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Agonistic behavior involves the displays that arise when conspecifics compete for valuable resources such as territory. After conflict resolution, dominants obtain priority access to the resource while subordinates lose it. We aimed to evaluate how agonistic encounters mediate the acquisition of different sized territories in the weakly electric fish, Gymnotus omarorum, a species that displays a well-documented non-breeding agonistic behavior very unusual among teleosts. When tested in intrasexual and intersexual dyads in small arenas, a sex-independent dominant-subordinate status emerged after highly aggressive contests in which subordinates signaled submission by retreating and emitting submissive electric signals. We staged dyadic agonistic encounters in a large arena, in which the initial interindividual distance resembled the one observed in nature. We observed the emergence of a dominant-subordinate status after longer but milder contests with rare electric signaling of submission. We found the persistence of dominance over time with no outcome reversion. We observed how dominants exclude subordinates from their conquered resource during all the recording time. Although the territorial behavior of Gymnotus has been put forth since pioneer reports, this is the first study to show how agonistic behavior depends on the territory size in this genus. Agonistic encounters of G. omarorum in the small arena resemble the characteristics of violent-like behaviors. The ease of shifting from mild to high levels of aggression due to confinement, together with the use of electrical signaling of submission, makes this species an excellent model to explore new perspectives in territoriality assessment.

The present study describes the life history of Gymnotus refugio, a species classified as Endangered in the last published list of threatened species of the Brazilian fauna. The study was conducted at a conservation unity that protect one of the last remaining semideciduous forests in the region. The reproductive period was estimated as occurring from the end of winter to the last summer months. Gymnotus refugio exibited fractional spawning, the lowest relative fecundity registered among the Gymnotifomes species studied at the present, and male parental care behavior. The analyses showed a seasonal pattern on the species diet, associating different food categories to winter, autumn, and spring. According to food items analysis and estimated intestinal quotient, G. refugio was classified as invertivorous, feeding mainly on autochthonous insects. The results obtained herein suggest that the position of G. refugio as an Endangered species might be influenced by its territoriality, habitat specificity, parental care behavior, and low fecundity, reinforcing the importance of swampy forest environment conservation as the only means of the species maintenance.

We describe chromosomal and electric signal diversity in three sympatric species of Gymnotus (Gymnotidae) fish from the Central Amazon Floodplain. Gymnotus arapaima presents a karyotype of 2n = 44 (24 m-sm + 20st-a), G. mamiraua 2n = 54 (42 m-sm + 12st-a), and G. jonasi 2n = 52 (12 m-sm + 40st-a). No evidence for a chromosomal sexual system was observed in two species for which both males and females were analyzed (G. mamiraua and G. arapaima). In all three species the constitutive heterochromatin is located primarily in pericentromeric regions, but also at some other sites. G. arapaima and G. mamiraua exhibit simple nucleolar organizing regions (NORs) on short arms of chromosome pairs 19 and 24, respectively. Gymnotus jonasi exhibits a multiple interstitial NOR on the long arm of pairs 9 and 10, and on the short arm of pair 11. G. arapaima and G. mamiraua exhibit several additional similarities in their karyotypic formulas—reflecting the phylogenetic proximity of these species within a G. carapo group clade (based on molecular phylogenetic evidence). The chromosomal differences among these three sympatric species imply complete post-zygotic reproductive isolation. A prominent pattern of partitioning of the peak power frequency of the electric organ discharge of these three species indicates pre-zygotic reproductive isolation of mate attraction signals. We conclude by discussing the evolutionary events that may have promoted signal divergence and reproductive isolation in Gymnotus of the Central Amazon, and the role that chromosomal rearrangements may place in diversification.

We characterize the karyotype of Gymnotus sp., an undescribed fish species from the Parana River. The diploid number is 2n = 40 chromosomes for females and 2n = 39 chromosomes for males, showing a multiple sex chromosome system of the X1X1X2X2/X1X2Y type, a first description in the genus Gymnotus. The nucleolar organizing regions sites are located in only one chromosome pair, a common feature among Gymnotiformes, and heterochromatin is present in the centromeric region of all chromosomes. Although the metacentric neo-Y chromosome appears to have arisen by centric fusion in three species from different Gymnotiformes families, the presence of the X1X1X2X2/X1X2Y sex chromosome system seems to be a homoplasic event in this group.

