Development of electrical signalling in larvae of the African fish, Pollimyrus adspersus (Mormyridae, Teleostei): the patterns of interdischarge intervals

Abstract

The ontogenetic development of the overt motor and the electrical signalling behaviour in larvae of a
West African elephantfish, Pollimyms adspersus, were studied. At an age of 35-40 days, hovering in
mid-water was first observed. Before that time, the larvae moved only occasionally and rested in the
nest that was guarded by the male parent. The very low electric organ discharge (EOD) rate of 2.4 ± 0.9 Hz observed in 8- to 10-day-old larvae (that generate their first EODs on day 8) increased to an
adult rate of 8.4 ± 1.7 Hz in 21- to 25-day-old larvae. Even 8- to 10-day-old larvae generated a
trimodal inter-EOD interval distribution (with three distinct discharge rates), similar to that observed
in adults, although larval interval modes were of much longer duration. For the first (high rate) mode,
the trend towards shortening stabilized already at the age of 21-25 days, whereas for the second and
third (low rate) modes, this occurred only at an age of around 61-70 days.
Inter-EOD interval patterns recorded during swimming behaviour of 8- to 10-day-old larvae closely
resembled that observed in juveniles (exceeding 100 days): there was a single mode only, and EOD
rate was increased (13.1 ± 3.2 Hz).
Artificial stimulation with natural inter-EOD interval pattems previously recorded from other larvae
or the nest-guarding male did not evoke any responses in 14-day-old larvae, apart from a brief EOD
stop response to stimulus onset. However, even in larvae as young as ll days, Preferred Latency
Responses of their EODs to an artificial series of stimulus pulses (constant rate of 5 Hz) were observed
at a stimulus intensity of 120 µVp-p/cm. In the youngest larvae, 36-ms latencies were most frequent;
this shortened to 19 ms in 31-day-old larvae (approaching the adult value; Kramer, 1978). The time
pattern oT EOD generation in P. adspersus larvae resembled that in mature specimens even before the
adult electric organ became functional.