Fish sense the world around them in many ways. While most fish possess sight, hearing, taste, and smell senses, all of which we can easily relate to, they also have sensory means for detecting stimuli, such as water particle displacement, and in some fish, electrical currents. These later sensory perceptions, take advantage of the physical and chemical properties of water, and work in conjunction with the more conventional sight, hearing, taste, and smell sensory modes. Let’s explore them!
The lateral line – In an effort to help you visualize the structures that make up a lateral line, picture the lateral line as being a river. On a fish, this river is a lateral line canal. The lateral line canal is filled with endolymph; the same fluid that’s in our inner ear. Below the river, running parallel to it is ground water. This, on a fish is nerves. At various locations along the river there are springs connecting the ground water to surface water. That point of connection is the spring heads, which on fish are called neuromasts. Neuromasts connect the nerves to the lateral line canal, and that connection through the neuromasts allow fish to sense mechanical changes in water.
Each neuromast is comprised of hair cells. Like all hair cells, those of the lateral line are contained in hair bundles. The hair bundles grow longer from one edge of the bundle to the other. These hair bundles are covered by a flexible and jellylike cupula (essentially a cup) that connects the bundles with canal fluid, or in some cases with the water surrounding the fish. The cupula are sensitive to movements of the watery endolymph fluid through the canal. Pressure changes bend the cupula, and in turn bend the hair cells inside.
There are actually two main varieties of neuromasts located in fish, canal neuromasts and superficial or freestanding neuromasts. Canal neuromasts are located along the lateral lines in fluid filled canals (the river), just under the skin, which usually open to the environment through a series of pores. You can see these pores if you look carefully at the scales along the lateral line. Superficial neuromasts are located externally on the surface of the body (around the head, trunk, and tail). These neuromasts work the same way as canal neuromasts, except instead of being in contact with endolymph fluid, they are in contact with the external water environment.
As fish swim they produce a flow field around their bodies. The lateral line system is able to detect distortions in this self-generated flow field due to the presence of objects. The distortions cause pressure changes which is received by the neuromasts. Pressure change information received by neuromasts is passed along to the brain. By integrating the information of many neuromasts, fish can detect different things, like movement, vibration, and pressure gradients in the water around it. This plays an essential role in orientation, predatory behavior, defense, and social schooling.
For instance, the lateral line system is necessary to detect vibrations made by prey, and to orient fish towards the source to begin predatory action. Surface feeding forage fish can detect the surface waves caused by struggling insects that have fallen into the water with their lateral line. They can also determine the direction and the distance to the surface wave source. Midwater fish use the lateral line for detection of moving objects. Not only can they detect the direction of movement, but they can also detect its speed, size and shape.
Electroreception – As mentioned earlier some fish have a sensory means for detecting electrical currents. Electroreception facilitates the detection of prey, objects, and is used by some species as a means of social communication. The electroreception ability is enabled by Ampullae of Lorenzini. The Ampullae of Lorenzini are made up of a large pore, filled with a jelly-like substance. Minute sensory cells line the walls of each pore. These sense even faint electrical impulses from the environment and transmit the message to the sensory nerve at the base of each pore. This nerve sends messages directly to the brain which in turn inform the fish of gravitational sensations or nearby prey. The Ampullae of Lorenzini are also able to detect changes in water pressure and to some degree, temperature.
Electroreception is most notable in cartilaginous fish (sharks and rays); however, they are also reported to be found in other fish such as sturgeon. The electroreception ability present in sharks is a significant survival tool as it allows them to seek out and find prey, even when they’re hiding in structure or in the sand, just from sensing the natural electrical signals emitted by all animals.
Electroreception occurs almost exclusively in aquatic animals, but it’s not restricted to fish. Most amphibians are electroreceptive during their aquatic larval phase, and many species continue to be electroreceptive as adults.
Albert, J.S. and W.G.R. Crampton. 2005. The Physiology of Fishes, Chapter 12: Electroreception and Electrogenesis, pgs 431-472.
Bleckmann, H. and R. Zelick. 2009. Lateral line system of fish, Integrative Zoology 4:13-25.
Kasumyan, A.O. 2003. The Lateral Line in Fish: Structure, Function, and Role in Behavior, Journal of Ichthyology, 43(2) 175–213.
Schwartz, E., Analysis of Surface-Wave Perception in Some Teleosts, Lateral Line Detectors, Cahn, P., Ed., Bloomington: Indiana Univ., 1967, pp. 123–134.
Moyle, P.B. and J.J. Check, Jr. 2000. Fishes: An Introduction to Ichthyology, 4th ed., Chapter 10: Sensory Perception, pgs 151-156.
Reviewed by Josh Patterson, UF Fisheries and Aquatic Sciences.