Electric Organ Discharge Rate Research Articles

Sexual and seasonal plasticity in the emission of social electric signals. Behavioral approach and neural bases

Behavior in electric fish includes modulations of a stereotyped electric organ discharge (EOD) in addition to locomotor displays. Gymnotiformes can modulate the EOD rate to produce signals that participate in different behaviors. We studied the reproductive behavior of Brachyhypopomus pinnicaudatus both in the wild and laboratory settings. During the breeding season, fish produce sexually dimorphic social electric signals (SES): males emit three types of chirps (distinguished by their duration and internal structure), and accelerations, whereas females interrupt their EOD. Since these SES imply EOD frequency modulations, the pacemaker nucleus (PN) is involved in their generation and constitutes the main target organ to explore seasonal and sexual plasticity of the CNS. The PN has two types of neurons, pacemakers and relays, which receive modulatory inputs from pre-pacemaker structures. These neurons show an anisotropic rostro-caudal and dorso-ventral distribution that is paralleled by different field potential waveforms in distinct portions of the PN. In vivo glutamate injections in different areas of the PN provoke different kinds of EOD rate modulations. Ventral injections produce chirp-like responses in breeding males and EOD interruptions in breeding females, whereas dorsal injections provoke EOD frequency rises in both sexes. In the non-breeding season, males and females respond with interruptions when stimulated ventrally and frequency rises when injected dorsally. Our results show that changes of glutamate effects in the PN could explain the seasonal and sexual differences in the generation of SES. By means of behavioral recordings both in the wild and in laboratory settings, and by electrophysiological and pharmacological experiments, we have identified sexual and seasonal plasticity of the CNS and explored its underlying mechanisms.

Environmental, seasonal, and social modulations of basal activity in a weakly electric fish

The electric organ discharge (EOD) of weakly electric fish encodes information about species, sex, behavioral, and physiological states throughout the lifetime. Its central command is crucial for sensory-motor coordination, and is therefore the target of plastic mechanisms that adapt fish to environmental and social challenges. The EOD waveform of Brachyhypopomus pinnicaudatus is modulated by environmental factors and the neuroendocrine system. In this study we investigate the effects of water temperature and day–night cycle upon EOD rate in this species during the breeding and non-breeding seasons. During the non-breeding season, EOD rate is a linear function of water temperature and exhibits counterclockwise hysteresis. During breeding, a thermal resistance strategy prevents the decrease of EOD rate to cooling. A nocturnal increase of EOD basal rate independent of water temperature and locomotor activity was demonstrated in isolated non-breeding adults and in male–female dyads all year round. An additional increase of nocturnal EOD rate, probably acting as a social courtship signal, was found in breeding dyads. This additional increase of nocturnal EOD rate could not be fully explained by gonadal maturation and was modulated by social stimuli. This study provides novel data on the complex interactions between environment, reproductive cycle, social behavior, and electromotor output in an advantageous model of the vertebrate central nervous system.

Analysis of behavior-related excitatory inputs to a central pacemaker nucleus in a weakly electric fish

Gymnotid electric fish explore their environment and communicate with conspecifics by means of rhythmic electric organ discharges. The neural command for each electric organ discharge arises from activity of a medullary pacemaker nucleus composed of two neuronal types: pacemaker and relay cells. During different behaviors as in courtship, exploration and agonistic interactions, these species display specific electric organ discharge frequency and/or waveform modulations. The neural bases of these modulations have been explained in terms of segregation of inputs to pacemaker or relay cells, as well as differential activation of the glutamate receptors of these cells. One of the most conspicuous electric organ discharge frequency modulations in Gymnotus carapo results from the activation of Mauthner cells, a pair of reticulospinal neurons that are involved in the organization of sensory-evoked escape responses in teleost fish. The activation of Mauthner cells in these animals produces a prolonged increase in electric organ discharge rate, whose neural mechanisms involves the activation of both N-methyl-d-aspartate (NMDA) and metabotropic glutamatergic receptors of pacemaker cells. Here we provide evidence which indicates that pacemaker cells are the only cellular target of the synaptic inputs responsible for the Mauthner cell initiated electric organ discharge modulation at the medullary pacemaker nucleus. Additionally, although pacemaker cells express both NMDA and non-NMDA ionotropic receptors, we found that non-NMDA receptors are not involved in this synaptic action which suggests that NMDA and non-NMDA receptor subtypes are not co-localized at the subsynaptic membrane. NMDA receptor activation of pacemaker cells seems to be an efficient neural strategy to produce long-lasting enhancements of the fish sampling capability during Mauthner cell-initiated motor behaviors.