The amplitude and the autocorrelation level of the noise affecting the interval between successive electric organ discharges were estimated in isolated fish and in socially interacting fish of the species Gymnotus carapo. Both quantities increased in the fish with the slower discharging rate of the pair during the interaction, and we aim to assess whether they have some functional implication for the efficiency of the jamming avoidance response performed by the fish having the faster discharging rate of the pair. For this purpose, the noisy variability of the intervals around its mean value was simulated using autoregressive models estimated from experimental recordings of isolated and interacting fish. The simulation was implemented using two autoregressive models, each representing one fish of the pair. The jamming avoidance response was included by adding transient interval shortenings to the train simulating the fish of the pair that discharges at a faster rate whenever the two trains were close to discharge simultaneously. The number of double coincidences (i.e., simultaneous discharges occurring in two successive firing cycles) of the two simulated trains was used to measure the efficiency of the jamming avoidance. This quantity was evaluated separately as a function of the autocorrelation level and amplitude of the simulated variability, in realizations with and without jamming avoidance response. Only if jamming avoidance response was included in the simulation have we found that (i) the number of coincidences decreased with the increasing of the autocorrelation and (ii) the increase in the amplitude determined a growth of the coincidence number at a rate that is inversely proportional to the autocorrelation level. We argue that the persistent correlations of the fish variability constitute an adaptation that improves the efficiency of transient interval shortenings as a jamming avoidance strategy. The long autocorrelation time prevents the disruption of the jamming avoidance performance due to increases in the variability amplitude.

This study was carried out in Uruguay (30–35°S), South America, with two complementary approaches. First, an extensive exploration of Uruguayan freshwaters allowed us to assess the distribution of the two major species of Gymnotiformes (out of 4) across sites. Gymnotus carapo was uniformly distributed in Uruguayan territory, whereas Brachyhypopomus pinnicaudatus was observed in the northern and eastern part of the country. There was a highly significant negative correlation between the relative abundance of Brachyhypopomus pinnicaudatus and pH and water conductivity. Moreover, these environmental factors are significant contributors to its spatial differences in relative abundance. Second, temperature, conductivity, photoperiod, and the structure of a Brachyhypopomus pinnicaudatus population were analyzed across seasons in a small lake over a two-year period. Water temperature and photoperiod exhibited important seasonal changes, whereas water conductivity remained low and relatively constant. The presence of sexually mature males, females, and the sudden increase of juveniles indicated the occurrence of the breeding season in November, December, and January, coinciding with high mean water temperatures and extreme photoperiod. These results agree with previous data that support the hypothesis of temperature as an important environmental factor for the onset of breeding in Gymnotiformes from the temperate zone.

The functional role of the dorsal portions of dorsolateral telencephalon in modulating the stable electric organ discharge was determined by microinjection of an agonist or antagonist GABAergic drug in the awake weak electric fish, Gymnotus carapo. The dorsolateral telencephalon, which is interconnected with the preglomerular complex and the dorsocentral telencephalic area was microinjected through a guide cannula previously implanted, with different doses of bicuculline, muscimol and saline. Microinjection of bicuculline into the dorsolateral telencephalon induced a complex response consisting of increase, decrease and abrupt interruptions in the frequency of electric organ discharges and an increase in motor activity. Motor activity and modulations in the electric organ discharge are dose dependent. The somatic, but not the electric, effect is abolished under anesthesia by urethane, suggesting that the two responses are parallel but unrelated in terms of occurrence. These data, together with former neuroanatomical findings by this laboratory, suggest two parallel pathways by which the blockage of GABAA receptors in the dorsolateral telencephalon causes modulations in the firing of the medullary pacemaker nucleus. A possible route for the motor effect through reticular projection from the torus semicircularis dorsalis is discussed.

