No. 89: Sep-Oct 1993
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.)