Auditory discrimination in a sound-producing electric fish (Pollimyrus): tone frequency and click-rate difference detection.

Pollimyrus adspersus is a fish that uses simple sounds for communication and has auditory specializations for sound-pressure detection. The sounds are species-specific, and the sounds of individuals are sufficiently stereotyped that they could mediate individual recognition. Behavioral measurements are presented indicating that Pollimyrus probably can make species and individual discriminations on the basis of acoustic cues. Interclick interval (ICI; 10-40 ms) and frequency (100-1400 Hz) discrimination was assessed using modulations of the fish's electric organ discharge rate in the presence of a target stimulus presented in alternation with an ongoing base stimulus. Tone frequency discrimination was best in the 200-600-Hz range, with the best threshold of 1.7% +/- 0.4% standard error at 500 Hz (or 8.5 Hz +/- 1.9 SE). The just noticeable differences (jnd's) were relatively constant from 100 to 500 Hz (mean 8.7 Hz), then increased at a rate of 13.3 Hz per 100 Hz. For click trains, jnd's increased linearly with ICI. The mean jnd's for 10- and 15-ms ICI were both 300 micros (SE= 0.8 ms at 10-ms ICI, SE= 0.11 ms at 15-ms ICI). The jnd at 20-ms ICI was only 1.1 ms +/- 0.25 SE.

Auditory thresholds in a sound-producing electric fish (Pollimyrus): behavioral measurements of sensitivity to tones and click trains.

In this report we present the first behavioral measurements of auditory sensitivity for Pollimyrus adspersus. Pollimyrus is an electric fish (Mormyridae) that uses both electric and acoustic signals for communication. Tone detection was assessed from the fish's electric organ discharge rate. Suprathreshold tones usually evoked an accelerated rate in naive animals. This response (rate modulation > or =25%) was maintained in a classical conditioning paradigm by presenting a weak electric current near the offset of 3.5-s tone bursts. An adaptive staircase procedure was used to find detection thresholds at frequencies between 100 and 1700 Hz. The mean audiogram from six individuals revealed high sensitivity in the 200-900 Hz range, with the best thresholds near 500 Hz (66.5+/-4.2 SE dB re: 1 microPa). Sensitivity declined slowly (about 20 dB/octave) above and below this sensitivity maximum. Sensitivity fell off rapidly above 1 kHz (about 60 dB/octave) and no responses were observed at 5 kHz. This behavioral sensitivity matched closely the spectral content of the sounds that this species produced during courtship. Experiments with click trains showed that sensitivity (about 83-dB peak) was independent of inter-click-interval, within the 10-100 ms range.

