Genetic drift does not sufficiently explain patterns of electric signal variation among populations of the mormyrid electric fish Paramormyrops kingsleyae

Communication signals serve crucial survival and reproductive functions. In Gabon, the widely distributed mormyrid fish Paramormyrops kingsleyae emits an electric organ discharge (EOD) signal with a dual role in communication and electrolocation that exhibits remarkable variation: populations of P. kingsleyae have either biphasic or triphasic EODs, a feature that characterizes interspecific signal diversity among the Paramormyrops genus. We quantified variation in EODs of 327 P. kingsleyae from nine populations and compared it to genetic variation estimated from microsatellite loci. We found no correlation between electric signal and genetic distances, suggesting that EOD divergence cannot be explained by drift alone. An alternative hypothesis is that EOD differences are used for mate discrimination, which would require P. kingsleyae be capable of differentiating between divergent EOD waveforms. Using a habituation-dishabituation assay, we found that P. kingsleyae can discriminate between biphasic and triphasic EOD types. Nonetheless, patterns of genetic and electric organ morphology divergence provide evidence for hybridization between these signal types. Although reproductive isolation with respect to signal type is incomplete, our results suggest that EOD variation in P. kingsleyae could be a cue for assortative mating.     

Single electrocytes produce a sexually dimorphic signal in South American electric fish,Hypopomus occidentalis (Gymnotiformes,Hypopomidae)

Summary Hypopomus occidentalis is a weakly electric Gymnotiform fish with a pulse-type electric organ discharge (EOD).Hypopomus used in this study were taken from one of the northernmost boundaries of this species, the Atlantic drainage of Panama where the animals breed at the beginning of the dry season (December). In normal breeding populations,Hypopomus occidentalis exhibit a sexual dimorphism in EOD and morphology. Mature males are large with a broad tail and have an EOD characterized by a low peak power frequency. Females and immature males are smaller, having a slender tail and EODs with higher peak power frequencies (Fig. 1). This study describes differences in the EOD and electric organ morphology between breeding field populations of male and femaleHypopomus. Changes in physiology, morphology and EOD shape which may accompany this seasonal change were examined in steroid injected fish, using standard histological and physiological techniques.A group of females were injected with hormones (5-dihydrotestosterone (DHT), estrogen or saline) to assess changes in their morphology and EOD. Animals treated with DHT developed characteristics which mimicked the sexually dimorphic characteristics of a male, while the other groups did not (see Fig. 5). Tissue from the tails of breeding males and females, and females treated with DHT, were sampled to measure the size of the electrocytes in the tail. The broader tail of males and DHT-females is composed of large electrocytes, whereas the slender tail of normal females is composed of smaller electrocytes. Therefore, the increase in the tail width in the female DHT group is caused by an enlargement of the electrocytes in this area.Intracellular recordings from the electrocytes of saline and DHT injected females show a difference in the responses of the rostral faces of the electrocytes from the two groups, which reflect the differences in their EODs. Saline-treated animals had symmetrical EODs (the first and second phase of the EOD were equal in duration and amplitude), while the physiological responses from each face of the electrocytes yielded responses that were similarly equal in duration and amplitude. DHT-treated animals had asymmetrical EODs (the first phase of the EOD was similar to that of saline treated fish and larger in amplitude and shorter in duration than the second phase) and the physiological responses of the electrocytes reflected this asymmetry. The differential recordings across the caudal face were similar to those from saline treated fish, while the responses from the rostral face were longer in duration and smaller in amplitude.These data suggest that the effects of androgens underlie the changes in single electrocytes which produce the sexually dimorphic signals and morphology present in natural breeding populations ofHypopomus occidentalis.     

