Production of aggressive electrocommunication signals to progressively realistic social stimuli in male Apteronotus leptorhynchus

Brown ghost knife fish, Apteronotus leptorhynchus, produce a continuous electric organ discharge (EOD) that they use for communication. While interacting aggressively, males also emit brief EOD modulations termed chirps. The simplicity of this behaior and its underlying neural circuitry has made it an important model system in neuroethology. Chirping is typically assayed by confining a fish in a tube (‘chirp chamber’) and presenting it with sine wave electrical stimuli that partially mimic EODs of other fish. We presented male fish with progressively more realistic social stimuli to examine whether some of the stimulus complexities during dyadic interaction influence the production of chirps. In a chirp chamber, fish chirped less to a recording of an EOD containing chirps than to a recording of an EOD alone and to sine wave stimuli. Free‐swimming fish chirped more to stimulus fish than to sine wave stimuli presented through electrodes. Fish chirped more when interacting directly than when interacting across a perforated barrier. Together, these studies demonstrate that the presence of chirps, electric field complexity, and/or non‐electric social stimuli are important in eliciting chirp production in brown ghosts.     

‘Recognition units’ at the top of a neuronal hierarchy?

The electric fish, Eigenmannia, is able to discriminate the sign of the frequency difference, Df, between a neighbor's electric organ discharges (EODs) and its own. The fish lowers its EOD frequency for positive Dfs and raises its frequency for negative Dfs to minimize jamming of its electrolocation ability by a neighbor's EODs of similar frequency. This jamming avoidance response (JAR) is controlled by a group of 'sign-selective' neurons in the prepacemaker nucleus (PPN) that is located at the boundary of the midbrain and the diencephalon (Fig. 1). Extracellular recordings from a total of 35 neurons revealed a great similarity between behavioral and neuronal response properties: 1. All neurons fired vigorously for negative Dfs and were almost silent for positive Dfs, regardless of the orientation of the jamming stimulus, and thus discriminated the sign of Df unambiguously (Fig. 2). 2. In accordance with behavioral observations, individual neurons failed to discriminate the sign of Df when the jamming stimulus had the same field geometry as the signal mimicking the animal's own EOD (Fig. 3). 3. Df magnitudes which evoke strongest JARs, usually 4 to 8 Hz, also induced most vigorous responses in sign-selective neurons (Fig. 5). 4. Behavioral and neuronal thresholds for the detection of small jamming signals were similar. Threshold for sign selectivity was reached when the amplitude ratio of the jamming signal to the EOD mimic, measured near the head surface, was 0.001. This value corresponds to a maximal temporal disparity (a necessary cue for performing a correct JAR) of 1 to 2 microseconds for signals received by the two sides of the body in a transverse jamming field (Fig. 7). 5. The effects of two jamming fields, offered orthogonally to each other, may interact nonlinearly at the behavioral as well as at the neuronal level. A positive Df presented in one field may suppress behavioral and neuronal responses to modulations of the sign of Df in the other field (Fig. 8c).     

Electric interactions through chirping behavior in the weakly electric fish, Apteronotus leptorhynchus

The weakly electric fish Apteronotus leptorhynchus produces wave-like electric organ discharges distinguished by a high degree of regularity. Transient amplitude and frequency modulations (“chirps”) can be evoked in males by stimulation with the electric field of a conspecific. During these interactions, the males examined in this study produced six types of chirps, including two novel ones. Stimulation of a test fish with a conspecific at various distances showed that two electrically interacting fish must be within 10 cm of each other to evoke chirping behavior in the neighboring fish. The chirp rate of all but one chirp type elicited by the neighboring fish was found to be negatively correlated with the absolute value of the frequency difference between the two interacting fish, but independent of the sign of this difference. Correlation analysis of the instantaneous rates of chirp occurrence revealed two modes of interactions characterized by reciprocal stimulation and reciprocal inhibition. Further analysis of the temporal relationship between the chirps generated by the two fish during electric interactions showed that the chirps generated by one individual follow the chirps of the other with a short latency of approximately 500–1000 ms. We hypothesize that this “echo response” serves a communicatory function.     