Sensory Processing in the Pallium of a Mormyrid Fish

To investigate the functional organization of higher brain levels in fish we test the hypothesis that the dorsal gray mantle of the telencephalon of a mormyrid fish has discrete receptive areas for several sensory modalities. Multiunit and compound field potentials evoked by auditory, visual, electrosensory, and water displacement stimuli in this weakly electric fish are recorded with multiple semimicroelectrodes placed in many tracks and depths in or near telencephalic area dorsalis pars medialis (Dm). Most responsive loci are unimodal; some respond to two or more modalities. Each modality dominates a circumscribed area, chiefly separate. Auditory and electrical responses cluster in the dorsal 500 micrometer of rostral and caudolateral Dm, respectively. Two auditory subdivisions underline specialization of this sense. Mechanoreception occupies a caudal area overlapping electroreception but centered 500 micrometer deeper. Visual responses scatter widely through ventral areas. Auditory, electrosensory, and mechanosensory responses are dominated by a negative wave within the first 50 msec, followed by 15-55 Hz oscillations and a slow positive wave with multiunit spikes lasting from 200 to 500 msec. Stimuli can induce shifts in coherence of certain frequency bands between neighboring loci. Every electric organ discharge command is followed within 3 msec by a large, mainly negative but generally biphasic, widespread corollary discharge. At certain loci large, slow ("deltaF") waves usually precede transient shifts in electric organ discharge rate. Sensory-evoked potentials in this fish pallium may be more segregated than in elasmobranchs and anurans and have some surprising similarities to those in mammals.

A New Warning Biomonitor Using a Weakly Electric Fish, Apteronotus Albifrons (Gymnotiformes), and the Effect of Temperature on the Bioelectric Responses

The weak electric wave discharges of the knifefish, Apteronotus albifrons (gymnotiformes, apteronotidae) are used as a new biomonitor to assess water quality deterioration. The principle consists in a real-time analysis of the Electric Organ Discharge frequency and waveform. These two characteristics are naturally stable under standard conditions. Detailed descriptions of the biological sensor and the test apparatus as well as the set-up for recording, processing and analysing the electric signals are presented. The relationship between Apteronotus electric activity level and temperature of the surrounding water is determined. The variation of the water temperature was increased – decreased and vice versa. Bioelectric responses are investigated in a 22–32 °C temperature range. The A. albifrons rate of electric organ discharge is temperature dependent rising very significantly with a unit increase in water temperature (t ≥ 246.9; df = 108; P ≤ 0.001) and decreasing also very significantly with a unit fall in temperature (t ≥ 310.3; df = 108; P ≤ 0.001). For the set of the individual fish tested, the observed frequency variations lie between 45.4 and 63.9 Hz/°C. Regression coefficients obtained for downward temperature variations are always bigger than the corresponding coefficients for upward temperature variations. Student t-test comparison of the two types of coefficients reveals a sharp significant difference in 75% of the cases considered (6.167 ≤ t ≤ 18.024; df = 216; P = 0.001). The raw data of the A. albifrons electric activity is shown to be characterized by a phenomenon of hysteresis in the 0.3 to 4.5% range. The Q10-values of the discharge rate over the 22–32 °C temperature range was found to be 1.6. The observations did not reveal any dependence of the A. albifrons signal waveform on the temperature of the surrounding water. The temperature dependence of the Electric Organ Discharge frequency imposes the need for a very strict control of the temperature variations of the water surrounding the fish in order to ensure the reliability of using electric fish as indicators.

Development of electrical signalling 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 signalling behaviour in larvae of a West African elephantfish, Pollimyrus 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 patterns 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 11 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 of EOD generation in P. adspersus larvae resembled that in mature specimens even before the adult electric organ became functional.

Short‐range Navigation of the Weakly Electric Fish, Gnathonemus petersii L. (Mormyridae, Teleostei), in Novel and Familiar Environments

