Hearing is the ability to detect sound.
The most obvious part of the human auditory system is the ear. This large protuberance on the outside of the head is just one part of a complicated and amazing structure.
The outer ear serves to capture sound and channel it in towards the auditory canal – the small tube that extends into the head. This canal transmits sound waves through to the tympanic membrane (eardrum), which causes three very small bones to vibrate – the hammer (malleus), the anvil (incus) and the stirrup (stapes). These intensify and transmit the sound waves into the cochlea, which is filled with fluid. The vibrations trigger specialised hair cells to respond. These cells transmit the signal via nerves to the brain and we ‘hear’ noise.
Part of the inner ear – the labyrinth – is devoted to balance rather than hearing. The labyrinth consists of three canals at different orientations (x, y and z axes). These canals are also filled with fluid, as in the cochlea, and are lined with hair cells. In these canals, movement of the head will cause the fluid to move, which is registered by the hair cells. Whenever the head moves, the hair cells fire, producing an accelerometer that detects motion and movement.
Hearing in fish
Fish rely on a structure much like the inner ear of humans to detect sound.
Sound is a pressure wave. In order to be transmitted through water, the wave must have quite a lot of energy – more so than to travel through air. Coupled with the fact that there is no air/water boundary to travel through, this means there is no need for the complicated structures of the outer and middle ear.
Fish only have an equivalent of the inner ear structure. Unlike the cochlea, it is not curved in a spiral but is a simple chamber structure (the saccule). Like the inner ear of humans, this cavity contains fluid and hair cells, but it also contains a bony structure called an otolith. This very small (sometimes less than 100 microns) ‘pebble’ floats on the tops of the hair cells.
When sound moves through the fish, the otolith remains stationary while the fluid around it moves. This relative motion is detected by the hair cells in a similar manner as in human ears. This provides not only a sense of hearing but also of gravity and balance – similar to the detection of motion in the human labyrinth.
Evidence suggests that the human inner ear has evolved from a structure very closely related to the saccule and otolith found in modern fish species. Studying the otolith has led scientists to understand the human sense of balance better and may be the justification for some drug tests to be conducted in fish rather than in humans.
Hearing in shrimps
Scientists have been able to demonstrate that very small animals such as shrimp and larval crabs are also able to respond to sound. This suggests that they are capable of hearing.
Recent research suggests that an organ known as the statocyst acts in a very similar manner to the saccule and otolith of a fish. First described as a balance organ, there is evidence to suggest that the statocyst is also capable of detecting sound as well as gravity and motion. This may be the basis of hearing in these very small invertebrates
There is still a lot to be understood about animal hearing, but it is certainly an area of science that is pushing the boundaries of our understanding.