Sex differences in the electrocommunication signals of Sternarchogiton nattereri (Gymnotiformes: Apteronotidae)

In this study we examined electrocommunication behavior in Sternarchogiton nattereri (Apteronotidae), a weakly electric fish from South America. We focused on variation between females and males lacking external dentition and used playbacks of simulated conspecifics to elicit chirps (modulations of their electric organ discharge, EOD). Chirp responses were not affected by the frequency of the playback stimulus. EOD frequency, chirp rate, and chirp duration were not sexually dimorphic; however, the amount of chirp frequency modulation was significantly greater in toothless males than in females. These results reinforce that sex differences in chirp structure are highly diverse and widespread in the Apteronotidae.     

Diversity of sexual dimorphism in electrocommunication signals and its androgen regulation in a genus of electric fish, Apteronotus

Gymnotiform electric fish emit an electric organ discharge that, in several species, is sexually dimorphic and functions in gender recognition. In addition, some species produce frequency modulations of the electric organ discharge, known as chirps, that are displayed during aggression and courtship. We report that two congeneric species (Apteronotus leptorhynchus and A. albifrons) differ in the expression of sexual dimorphism in these signals. In A. leptorhynchus, males chirp more than females, but in A. albifrons chirping is monomorphic. The gonadosomatic index and plasma levels of 11-ketotestosterone were equivalent in both species, suggesting that they were in similar reproductive condition. Corresponding to this difference in dimorphism, A. leptorhynchus increases chirping in response to androgens, but chirping in A. albifrons is insensitive to implants of testosterone, dihydrotestosterone or 11-ketotestosterone. Species also differ in the sexual dimorphism and androgen sensitivity of electric organ discharge frequency. In A. leptorhynchus, males discharge at higher frequencies than females, and androgens increase electric organ discharge frequency. In A.␣albifrons, males discharge at lower frequencies than females, and androgens decrease electric organ discharge frequency. Thus, in both chirping and electric organ discharge frequency, evolutionary changes in the presence or direction of sexual dimorphism have been accompanied and perhaps caused by changes in the androgen regulation of the electric organ discharge. Accepted: 18 February 1998     

Social interactions and cortisol treatment increase the production of aggressive electrocommunication signals in male electric fish,Apteronotus leptorhynchus

Brown ghost knife fish, Apteronotus leptorhynchus, continually emit a weakly electric discharge that serves as a communication signal and is sensitive to sex steroids. Males modulate this signal during bouts of aggression by briefly (approximately 15 ms) increasing the discharge frequency in signals termed "chirps." The present study examined the effects of short-term (1-7 days) and long-term (6-35 days) male-male interaction on the continuous electric organ discharge (EOD), chirping behavior, and plasma levels of cortisol and two androgens, 11-ketotestosterone (11KT) and testosterone. Males housed in isolation or in pairs were tested for short-term and long-term changes in their EOD frequency and chirping rate to standardized sinusoidal electrical stimuli. Within 1 week, chirp rate was significantly higher in paired fish than in isolated fish, but EOD frequency was equivalent in these two groups of fish. Plasma cortisol levels were significantly higher in paired fish than in isolated fish, but there was no difference between groups in plasma 11KT levels. Among paired fish, cortisol levels correlated positively with chirp rate. To determine whether elevated cortisol can cause changes in chirping behavior, isolated fish were implanted with cortisol-filled or empty Silastic tubes and tested for short-term and long-term changes in electrocommunication signals and steroid levels. After 2 weeks, fish that received cortisol implants showed higher chirp rates than blank-implanted fish; there were no difference between groups in EOD frequency. Cortisol implants significantly elevated plasma cortisol levels compared to blank implants but had no effect on plasma 11KT levels. These results suggest that male-male interaction increases chirp rate by elevating levels of plasma cortisol, which, in turn, acts to modify neural activity though an 11KT-independent mechanism.     

Sex differences in the electrocommunication signals of the electric fish Apteronotus bonapartii

The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior.     

Serotonergic activation of 5HT1A and 5HT2 receptors modulates sexually dimorphic communication signals in the weakly electric fish Apteronotus leptorhynchus

