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.     

Signal variation and its morphological correlates in <Emphasis Type="Italic">Paramormyrops kingsleyae</Emphasis> provide insight into the evolution of electrogenic signal diversity in mormyrid electric fish

We describe patterns of geographic variation in electric signal waveforms among populations of the mormyrid electric fish species Paramormyrops kingsleyae. This analysis includes study of electric organs and electric organ discharge (EOD) signals from 553 specimens collected from 12 localities in Gabon, West-Central Africa from 1998 to 2009. We measured time, slope, and voltage values from nine defined EOD “landmarks” and determined peak spectral frequencies from each waveform; these data were subjected to principal components analysis. The majority of variation in EODs is explained by two factors: the first related to EOD duration, the second related to the magnitude of the weak head-negative pre-potential, P0. Both factors varied clinally across Gabon. EODs are shorter in eastern Gabon and longer in western Gabon. Peak P0 is slightly larger in northern Gabon and smaller in southern Gabon. P0 in the EOD is due to the presence of penetrating-stalked (Pa) electrocytes in the electric organ while absence is due to the presence of non-penetrating stalked electrocytes (NPp). Across Gabon, the majority of P. kingsleyae populations surveyed have only individuals with P0-present EODs and Pa electrocytes. We discovered two geographically distinct populations, isolated from others by barriers to migration, where all individuals have P0-absent EODs with NPp electrocytes. At two sites along a boundary between P0-absent and P0-present populations, P0-absent and P0-present individuals were found in sympatry; specimens collected there had electric organs of intermediate morphology. This pattern of geographic variation in EODs is considered in the context of current phylogenetic work. Multiple independent paedomorphic losses of penetrating stalked electrocytes have occurred within five Paramormyrops species and seven genera of mormyrids. We suggest that this key anatomical feature in EOD signal evolution may be under a simple mechanism of genetic control, and may be easily influenced by selection or drift throughout the evolutionary history of mormyrids.     

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.     

Sex recognition and neuronal coding of electric organ discharge waveform in the pulse-type weakly electric fish, Hypopomus occidentalis

1. Hypopomus occidentalis, a weakly electric gymnotiform fish with a pulse-type discharge, has a sexually dimorphic electric organ discharge (Hagedorn 1983). The electric organ discharges (EODs) of males in the breeding season are longer in duration and have a lower peak-power frequency than the EODs of females. We tested reproductively mature fish in the field by presenting electronically generated stimuli in which the only cue for sex recognition was the waveshape of individual EOD-like pulses in a train. We found that gravid females could readily discriminate male-like from female-like EOD waveshapes, and we conclude that this feature of the electric signal is sufficient for sex recognition. 2. To understand the possible neural bases for discrimination of male and female EODs by H . occidentalis, we conducted a neurophysiological examination of both peripheral and central neurons. Our studies show that there are sets of neurons in this species which can discriminate male or female EODs by coding either temporal or spectral features of the EOD. 3. Temporal encoding of stimulus duration was observed in evoked field potential recordings from the magnocellular nucleus of the midbrain torus semicircularis. This nucleus indirectly receives pulse marker electroreceptor information. The field potentials suggest that comparison is possible between pulse marker activity on opposite sides of the body. 4. From standard frequency-threshold curves, spectral encoding of stimulus peak-power frequency was measured in burst duration coder electroreceptor afferents. In both male and female fish, the best frequencies of the narrow-band population of electroreceptors were lower than the peak-power frequency of the EOD. Based on this observation, and the presence of a population of wide-band receptors which can serve as a frequency-independent amplitude reference, a slope-detection model of frequency discrimination is advanced. 5. Spectral discrimination of EOD peak-power frequency was also shown to be possible in a more natural situation similar to that present during behavioral discrimination. As the fish's EOD mimic slowly scanned through and temporally coincided with the neighbor's EOD mimic, peak spike rate in burst duration coder afferents was measured. Spike rate at the moment of coincidence changed predictably as a function of the neighbor's EOD peak-power frequency. 6. Single-unit threshold measurements were made on afferents from peripheral burst duration coder receptors in the amplitude-coding pathway, and midbrain giant cells in the time-coding pathway.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effects of androgens and estrogen on the external morphology and electric organ discharge waveform of Gnathonemus petersii (Mormyridae, Teleostei)

