DEVELOPMENT OF ELECTRICAL SIGNALING IN LARVAE OF THE AFRICAN FISH, POLLIMYRUS-ADSPERSUS (MORMYRIDAE, TELEOSTEI) - THE PATTERNS OF INTERDISCHARGE INTERVALS

The ontogenetic development of the overt motor and the electrical sign alling behaviour in larvae of a West African elephantfish, Pollimyrus adspersus, were studied. At an age of 35-40 days, hovering in mid-wate r was first observed. Before that time, the larvae moved only occasion ally and rested in the nest that was guarded by the male parent. The v ery 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 larv ae. Even 8- to 10-day-old larvae generated a trimodal inter-EOD interv al distribution (with three distinct discharge rates), similar to that observed in adults, although larval interval modes were of much longe r duration. For the first (high rate) mode, the trend towards shorteni ng 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 behavio ur of 8- to 10-day-old larvae closely resembled that observed in juven iles (exceeding 100 days): there was a single mode only, and EOD rate was increased (13.1 +/- 3.2 Hz). Artificial stimulation with natural i nter-EOD interval patterns previously recorded from other larvae or th e 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 11 days, Preferred Latency Responses of their E ODs to an artificial series of stimulus pulses (constant rate of 5 Hz) were observed at a stimulus intensity of 120 mu Vp-p/cm. In the young est larvae, 36-ms latencies were most frequent; this shortened to 19 m s in 31-day-old larvae (approaching the adult value; Kramer, 1978). Th e time pattern of EOD generation in P. adspersus larvae resembled that in mature specimens even before the adult electric organ became funct ional.