Hearing in fish
Fish ears are embedded in the bones of the skull, one on each side of the head.
They consist of one saccule, one utricle and three semicircular canals.
Because of their location they are called inner ears.
Sound waves transmitted by water reach the fish's body, spreading through the skull bones to the saccule. The sound vibrations are transmitted to the fluid that fills the saccule and, as a result, sensory cells located in the saculum wall are stimulated. The nerve impulses generated in these cells are carried by the auditory nerve to the brain, which interprets them as sounds.
The swim bladder also vibrates in response to sounds and can transfer these vibrations to the saccule. Some fish have a series of strategically arranged bones that drive the vibrations of the swimming bladder to the inner ear.
Fish's ears pick up only high frequency sounds; Low frequency sounds are perceived by a set of sensors located on the sideline.
The lateral line consists of a series of channel-interconnected pores arranged under the skin on the sides of the body. Within these channels there are clusters of ciliated sensory cells that respond to water movement. When water moves within the channels, the sensory hairs bend generating nerve impulses that are sent to the central nervous system.
Through the information received on the sideline, a fish detects water currents and small vibrations related to the presence of obstacles and moving bodies such as predators, prey, etc.
The inner ear of amphibians is similar to that of fish. Amphibians, however, also have middle ear - an air-filled tube communicating the saccule wall with the body surface. The outer opening of the middle ear is provided by the tympanic membrane. Inside the middle ear of the amphibians is a small bone whose ends are attached to the tympanic membrane and the saccule wall.
Sounds cause the tympanic membrane to vibrate, and so the middle ear ossicle also vibrates, transmitting the vibration to the saccule wall and the fluid within it. This stimulates the hair sensory cells to generate nerve impulses that, reaching the auditory center of the brain, produce the sensation of sounds.