The effects of androgens and estrogen on the external morphology and electric organ discharge (EOD) waveform in Gnathonemus petersii, a weakly discharging electric fish, were investigated. Following preimplant data collection, juvenile and adult fish were gonadectomized and implanted with silastic capsules containing either high or low doses of testosterone (T), dihydrotestosterone (DHT), estradiol-17 beta (E2), or cholesterol. One group of fish was treated with high doses of DHT + E2. Radioimmunoassays revealed that low-dose implants resulted in plasma T levels comparable to and high-dose implants about sixfold greater than those found in adult males imported during breeding season. High-dose E2 implants resulted in higher plasma E2 levels in adults than those in juveniles. At either dose, both androgens induced male-like indentations in the dorsal margin of the anal fin of juveniles and adult females by 4 weeks postimplant. Both low and high doses of T decreased the peak power spectrum frequency (PPSF) of Fourier transformations of EODs and increased the durations of phases 2 and 3 of the EOD in juveniles and adults, but the high doses caused more rapid and profound effects. The two doses of T caused opposite effects on the durations of phases 1 and 4 juveniles. The low dose of T decreased the durations of phases 1 and 4, while the high dose increased them. In adults, the high dose of T increased the duration of phase 1, but had inconsistent effects on the duration of phase 4. Total EOD durations were increased by both doses of T in juveniles, while adults showed inconsistent effects possibly due to individual variability in hormone sensitivity. Compared to T, DHT exerted similar, but less dramatic effects on all measures, but only at high doses. E2 significantly increased adult PPSFs, the first such finding in a mormyrid species. E2 had no effects on juvenile PPSFs, or on adult or juvenile EOD phase durations. The effects of DHT + E2 on PPSF and phases 2 and 3 were similar to those of DHT alone. These findings demonstrate quantifiable steroid-dependent plasticity in the durations of individual phases of EODs in an electric fish and are the first to show that the external morphology in Gnathonemus petersii is androgen-dependent. The results are discussed with regard to methodological considerations and hormone studies involving sex differences in EODs reported for this and other species.     

Androgens enhance plasticity of an electric communication signal in female knifefish, Brachyhypopomus pinnicaudatus

Sex steroids were initially defined by their actions shaping sexually dimorphic behavioral patterns. More recently scientists have begun exploring the role of steroids in determining sex differences in behavioral plasticity. We investigated the role of androgens in potentiating circadian, pharmacological, and socially-induced plasticity in the amplitude and duration of electric organ discharges (EODs) of female gymnotiform fish. We first challenged female fish with injections of serotonin (5-HT) and adrenocorticotropic hormone (ACTH), and with social encounters with female and male conspecifics to characterize females' pre-implant responses to each treatment. Each individual was then implanted with a pellet containing dihydrotestosterone (DHT) concentrations of 0.0, 0.03, 0.1, 0.3, or 1.0 mg 10 g^− 1 body weight. We then repeated all challenges and compared each female's pre- and post-implant responses. The highest implant dose enhanced EOD duration modulations in response to all challenge types, responses to male challenge were also greater at the second highest dose, and responses to ACTH challenge were enhanced in females receiving all but the smallest dose (and blank) implants. Alternatively, amplitude modulations were enhanced only during female challenges and only when females received the highest DHT dose. Our results highlight the differential regulation of EOD duration and amplitude, and suggest that DHT enhanced the intrinsic plasticity of the electrogenic cells that produce the EOD rather than modifying behavioral phenotypes. The relative failure of DHT to enhance EOD amplitude plasticity also implies that factors other than androgens are involved in regulating/promoting male-typical EOD circadian rhythms and waveform modulations displayed in social contexts.     

Androgen correlates of socially induced changes in the electric organ discharge waveform of a mormyrid fish

Weakly electric fish from the family Mormyridae produce pulsatile electric organ discharges (EODs) for use in communication. For many species, male EODs are seasonally longer in duration than those of females, and among males, there are also individual differences in EOD duration. While EOD elongation can be induced by the administration of exogenous androgens, androgen levels have never before been assessed under natural or seminatural conditions. By simulating the conditions occurring during the breeding season in the laboratory, we provide evidence of a sex difference in EOD duration as well as document levels of circulating androgens in males. In this study, we analyzed the nature of social influences on male EOD duration and plasma androgen levels in Brienomyrus brachyistius. Individual males, first housed with a single female and then placed into social groups consisting of three males and three females, showed status-dependent changes in EOD duration. Top-ranking males experienced a relatively large increase in EOD duration. Second-ranking males experienced a more modest increase, and low-ranking males experienced a decrease in EOD duration. These changes were paralleled by differences in circulating levels of plasma 11-ketotestosterone (11-KT), but not testosterone, suggesting that the changes in EOD duration may have been mediated by changes in plasma 11-KT levels. Thus, it appears that EOD duration is an accurate indicator of male status, which is under social and hormonal control.     