AbstractWe investigated the electrolocation performance of the weakly electric fish, Gnathonemus petersii, in novel and familiar environments. By selectively interfering with the fish's sensory input, we determined the sensory channels necessary for navigation and orientation. The fish's task was to locate a circular aperture (diameter: 64 mm) in a wall dividing a 200–1 aquarium into two equal compartments. To assess the fish's performance, we measured (1) the time it took the fish to locate the aperture, (2) the height at which it contacted the divider, (3) its electric organ discharge rate, and (4) the frequency of divider crossings. In the first experiment (novel environment), 50 naive G. petersii assigned to five groups of 10 fish each (intact, blind, electrically “silent,” blind and “silent,” and shamoperated animals) were tested with the aperture presented randomly in one of three positions (aperture center: 7.6, 17.7, 27.8 cm from the bottom). In a novel environment, G. petersii depend on active electrolocation. Despite the changing aperture position, over the 15 trials, fish with a functioning electric organ found the aperture, whereas those without one did not. The electric organ discharge rate was inversely correlated with the amount of time spent searching for the aperture. In a second experiment (familiar environment) 20 intact fish learned the position of a fixed aperture. When we subsequently denervated the electric organ in 10 of these animals, their performance did not differ significantly from that of their conspecifics. Thus, once the fish were familiar with the aperture's position, they no longer depended on active electrolocation. We interpret and discuss this behavior as evidence for a “central expectation” and discuss its possible role in electronavigation.

Responses of electric fish (family mormymdae) to chemical changes in water quality: I. cyanide

Abstract The effect of cyanide on the electric organ activity of two species of the tropical monnyrid fish, Gnathonemus petersi and Gnathonemus tamanduá have been examined. Significant responses in the mean electric organ discharge rate (EOD) are measured following the introduction of cyanide at a concentration of 25μg l−1. This corresponds with an increase in the variance. There is also a significant response detected at a concentration of 5μg l−1 but in this case the time to react is increased.

Responses of the electric fishGnathonemus tamandua(family mormyridae) to changes in ph

Abstract The response of the tropical mormyrid fish, Gnathonemus tamandua, to variations in pH (within a range of 4.5 to 9.0) at a constant temperature of 27.5°C ± 0.5°C has been evaluated. Fluctuations in electric organ discharge (EOD) were detected using plate electrodes and a microcomputer produced counts as pulses per minute (PPM). The introduction of alkaline conditions (up to pH 9.0) significantly increased the rate of EOD while acidity (down to pH 4.5) acted to reduce it. The optimum pH for stable pulsing was established at 7.1.

Electric Organ Discharge Displays during Social Encounter in the Weakly Electric FishBrienomyrus nigerL. (Mormyridae)

AbstractWe investigated the electric organ discharge (EOD) activity of the mormyrid fishBrienomyrus nigerduring social encounters. The fish were contained in porous ceramic shelters and tested alone and in pairs in an experimental tank designed to restrict communication to the electrosensory modality. We moved one fish toward and away from a stationary conspecific, beginning at a distance known to be outside the range of communication (250 cm). Baseline EOD activity was recorded prior to interaction and categorized as ‘variable’, ‘regular’, and ‘scallop’. When moved closer together, the fish modulated this baseline activity in four ways: (1) At 100–130 cm apart, the stationary fish emitted a maximum of sudden EOD rate increases which defined the outer limit of its communication range. (The associated Electric Field Gradient was 1 μV/cm). (2) Long EOD cessations, which we called social silence, lasted from 5–130 s and occurred most frequently when the fish were 36 to 55 cm apart (EFG: 100 μV/cm). The duration of social silence was negatively correlated (r = − 0.862) with the responding fish's size, and was independent of the partner's sex and size. Fish whose EOD baseline pattern was ‘scallop’ were least likely to fall electrically silent, and those that were categorized as ‘regular’ or ‘variable’ were most likely to cease discharging. (3) Within electrolocation range, fish ‘regularized’ their EOD activity while the partner was ‘silent’ (EFG: 1 mV/cm). (4) Following long EOD cessations the fish resumed discharging with characteristic EOD rebound patterns. The possible ethological significance of these findings is discussed.

