No. 89: Sep-Oct 1993
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?