How do fish hear?
Sound is important in the marine environment and fishes have developed sensory mechanisms for detecting, localizing, and interpreting sounds. Two independent but related sensory systems used by fish to detect sound are the inner ear (the auditory system) and, to a lesser extent, the mechanosensory lateral line system, which is generally used to detect vibration and water flow.
One interesting question is why hearing evolved. While the sound is used for communication, it is likely that hearing evolved well before animals could produce sounds to communicate. Instead, it is likely that hearing evolved to help animals learn about their environment. In considering all of the sensory abilities an animal has, it becomes apparent that each provides a particular type of information and thus has special roles that enable an animal to survive and thrive in its environment. For example, chemical signals are long lasting (especially in air) but they do not provide very good directional information and work best when the receiving animal is very close to the chemical source. Similarly, touch is only useful when the animal is very close to a stimulus. Vision can give information about objects at greater distances, but it only works if an animal is looking directly at the object, and in low light environments or at night, vision does not work very well. In contrast, sound provides animals with information about objects at very great distances and in all directions. In other words, sound provides an animal with a three-dimensional "view" of its world, and this view is not hindered by currents, light levels, or even the presence of most objects (e.g., other organisms) in the environment.
Indeed, if you think about what senses give you the broadest picture of your environment, you will realize that it is hearing, and not vision. While vision is very important, when you walk into a dark room, you can sense a good deal about the room from the sounds you hear, even if you can see nothing. The same goes for fish and other animals -- they get a great deal of information about the "acoustic scene" from their sense of hearing. Thus, it becomes clear that hearing, in evolving very early in the history of vertebrates, provided fish with the ability to learn a great deal about their environment that would not be available from the other senses. It was only later, as fish evolved the ability to make sounds, that hearing became useful for communication.
The Inner Ear
The bodies of fish have approximately the same density as water, so sound passes right through their bodies, which move in concert with the traveling sound wave. Fish have bones in the inner ear, called otoliths, which are much denser than water and the fish's body. As a result, these ear bones move more slowly in response to sound waves than does the rest of the fish. The difference between the motion of the fish’s body and the otoliths bend or displace the cilia on the hair cells that are located in the inner ear. This differential movement between the sensory cells and the otolith is interpreted by the brain as sound. Otoliths are made of calcium carbonate and their size and shape is highly variable among species.
Sensitivity to sound differs among fish species. One factor affecting this is the proximity of the inner ear to the swim bladder. The density of the gas within the swim bladder is much lower than that of seawater and the fish’s body. As a result, the gas in the swim bladder can be easily compressed by sound pressure waves. The swim bladder changes in volume cyclically in reaction to passing sound waves. If the movements of the swim bladder wall are transmitted to the ear, this results in the stimulation of the hair cells of the inner ear. Species lacking a swim bladder (e.g. elasmobranchs), those that have a small or reduced swim bladder (many bottom-dwelling species, including flatfish), or those that have a swim bladder that is not in close proximity, or mechanically connected to the ears (e.g., oyster toadfish) tend to have relatively poor auditory sensitivity, and generally cannot hear sounds at frequencies above 1 kHz. In contrast, fishes with swim bladders that are in close proximity to the inner ear and/or are connected to the inner ear (e.g., they have an otophysic connection), have increased hearing sensitivity. Such fishes can hear up to 3 kHz or more.
Several types of otophysic connections are found among fishes. In the otophysan fishes (e.g., the carps, minnows, catfishes, and characins; the majority of freshwater fishes worldwide), the swim bladder is mechanically linked to their inner ears via a series of bones called the Weberian ossicles, which are modified bones of the vertebral column (the first few vertebrae of the the backbone). The Weberian ossicles are thought to facilitate sound transmission and generally improve hearing sensitivity. For instance, goldfish hear up to 4Khz with best hearing between 500-800Hz.
The clupeiform fishes (e.g., herrings, shads, sardines, anchovies) have a pair of elongated gas ducts ending in “bullae” that extend from the swim bladder and invade the skull coming in direct contact with the inner ear. The presence of a bubble of compressible gas in the bullae in close proximity to the inner ears enhances stimulation of the ear and thus increases hearing sensitivity. One clupeid, the American Shad can detect ultrasonic frequencies up to 180 kHz. Thus, it is possible that these fish can detect the ultrasonic clicks of dolphins.
For a more detailed look at the inner ears of fish and resources on bioacoustics visit Dr. Arthur N. Popper's Laboratory of Aquatic Bioacoustics,http://www.life.umd.edu/biology/popperlab/.
The Lateral Line System
Sound pressure waves passing through water also creates particle motion close to the source of the sound. Fishes have organs called neuromasts on the skin or in canals below the skin’s surface. These are composed of hair cells, like the inner ear. They detect the relative motion between themselves and the surrounding water. Fishes can use the lateral line system to detect acoustic signals at short range, over a distance of one to two body lengths, and at low frequencies (lower than 160 to 200 Hz).
Neuromasts are located on the skin (superficial neuromasts) or in canals (canal neuromasts) on both the head and body. In the lateral line canals, neuromast organs are located in linear series, with one organ between two adjacent canal pores. The pores link the outside environment to the fluid inside the lateral line canals where changes in the flow field (water movement) around the fish are detected. The cilia of all of the hair cells in a neuromast are embedded in a gelatinous cupula. Water movements within the canal, caused by water flows outside the canal, cause the stereociliary bundles of hair cells to bend alerting the fish to nearby movement.
Have you ever seen fish swimming in a school? All the fish in the school seem to move exactly in the same direction and as one large mass. As well as using vision, the coordinated movements of fishes within a school are the result of input of water flow information to the lateral line system. As one fish moves in a certain direction, it creates a flow of water that provides information to the fish next to it or behind it. Each fish in the school is able to use this information to accurately maintain their position within a rapidly moving school.