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No. 89: Sep-Oct 1993

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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 at its tail. The fish must swim backwards to be in a position to eat the food."

(Day, Stephen; "Why Do Electric Fish Swim Backwards?" New Scientist, p. 13, April 17, 1993.)

From Science Frontiers #89, SEP-OCT 1993. 1993-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987