Morphological variation in the electric organ of the little skate (Leucoraja erinacea) and its possible role in communication during courtship

Skates discharge an electrical current too weak to be used for predation or defense, and too infrequent and irregular to be used for electrolocation. Additionally, skates possess a specialized sensory system that can detect electrical stimuli at the same strength at which they discharge their organs. These two factors are suggestive of a communicative role for the electric organ in skates, a role that has been demonstrated in similarly weakly electric teleosts (e.g., mormyrids and gymnotiforms). There is evidence that the sexual and ontogenetic variations in the electric organ discharge (EOD) in these other weakly electric fishes are linked to morphological variations in electric organs and the electrogenerating cells of the organs, the electrocytes. Little work has been done to examine possible sexual and ontogenetic variations in skate EODs or variations in the electrocytes responsible for those discharges. Electric organs and electrocyte morphology of male and female, and mature and immature little skates, Leucoraja erinacea, are characterized here. Female electric organs were bigger than male electric organs. This is suggestive of a sexually dimorphic EOD waveform or amplitude, which might be used as a sex-specific identification signal during courtship. The shapes of electrocytes that make up the organ were found to be significantly different between mature and immature individuals and, in some cases, posterior membrane surface area of the electrocytes increased at the onset of maturity due to the formation of membrane surface invaginations and papillae. This is evidence that the EOD of skates may differ in its waveform or amplitude or frequency between mature and immature skates, and act as a signal for readiness to mate. This study supports a communicative role during courtship for the weak electric organs of little skates, but studies that characterize skate EOD dimorphisms are needed to corroborate this speculation before conclusions can be drawn about the role the electric organ plays in communication during courtship.     

Petrocephalus of Odzala offer insights into evolutionary patterns of signal diversification in the Mormyridae, a family of weakly electrogenic fishes from Africa

Electric signals of mormyrid fishes have recently been described from several regions of Africa. Members of the Mormyridae produce weak electric organ discharges (EODs) as part of a specialized electrosensory communication and orientation system. Sympatric species often express distinctive EODs, which may contribute to species recognition during mate choice in some lineages. Striking examples of interspecific EOD variation within assemblages have been reported for two monophyletic radiations: the Paramormyrops of Gabon and the Campylomormyrus of Lower Congo. Here, we describe a speciose assemblage of Petrocephalus in the Lékoli River system of Odzala National Park, Republic of Congo. This widespread genus comprises the subfamily (Petrocephalinae) that is the sister group to all other mormyrids (Mormyrinae). Eleven Petrocephalus species were collected in Odzala, five of which are not described taxonomically. We quantify EOD variation within this assemblage and show that all eleven species produce EOD waveforms of brief duration (species means range from 144 to 663 μs) compared to many other mormyrids. We also present reconstructed phylogenetic relationships among species based on cytochrome b sequences. Discovery of the Odzala assemblage greatly increases the number of Petrocephalus species for which EODs and DNA sequence data are available, permitting a first qualitative comparison between mormyrid subfamilies of the divergence patterns that have been described within lineages. We find that the Petrocephalus assemblage in Odzala is not a monophyletic radiation. Genetic divergence among Petrocephalus species often appears higher than among Paramormyrops or Campylomormyrus species. In contrast, results of this study and others suggest that Petrocephalus may generally exhibit less interspecific EOD divergence, as well as smaller sex differences in EOD waveforms, compared to Paramormyrops and Campylomormyrus. We discuss possible causes and consequences of EOD diversification patterns observed within mormyrid subfamilies as a framework for future comparative studies of signal evolution using this emerging model system.     

Spatial distribution of the medullary command signal within the electric organ of Gymnotus carapo

The pacemaker nucleus of Gymnotus carapo contains two types of neurons: pacemaker cells which set up the frequency of the electric organ discharge (EOD) and relay cells which convey the command signal to the spinal cord. Direct activation of a single relay cell provides enough excitation to discharge a pool of spinal electromotor neurons and electrocytes, generating a small EOD (unit EOD). Different relay cells generate unit EODs of variable size and waveform, indicating the involvement of different groups of electrocytes. A special technique of EOD recording (multiple air-gap) was combined with intracellular stimulation of relay cells to study the spatial distribution within the electric organ (EO) of the command signal arising from different relay cells. Three types of relay cells could be identified: type I commanding the rostral 10% of the EO, type II which distribute their command all along the EO and type III driving the caudal 30%. Waveform analysis of unit EODs indicates that doubly innervated electrocytes which are the most relevant for attaining the specific EOD waveform, receive a favored command from the pacemaker nucleus.Abbreviations CV conduction velocity - EMF electromotive force - EMN electromotor neuron - EO electric organ - EOD electric organ discharge - PN pacemaker nucleus - uEOD unit electric organ discharge     

