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An Anecdote
In college, I had a summer job on a trout lake in Colorado. Rainbow trout over about 12" were generally blind (they had opaque white irises) because of the presence of an eye parasite. Yet they continued to grow well and maintain good condition, feeding on the principle food source which was a inch-long, freshwater shrimp (Mysis). Moreover, anglers regularly caught these large trout on wet flies. How could blind trout feed so effectively? The answer must be the lateral line sense. Certainly, they could not smell or hear an artificial fly. |
Because water is so dense, it propagates pressure waves very well. Fish have evolved a lateral line sense that detects and interprets pressure waves. This sense allows fish to detect very low frequency vibrations such as those generated by a tail beat or nearby relative movement. To understand the detection of relative movement by pressure wave perception, move your hand slowly toward the top of your desk without quite touching it. If the air was dense enough, and your hand sensitive enough, you could feel the higher pressure above the desk relative to the lower pressure at the edge where the air could flow away. This is how the lateral line works in fish and why it has been termed "distant touch". For the lateral line sense to work, either the fish, the object it is detecting, or both must be moving. Experimental evidence suggests that the lateral line sense can be quite precise in detecting relatively small objects, but primarily at close range. Under poor visibility conditions, which are quite common underwater, the lateral line sense is quite valuable in feeding (see sidebar).
The receptor for the lateral line system is the neuromast. This small structure consists of a flexible, jellylike cupula resting on a mass of sensory cells. The sensory cells have hairs embedded in the cupula. A pressure wave causes the cupula to move and the hairs transmit the direction and extent of movement to the sensory cells which then generate nerve impulses carrying this information to the brain where it is interpreted. Neuromasts may be located on the skin, in pits, or in canals that run in the superficial bone of the head and through the scales along the side of the fish (hence: lateral line sense).
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Water is an excellent conductor of electricity. It is not surprising then that many fish have the ability to sense low levels of electrical current such as those generated by muscle contractions and movement through the earth's magnetic field. Electrical reception in many fish is accomplished by pit organs in the skin. Sharks have a highly developed electrical detection system consisting of canals of electrical conducting gel joining many sensory pits, termed ampullae of Lorenzini. Interestingly, electrical reception is more widespread in primitive fishes like lampreys and sharks, than in teleosts.
A few teleosts have taken weak electrical sense a step further. Members of the family Gymnotidae (knifefish) and Morymidae (elephantfish) not only receive electrical signals but transmit them, as well. Elephant fish typically live in turbid environments where sight is of limited value and feed on small worms buried in the substrate. Elephant fish generate a weak electrical field from electric organs located in the caudal region. This field has a potential of only about 10 volts, far less than the 100's of volts that strongly electric fishes (eels and catfish) use to stun their prey. The elephant fish transmits and receives its own signal and is able to tell when a hidden body, with a different conductivity than water or mud, changes the shape of the field. In this way, it uses the electrical field as a probe for food. It apparently takes a lot of neural processing to execute this type of detection because morymids have the largest relative brain weight of any fish...even exceeding humans! The bulk of this brain mass is in the cerebellum.
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