Physiology of Finfish and Shellfish (2+1)

3.3.3.1. Hearing

The nature of sound transmission in water had an important influence on the evolution of hearing in fishes. Because of its greater density water is a much more efficient conductor of sound pressure waves than air. In water, sound travels more than 4.8 times the speed of sound through air.

Sound perception and balance are intimately associated senses in a fish.
The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. A fish's soft body tissue has about the same acoustic density as water. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see the fisherman. Compared to humans, however, the range of sound frequencies heard by fishes is greatly restricted. It is thought that many fishes communicate with each other in a crude way by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways. Sound vibrations are detected both by the teleosts and elasmobranchs through the inner ear and or the lateral line.

The ears of a bony fish function in equilibrium, detecting acceleration, and hearing. There are no external openings to the ears. Sound waves travel through soft tissue to the ears. The organs of equilibrium and hearing i.e., the inner ear are divided into pars superior and pars inferior. The pars superior is composed of the semicircular canals and their ampullae (fluid inertia sensing chambers) and a sac like vesicle, the utriculus. The pars inferior, the structure of sound reception proper, consists of 2 more vesicles, the sacculus and the lagena (Fig.). The pars inferior of the inner ear lagena and sacculus with their otoliths, astericus and sagitta is the seat of sound reception.

The physiological mechanism of discrimination of different sound frequencies is not known but it is seated mainly in the utriculus. Among the minnows, catfishes and suckers, the inner ear is connected uniquely to the gas bladder through a chain of bones, the weberian ossicles.

There is great variation in hearing sensitivity, bandwidth, and upper frequency limit among bony fish species. The hearing range of the cod Gadus morhua is about 2 to 500 Hz, with peak sensitivity near 20 Hz - probably typical for most bony fish species that lack the adaptations described below.

In some bony fish species, the swim bladder is associated with adaptations for enhanced sound reception at higher frequencies. In some, the swim bladder lies against the ear and acts as an amplifier to enhance sound detection. In other species, such as goldfish (Carassius auratus), a series of small bones connects the swim bladder to the ear. The hearing range of the goldfish is about 5 to 2,000 Hz - with peak sensitivity near 400 Hz. Recently researchers have discovered that the American shad (Alosa sapidissima) and certain related species can detect sounds from 200 to 180,000 Hz. The researchers theorize that this ultrasonic hearing is an adaptation for avoiding echolocating dolphins, which typically produce clicks at about 100,000 Hz.