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.     

Electroreceptor model of weakly electric fish Gnathonemus petersii: II. Cellular origin of inverse waveform tuning.

In part I (. Biophys. J. 75:1712-1726), we presented a cellular model of the A- and B-electroreceptors of the weakly electric fish Gnathonemus petersii. The model made clear the cellular origin of the differences in the response functions of A- and B-receptors, which sensitively code the intensity of the fish's own electric organ discharge (EOD) and the variations in the EOD waveform, respectively. The main purpose of the present paper is to clarify the cellular origin of the inverse waveform tuning of the B-receptors by using the receptor model. Inverse waveform tuning means that B-receptors respond more sensitively to the 180 degrees inverted EOD than to undistorted or less distorted EODs. We investigated how the A- and B-receptor models respond to EODs with various waveforms, which are the phase-shifted EODs, whose shift angle is varied from -1 degrees to -180 degrees, and single-period sine wave stimuli of various frequencies. We show that the tuning properties of the B-receptors arise mainly from the combination of two attributes: 1) The waveform of the stimuli (Bstim) effectively sensed by the B-receptor cells. This consists of a first smaller and a second larger positive peak, even though in the original phase-shifted EOD stimuli, the amplitudes of the two positive peaks are reversed. 2) The effective time constant of dynamical response of the receptor cells. It is on the order of the duration of a single EOD pulse. We also calculated the response properties of the A- and B-receptor models when stimulated with natural EODs distorted by various capacitive and resistive objects. Furthermore, we investigated the effect of EOD amplitude on the receptor responses to capacitive and resistive objects. The models presented can systematically reproduce the experimentally observed response properties of natural A- and B-receptor cells. The mechanism producing these properties can be reasonably explained by the variation in the stimulus waveforms effectively sensed by the A- and B-receptor cells and by time constants.     

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 evolutionary origins of electric signal complexity.

This study explores the evolutionary origins of waveform complexity in electric organ discharges (EODs) of weakly electric fish. I attempt to answer the basic question of what selective forces led to the transition from the simplest signal to the second simplest signal in the gymnotiform electric fishes. The simplest electric signal is a monophasic pulse and the second simplest is a biphasic pulse. I consider five adaptive hypotheses for the evolutionary transition from a monophasic to a biphasic EOD: (i) electrolocation, (ii) sexual selection, (iii) species isolation, (iv) territory defense, (v) crypsis from electroreceptive predators. Evaluating these hypotheses with data drawn largely from the literature, I find best support for predation. Predation is typically viewed as a restraining force on evolution of communication signals, but among the electric fishes, predation appears to have served as a creative catalyst. In suppressing spectral energy in the sensitivity range of predators (a spectral simplification), the EOD waveforms have become more complex in their time domain structure. Complexity in the time domain is readily discernable by the high frequency electroreceptor systems of gymnotiform and mormyrid electric fish. The addition of phases to the EOD can cloak the EOD from predators, but also provides a substrate for subsequent modification by sexual selection. But, while juveniles and females remain protected from predators, breeding males modify their EODs in ways that enhance their conspicuousness to predators.     

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     

EOD modulations of brown ghost electric fish: JARs, chirps, rises, and dips.

Weakly electric "wave" fish make highly regular electric organ discharges (EODs) for precise electrolocation. Yet, they modulate the ongoing rhythmicity of their EOD during social interactions. These modulations may last from a few milliseconds to tens of minutes. In this paper we describe the different types of EOD modulations, what they may signal to recipient fish, and how they are generated on a neural level. Our main conclusions, based on a species called the brown ghost (Apteronotus leptorhynchus) are that fish: (1) show sexual dimorphism in the signals that they generate; (2) make different signals depending on Whether they are interacting with a fish of the opposite sex or, within their own sex, to a fish of that which is dominant or subordinate to it; (3) are able to assess relative dominance from electrical cues; (4) have a type of plasticity in the pacemaker nucleus, the control center for the EOD, that occurs after stimulation of NMDA receptors that causes a long-lasting (tens of minutes to hours) change in EOD frequency; (5) that this NMDA receptor-dependent change may occur in reflexive responses, like the jamming avoidance response (JAR), as well as after certain long-lasting social signals. We propose that NMDA-receptor dependent increases in EOD frequency during the JAR adaptively shift the EOD frequency to a new value to avoid jamming by another fish and that such increases in EOD frequency during social encounters may be advantageous since social dominance seems to be positively correlated with EOD frequency in both sexes.     