Electrocommunication and Social Behaviour in Marcusenius senegalensis (Mormyridae,Teleostei)

The electric organ discharges (EODs) of Marcusenius senegalensis, a West African freshwater fish, are bipolar pulses of short duration (220 ± SE 13 μs). In males (n = 10; 10.1–13.1 cm standard length — which is around the size of getting mature), the duration of EOD pulses was of significantly greater variance than in females (n = 9; 9.8–12.8 cm standard length). Male EODs also showed a tendency for a longer duration than female EODs. Groups of three as well as of 14 M. senegalensis formed temporary schools in a ‘naturally’ equipped 720-1 tank. While swimming slowly in a loose school during their nocturnal active phase, fish discharged in irregular long-short-long inter-EOD interval patterns. Near neighbours displayed a tendency to discharge in intervals of similar duration (nearest neighbour distance < 1/2 fish length). On removal of a plastic partition that had separated a pair of fish for at least 3 days, mutual threat displays followed by fighting were observed. During threatening, the fish alternated regularly between bursts of a high discharge rate and short discharge breaks; the rate of change was 4/s. The subdominant animal in a group of two was attacked frequently and often ceased discharging when the dominant fish approached. Courtship behaviour involving gonadally mature fish was accompanied by high-discharge-rate displays with intervals of constant duration in both fish, and the reciprocal display of ‘preferred’ EOD latencies in the 12 ms range. The results demonstrate electric communication by distinct inter-discharge interval patterns in the social behaviour of this mormyrid fish.     

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     

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.     

Mate preference in female electric fish, Brachyhypopomus pinnicaudatus

Weakly electric fish communicate with brief electrostatic field pulses called electric organ discharges (EODs). EOD waveforms are sexually dimorphic in most genera, a condition thought to result from mate choice acting to shape the electric signal's constituent action potentials. We have no direct behavioural evidence that sexual selection by either mate choice or intrasexual competition is responsible for sex differences in the EOD waveforms of electric fish. We explored sexual selection in electric fish by conducting two-choice unforced preference tests with live, unaltered gymnotiform electric fish,Brachyhypopomus pinnicaudatus , which are sexually dimorphic. In the initial test, gravid females selected males over females only when the males were larger than average. Gravid females in later tests preferred larger males to smaller males in a significant majority of those trials in which they showed a preference. In about one-third of those trials, females spawned with their preferred male, confirming their preference. We concluded that passage through the choice apparatus was related to mate choice. The signals of chosen males had larger EOD amplitudes and longer EOD durations. These findings show that femaleB. pinnicaudatus do have a preference for a certain male phenotype. The system requires additional study to dissociate correlated male phenotypic characters to identify which male traits the female prefers. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Frequency response characteristics of electroreceptors in weakly electric fish (Gymnotoidei) with a pulse discharge

Summary Three species of Gymnotid fish, two species ofHypopomus andRhamphichthys rostratus, each having pulse type electric organ discharges (EOD) of different durations were studied to learn if any correlation exists between the spectral composition of the species specific EOD pulse and the frequency response characteristics of that species' electroreceptors. The receptor population consisted of two major categories (examples in Fig. 3). One category, termed pulse marker receptors, responded to suprathreshold stimulus pulses with a single spike at a short (<2 ms) latency. These receptors were tuned to the higher frequency components of a species' EOD (Fig. 4A) and were always 5 to 10 dB less sensitive than any other electroreceptors within a given species. The second major receptor category, burst duration coders, responded to an electrical stimulus with a burst of spikes at a longer latency, burst length was a function of stimulus amplitude. This second category could be further divided into three sub-categories according to the receptors' frequency response characteristics. The most commonly seen subcategory, wide band receptors (Fig. 4B), responded best to stimuli having frequencies equal to the dominant frequency component of the species' EOD in the two species ofHypopomus studied. A second subcategory, narrow band receptors (Fig. 4 A), had frequency response characteristics similar to those of the pulse marker receptors; however, these had thresholds 10 dB lower than those of the pulse marker. The third subcategory of burst duration coders, low frequency receptors (Fig. 4 C, D), responded best to stimulus frequencies ranging from about 50 to 150 Hz. Mechanisms of coding stimulus amplitude and responses to prolonged sinusoidal electrical stimuli were also studied in the various receptor types.It is suggested that the differences in the major receptor types and the different frequency response characteristics of the electroreceptors within a given species allows the animals to identify and evaluate signals resulting from their own EOD, the EODs of conspecifics and electrical stimuli generated by other species of electric fish.Supported by NIH Grant #1 RO1 NS 12337-01     

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.     

