate auditory capacity of fish. Behavioral training paradigms (with either punishment or reward as incentives to the tested subjects) and various electrophysiological methods have been used to obtain audiograms, i.e., the auditory sensitivity curves of fish. Each method has its practical limitations: behavioral methods take long training time while the electrophysiological methods are invasive. This chapter first describes a novel, non-invasive electrophysiological recording method, i.e., the auditory brainstem response (ABR), which was first developed in my laboratory with the purpose of obtaining audiograms within short time for a comprehensive study on fish hearing ability. The setup of the system as well as technical details involved in using the ABR system are discussed in-depth with inputs from various users over the past 5 years since the debut of the ABR method. The second part of this chapter is focused on understanding how gas-holding structures (e.g., gasbladder, suprabranchial chamber, otic gasbladder) play a role in overall hearing ability of fish. The gas-holding structure, e.g., gasbladder, is generally regarded as an underwater resonant bubble. It is assumed that the resonant frequency created by the passing sound wave to gas-holding structure could aid in overall hearing ability of fish. This chapter reviews the work carried out by the ABR method in the author’s laboratory investigating how gas-holding structure plays a role in hearing ability of fish. The audiograms are obtained with the use of the ABR method before and after gas removal from: 1) gasbladder of goldfish, toadfish, and blue gourami; 2) suprabranchial chambers of blue gourami, kissing gourami, dwarf gourami; and 3) otic gasbladder of a mormyrid weakly electric fish. The general conclusion is that those fish (e.g., goldfish, gourami, mormyrid) that have direct coupling between gas-holding structure and the inner ear benefits greatly from such a coupling to have better hearing ability. While the gasbladder of toadfish, albeit large in size and is placed in close proximity to the inner ear does not contribute to the overall hearing ability. The finding is in general agreement with how a fish is either classified as a hearing specialist (with coupling device between ear and gas-holding structure) or a hearing generalist (without such a coupling device). This study also reveals the possible existence of different populations of sensory hair cells in specialist species.