Electric Organ Discharge Rate Research Articles

Agonistic behaviour and electric signalling in a mormyrid fish, Gnathonemus petersii

1. Agonistic motor behaviour and concurrent electric signalling were studied in individually held, residential Gnathonemus petersii. Aggressive behaviour was elicited by presenting a specimen of a closely related species, Mormyrus rume, for 3 min a day. 2. The principal agonistic motor patterns are described (Fig. 2). Among them head butt, approach and lateral display were further analysed. 3. The electrical activity displayed during agonistic behaviour was found to differ fundamentally both from isolated resting and swimming conditions. The mean discharge rate recorded during aggressive behaviour (31 Hz, Fig. 3 c) is approximately twice the rate observed in an isolated swimming fish (Fig. 3b) and three times the rate displayed by a resting animal (Fig. 3a). An attacking G. petersii exhibits a much greater range of electric organ discharge (EOD) intervals than isolated swimming or resting individuals. EOD-interval histograms recorded from attacking fish show two sharp modes at high discharge rate; there are no intermediate intervals. 4. During the course of an attack, the initially low and variable discharge rate increases fairly linearly as the distance from the attacked fish diminishes (Fig. 9). The EOD rate associated with physical contact (head butt) comprises between 60 and 80 Hz in 24 of 28 attacks analysed; the dominant mode of the distribution is 61 Hz (Fig. 8). 5. During subsequent lateral display, G. petersii emits a high discharge rate pattern consisting of two types of “steady-state” activities which may last up to a few seconds: the first is a fairly regular alternation of approx. 16 and 8 ms intervals (paired pulses); this pattern gives rise to the two peaks of high discharge rate in the interval histogram (Fig. 3c). The second is a regular sequence of either 16 or 8 ms intervals (Fig. 4A). The only female among the animals used in our study showed the same display but did not exhibit the highest possible discharge rate (i.e. a regular sequence or 8 ms intervals; Fig. 4B). The high discharge rate is terminated by a sudden discharge break (Figs. 4A and 6). 6. It is suggested that the attack-associated EOD rate increase is a remnant of an ordinary locomotory pattern which has changed its function to a ritualised aggressive signal that occurs in a socially significant and well-defined context. The high discharge rate might serve three functions: (i) behavioural isolation of closely related, sympatrically living mormyrids (perhaps by character displacement); (ii) recognition of sexes; (iii)_synchronisation of mates during courtship.

The echo response inGnathonemus petersii (Mormyridae)

1. Gnathonemus petersii respond to each other's electric organ discharge (EOD) with an “echo” discharge of their own at a latency of about 12 msec. This response persists until interfish distances reach about 30 cm (Fig. 4). 2. Artificial electrical stimuli were used to further characterize the response. Response threshold, latency dependence on stimulus intensity, polarity characteristics (Figs. 5–7), and differential regional sensitivity indicate that “medium” electroreceptors in the anterior region of the animal underlie the response. 3. Response probability depends upon the delay of the stimulus after the last EOD and also upon the instantaneous EOD rate (Figs. 8, 9). The echo response in turn resets the EOD rhythm of the responding animal (Fig. 10). These results suggest that the echo input pathway terminates on the presumed mesencephalic command center.

‘Communication’ in weakly electric fish, Gnathonemus petersii (Mormyridae) II. Interaction of electric organ discharge activities of two fish

The electric organ discharges (EODs) of pairs of weakly electric fish, Gnathonemus petersii, were simultaneously recorded to study the significance of the EODs as communication signals. In a 400-litre tank a larger fish (12 to 15 cm) was passively moved within a shelter tube toward a smaller specimen (6 to 9 cm), either in steps or a continuous move. The movement was stopped at that distance when at least one fish significantly lowered or ceased its EOD activity. From this ‘threshold interfish distance’ the spatial range of a ‘communication field’ was found to extend about 30 cm from the fish. At threshold distances an EOD frequency increase caused a temporary EOD activity cessation in the second fish. The spontaneous irregular EOD pattern of the fish displaying the increased EOD rate changed into a regular one with almost equal time intervals between fish pulses.

Modification of electric organ activity of the transparent knifefish by benzocaine

1. 1. Para-ethylaminobenzoate (benzocaine) reduced the rate of electric organ discharge in the transparent knifefish, Eigenmannia virescens, without altering the amplitude of individual pulses. 2. 2. A high concentration of benzocaine, 0·2 m M, always depressed the discharge frequency by about 50 per cent initially, and then suddenly and abruptly abolished the remaining level of electromotor activity completely. 3. 3. Following the total cessation of electric organ activity, and after the removal of benzocaine, recovery of the frequency of firing was rapid but was accompanied by a brief period during which the amplitudes of individual pulses were severely distorted.

Avoidance conditioning of the rate of electric organ discharge in mormyrid fish

In Mormyrid fish the electric organ appears to discharge as a single unit, and the frequency of discharge is a simply quantifiable behavioural variable. Following a study showing that acceleration in discharge rate can be conditioned classically, demonstration of operant conditioning was undertaken using a yoked control procedure. For each pair of fish six to twelve series of trials were run, and in successive series the avoidance and control contingencies were reversed. The procedure permitted comparison of the contingencies successively in the same fish and simultaneously between fish. The results from eight Gnathonemus indicated that avoidance conditioning can control electric organ activity. In naive subjects avoidance conditioning produced significantly more responding than yoked classical conditioning ( P > 0·02). The occurrence of avoidance conditioning was confirmed by the effects of subsequent reversals of avoidance and control schedules.

Classical conditioning of electric organ discharge rate in mormyrids.

Weakly electric fish of the African family Mormyridae emit pulses at variable intervals with a distribution skewed toward longer intervals. Fourteen specimens of the genera Mormyrops, Gnathonemus, and Marcusenius were classically conditioned to increase briefly their discharge frequency. The unconditioned stimulus was electric shock and the conditioned stimulus was light. These results are novel in that the overt conditioned response involves neither secretion nor movement.