Electrosensory basis for individual recognition in a weakly electric, mormyrid fish, Pollimyrus adspersus (Günther, 1866)

Pollimyrus adspersus discriminates the individually variable waveforms of Electric Organ Discharges (EODs) of conspecifics of only 150–250 μs duration. We examined: (1) the discrimination threshold for artificially generated EODs of similar waveform, (2) the mechanism of signal analysis (spectral vs temporal) present, by determining the discrimination between different waveforms of identical amplitude spectra, and (3) the threshold field intensity and reach of discrimination. The triphasic P. adspersus EOD waveform was artificially generated by superimposing two Gaussians, one wide, the second narrow, inverted, and of threefold amplitude. The natural variability among individual EOD waveforms was simulated by phase-shifting one Gaussian relative to the other. The symmetrical waveform where the peaks of the two Gaussians coincided was used as a reference (phase shift=0, rewarded stimulus S+). Results were: (1) in food-rewarded conditioning experiments, trained fish (N=7) detected a phase-shift in artificial EOD stimuli as low as 2 μs (N=2 fish), 6 μs (N=1) and 10 μs (N=1). (2) All fish tested (N=3) discriminated between artificial EODs of identical amplitude spectra but different waveforms (hence, different phase spectra), demonstrating a temporal mechanism of signal analysis. (3) The maximum reach of waveform discrimination was 130 cm at 4.9 μVp-p/cm and 100 μS/cm water conductivity (test signal generated at natural amplitude), that is, similar to the reach of EOD detection. Therefore, among the three kinds of electroreceptor organ present in mormyrids, we consider Knollenorgane the relevant sensory organs for EOD waveform discrimination.