Serotonin modulates agonistic and reproductive behavior across vertebrate species. 5HT_1A and 5HT_1B receptors mediate many serotonergic effects on social behavior, but other receptors, including 5HT₂ receptors, may also contribute. We investigated serotonergic regulation of electrocommunication signals in the weakly electric fish Apteronotus leptorhynchus. During social interactions, these fish modulate their electric organ discharges (EODs) to produce signals known as chirps. Males chirp more than females and produce two chirp types. Males produce high-frequency chirps as courtship signals; whereas both sexes produce low-frequency chirps during same-sex interactions. Serotonergic innervation of the prepacemaker nucleus, which controls chirping, is more robust in females than males. Serotonin inhibits chirping and may contribute to sexual dimorphism and individual variation in chirping. We elicited chirps with EOD playbacks and pharmacologically manipulated serotonin receptors to determine which receptors regulated chirping. We also asked whether serotonin receptor activation generally modulated chirping or more specifically targeted particular chirp types. Agonists and antagonists of 5HT_1B/1D receptors (CP-94253 and GR-125743) did not affect chirping. The 5HT_1A receptor agonist 8OH-DPAT specifically increased production of high-frequency chirps. The 5HT₂ receptor agonist DOI decreased chirping. Receptor antagonists (WAY-100635 and MDL-11939) opposed the effects of their corresponding agonists. These results suggest that serotonergic inhibition of chirping may be mediated by 5HT₂ receptors, but that serotonergic activation of 5HT_1A receptors specifically increases the production of high-frequency chirps. The enhancement of chirping by 5HT_1A receptors may result from interactions with cortisol and/or arginine vasotocin, which similarly enhance chirping and are influenced by 5HT_1A activity in other systems.     

Testosterone modulates female chirping behavior in the weakly electric fish,Apteronotus leptorhynchus

The weakly electric fish, Apteronotus leptorhynchus, produces a wave-like electric organ discharge (EOD) utilized for electrolocation and communication. Both sexes communicate by emitting chirps: transient increases in EOD frequency. In males, chirping behavior and the jamming avoidance response (JAR) can be evoked by an artificial EOD stimulus delivered to the water at frequencies 1–10 Hz below the animal's own EOD. In contrast, females rarely chirp in response to this stimulus even though they show consistent JARs. To investigate whether this behavioral difference is hormone dependent, we implanted females with testosterone (T) and monitored their chirping activity over a 5 week period. Our findings indicate that elevations in blood levels of T cause an enhancement of chirping behavior and a lowering of basal EOD frequency in females. Elevated blood levels of T also appear to modulate the quality of chirps produced by hormone treated females. The effects of T on female chirping behavior and basal EOD frequency appear specific, since the magnitude of the JAR was not affected by the hormonal treatment. These findings suggest that seasonal changes in circulating concentrations of T may regulate behavioral changes in female chirping behavior and basal EOD frequency.Abbreviations DHT dihydrotestosterone - E estradiol - EOD elecdric organ discharge - GSI gonadal size index - JAR jamming avoidance response - PPn prepacemaker nucleus - T testosterone     

Neural coding of difference frequencies in the midbrain of the electric fishEigenmannia: Reading the sense of rotation in an amplitude-phase plane

Eigenmannia is able to discriminate the sign of the difference, Df, between the frequency of a neighbor's electric organ discharge (EOD) and that of its own EOD. This discrimination can be demonstrated at the level of individual neurons of the midbrain. Intracellular and extracellular recordings of such sign-selective cells revealed the following: Units preferring positive Dfs and units preferring negative Dfs were found with equal frequency. The degree of selectivity was also similar for these two classes of neurons. All sign-selective units were sensitive to the magnitude of the frequency difference, i.e. the beat rate. Most units responded best to beat rates in the 4-8 Hz range. Sign-selectivity was observed only when the jamming signal (S2) was presented through electrodes other than those used to deliver the mimic (S1) of the fish's EOD, i.e. only when amplitude modulations were accompanied by modulations of differential phase. Intracellular studies suggest that most sign-selective neurons of the tectum are large, multipolar cells in the stratum album centrale. These cells send projections to the reticular formation, to lamina 9 of the torus semicircularis and to the N. electrosensorius.     

Serotonin in a diencephalic nucleus controlling communication in an electric fish: sexual dimorphism and relationship to indicators of dominance

