Emergence and development of the electric organ discharge in the mormyrid fish,Pollimyrus isidori

Summary

The paper describes the first part of a longitudinal study of the development of electric organ discharge (EOD) in the mormyrid fishPollimyrus isidori. Laboratory bred larvae were continuously monitored for EOD onset in a small glass recording cell fitted with recording electrodes (Fig. 1). Temperature was held at 27 °C+-0.05°.

• 1. First recognisable EODs were recorded on Day 8 following spawning (Fig. 4). The EOD was about 10 ms in duration in contrast to 50 μs in the adult (Fig. 2). The head positive main phase of the larval EOD soon appears (Fig. 10) and is thus of the opposite polarity to the adult discharge.

• 2. During the first 6 h following the first EOD the amplitude and discharge patterns changed very rapidly. The EOD of a reference fish was analysed discharge by discharge over this period (Fig. 5). Amplitude increased by over 400% from an initial 50 μV (p-p) recorded potential. Most discharges occurred in trains of up to 14 EODs with relatively constant intra-train intervals of 100–150 ms (Figs. 7, 8, 13).Inter-train intervals decreased very rapidly from minutes to tens of seconds over the first 6 h (Fig. 9). Longer trains tended to occur more often during swimming activity (Fig. 6).

• 3. The first clear changes in form of discharge occurred between 8 and 10 h post-EOD onset (Fig. 10). Secondary and tertiary discharges at 6 ms and 12 ms intervals from the main EOD commenced during the 12th and 18th h respectively. By 24 h all fish in the peer group (Fig. 11) were producing multiple EODs. At 36 h all these discharges had stopped but were soon replaced by large, secondary EODs (Fig. 14). These were in turn replaced by 8 ms multiples (Fig. 15) for a transitory period and by 70 h all multiple EODs had ceased.

• 4. Morphological studies show that a primitive larval electric organ is the only generating structure present during the period of larval discharge.

• 5. Physiological models are proposed to account for the observed discharge pattern variations during the initial developmental period. 12 ms multiple EODs appear to be ‘self-echoing’ -an oscillation of the electrosensory system due to the incomplete establishment of the afferent control system which normally inhibits such feedback.