A hormone-sensitive communication system in an electric fish

The electric communication system includes both special muscle-derived cells or electrocytes that produce species-typical electric signals, or electric organ discharges (EODs), and specialized sensory receptors, or electroreceptors, that encode the electric fields set up by EODs. Steroid hormones can influence the characteristic properties of both EODs and electroreceptors. Steroids appear to directly effect the anatomy and physiology of the electrocytes that generate an EOD. In contrast, the steroid effect on electroreceptors may be predominantly via an indirect mechanism whereby changes in the spectral characteristics of the EOD appear to induce changes in the spectral sensitivity of electroreceptors. Continued studies of electrosensory and electromotor systems will offer insights into the cellular bases for the development and evolution of steroid-sensitive pathways in the vertebrate nervous system.     

Behavioral responses of weakly electric fish to complex impedances

Summary Members of the family of African electric fish, Mormyridae, exhibit a novelty response, consisting of an acceleration in the rate of electric organ discharges (EODs), when faced with changes in feedback arising from their EODs. In this study, the novelty responses of three different species of mormyrids to shunts with different electrical characteristics were noted. The three species differed in the frequency contents of their EODs: two species had relatively high spectral frequencies in their EODs (>10 kHz), while the third species had only lower spectral frequencies (< 10 kHz). Primarily resistive shunts elicited novelty response accelerations in all three species, and the magnitudes of these responses, when normalized to the responses obtained for a shunt with no introduced resistance, were comparable for all three species. For primarily capacitive shunts, however, the magnitudes of the normalized responses were different for the three species: the two species with high spectral frequencies in their EODs showed larger normalized responses than the third species which had only low EOD spectral frequencies.The differences in species responses for capacitive shunts, and the similarities in species responses for resistive shunts, suggest that electric fish detect the complex impedance of objects in their near field environment: a circuit model consisting of a fish emitting discharges into the surrounding water, which can be shunted by a variable complex impedance, conforms well to the data. Thus, electrolocation is a frequency dependent sensory process, and this frequency dependency should be considered in any speculation about the adaptive value of different EOD waveforms.Abbreviation EOD electric organ discharge     

Trained Weakly-electric Fishes Pollimyrus isidori and Gnathonemus petersii (Mormyridae,Teleostei) Discriminate between Waveforms of Electric Pulse Discharges

Fish of the family Mormyridae emit weak, pulse-like electric organ discharges (EODs). The discharge rhythm is variable, but the waveform of the EOD is constant for each fish, with species- and individual characteristics. The ability of Pollimyrus isidori and Gnathonemus petersii (Mormyridae) to discriminate between different EOD waveforms was tested using a differential conditioning procedure. Fish were first trained to respond to a reference signal in swimming to a dish to receive a bloodworm (food reward). The reference signal consisted of a 10-Hz train of the digitally recorded EOD of a conspecific. Second, an alternative signal (10-Hz train of a different EOD, either from another species, or from a conspecific of the other sex) was associated with air bubbles as punishment. The two signals were played at successive trials in random order. On each trial the latency was measured between the onset of the signal and the response. 7 out of the 8 P. isidori tested and both of the two G. petersii tested associated the reference EOD with food. Among these, five P. isidori and two G. petersii responded differentially (p < 0.01) to EODs of different species. P. isidori similarly discriminated between conspecific EODs of different sexes. The quantity of different alternative EODs which could be tested was limited when fish eventually habituated to the punishment. Even when the amplitude of the EODs was randomly changed at each trial, two out of two G. petersii differentiated between EODs of the two species, and three out of three P. isidori tested differentiated between EODs within their own species. Response latencies to the rewarded signal during the basic training and during discrimination (when it had to be distinguished from the S-) were similar. G. petersii showed differential responses for S+ and S- also in the rhythm of discharge exhibited during playback, after five EOD pulses for one fish, and after a single pulse for the other. Mormyrids may therefore distinguish between conspecifics and members of other species, and even between individual conspecifics, by their EOD waveform.     

