Fish – touch and hearing all in one

There is a lot going on underwater, even if we humans are only rarely able to hear it – and in any case, we are able to perceive only a portion of it, even if we are under water ourselves. Fish produce several different sounds to communicate with their kind: they make grinding noises, blow bubbles in the water, and generate sounds when air is released from their swim bladders. These calls can be essential for survival, especially for schooling fish – but how do fish hear?

They have small, liquid-filled tubes behind their eyes which function in a similar way to the inner ear of land vertebrates: there are tiny otoliths made of calcium carbonate suspended in this fluid, and incoming sound waves cause them to vibrate. The vibrations stimulate fine sensory cells that in turn relay this information to the brain. Hearing quality differs between species, however; a lot of aquarium fish have only rudimentary sound perception, whereas in some breeds, the inner ear is connected to the swim bladder so that vibrations received there are transmitted to the auditory organs. 

On land, humans know what direction a sound is coming from because the noise reaches the ear angled more closely towards the source a little sooner than the other ear. Water, however, presents very different challenges to sense organs than air. “For example, the water near surf is murky and churned up – so fish cannot see clearly and have to rely on acoustic signals to find their way about,” explains Stefan Launer, Sonova’s resident expert in audiological research and Senior Vice President Audiology and Health Innovation. They have made use of highly specialized mechanisms to adapt to such challenges. “Generally speaking, animals have far more specialized hearing than humans,” says Launer. 

When submerged, a “normal” ear is not able to locate the precise direction of a noise; sound waves travel four times faster in water than in air, and the difference in time the sound takes to reach the ears is too small to locate its source. This is why fish have developed a lateral line system, an additional, highly specialized organ with which they are able to perceive incoming pressure waves from their surroundings. This organ is often visible from outside as fine stripes running lengthways down the side of the fish’s body under the skin; it consists of a mucus-filled tube connected to the outside world via narrow pores. This highly sensitive remote-sensing array enables fish to perceive shockwaves, currents, and sounds in the water – and also determine where these signals are coming from. “What’s more, they use the lateral line system to maintain their stability,” explains Launer. “The human balance system is derived from this to a certain extent.”

As with the fish’s ear, this second, fluid-filled tube contains fine hair cells that react to pressure waves: a fish swimming normally pushes a mass of water in front of it, but if this encounters an obstacle (such as a mussel, some prey, or a hostile animal), the pressure wave is bounced back to the lateral line sensor. The power and direction of the vibrations here will indicate to the fish how far away the obstacle is (along with its shape and size) far more precisely than it could hear with its “ear”, as the dense array of sensory hair cells allows for far sharper resolution in timing the stimulus. This ability is also crucially important in schools, enabling fish to avoid collisions. 

Audiological researchers cannot copy directly from the animal world as water and air – the habitats of fish and humans respectively – are so different, but fish are nonetheless proving useful to pharmacologists looking for ways to treat hearing loss, says Launer: “The zebra fish is very popular, for example – it’s a simple creature and easy to examine, which makes it possible for us to test active ingredients.”