Fine Analysis of Predatory and Defensive Motor Events in Torpedo Marmorata (Pisces)

ABSTRACT Predation and defence of Torpedo marmorata, elicited with biological., and inanimate stimuli, were studied in experimental conditions. Patterns of responses of neonates and adults are described, and chains of separate, coordinated motor acts are defined. Jumping predation, creeping predation, defence of the disc and defence of the tail are distinguished. Each predatory behaviour consists of stimulus reaction chains, whereas defensive types of behaviour involve single stimulus reactions. During jumping behaviour, tail strokes provoked by the initial response in the chain are displayed during the whole period of predation; these ‘delayed’ tail strokes provoke displacements of the prey, thus probing the substrate and furnishing biological cues. Large behavioural variations are reported, including incomplete chains of responses, repetition of responses, oriented responses, stimulus intensity/response magnitude relationships and missing of certain motor acts. Ontogenetic maturation of the behaviour patterns is observed in embryos and in immature neonates. Sets of electric organ discharges (EODs), which accompany the defensive responses and certain predatory responses, show a stereotyped pattern, characteristic for each EOD motor act. Variations found in the EOD pattern are often related to the effects of fatigue on the EOD rate. Electric shock provokes the immobilization of the prey, or the flight of the aggressor. Effective use of the EOD by neonates as a weapon against prey is reported. The possible neuronal basis of the sensorimotor organization which may account for the results is discussed.

Locomotor and electric displays associated with electrolocation during exploratory behavior in mormyrid fish

In the presence of a novel stimulus such as an aluminum or a plastic rod mormyrid fish ( Marcusenius cyprinoides and Gnathonemus petersii) exhibit characteristic motor probing acts (PMAs): ‘chin probing’, ‘radial’ and ‘lateral va-et-vient’, ‘lateral probing’, ‘tangential probing’, and ‘stationary probing’. During the display of these PMAs the fish maintain characteristic probing distances from the object which were 4.5 cm for the metal and 2.5–3.7 cm for the plastic stimulus. G. petersii with their electric organ rendered inoperative (‘silent fish’) no longer exhibited ‘radial’ and ‘lateral va-et-vient’. Regardless of the nature of the stimulus the probing distances were shorter in ‘silent’ fish and ranged from 1.8 to 2.6 cm. During the display of PMAs intact fish changed their variable electric organ discharge rate to a unique and stable, regularized rate, with the interdischarge interval maintained at 28–30 ms. The fish's electric and non-electric (motor) probing behavior in the presence of novel objects (and following their removal during phantom PMAs) is discussed in light of theories on exploratory behavior.

The neural basis of a sensory filter in the Jamming Avoidance Response: No grandmother cells in sight

The Jamming Avoidance Response (JAR), during which weakly electric fish modulate their electric organ discharge rate in response to a foreign electric signal of nearly the same frequency is strongest for frequency differences (Δfs) between 3–8 Hz. We have searched for neural correlates of this behavioral specificity. Single unit recordings in the anterior lateral line ganglion (ALLG), the posterior lateral line lobe (PLLL) and the torus semicircularis (TS) ofEigenmannia virescens were made during electrical stimulation simulating jamming by a nearby conspecific.

Electric and Motor responses of the Weakly electric fish, Gnathonemus petersii (Mormyridae), to play-back of social signals