The backwards swimming behavior exhibited by American weakly electric fishes (Gymnotiformes) is thought to be an important component of foraging, particularly in the electrolocation of prey items. Previous studies of Eigenmannia virescens and Apteronotus albifrons have shown that backwards swimming appears to allow a fish to scan a potential prey item across its cutaneous electroreceptor array, then put itself in position for a short, forward lunge preceding ingestion. Adult gymnotiforms exhibit considerable variation in size, shape, and electric organ characteristics. For example, gymnotiforms produce either a wave or a pulse electric organ discharge (EOD). Given this variation, we ask whether the results reported previously can be completely generalized to all gymnotiforms. To address this question we observed the foraging patterns of phylogenetically and ecologically distinct gymnotiforms: three wave species, E. virescens, A. albifrons and Sternopygus macrurus; and three pulse species, Gymnotus carapo, Brachyhypopomus cf. brevirostris, and Rhamphichthys rostratus. Electric organ placement and body shape were also noted in these species to determine if morphological differences correlate with variations in foraging behaviors. Results demonstrate that following prey detection the wave species examined primarily swim backwards during prey approach, prior to lunging forward and ingesting prey. This result is similar to previous findings. In contrast, the pulse species examined detect, approach, and ingest prey primarily in the forward direction, swimming backwards only to reposition themselves.

The electric fish Gymnotus carapo emits brief (4 ms) electric pulses separated by much longer intervals of high regularity (coefficient of variation 0.01–0.02). Two main changes in the firing patterns of electric organ discharge were observed when two fish were placed together. (1) All fish pairs showed an increase in the frequency difference between the two fish, in comparison with the value observed in isolated fish, prior to the interaction. This change increased the number of beats per second between both discharge trains, i.e., the number of times per second that the higher rate discharge sweeps the lower rate one when displayed on an oscilloscope. (2) When the sweeping velocity fell below 2–3 sweeps/s, transient frequency increases were also observed in the electric organ discharge of the higher rate fish when they were about to discharge simultaneously. The contribution to jamming avoidance of these two changes was analyzed by comparing recordings of behavioral interactions with simulations produced by a computational model. The jamming effect of the firing of a conspecific located in the same tank was evaluated by counting the number of coincidences between both trains (occurrence of discharges of the two fish within 2 ms of one another). The number of coincidences was evaluated as a function of the sweeping velocity in both simulations (with and without transient frequency increases) and real fish. As the sweeping velocity increased, single coincidences increased slightly in simulations without transient frequency increases, whereas the successive coincidences (coincidences repeated in successive discharges) decreased abruptly. The simulation including transient frequency increases eliminated the successive coincidences and decreased the single ones. Only when the sweeping velocity was less than 2–3 sweeps/s, did transient frequency increases improve the coincidence-avoiding performance of the simulation. The number of coincidences observed in natural behavioral interactions for the different sweeping velocities coincided with the distributions obtained with the simulations. As successive coincidences are known to be more detrimental for electrolocation than single ones, the increase in the sweeping velocity may be considered a jamming avoidance strategy in Gymnotus carapo, in addition to the already described transient frequency increases.

Understanding how electrosensory images are generated and perceived in actively electrolocating fish requires the study of the characteristics of fish bodies as electric sources. This paper presents a model of Gymnotus carapo based on measurements of the electromotive force generated by the electric organ and the impedance of the passive tissues. A good agreement between simulated and experimentally recorded transcutaneous currents was obtained. Passive structures participate in the transformation of the electromotive force pattern into transcutaneous current profiles. These spatial filtering properties of the fish's body were investigated using the model. The shape of the transcutaneous current profiles depends on tissue resistance and on the geometry and size of the fish. Skin impedance was mainly resistive. The effect of skin resistance on the spatial filtering properties of the fish's body was theoretically analyzed. The model results show that generators in the abdominal and central regions produce most of the currents through the head. This suggests that the electric organ discharge (EOD), generated in the abdominal and central regions is critical for active electrolocation. In addition, the well-synchronized EOD components generated all along the fish produce large potentials in the far field. These components are probably involved in long-distance electrocommunication. Preliminary results of this work were published as a symposium abstract.

To investigate the synchronization mechanisms operating in the electromotor system, electric organ discharge related field potentials of neural origin were recorded in intact fish. Components corresponding to the relay nucleus, the bulbar-spinal electromotor tract, the electromotoneurons and the peripheral nerves were identified. Delays between components were used to estimate the following intervals: (1) the conduction time along the cord (central conduction interval), (2) the interval between the local activation of the tract and the electromotoneuron firing within a restricted portion of the cord (coupling interval) and (3) the conduction time along the peripheral axons plus the time taken by synaptic activation of the electrocytes (peripheral interval). While central conduction interval increases with the distance from the pacemaker, the coupling interval diminishes. Peripheral interval also diminishes from rostral to caudal targets in the electric organ. It is concluded that the electrocyte synchronization, resulting from matching the three above-defined intervals, is achieved by a cascade of synergetic mechanisms.