Serotonin regulates aggressive behavior. The production or release of serotonin is sexually dimorphic and related to social rank in many species. We examined serotonin expression in the central posterior/prepacemaker nucleus (CP/PPn) of the electric fish Apteronotus leptorhynchus. The CP/PPn is a thalamic nucleus that controls agonistic and reproductive electrocommunication signals known as chirps and gradual frequency rises. In parts of the CP/PPn that control chirping, females had more than twice as many serotonergic fibers and terminals as did males. Serotonin immunoreactivity in chirp-controlling areas of the CP/PPn was also negatively correlated with two indicators of dominance: electric organ discharge (EOD) frequency and body mass. Within sexes, the negative correlation between EOD frequency and serotonergic innervation of the PPn was significant in females, but not in males. Females with higher EOD frequencies had less serotonin in the CP/PPn than did females with lower EOD frequencies. Thus, the CP/PPn contained more serotonin in females than in males, and in particular, more serotonin in females with EOD frequencies typical of social subordinates than in females with EOD frequencies typical of social dominants. These results, combined with previous findings that serotonin inhibits chirping and that females chirp much less than males, suggest that serotonin may link sex, social rank, and the production of agonistic communication signals. The relative simplicity of the neural circuits that control the EOD and chirping make the electromotor system well-suited for studying the cellular, physiological, and behavioral mechanisms by which serotonin modulates agonistic communication.     

Evidence for a direct effect of androgens upon electroreceptor tuning

Tuberous electroreceptors of individual wave type weakly electric fish are tuned to the fundamental frequency of that fish's electric organ discharge (EOD). EOD frequency and receptor best frequency (BF) are both lowered following systemic injection of 5-alpha-dihydrotestosterone (DHT). A previous study (Meyer et al. 1984) showed that the effect of DHT on the EOD generating circuitry was independent of an ongoing EOD and suggested that its effect on electroreceptor tuning was indirect, possibly mediated by the electric field. We have continued these studies to determine the factors which influence electroreceptor tuning. Baseline recordings of EOD frequency, receptor oscillations, and single afferent tuning curves were taken. After fish were electrically silenced by spinal cord transection they were injected daily with either DHT or saline or were implanted with either DHT-filled or empty silastic capsules. As previously reported, the EOD frequency (determined from pacemaker nucleus recordings) was lowered in DHT-treated, transected fish and increased in control fish. Similarly, receptor tuning was lowered in the DHT-treated, silenced fish. Oscillation frequencies decreased in both treated and control groups, but significantly more in the hormone group. Single afferent best frequencies were lowered in both DHT groups and raised in their respective control groups. In another series of experiments exogenous electric fields capable of driving receptors in a 1-to-1 phase-locked manner were placed around silenced fish. We were unable to elicit any shift in pacemaker frequency or electroreceptor tuning regardless of stimulus field geometry. Four transected fish were injected with DHT and placed in exogenous electric fields of higher frequency than their original EOD. Even in the presence of a higher frequency electric field, DHT lowered EOD frequency and afferent BF. We conclude that androgens produce effects both on the EOD generating circuitry, probably at the level of the pacemaker nucleus, and on electroreceptors, probably, ultimately, on receptor cell membrane conductances. These effects occur in parallel allowing the two parameters to remain well matched. In contrast to former predictions, exogenous electric fields alone appear unable to shift receptor tuning.     

Court and spark: electric signals in the courtship and mating of gymnotoid fish

By mimicking tropical rainy season conditions in aquaria, we stimulated two species of gymnotoid electric fish, Eigenmannia virescens and Apteronotus leptorhynchus, to spawn in captivity. Their courtship activity, breeding behaviour and electric social communication were monitored in several groups over 2 years. Groups of both species established dominance hierarchies correlated with electric organ discharge frequency, aggressiveness and size. Spawning was preceded by several nights of courtship during which the male modulated its electric organ discharge to produce ‘chirps’. Continual bouts of chirping lasted for hours on evenings prior to spawning. These electrical signals play a significant role in courtship and spawning, as gravid E. virescens females could be stimulated to spawn by playing back into the tank a tape recording of male courtship chirps. In both species the chirp invovves a slight increase in frequency followed by a cessation of the dominant frequency. This suggests a common mode of signal production in these two different genera of fish. Chirps are short and abrupt during aggressive encounters, but assume a softer and more raspy quality during courtship.     

Individual prepacemaker neurons can modulate the pacemaker cycle of the gymnotiform electric fish,Eigenmannia