Sound production to electric discharge: sonic muscle evolution in progress in Synodontis spp. catfishes (Mochokidae)

Elucidating the origins of complex biological structures has been one of the major challenges of evolutionary studies. Within vertebrates, the capacity to produce regular coordinated electric organ discharges (EODs) has evolved independently in different fish lineages. Intermediate stages, however, are not known. We show that, within a single catfish genus, some species are able to produce sounds, electric discharges or both signals (though not simultaneously). We highlight that both acoustic and electric communication result from actions of the same muscle. In parallel to their abilities, the studied species show different degrees of myofibril development in the sonic and electric muscle. The lowest myofibril density was observed in Synodontis nigriventris, which produced EODs but no swim bladder sounds, whereas the greatest myofibril density was observed in Synodontis grandiops, the species that produced the longest sound trains but did not emit EODs. Additionally, S. grandiops exhibited the lowest auditory thresholds. Swim bladder sounds were similar among species, while EODs were distinctive at the species level. We hypothesize that communication with conspecifics favoured the development of species-specific EOD signals and suggest an evolutionary explanation for the transition from a fast sonic muscle to electrocytes.     

Electrolocation of Capacitive Objects in Four Species of Pulse-type Weakly Electric Fish I. Discrimination Performance

The great variety of species-typical electric signals (electric organ discharges, EOD) emitted by weakly electric mormyrid fish might be the result of evolutionary pressures stemming from the two main functions of the electro-sensory-motor system: electrocommunication and electrolocation. Employing a conditioned discrimination task we tested four species of mormyrids, emitting EODs differing in waveform, for their ability to detect capacitive properties of objects during electrolocation. Each fish could discriminate capacitive objects within a certain range of capacitive values, which was species specific. The upper and lower limits (upper and lower thresholds) of this detectable range were determined for each fish. In fish species emitting long duration EODs composed of mainly low spectral frequencies both the lower and the upper thresholds were shifted to larger capacitive values compared to fish species emitting shorter EODs. The upper limit of the detectable range was much more variable between species than the lower limit, which was relatively low in all fish. We interpret this as an adaptation of mormyrids to detect small capacitive objects, for example food items. All mormyrids could discriminate between a resistive object and a capacitive object even if the complex impedances of the two objects were identical. This implies that the fish are highly sensitive to small waveform distortions of their self produced EODs.     

Variation of electric organ discharge in Mormyrus kannume (Mormyridae) from south-western Ethiopia in relation to individual size,sex and reproductive status

Five variables of the electric organ discharge (EOD) were studied in 132 freshly caught individuals of Mormyrus kannume from the Gibe and Gojeb rivers (the enclosed Omo-Turkana system) and Baro River (the White Nile system) in south-western Ethiopia, East Africa. Most individuals exhibited the typical biphasic EODs with an initial head-positive phase followed by a head-negative phase exceeding initial one in the relative amplitude. In three repeated samples from the Gibe, stable differences in the discharge variables between females and males were found, while no sex differences were observed in other samples. In three cases, a positive correlation between the individual size and discharge duration was found: in two male subsamples from the Gibe and in a mixed sample of both sexes from the Gojeb. In most samples studied, some males exhibited the outstandingly long EOD duration. At the same time, no relationships between the discharge variables and reproductive status of individuals were found. The possible influence of individual and intra-individual variations on the results of inter-population and interspecific comparisons of the discharge variables in mormyrids are highlighted.     

