How fish hear sounds: Animals and Sound in the Sea

Sound is important in the marine environment and fishes have developed special sensory adaptations for detecting and interpreting sounds. Two independent but related sensory systems used by fish to "hear" are the inner ear and the lateral line, which together make up the octavo-lateralis system.

One interesting question is why hearing evolved. While the first response one normally gives is that sound is used for communication, it is likely that hearing evolved well before animals could produce sounds to communicate (Fay and Popper, 2000). 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 has a special role to enable an animal to survive and thrive in its environment, and each provides a particular type of information. For example, chemical signals are long lasting (especially in air) but they are not very directional 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 the stimulus. Vision can give information about objects at greater distances, but it only works for things at which the animal is looking, and in the dark vision does not work very well. In contrast, sound provides animals with information about objects (predators and prey, for example) at very great distances and from 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 objects 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 hearing. Thus, it becomes clear that hearing, in evolving very early in the history of vertebrates, provided fish and their ancestors with the ability to learn a great deal about their environment that would be available from no other sense. It was only later, as fish started to make sounds, that hearing became useful for communication.

The bodies of fish have approximately the same density as water; therefore, sound waves cause the entire fish to move with the water and sound passes right through their bodies. Fish have bones in the inner ear, called otoliths, which are much denser than water and the rest of the fish's body. 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 and the otoliths stimulate cilia on the sensory hair cells that are located in the inner ear. This differential movement between the sensory cells and the otolith is interpreted as sound. Otoliths are made of calcium carbonate and their size and shape is highly variable among species.

This drawing shows the ear of the blue gourami. The ear has three otolithic organs and three semicircular canals. Abbreviations: A, H, P- anterior, horizontal, posterior semicircular canals; AN- auditory nerve to saccule; CC- crus commune; L- lagena; LM- lagena macula; LN- eighth nerve to lagena; LO -- lagenar otolith; S -- saccule; SM -- saccular macula; SO- saccular otolith; U- utricle; UO- utricular otolith. Copyright Dr. Arthur N. Popper, Laboratory of Aquatic Bioacoustics, University of Maryland. http://www.life.umd.edu/biology/popperlab/background/anatomy.htm.

These are pictures of the left and right ears of the blue antimora (Antimora rostrata), a deep-sea cod. In the picture of the right ear (on the right), you can clearly see the three otolith organs as white objects. The saccular otolith in this species is very large and heavy. Copyright Xiaohong Deng, Neuroscience and Cognitive Science Program, University of Maryland. http://www.life.umd.edu/biology/popperlab/research/deepsea.htm.

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 swim bladder is very different from that of seawater because it is filled with gas. The gas in the swim bladder can be easily compressed by sound pressure waves. The swim bladder pulses in reaction to passing sound waves and this is transmitted to the ear, and results in stimulation of the sensory cells. Species with no swim bladder (e.g. elasmobranchs), a much reduced swim bladder (many bottom-dwelling species, including flatfish), or well developed swim bladder that are not directly connected to the ears (such as the oyster toadfish) tend to have relatively poor auditory sensitivity. These fishes are called auditory or hearing generalists. Swim bladders in auditory generalists do not appear to aid significantly in hearing (Popper et al. 2003). Other fishes have swim bladders that are connected directly to the inner ear, which increases their hearing sensitivity. These fishes are called hearing specialists. In some fish species, such as carps, catfishes, and characins, the swim bladder is connected to their inner ear via a system of bones called the Weberian ossicles which are modified parts of the vertebral column (the backbone).

The clupeids (herring family) have a pair of elongated gas ducts that directly connect the swim bladder to the inner ear. One clupeid, the American shad (Alosa sapidissima) can detect ultrasonic frequencies up to 180 kHz (Mann et al. 1997). Most other hearing specialists can hear sounds above 3 kHz. Hearing generalists usually cannot hear sounds at frequencies above 1 kHz. 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/.

Sound passing through water also creates particle motion. Fishes have organs called neuromasts that detect the relative motion between themselves and the surrounding water. Like the inner ear, the neuromasts have hair cells that are stimulated during movement, sending nervous signals to the brain. Fishes can use the lateral line system to detect acoustic signals over a distance of one to two body lengths, and at low frequencies (lower than 160 to 200 Hz).

All fish have some neuromasts, and most bony fish and elasmobranches have lateral line systems that are a collection of neuromasts located on the skin or just beneath it in fluid-filled canals. The lateral line system is composed of small canals located on the head and sides of the body. These canals are open to the environment via a series of pores in the lateral line. Neuromast organs are located inside the canal, 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. These changes in flow cause the ciliary bundles of hair cells, embedded in the cupula, on the neuromasts to bend alerting the fish to nearby movement. Stimulation of the hair cells in lateral lines and inner ears involves the same basic mechanisms that all vertebrates, including humans use in hearing.

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 their eyesight, the coordinated movement is due to the lateral line system providing information about water flow created by each fish. As one fish moves in a certain direction, it creates a flow of water that triggers the fish next to it to follow the movement that has occurred. Each fish in the school is able to use this information to maintain group cohesion while swimming.