Stimulation of the spinal cord of the electric fish Gymnotus carapo, evoked an abrupt increase in the discharge rate of the electric organ. At the maximum of this response, the rate increased an average of 26 ± 11.8%. The duration of the response was 4.9 ± 2.12 s; its latency was 10.4 ± 1.1 ms. Activation of the Mauthner axon played a decisive role in this phenomenon as indicated by the following: (1) recordings from the axon cap of the Mauthner cell demonstrated that the response was evoked if the Mauthner axon was antidromically activated and (2) a response that was similar to that produced by spinal cord stimulation, was elicited by intracellular stimulation of either Mauthner cell. Stimulation of the eighth nerve could also increase the discharge rate of the electric organ. The effect was greater if a Mauthner cell action potential was elicited. The findings described in the present report, indicate the existence of a functional connection between the Mauthner cell and the electromotor system in Gymnotus carapo. This connection may function to enhance the electrolocative sampling of the environment during Mauthner-cell mediated behaviors. This is a novel function for the Mauthner cell.

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.

A new technique of multiple-air-gap recording was developed to study the EO activation process in Gymnotus carapo. Using this technique, the spatiotemporal pattern of electromotive force generation was investigated in normal and spinal-lesioned animals.

The duration of the electric organ discharge (EOD) in Gymnotus carapo is brief and independent of fish size. Spinal mechanisms involved in electrocyte synchronization were explored by recording spontaneous action potentials of single fibers from the electromotor bulbospinal tract (EBST). Using the field potential of the medullary electromotor nucleus (MEN) as a temporal reference we calculated the orthodromic conduction velocity (CV) of these fibers (range: 10.7–91 m/s). The CVs (in m/s) of fibers recorded at the same level of the spinal cord were significantly different in small and large fish; this difference disappeared when CV were expressed as percentage of body length/ms. Plotting these values against conduction distance (also in %) showed that low CV fibers predominate in the rostral cord while only fast fibers are found at distal levels. Moreover, antidromic stimulation of the distal cord was only effective on high CV fibers. The orthodromic CVs in the distal portion of the recorded fibers were calculated by collision experiments; no significant differences were found between proximal and distal portions. The spatial distribution of CV values within the EBST is proposed to play the main role in synchronizing the electromotoneurons' activity along the spinal cord.

Resource utilization by nine abundant piscivores from a diverse tropical fish assemblage was examined over the course of a year. All nine species exhibited peak reproduction during the early wet season and a similar sequence of size-dependent shifts from a diet composed primarily of microcrustacea, to aquatic insects, and finally fishes. Three piranha species specialized on fish fins, particularly at subadult size classes (SL 30–80 mm). Gradual dessication of the floodplain during the transition season was associated with fish growth, increased fish density, and decreased aquatic primary productivity and availability of invertebrate prey. Based on 118 resource categories, average pairwise diet overlap was low during all three seasons: wet, transition, and dry. Of 72 species pairings, only one pair of fin-nipping piranhas exhibited high overlap simultaneously on three niche dimensions: food type, food size, and habitat. Adults of two species, a gymnotid knifefish and pimelodid catfish, were largely nocturnal. Patterns of habitat utilization indicate that piranhas may restrict diurnal use of the open-water region by other piscivores. Collective diet overlap of individual piscivore species with the other eight feeding guild members and collective overlaps with the entire fish community each revealed two basic seasonal trends. Four species that showed an early switch to piscivory also showed a high degree of diet separation with both the guild and community at large on a year-round basis. The five remaining species showed lowest collective diet overlaps during the transition season when availability of invertebrates was reduced and fish densities were maximal. Whereas predation may play a role in habitat separation, diffuse competition for food resources during the approximately four-month transition season probably is the principal factor yielding patterns of diet specialization.