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.     

Temporal patterning of electric organ discharges in the African electric catfish, Malapterurus electricus (Gmelin)

This paper is the first detailed analysis of situation-specific temporal patterning of electric organ discharges (EODs) in a strong electric fish. Using a resident-intruder paradigm EODs were recorded during interactions between dyads composed of Malapterurus electricus (Gmelin) and four different types of fish: (1) conspecifics; (2) large prey-type mid-water fish, goldfish ( Carassius auratus , Linnaeus 1758) and tilapia ( Oreochromis melanotheron , Rüppel, 1852); (3) a sympatric competitor, Polypterus palmas (Ayres 1850) and (4) a larger, threatening catfish, Clarias sp.
An analysis of the EODs emitted showed that in the presence of conspecifics the average EOD volley consisted of a single long-duration, low frequency train of EODs. The presence of the midwater fish (goldfish and Tilapia) elicited volleys consisting of two short trains, and P. palmas elicited long duration volleys with two trains and long inter-train intervals. Finally, an attacking Clarias resulted on average in volleys consisting of two high-frequency trains of EODs. With nonconspecific partner species resident electric catfish emitted volleys with more pulses, more trains that were longer in duration and higher in frequency than the EODs in volleys emitted by intruder electric catfish with the same species stimulus fish.     

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     

Imaging of objects through active electrolocation in Gnathonemus petersii.

The weakly electric fish Gnathonemus petersii detects, localizes, and analyzes objects during active electrolocation even in complete darkness. This enables these fish to lead a nocturnal life and find and identify their prey (small insect larvae) on the ground of their freshwater habitat. During active electrolocation, fish produce a series of brief electric signals, electric organ discharges (EOD), with an electric organ in their tail. Each EOD builds up a stable electric field around the fish, which is distorted only by nearby objects. Field distortions lead to changes of the transepidermal electric current flow at a region of the fish's electroreceptive skin surface called the 'electric image'. Within the electric image, locally perceived EODs can be either altered in amplitude or waveform by an object. Fish measure both parameters to assess object properties, such as the capacitive and resistive components of the object's complex impedance. the object's size and shape, and its distance from the fish. None of these object properties can be evaluated in isolation, but have to be inferred during parallel processing of electric image spatial and qualitative parameters. Two anterior skin regions of G. petersii appear to possess particular properties for special electrolocation tasks and we therefore refer to them as 'foveal' regions. Because of its high electroreceptor density, the electric field geometry around it, and its behavioral use, the 'nasal region' between the nares and the mouth at the head of the fish is suggested to be a fovea for long-range guidance and object detection. We propose that the 'Schnauzenorgan', a long and flexible chin appendix covered densely with electroreceptor organs, is a second electroreceptive fovea associated with a short-range (food) identification system. Together, these two electric foveae constitute an effective prey detection and identification system.     