Summary The prepacemaker nucleus (PPN) in the midbrain of the gymnotiform electric fishEigenmannia provides the only known neuronal input to the medullary pacemaker nucleus, which triggers each electric organ discharge (EOD) cycle by a single command pulse. Electrical stimulation of the PPN elicited two distinct forms of modulations in the pacemaker activity, brief accelerations, hence referred to as chirps, and gradual frequency shifts with a time constant of approximately one second. The associated EOD modulations were indistinguishable from natural communication signals. Depending upon the site of stimulation, the two forms of modulation could be elicited alone or superimposed (Fig. 1). Stimulation sites eliciting only chirps could be separated from sites eliciting only gradual shifts by as little as 60 m. The magnitude of the elicited chirps depended upon the timing of the pulse stimulus with reference to the phase of the pacemaker cycle (Figs. 2, 3).Extracellular and intracellular recordings of single PPN neurons revealed that an action potential from a single neuron generates a chirp, and that the magnitude of the chirp depends upon the timing of the action potential with reference to the phase of the pacemaker cycle (Figs. 4, 5). The spike activity of these neurons had no relation to the jamming avoidance response (JAR), suggesting independent neuronal mechanisms for chirps and the JAR. Depolarization of such neurons by current injection produced bursts of chirps (Fig. 6), and intracellular injection of Lucifer Yellow identified these cells as a large type of PPN neuron which could also be retrogradely labeled from the pacemaker with horseradish peroxidase (HRP) (Fig. 7). We were unable to record from neurons linked to gradual shifts of the pacemaker frequency, although the JAR was elicited continually during the experiments. A smaller cell type of the PPN which can be retrogradely labeled with HRP but so far could not be recorded may control gradual frequency shifts.Abbreviations PPN prepacemaker nucleus - JAR jamming avoidance response - EOD electric organ discharge - Df neighbor's EOD frequency (or its mimic) minus animal's own EOD frequency (or its mimic)     

Communication in the weakly electric fish Sternopygus macrurus

There is a sexual dimorphism in the frequency of the quasi-sinusoidal electric organ discharge (EOD) of Sternopygus macrurus, with males, on average, an octave lower. EODs are detected by tuberous electroreceptor organs, which exhibit V-shaped frequency tuning with maximal sensitivity near the fish's own EOD frequency. This would seem to limit the ability of a fish to detect the EODs of opposite-sex conspecifics. However, electroreceptor tuning has always been based on single-frequency stimulation, while actual EOD detection involves the addition of a conspecific EOD to the fish's own. In the present study, recordings were made from single electroreceptive units while the fish were stimulated with pairs of sine waves: one (S1) representing the fish's own EOD added to a second (S2) representing a conspecific EOD. T unit response was easily predicted by assuming that the electroreceptor acts as a linear filter in series with a threshold-sensitive spike initiator. P unit response was more complex, and unexpectedly high sensitivity was found for frequencies of S2 well displaced from the fish's EOD frequency. For both P and T units, detection thresholds for S2 were much lower when added to S1, than when presented alone.     

Social electric signals in freely moving dyads of <Emphasis Type="Italic">Brachyhypopomus pinnicaudatus</Emphasis>

Brachyhypopomus pinnicaudatus (pulse-type weakly electric fish) is a gregarious species that displays reproductive behavior and agonistic encounters between males only during the breeding season. During social interactions, in addition to its basal electric organ discharge (EOD), fish emit social electric signals (SESs) in the contexts of reproduction and intrasexual aggression. We reproduced natural behavior in laboratory settings: SESs recorded in the field are indistinguishable from those observed in our experimental setup. SESs are nocturnal, change seasonally and exhibit sexual dimorphism. This study provides an exhaustive characterization and classification of SESs produced by males and females during the breeding season. In male–female dyads, males produce accelerations and chirps while females interrupt their EODs. The same SESs are observed in male–male dyads. We present a novel, thorough classification of male chirps into four independent types (A, B, C, and M) based on their duration and internal structure. The type M chirp is only observed in male–male dyads. Chirps and interruptions, both in male–female and male–male dyads, are emitted in bouts, which are also grouped throughout the night. Our data suggest the existence of a sophisticated electric dialog during reproductive and aggressive interaction whose precise timing and behavioral significance are being investigated.     