The electric organ (EO) ofGymnotus carapo was studied using different neurohistological techniques including conventional electron microscopy. The electric tissue extends along the fish body from the pectoral girdle to the tip of the tail, constituting a single, undivided organ. However, taking into account the number, arrangement, and innervation of the electrocytes, it is possible to divide the EO into three different portions. The more rostral portion is included within the ventral wall of the abdominal cavity. It consists of singly and doubly innervated electrocytes arranged in two rows at each side of the midline. Innervation of this zone is supplied by the first 5–7 segmental nerves and by the ‘anterior electromotor nerves.’ Segmental nerves terminate on the rostral faces of doubly innervated electrocytes; axons stemming from the anterior electromotor nerves end on the caudal faces of both doubly and singly innervated electrocytes. There is an intermediate body-tail region in which the electrocytes are arranged in four dorsoventral tubes (tubes 1 to 4) on each side of the midline. In this zone, doubly innervated electrocytes (confined within tube 1) coexist together with singly innervated ones, receiving nerve terminals on their caudal faces (tubes 2, 3, and 4). The innervation characteristics appear modified at more distal portions of the tail where the doubly innervated electrocytes of tube 1 are replaced by singly innervated units. The most distal portion of the EO (approximately its terminal 30%) consists of numerous, homogeneously innervated electrocytes with nerve endings distributed exclusively on their caudal faces. Nerve supply to the intermediate and distal regions derives from the posterior electromotor nerves (PENs) which appear as well-defined anatomical entities beyond the level of metamere XXVII. At the bodytail and more distal regions the innervation pattern of the EO is particularly complex. Thin nerve trunks arise from the PENs and project ventrally toward the electrocyte tubes. Before reaching the electric tissue the electromotor axons branch frequently. Our anatomical studies indicate that the EO is heterogeneous, a feature consistent with most recent electrophysiological and biophysical experiments.

Frequency response characteristics of phasic electroreceptors in the weakly electric pulse fish,Gymnotus carapo, were related to the character of a receptor signal (RS) recorded externally over the receptor pore. Receptors that were equally sensitive to a broad range of frequencies produced RS's of a single hump, whereas sharply tuned receptors produced oscillatory RS's (Fig. 4A). Furthermore, the frequency of the recorded oscillation provided a reliable estimate of the receptor's best frequency (Fig. 4B). The two major classes of electroreceptors, burst duration coders and pulse markers, could not be distinguished on the basis of best frequency (BF), Q5, or threshold at BF (Fig. 1A-C). These parameters were widely distributed in both classes. Pulse markers, however, never responded to a pulse stimulus with more than one spike and tended to have shorter latencies than burst duration coders (Fig. 2). Power spectra were calculated for local EOD waveforms recorded with three different electrode configurations (Figs. 6B, 7A-J). The local spectral peak power measured transepidermally in the rostral half of the body was 540±52 Hz, whereas the spectral peak power of the H-T waveform was 1127±90 Hz. Electroreceptors were not sharply tuned to the fish's own EOD, but showed a decrease in median BF from the rostral region where it was 950 Hz to the middle region where it was 350 Hz (Fig. 8A-D).

1. The electrical interactions of a pair of Gymnotus carapo of known dominance relationship were studied in a large semi-natural aquarium while resting during the daytime (Fig. 1). 2. Synchronisations of the two discharge frequencies were found to occur predominantly in bouts of 15–20 consecutive intervals although very close frequencies with beat periods lasting more than 100 intervals were recorded. 3. The delay of a fish's pulses with respect to those of its partner during a synchronisation showed specific dominance-related tendencies indicating that the dominant fish places its discharges in the period of maximum electroreceptive sensitivity of the submissive animal (Figs. 2, 3, and 4). Such a synchronisation effectively protects the first fish from the jamming effect of its partner's pulses. 4. The analysis of 4-h recordings demonstrated the existence of clear jamming-avoidance manoeuvres in Gymnotus. Examination of these responses showed that when jamming-avoidance frequency increases occurred at near discharge coincidence, it was the lower frequency fish which readjusted its frequency (Fig. 5) 5. Interactions at close discharge frequency (Figs. 6, 7, and 8) show that frequency following takes place in certain situations and a struggle for ‘frequency dominance’ is seen which, it is suggested, is the best strategy for the preservation of optimal electrolocation. 6. A model is proposed (Fig. 9) to account for the different sensitivity of Gymnotus to foreign pulses sweeping through its own discharge in either the time-positive (lower frequency partner) or time-negative (higher frequency partner) directions. The model is based on the previous finding of a phaselocked attenuation of electroperceptive information in this species. 7. Whether or not the jamming avoidance in Gymnotus is related to social communication remains an open question. Efficient jamming avoidance requires the determination of the sign and magnitude of the frequency difference between the interacting fish. This would also communicate information related to social dominance, previously shown to correlate well to overall discharge frequency. The present results indicate that the dominant fish was in the more favourable position, as far as efficient electrolocation is concerned, for most of the time.