Behavioral detection of electric signal waveform distortion in the weakly electric fish,Gnathonemus petersii

The novelty response of weakly electric mormyrids is a transient acceleration of the rate of electric organ discharges (EOD) elicited by a change in stimulus input. In this study, we used it as a tool to test whether Gnathonemus petersii can perceive minute waveform distortions of its EOD that are caused by capacitive objects, as would occur during electrolocation. Four predictions of a hypothesis concerning the mechanism of capacitance detection were tested and confirmed: (1) G. petersii exhibited a strong novelty response to computer-generated (synthetic) electric stimuli that mimic both the waveform and frequency shifts of the EOD caused by natural capacitive objects (Fig. 3). (2) Similar responses were elicited by synthetic stimuli in which only the waveform distortion due to phase shifting the EOD frequency components was present (Fig. 4). (3) Novelty responses could reliably be evoked by a constant amplitude phase shifted EOD that effects the entire body of the fish evenly, i.e., a phase difference across the body surface was lacking (Figs. 3, 4). (4) Local presentation of a phase-shifted EOD mimic that stimulated only a small number of electroreceptor organs at a single location was also effective in eliciting a behavioral response (Fig. 5).Our results indicate that waveform distortions due to phase shifts alone, i.e. independent of amplitude or frequency cues, are sufficient for the detection of capacitive, animate objects. Mormyrids perceive even minute waveform changes of their own EODs by centrally comparing the input of the two types of receptor cells within a single mormyromast electroreceptor organ. Thus, no comparison of differentially affected body regions is necessary. This shows that G. petersii indeed uses a unique mechanism for signal analysis, which is different from the one employed by gymnotiform wavefish.Abbreviations EOD electric organ discharge - p-p-amplitude peak-to-peak amplitude     

Electric organ morphology of Sternopygus macrurus,a wave-type,weakly electric fish with a sexually dimorphic EOD

In several species of electric fish with a sex difference in their pulse-type electric organ discharge (EOD), the action potential-generating cells of the electric organ (electrocytes) of males are larger and more invaginated compared to females. Androgen treatment of females and juveniles produces a longer-duration EOD pulse that mimics the mature male EOD, with a concurrent increase in electrocyte size and/or membrane infolding. In Sternopygus macrurus, which generates a wave-type EOD, androgen also increases EOD pulse duration. To investigate possible morphological correlates of hormone-dependent changes in EOD in Sternopygus, we examined electric organs from both fish collected in the field, and untreated and androgen-treated specimens in the laboratory. The electrocytes are cigar shaped, with prominent papillae on the posterior, innervated end. Electrocytes of field-caught specimens were significantly larger in all parameters than were electrocytes of specimens maintained in the laboratory. EOD pulse duration and frequency were highly correlated, and were significantly different between the sexes in sexually mature fish. Nevertheless, no sex difference in electrocyte morphology was observed, nor did any parameters of electrocyte morphology correlate with EOD pulse duration or frequency. Further, whereas androgen treatment significantly lowered EOD frequency and broadened EOD pulse duration, there was no difference in electrocyte morphology between hormone-treated and control groups. Thus, in contrast to results from studies on both mormyrid and gymnotiform pulse fish, electrocyte morphology is not correlated with EOD waveform characteristics in the gymnotiform wave-type fish Sternopygus. The data, therefore, suggest that sex differences in EOD are dependent on changes in active electrical properties of electrocyte membranes. © 1992 John Wiley & Sons, Inc.     

Electric signaling behavior and the mechanisms of electric organ discharge production in mormyrid fish.