Electrolocation in the presence of jamming signals: electroreceptor physiology

Responses of ampullary and tuberous electroreceptor afferents were studied using moving electrolocation targets and electrical modulations of the animal's electric organ discharge as stimuli. The ability of the electroreceptors to encode these stimuli was measured with and without various forms of electrical jamming signals. The goal of this study was to measure the deterioration in electroreceptor responses due to the jamming signals, and to compare these results with the behavioral measures of electrolocation under the same conditions of jamming as described in the preceding report (Bastian 1987). 1. Three types of jamming stimuli were used to interfere with the tuberous electroreceptor afferents' ability to respond to the test stimuli mentioned above: Broad-band noise, high frequency stimuli consisting of a sinusoidal waveform having a frequency maintained at a chosen difference frequency (DF) from the EOD frequency of the fish being studied, and 5 or 50 Hz sinusoidal stimuli. 2. The tuberous receptor afferents' spontaneous frequency was sensitive to continuous presentation of all but the 5 Hz jamming signals. The 4 Hz DF signal caused the largest increase in spontaneous activity, the 50 Hz stimulus was intermediate in effectiveness, and the noise stimulus caused the smallest increase. Estimates of the variability of the ongoing receptor activity were also made, and both the 4 Hz DF and the 50 Hz stimuli reduced the coefficient of variation of the receptor activity, but noise had no significant effect on this parameter. Noise, 4 Hz DF, and 50 Hz jamming signals also reduced the tuberous receptors' responses to a 100 ms EOD amplitude modulation, and the 5 Hz stimulus was again ineffective. 3. Noise and 4 Hz DF jamming were also effective in reducing tuberous receptor afferents' responses to a moving metal electrolocation target. The 4 Hz DF stimulus was most effective in reducing the receptor's ability to encode information about the target. Receptor responses showed about a three-fold larger decrease per 10 dB increase in DF jamming amplitude as compared to similar sized increases in noise amplitude. Threshold target distances were also determined with and without noise and DF jamming, and again, the noise stimulus was less effective in reducing the distance at which electrolocation targets were just detectable. 4. Recordings from ampullary receptor afferents confirmed that the galvanic potentials produced by metal electrolocation targets stimulate these receptors while EOD distortions caused by such objects probably do not.(ABSTRACT TRUNCATED AT 400 WORDS)     

The electric organ discharges of the gymnotiform fishes: I. Apteronotus leptorhynchus

We present high temporal and spatial resolution maps in 3-dimensions of the electric field vector generated by the weakly electric fish, Apteronotus leptorhynchus. The waveforms and harmonic composition of the electric organ discharge (EOD) are variable around the fish but highly stable over long times at any position. We examine the role of harmonics on the temporal and spatial characteristics of the EOD, such as the slew rate and rostral-to-caudal propagation. We also explore the radial symmetry of the fish's field. There are major differences in the direction of the electric field vector at the head and caudal body. In the caudal part of the fish, the electric field vector rotates during the EOD cycle. However, rostral of the pectoral fin, the field magnitude and sign oscillate while maintaining relatively constant orientation. We discuss possible functional ramifications of these electric field patterns to electrolocation, communication, and electrogenesis.Abbreviations EOD electric organ discharge - EO electric organ - RMS root mean square - ADC analog-to-digital converter     

Discrimination of the sign of frequency differences bySternopygus,an electric fish without a jamming avoidance response

Several species of weakly electric fish reflexively change their frequency of electric organ discharge (EOD) in response to sensing signals of similar frequency from conspecifics; that is, they exhibit jamming avoidance responses (JAR).Eigenmannia increases its EOD frequency if jammed by a signal of lower frequency and decreases its EOD frequency if jammed by a signal of higher frequency. This discrimination is based on an analysis of the patterns of amplitude modulations and phase differences resulting from signal interference. Fish of the closely related genus,Sternopygus, however, do not exhibit a JAR. Here we show that despite lacking this behavior,Sternopygus shares many sensory processing capacities withEigenmannia:

Hormone-induced and maturational changes in electric organ discharges and electroreceptor tuning in the weakly electric fishApteronotus

Summary Plasticity in the frequency of the electric organ discharge (EOD) and electroreceptor tuning of weakly electric fish was studied in the genusApteronotus. Both hormone-induced and maturational changes in EOD frequency and electroreceptor tuning were examined.Apteronotus is different from all other steroid-responsive weakly electric fish in that estradiol-17, rather than androgens, induces discharge frequency decreases. This result can account for the reversed discharge frequency dimorphism found inApteronotus in which, counter to all other known sexually dimorphic electric fish, females have lower discharge frequencies than males. Studies of electroreceptor tuning inApteronotus indicate that electroreceptors are closely tuned to the frequency of the EOD. This finding was noted not only in adult animals, but also in juvenile animals shortly after the onset of their EODs. Tuning plasticity inApteronotus, as in other species studied, is associated with altered EOD frequencies and was noted in both maturational EOD changes and in estrogen-induced changes. Thus, tuning plasticity appears to be a general phenomenon which occurs concurrent with a variety of EOD changes.