1. A survey of the species distribution, discharge characteristics, ecology and behaviour of South American Gymnotiforme electric fish was carried out during two field trips to sites in the vicinity of Kourou, French Guiana, in 1983 and 1985. 2. Measurements of water conditions, collected over a twelve month period, highlight the strong ecological pressure on species exploiting the rapidly fluctuating and often temporary coastal streams. Adaptation of the reproductive cycle to the rainy seasons is particularly marked and the possible zeitgebers are considered. Given the almost invariant water conductivity, its suggested use as an environmental trigger is rejected in favour of acoustic or mechanical cues. 3. The two main study areas were inhabited principally by pulse gymnotoids of the genera Gymnotus and Hypopomus. A new Hypopomus species was discovered as well as specimens of G. pantherinus, previously undescribed in French Guiana. Larvae of H. beevei were found prior to the start of the 1985 rainy season. Developmental data for this species are presented which support the view that, as in other weakly electric gymnotoids, a primitive larval electric organ probably precedes the adult structure. The larval discharge also strongly resembles that of the adult electric eel, lending weight to published theories of an Electrophorus-like ancestor to the wekly electric fish. 4. The predatory behaviour of the strongly electric eel was recorded in detail. The field results indicated that this species preyed on weakly electric fish whose responses to the foraging eel are also reported. The probable use of electrical cues by hunting eels was further investigated in a laboratory experiment in which predatory responses were compared in the presence of non-electric and electric live prey, and models mimicking electric prey. It is concluded that electrical cues are probably of paramount importance in the eel's normal prey capture behaviour.

Des substances muqueuses ont ete examinees par des methodes histochimiques au niveau des organes recepteurs cutanes de type ampullaire (organe de type A, Szabo, 1965) faisant partie du systeme de la ligne laterale des poissons electriques a faible decharge Gnathonemus petersii (Mormyrides), Gymnotus carapo (Gymnotides) et Gymnarchus niloticus (Gymnarchides).

The signal value of 4 distinct discharge patterns inGymnotus carapo was investigated by means of a model electric fish. The model produced an electrical approximation to theGymnotus discharge and field but bore no physical resemblance to the real animal (Figs. 1, 2). Three of the sequences used: Bursts, Offs and Shifts (Fig. 3) were simulations of patterns known to be involved in the agonistic behaviour ofGymnotus. The fourth was that characteristic of a resting fish and was used as a control. 1. No aggressive behaviour was shown towards the model when it was electrically inactive. Components directed to the head region during normal interaction (Head Butt, Bite, etc.) were invariably directed to the electrical “head” of the otherwise symmetrical model. Polarity inversion resulted in a corresponding reversal of the end attacked. 2. Comparison of the frequency distributions and rate of occurrence of behaviour components during interaction with the model and conspecifics revealed great similarity-the differences found being due to the immobility of the model (Fig. 4). 3. A repeated measures analysis of variance (AOV) was used to examine the 6 replications of the experiment. The factors in the analysis were: Playback Sequence (Conditions), Amplitude (Level), Dominance and Days. Two groups of agonistic components, Approaches and Contacts were examined. 4. The Days factor was significant for Contact scores only, due to the rapid habituation of high intensity (Contact) components; Approach scores remained at the same level (Fig. 5). 5. Animals in a given dominance group showed the highest level of aggression towards the model when the amplitude was closest to their own (Fig. 6). 6. High score for dominant and low for submissive in the Shifts condition, compared with the control, suggest that this is a “dominance” signal. 7. Contact scores in the Offs condition were very low (Fig. 7) while Approaches remained at a high level (Fig. 6)-demonstrating the extreme effectiveness of the Off as a submissive signal. 8. Contact scores in the Bursts condition showed a highly significant difference for the dominant group between the two playback amplitudes. Highest scores were for high level playback, the fish showing little interest in an apparently small fish (low amplitude) behaving aggressively (producing Bursts). These scores were even lower than for submissive animals at either amplitude (Fig. 7).

