ELECTROSENSORY THRESHOLDS IN LARVAE OF THE WEAKLY ELECTRIC FISH POLLIMYRUS-ISIDORI (MORMYRIDAE, TELEOSTEI) DURING ONTOGENY

Electrosensory thresholds and tuning were determined from behavioural studies in larvae of Pollimyrus isidori using the stop response of the ir electric organ discharge to weak electrical stimuli. Two age groups were studied: (1) 10- to 15-day-old larvae in which the electric orga n discharge (EOD), produced by a distinct larval electric organ, had j ust stabilized; (2) 54- to 60-day-old larvae, just before the advent o f the adult EOD (an adult electric organ functionally replaces that of the larva between about 60 and 80 days). Three stimulus pulse wavefor ms were used: (1) single-cycle, bipolar sine-wave pulses; (2) single-c ycle, monopolar sine-wave pulses and (3) monopolar square-wave pulses. The younger larvae were exceedingly sensitive to weak electrical stim uli, down to the 10 mu V(p-p)cm(-1) range. Stimulus pulse duration had a significant effect on threshold for all three pulse waveforms, but the shapes of the tuning curves were quite different. Thresholds at th e 'best' pulse duration were lower and the tuning sharper (with a V-sh aped curve) with monopolar sine-wave pulses than with bipolar sine-wav e pulses. The 'best' pulse duration was 1 ms for both sine-wave pulses , corresponding well to the spectral peak amplitude of larval EODs (96 4+/-22 Hz). The threshold curve for monopolar sine-wave pulses appeare d to be perfectly adapted for sensing larval rather than adult EODs. W ith square-pulse stimuli, thresholds increased monotonically with dura tion and there was no evidence of tuning for this kind of stimulus. Th ese results suggest that both conventional spectral