Mormyrid fish communicate and navigate using electric organ discharges (EODs). The EOD is highly stereotyped and provides information on sender identity, including species, sex, reproductive condition, and possibly relative status and individual identity. By contrast, the sequence of pulse intervals (SPI) is variable and plays more of a role in signaling behavioral states. Various types of SPI displays may be produced, including tonic patterns such as 'random' and 'regularized', and phasic patterns such as 'bursts' and cessations'. Certain displays have been linked to specific behaviors such as aggression, submission, courtship and active exploration. In addition, interacting pairs of fish may produce stereotyped displays involving the relative timing of their EODs. The EOD waveform is controlled by the morphological and physiological properties of cells in the electric organ termed electrocytes. Differences in the innervation, morphology, size and membrane characteristics of electrocytes have been directly linked to species and sex differences in the EOD. The generation of each EOD is initiated in the medullary command nucleus (CN), which thereby determines the timing of EOD output. CN does not have any properties of a pacemaker, but rather appears to integrate descending inputs that affect the probability of EOD production. The precommand nucleus (PCN) provides a major source of excitatory input to CN and is itself inhibited by corollary discharge feedback following the production of each EOD. Changes in the activity of PCN and its inhibitory feedback neurons modify EOD output, and therefore drive the generation of SPI patterns. Current studies are addressing the mechanisms underlying the generation of these patterns and preliminary results suggest that different types of signals may be controlled by distinct components of the electromotor system. This is similar to findings in other electrogenic teleosts, suggesting that it may be a general feature in the motor control of signaling behavior.     

An internal current source yields immunity of electrosensory information processing to unusually strong jamming in electric fish

Summary The electric organ of a fish represents an internal current source, and the largely isopotential nature of the body interior warrants that the current associated with the fish's electric organ discharges (EODs) recruits all electroreceptors on the fish's body surface evenly. Currents associated with the EODs of a neighbor, however, will not penetrate all portions of the fish's body surface equally and will barely affect regions where the neighbor's current flows tangentially to the skin surface. The computational mechanisms of the jamming avoidance response (JAR) in Eigenmannia exploit the uneven effects of a neighbor's EOD current to calculate the correct frequency difference between the two interfering EOD signals even if the amplitude of a neighbor's signal surpasses that of the fish's own signal by orders of magnitude. The particular geometry of the fish's own EOD current thus yields some immunity against the potentially confusing effects of unusually strong interfering EOD currents of neighbors.Abbreviations DF frequency difference - ELL electrosensory lateral line lobe - EOD electric organ discharge - JAR jamming avoidance response     

Observations on the electric organ discharge of two skate species (Chondrichthyes: Rajidae) and its relationship to behaviour

Synopsis The electric organ discharge (EOD) of the little skate,Raja erinacea and winter skate,R. ocellata was recorded both from isolated individuals and from small groups using methods that allowed for the identification of individuals producing EODs. Pulse duration, train lengh, frequency, and pulse patterns are characterized and correlated with behaviour. The two species,R. erinacea andR. ocellata, were found to have characteristically different EOD pulse durations of 70 ms and 217 ms respectively. Isolated skates rarely discharged whereas groups of skates were found to discharge regularly. The EOD was evoked by tactile prodding, physical contact with other skates and electrical stimulation. Skates also discharged reflexively in response to an artificially induced head-positive DC stimulus, sine wave and monopolar square pulses. During approach and contact, skates responded to each other with interacting EOD displays. EOD interaction and pulse duration differences between other species suggest a possible intra-specific communication function of the EOD inRaja.     

Interruption of pacemaker signals by a diencephalic nucleus in the African electric fish, Gymnarchus niloticus

The African electric fish Gymnarchus niloticus rhythmically emits electric organ discharges (EODs) for communication and navigation. The EODs are generated by the electric organ in the tail in response to the command signals from the medullary pacemaker complex, which consists of a pacemaker nucleus (PN), two lateral relay nuclei (LRN) and a medial relay nucleus (MRN). The premotor structure and its modulatory influences on the pacemaker complex have been investigated in this paper. A bilateral prepacemaker nucleus (PPn) was found in the area of the dorsal posterior nucleus (DP) of the thalamus by retrograde labeling from the PN. No retrogradely labeled neurons outside the pacemaker complex were found after tracer injection into the LRN or MRN. Accordingly, anterogradely labeled terminal fibers from PPn neurons were found only in the PN. Iontophoresis of l-glutamate into the region of the PPn induced EOD interruptions. Despite the exclusive projection of the PPn neurons to the PN, extracellular and intracellular recordings showed that PN neurons continue their firing while MRN neurons ceased their firing during EOD interruption. This mode of EOD interruption differs from those found in any other weakly electric fishes in which EOD cessation mechanisms have been known.     

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.     

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.