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

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

Capacitance detection in the wave-type electric fish Eigenmannia during active electrolocation

Weakly electric fish can detect nearby objects and analyse their electric properties during active electrolocation. Four individuals of the South American gymnotiform fish Eigenmannia sp., which emits a continuous wave-type electric signal, were tested for their ability to detect capacitive properties of objects and discriminate them from resistive properties. For individual fish, capacitive values of objects had to be greater than 0.22–1.7 nF (`lower threshold') and smaller than 120–680 nF (`upper threshold') in order to be detected. The capacitive values of natural objects fall well within this detection range. All fish trained could discriminate unequivocally between capacitive and resistive object properties. Thus, fish perceive capacitive properties as a separate object quality. The effects of different types of objects on the locally occurring electric signals which stimulate electroreceptors during electrolocation were examined. Purely resistive objects altered mainly local electric organ discharge (EOD) amplitude, but capacitive objects with values between about 0.5 and 600 nF changed the timing of certain EOD parameters (phase-shift) and EOD waveform. A mechanism for capacitance detection in wave-type electric fish based on time measurements is proposed and compared with the capacitance detection mechanism in mormyrid pulse-type fish, which is based on waveform measurements. Accepted: 31 July 1997     

Sensory cues for the gradual frequency fall responses of the gymnotiform electric fish, Rhamphichthys rostratus

The sensory cues for a less known form of frequency shifting behavior, gradual frequency falls, of electric organ discharges (EODs) in a pulse-type gymnotiform electric fish, Rhamphichthys rostratus, were identified. We found that the gradual frequency fall occurs independently of more commonly observed momentary phase shifting behavior, and is due to perturbation of sensory feedback of the fish's own EODs by EODs of neighboring fish. The following components were identified as essential features in the signal mixture of the fish's own and the neighbor's EOD pulses: (1) the neighbor's pulses must be placed within a few millisecond of the fish's own pulses, (2) the neighbor's pulses, presented singly at low frequencies (0.2–4 Hz), were sufficient, (3) the frequency of individual pulse presentation must be below 4 Hz, (4) amplitude modulation of the sensory feedback of the fish's own pulses induced by such insertions of the neighbor's pulses must contain a high frequency component: sinusoidal amplitude modulation of the fish's own EOD feedback at these low frequencies does not induce gradual frequency falls. Differential stimulation across body surfaces, which is required for the jamming avoidance response (JAR) of wave-type gymnotiform electric fish, was not necessary for this behavior. We propose a cascade of high-pass and low-pass frequency filters within the amplitude processing pathway in the central nervous system as the mechanism of the gradual frequency fall response.Abbreviations EOD electric organ discharge - f frequency of EOD or pacemaker command signal - JAR jamming avoidance response - S ₁ stimulus mimicking fish's own EOD - f ₁ frequency of S₁ - S ₂ stimulus mimicking neighbor's EOD - f ₂ frequency of S₂     

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.     

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.     

Food deprivation reduces and leptin increases the amplitude of an active sensory and communication signal in a weakly electric fish

Energetic demands of social communication signals can constrain signal duration, repetition, and magnitude. The metabolic costs of communication signals are further magnified when they are coupled to active sensory systems that require constant signal generation. Under such circumstances, metabolic stress incurs additional risk because energy shortfalls could degrade sensory system performance as well as the social functions of the communication signal. The weakly electric fish Eigenmannia virescens generates electric organ discharges (EODs) that serve as both active sensory and communication signals. These EODs are maintained at steady frequencies of 200–600 Hz throughout the lifespan, and thus represent a substantial metabolic investment. We investigated the effects of metabolic stress (food deprivation) on EOD amplitude (EODa) and EOD frequency (EODf) in E. virescens and found that only EODa decreases during food deprivation and recovers after restoration of feeding. Cortisol did not alter EODa under any conditions, and plasma cortisol levels were not changed by food deprivation. Both melanocortin hormones and social challenges caused transient EODa increases in both food-deprived and well-fed fish. Intramuscular injections of leptin increased EODa in food-deprived fish but not well-fed fish, identifying leptin as a novel regulator of EODa and suggesting that leptin mediates EODa responses to metabolic stress. The sensitivity of EODa to dietary energy availability likely arises because of the extreme energetic costs of EOD production in E. virescens and also could reflect reproductive strategies of iteroparous species that reduce social signaling and reproduction during periods of stress to later resume reproductive efforts when conditions improve.     

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.     

‘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).     

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.     

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.