THE subject of this communication may seem remote and uninteresting, but it will not be difficult to show that questions of the highest importance for physiology, anatomy, and the Darwinian theory are closely related to those touching the structure of the electric organs of certain fishes and the laws of their functions.

Undisturbed specimens of Gymnotus carapo discharge their electric organs at a rate of approximately 50 Hz, with a standard deviation of 3 Hz; stimuli of several sensory modalities (touch, vibration, light, electric fields) are capable of eliciting transient increases in discharge frequency rising to a maximum near 30 Hz above resting frequency and returning exponentially over several seconds. Such frequency transients may be evoked by sudden increases or decreases in the electrical resistance of the environment caused by shunting recording electrodes on either end of the fish. Threshold responses are seen to resistance changes of 10 to 20 ohms, which create changes of approximately 50 μV/cm in the amplitude of the electric discharge measured along the length of the fish. Gymnotus proves remarkably sensitive to electrical stimuli occuring in phase with the discharge train of its own electric organs, with a threshold near 50 μV/cm; out-of-phase stimuli ten times this amplitude fail to elicit frequency transients. Responses are also seen to small changes in latency between stimulus and discharge trains within the time span of an individual organ discharge. Phase-sensitivity is apparently a consequence of the sigmoid stimulus/response curve common to most receptors; a large “activating” pulse, normally generated by the electric organs, raises electroreceptor activity into the receptor's dynamic range, after which a relatively small stimulus is capable of causing a significant change in output. Such a mechanism probably serves as a peripheral filter, eliminating low-level interference from other electric fish. The frequency transients themselves are interpreted as arousal responses, momentarily increasing the fish's sensory resolution in the face of potential hazards or prey.

Eight adult Gymnotus carapo were used in an experiment designed to examine electrical and other behavioral interactions within 22 different pairings of animals. High scores on aggressive behavior were inversely related to the time spent in electrical silence. Approach and threat movement scores were positively correlated. Of particular interest in this study was the perfect positive correlation between electrical pulse frequency and subsequent display of threat movements in social encounters. In general the results confirmed a suggestion that dominance in this type of social grouping can be reliably predicted from the electrical characteristics of the interacting individuals.

1. A particular area of the midbrain called magnocellular mesencephalic nucleus (MMN), characteristic of several weakly electric fishes, was examined by different histological methods in Gymnotus carapo, Gymnotidae. 2. This area contains a dense network of thick fibers in which large neurons are scattered: it contains also a great number of small neurons. 3. The large neurons (25–35 μ) are unipolar with a profusely branching axon that gives rise to the main part of the network within the nucleus. At least one branch of the axonal arborisation leaves the nucleus towards the torus semicircularis, i.e. in the ventral direction. The small neurons (5–12 μ) are uni- or bipolar and display a profuse arborisation of their axon near the soma. 4. Large and small neurons are preferentially localized in the postero-ventral part of the nucleus. 5. Large neurons are contacted by multiple club endings originating partly from the rhombencephalon. The unipolar small neurons receive a single cup-like terminal. 6. The total number of club endings extablishing synaptic contacts on the surface of a large neuron was counted on serial semithin sections. 7. The present findings provide new information about the mesencephalic nucleus, which according to previous electrophysiological observations (Szabo, 1967; Szabo and Sakata, 1967) has been shown to represent a relay in the rapid electrosensory pathway.

Four specimens of the electric fish, Gymnotus carapo, were used in an experiment to investigate the social interactions of members of this species. Four different two-member groups were established and observed for 20-min sessions every day for 8 days. Two categories of aggressive behavior, approach and threat movements, were found to be inversely related to the phenomenon of “turning off” of electrical pulses. Further, the animal that became the more aggressive in any pairing could be predicted from the mean pulse frequency measured prior to the social experiment.

Electrophorus electricus, or Gymnotus electricus, is a teleost of the family of Gymnotidae, popularly known as the electric eel, and lives in the shallow waters of the South American equatorial rivers. It is the most powerful fresh-water electric fish ; mature specimens can discharge trains of pulses of more than 600 V and 1 amp1,2.

IN Sir Horace Rumbold's “Great Silver River” (London, 1887), the author, when on the Upper Uruguay above Uruguayana, speaks of a “kind of Electric Eel (Gymnotus) called here Rayo or Lightning, of the effects of contact with which, very curious and unrelateable stories are told.”