1. Seven isolated G. petersii resting in their daytime hiding-places were stimulated via a dipole model (Fig. 1a) with previously tape-recorded electric organ discharge (EOD) patterns in an attempt to determine whether G. petersii distinguishes two different intraspecific EOD patterns, rest and attack. 2. Rest pattern A was characterized by a broad distribution of EOD intervals, a low mean discharge rate (8 Hz, Fig. 3), and a long period of significantly positive autocorrelation (2 s, Fig. 4a). Accordingly, the spectrum of EOD rate fluctuations showed a low frequency range (0.005 to 0.12 Hz, Fig. 5a). Attack pattern B was a considerably different EOD interval distribution of high mean discharge rate (25 Hz, Fig. 3), showing a short period of significantly positive autocorrelation (0.8 s, Fig. 4b), only. Here, the spectrum of EOD rate fluctuations was at a considerably higher frequency range (0.09 to 0.47 Hz, Fig. 5b). 3. Play-back of attack pattern B elicited significantly more bodily startle responses from the experimental fish (Fig. 6) than did the rest pattern A (Table 1). Also the number of attacks directed at the dipolemodel was significantly greater during stimulation with attack pattern B (Table 2, Fig. 6). 4. The EOD responses of the experimental fish differed in several respects depending on which stimulation pattern was used. The modes of the pulse rate histograms as well as their spans were lower during play-back of rest pattern A than during stimulation with attack pattern B (average 12.3 vs 16.2 Hz, and average 47 vs 56 Hz, respectively; Fig. 11). Shortterm (0.2 s) EOD rate correlations were stronger when the fish were stimulated with rest pattern A than when they were stimulated with attack pattern B (average correlations 0.67 and 0.61, respectively; Figs. 10 and 11). Significant positive correlations were maintained for longer periods of time during rest pattern stimulation than during attack pattern stimulation (average 1.94 and 1.24 s, respectively; Figs. 10 and 11). The spectra of EOD rate fluctuations of the stimulated fish were at lower frequency ranges during rest pattern stimulation than during attack pattern stimulation (average amplitude-spectrum peak frequencies 0.02 and 0.07 Hz, respectively; Figs. 12 and 13). 5. Although maximal cross-correlations from the EOD rates to the stimulus pulse rates were weaker during rest pattern stimulation (average 0.2) than during attack pattern stimulation (average 0.33), significant cross-correlations were maintained for longer periods of time during rest pattern stimulation than during attack pattern stimulation (average 1.78 and 0.92 s, respectively). The lags of maximal cross-correlations were greater during rest pattern stimulation than during attack pattern stimulation (average 2.6 and 0.8 s, respectively; Figs. 14 and 15). 6. The results clearly showed that at least two specific EOD time patterns encode different ‘messages’ in the intraspecific communication system of G. petersii.

Ethological observations on the electric organ discharge behaviour of the electric catfish, Malapterurus electricus (Pisces)

1. Electric organ discharge (EOD) volleys emitted by the electric catfish, Malapterurus electricus, were recorded and sampled for 108 consecutive hours from the fish's natural habitat, a selected study area in the Swashi River, a tributary of the man-made Lake Kainji in Nigeria. 2. M. electricus intermittently generates different types of EOD volleys that start with a high-frequency phase, and range in duration from 2 ms to over 8 s with from 2 to over 600 pulses (EODs). 3. The catfish's EOD activity exhibited a daily activity rhythm which was correlated with local changes in light intensity. The EOD activity remained at a constant level throughout the day from 0700 h to 1800 h. Shortly after sunset, between 1800 h and 1900 h, it increased fivefold to its peak value at 1900 h. The occurrence of EOD volleys declined throughout the night with a noticeable drop to the daytime low after sunrise between 0600 h and 0700 h. During the first 6 h following sunset, the volleys were significantly longer and consisted of more EODs than the daytime volleys. It is inferred that M. electricus feeds most successfully during the early hours of the night. 4. A hitherto unknown type of Malapterurus EOD activity was discovered: during night-time, volleys consisting of more than 16 EODs were occasionally preceded by a low-frequency train of from 1 to 11 EODs. The occurrence rate of these pre-volley EODs was positively correlated with the duration of the ensuing volley (and its number of EODs). The peak pre-volley activity occurred during the first 4 h after sunset, with an average of 38% of all volleys preceded by this type of activity. The adaptive significance of the nocturnal pre-volley EODs as prey detection mechanism is discussed.

