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... Science Frontiers ONLINE No. 89: Sep-Oct 1993 Issue Contents Other pages Home Page Science Frontiers Online All Issues This Issue Sourcebook Project Sourcebook Subjects Why do electric fish swim backwards?This is not a trick question like the one about the chicken crossing the road. To understand the answer to the electric fish puzzle, we must restrict the discussion to those fish with active electric sensing systems. This group includes electric eels, South American knife fish, and African elephant snout fish. All of these have evolved, in a remarkable instance of parallel evolution, the capability of generating pulses of electricity. These pulses (up to 1,000 per second) radiate through the surrounding water. Prey and other nearby objects distort these oscillating electric fields. Electroreceptors on the fish and a sophisticated data processing system convert the field distortions into an "image" of the surroundings. M. and S.J . Lannoo, of Ball State University, have watched the black ghost knife fish, which plies murky Amazon waters, approach likely prey tail first. Swimming backward using an elongated belly fin, the knife fish slowly cruises past its potential victim. If the electrical image looks appetizing, the knife fish grabs its dinner with a forward lunge as it appears in front of it. "The researchers suggest that the fish swims past objects in order to scan them with its electroreceptors. This is the only way the fish can identify prey because an electric sense cannot be focussed like an eye. But if the fish carried out its scan by swimming forwards, the prey would end up ...
Terms matched: 1 - Score: 24 - 15 May 2017 - URL: /sf089/sf089b05.htm
... . 43: Jan-Feb 1986 Issue Contents Other pages Home Page Science Frontiers Online All Issues This Issue Sourcebook Project Sourcebook Subjects Piscatorial Data Processing Mammals such as bats and porpoises have their acoustical navigational gear, while many fish have opted for electrical methods of scanning their surroundings. The short-range "radars" of these fish are marvelously sophisticated, considering the low limb fish occupy on the Tree of Life. In fact, the following introductory paragraph from an article in Nature sounds almost as if it came from a textbook on electronic signal processing. "Behavioural experiments have demonstrated that certain species of fish can perform remarkable analyses of the temporal structure of electrical signals. These animals produce an electrical signal within a species-specific frequency range via an electric organ, and they detect these signals by electroreceptors located throughout the body surface. In the context of one electrosensory behaviour, the jamming avoidance response (JAR), the fish Eigenmannia determines whether a neighbour's electric organ discharge (EOD), which is jamming its own signal, is higher or lower in frequency than its own. The fish then decreases or increases its frequency, respectively. To determine the sign of the frequency difference, the fish must detect the modulations in the amplitude and in the differential timing, or temporal disparity, of signals received by different regions of its body surface. The fish is able to shift its discharge frequency in the appropriate direction in at least 90% of all trials for temporal disparities as small as 400 ns." (Rose, Gary, and Heiligenberg, Walter; "Temporal Hyperacuity ...
Terms matched: 1 - Score: 15 - 15 May 2017 - URL: /sf043/sf043p12.htm
... In Data Processing The incredible sophistication of the brain and nervous system of fish using active electric sensing is evident in the South American fish Eigenmannia . This fish (different from the knife fish above) emits electric pulses at frequencies betwen 250 and 600 per second for electrolocation and communication. M. Kawasaki, at the University of Virginia, has investigated what happens when two of these fish operating on similar frequencies meet. Ordinarily, the fish would jam each other's sensory apparatus and "blind" each other. To circumvent this Eigenmannia has evolved a "jamming avoidance response," in which they both shift their pulse frequencies away from each other. To accomplish this, the fish must be able to detect time disparities between the two sets of signals less than 1 microsecond long. Their individual electroreceptors are not capable of handling such small time differences. Kawasaki has concluded that the jamming avoidance response can come only from highly sophisticated signal processing in the fish's central nervous system. (Kawasaki, Masashi; "Temporal Hyperacuity in the Gymnotiform Electric Fish Eigenmannia ," American Zoologist , 33:86, 1993.) Comment. Echo-locating bats and dolphins also possess sophisticated data processing apparatus for analyzing the echos they receive back from their prey and surroundings. It will be interesting to discover if evolution has come up with similar organic "components" for handling acoustic and electric signals. Further, we know that some insects have developed ears and sound generators to detect and jam hunting bats. Have the prey of electric fish evolved corresponding countermeasures? If not, why not? From ...
Terms matched: 1 - Score: 14 - 15 May 2017 - URL: /sf089/sf089b06.htm