While fish lack the visible, external ears we are familiar with, they possess an internal auditory system for perceiving sound in their aquatic environment. Their hearing method differs from terrestrial animals, as water transmits sound directly to these hidden sensory structures.
A Look at a Fish’s Internal Ears
Hidden within a fish’s skull, just behind the eyes, are its inner ears. Unlike mammals, fish lack both an outer ear to capture sound and a middle ear to amplify it. The primary components for hearing are three dense, bone-like structures called otoliths, or “ear stones.” These otoliths are housed in fluid-filled sacs near a patch of sensory cells called hair cells, which have tiny projections that are stimulated by the otoliths’ movement.
How Fish Perceive Sound
Sound travels approximately 4.8 times faster through water than through air, and its vibrations pass with little resistance through a fish’s body, which has a density similar to water. This allows sound waves to travel directly to its inner ears. Because the otoliths are about three times denser than the fish’s body, they lag behind the vibrations passing through the softer tissues.
This difference in movement causes the otoliths to shift and bend the delicate hair cells they rest upon. This bending action converts the physical vibrations into nerve signals that are sent to the brain for interpretation as sound. Some species, known as hearing specialists, have enhanced abilities. In fish like carp and catfish, a series of tiny bones called Weberian ossicles connects the gas-filled swim bladder to the inner ear, using the bladder as a resonating chamber to amplify sound vibrations.
The Lateral Line System
In addition to their inner ears, fish possess another sensory system for perceiving their environment: the lateral line. This system is visible as a faint line of pores running along each side of the fish’s body, from head to tail. These pores lead to a canal just beneath the skin that is filled with water and contains a series of sensory organs called neuromasts.
Each neuromast contains a cluster of hair cells, similar to those in the inner ear, but these are designed to detect water displacement and pressure gradients. This system allows a fish to “feel” the movement of water around it, helping it to detect nearby predators, find prey, and navigate in tight formations within a school. While it senses low-frequency vibrations, the lateral line is distinct from the auditory system, providing a sense of touch at a distance.
Why Hearing is Crucial for Fish
A fish’s sense of hearing is important for its survival, especially in dark or murky water where vision is limited. It provides information from all directions, allowing fish to detect the low-frequency sounds of approaching predators long before they are visible.
Hearing is also used for finding food, as many fish can locate prey by listening for the sounds they produce, such as the movements of other small fish or invertebrates. Sound also plays a part in communication. Many species produce distinct sounds, like grunts or thumps, to attract mates during courtship rituals or to warn away rivals from their territory.