Influence of Water Temperature on the Electric Organ Discharge (Eod) of the Weakly Electric Fish Marcusenius Cyprinoides (Mormyridae)

1. The influence of different water temperatures on the electric organ discharge (EOD) of a mormyrid fish Marcusenius cyprinoïdes was studied. The range of the water temperatures was fixed according to the seasonal temperature variations of the rivers in Central Africa, the natural habitat of this species. 2. The EOD activity was characterized using the following parameters: mean EOD rate, EOD pattern in the form of Interpulse Interval Histograms (IIH), IIH range, and shortest pulse interval. These parameters remained constant during control experiments at constant temperature (27 degrees C) for 4 days. 3. The mean EOD rate increases with increasing water temperatures. The lowest mean EOD rate is always found at 17 degrees C, the highest between 26 and 33 degrees C. The characteristics of the IIH are modified by stepwise temperature increases. These IIH show during high temperatures (26-33 degrees C) similar patterns to those previously observed during high level motor activity and excitement. The IIH range diminishes with stepwise temperature increases. The shortest pulse interval has a negative, linear correlation with water temperature. 4. The possible role of water temperature in the reproduction of the mormyrids is discussed. The high discharge rate of M. cyprinoïdes produced by high water temperatures during the rainy season could serve to improve the resolution of the electroreceptors during this period.

Evolutionary designs for electric signals and electroreceptors in gymnotoid fishes of Surinam

1. Field collections of gymnotoid electric fish in coastal Surinam streams; 23 March–6 April, 1976, yielded 11 species belonging to two families (Rhamphichthyidae and Gymnotidae). Electric organ discharges (EODs) were recorded and power spectra were generated by Fourier analysis. Each species could be classified as either wave or pulse type. Although EODs varied from species to species, individuals had species-typical discharges. EODs appear to function in electrolocation and electric communication. Coexisting pulse fish diverged in either pulse spectrum or pulse repetition rate. Although each species showed ecological preferences for one habitat or another, no general correlation was found between EOD form and habitat type (Fig. 1). 2. Three species of coexisting Hypopomus showed widely differing EOD durations. Peak spectral energies were nonoverlapping (Fig. 4a). 3. Electrophysiologic studies of the electroreceptors in Hypopomus species revealed five types of electroreceptors. Two types of units appear to act as EOD filters, responding maximally to spectral frequencies characteristic of the peak power of the species-specific EOD (Fig. 7). 4. Relationships between EOD rate and spectrum are discussed with reference to their roles in communication and in electrolocation. A theory for the evolution of EOD wave forms is presented (Fig. 9).

The jamming avoidance response in gymnotoid pulse-species: A mechanism to minimize the probability of pulse-train coincidence

Gymnotoid electric fish with pulse-type electric organ discharges (EODs) shorten (lengthen) their EOD intervals as pulses of a slightly slower (faster) train scan their EODs (Figs. 1, 2). They thus minimize the chance of pulse coincidence by transient accelerations (decelerations) of their EOD rate. Studies in curarized preparations demonstrate that this Jamming Avoidance Response (JAR) is controlled by electroreceptive input alone and without reference to an internal electric organ pacemaker-related signal (Fig. 8). A sufficient stimulus input consists of a train of strong, EOD-like stimulus pulses (S1), which mimic the animal's experience of its own EOD, and a train of small pulses (S2) of slightly different repetition rate, which mimic EODs of a neighbor. Correct behavioral responses require S1 pulses of sufficient intensity to recruit pulse-markertype receptors; also spatial and temporal patterns must closely resemble those of the animal's EOD. These features are of little significance for S2 pulses which, while scanning S1 pulses, only provide a small perturbation of electroreceptive feedback from S1 pulses. Inappropriate S1 stimulation impairs and sometimes reverses (Fig. 7) the behavioral discrimination of scan directions. The JAR is explained in terms of excitatory and inhibitory processes (Fig. 3) which are triggered by S2 stimulation, at specific phases within the S1 cycle (Figs. 4–6). The JAR in pulse species strongly resembles the JAR in wave-species (Bullock et al., 1972) and could be considered an evolutionary ancestor of the latter. It is a response to a particular novelty in electroreceptive feedback.