Shifts in frequency tuning of electroreceptors in androgen-treated mormyrid fish

Summary Several species of mormyrid electric fish have a sex difference in the pulse waveform of their electric organ discharge (EOD). Field studies in Gabon, West Africa have shown for one such species,Brienomyrus brachyistius (triphasic), that the sexually mature male EOD differs in shape and is nearly twice the duration of the EODs of females and juveniles. Fourier analysis reveals that differences in EOD duration correlate with those in the EOD power spectrum which has a peak at 0.3 kHz in males and 1.3 kHz in females and juveniles. We find a corresponding sex difference in the frequency tuning of at least one class of electroreceptors known as Knollenorgans. The average best or characteristic frequency of Knollenorgans is lower in males compared to females and juveniles. This correlates with a lower peak in the power spectrum of the male's pulse. When females are treated with gonadal androgens, their EODs increase 2–3 fold in duration, and the power spectra of their pulses are correspondingly lowered to match that of mature males. The average best frequency of Knollenorgans decreases by nearly 1 kHz which matches the downward shift of their EOD's power spectrum.For a second species ofBrienomyrus (sp. 2) which is commercially imported from Nigeria, we have not detected a sex difference in the power spectrum or duration of the EOD. The power spectrum peaks at about 4.2 kHz in males, females, and juveniles. Androgens, however, do cause a coincident downward shift in the average peak of the EOD power spectrum (from 4.2 to 1.3 kHz) and the average best frequency of Knollenorgans (from 2.3 to 1.4 kHz).Specimens ofBrienomyrus (sp. 2) that have been electrically silenced by surgical means are tuned, on the average, only 0.2 kHz higher than control animals. Silenced animals that have been treated with androgens are tuned, on the average, 0.2 kHz below controls. The results suggest that electroreceptor tuning is only partially modifiable during androgen treatment if the electroreceptors arenot being stimulated by an external electrical stimulus, i.e. the animal's own EOD. Since androgen treatment has a dramatic effect on receptor tuningonly in intact fish, it seems likely that retuning isnot due to a direct action of androgens on receptors, but rather due to the action of the principal electrical stimulus upon the receptors, i.e. the EOD. The implications of such results for the development of species and sex differences in electro-receptor tuning is discussed.     

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.     

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.     

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.     

Androgen modulates the kinetics of the delayed rectifying K+ current in the electric organ of a weakly electric fish

Weakly electric fish such as Sternopygus macrurus utilize a unique signal production system, the electric organ (EO), to navigate within their environment and to communicate with conspecifics. The electric organ discharge (EOD) generated by the Sternopygus electric organ is quasi-sinusoidal and sexually dimorphic; sexually mature males produce long duration EOD pulses at low frequencies, whereas mature females produce short duration EOD pulses at high frequencies. EOD frequency is set by a medullary pacemaker nucleus, while EOD pulse duration is determined by the kinetics of Na+ and K+ currents in the electric organ. The inactivation of the Na+ current and the activation of the delayed rectifying K+ current of the electric organ covary with EOD frequency such that the kinetics of both currents are faster in fish with high (female) EOD frequency than those with low (male) EOD frequencies. Dihydrotestosterone (DHT) implants masculinize the EOD centrally by decreasing frequency at the pacemaker nucleus (PMN). DHT also acts at the electric organ, broadening the EO pulse, which is at least partly due to a slowing of the inactivation kinetics of the Na+ current. Here, we show that chronic DHT treatment also slows the activation and deactivation kinetics of the electric organ's delayed rectifying K+ current. Thus, androgens coregulate the time-varying kinetics of two distinct ion currents in the EO to shape a sexually dimorphic communication signal.     

Innervation pattern and electric organ discharge waveform in Gymnotus carapo (Teleostei; Gymnotiformes)

The electrogenic organ (EO) of Gymnotus carapo has two main portions: a posterior region consisting of four bilaterally arranged electrocyte rows; and an anterior portion composed of only two. The lateral row (LR) of the anterior portion contains doubly innervated electrocytes with axon terminals from different nerves on their rostral and caudal faces. The LR is continuous with the most dorsal row of the caudal region. This row also contains doubly innervated electrocytes. The medial row (MR) electrocytes of the anterior region and ventral rows of the caudal region are exclusively caudally innervated. All caudal faces of the anterior or abdominal region are supplied by two nerves which originate from spinal roots VIII to XXI. Roots I to VII give origin to pure rostral nerves whose electromotor axons terminate on the rostral surfaces of the first seven LR electrocytes. A given doubly innervated electrocyte is supplied on its caudal face by a nerve originating several segments (usually seven) posterior to the spinal root supplying its rostral face. Transections of the spinal cord at the level of root VIII isolate the activity of the rostral surfaces of the first electrocytes. The EO discharge (EOD) then appears as a head negative deflection which arises from abdominally located electrocytes. Its monophasic character reveals that the activity remains restricted to the rostral electrocyte surfaces. Damage of the abdominal portion of the EO abolishes the first negative deflection of the normal pulse. Transections of the spinal cord at the level of root XXI isolate the activity of the whole abdominal portion of the EO. Since both doubly and singly innervated electrocytes remain active, the EOD appears biphasic. Comparative studies have shown that the EOD of Hypopomus sp. lacks any early negative wave and correspondingly all its electrocytes are exclusively caudally innervated.     

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.