Circadian rhythms in electric waveform structure and rate in the electric fish Brachyhypopomus pinnicaudatus

Understanding daily rhythms in weakly electric fish: the role of melatonin on the electric behavior of Brachyhypopomus gauderio

Living organisms display molecular, physiological and behavioral rhythms synchronized with natural environmental cycles. Understanding the interaction between environment, physiology and behavior requires taking into account the complexity of natural habitats and the diversity of behavioral and physiological adaptations. Brachyhypopomus gauderio is characterized by the emission of electric organ discharges (EOD), with a very stable rate modulated by social and environmental cues. The nocturnal arousal in B. gauderio coincides with a melatonin-dependent EOD rate increase. Here, we first show a daily cycle in both the EOD basal rate (EOD-BR) and EOD-BR variability of B. gauderio in nature. We approached the understanding of the role of melatonin in this natural behavior through both behavioral pharmacology and in vitro assays. We report, for the first time in gymnotiformes, a direct effect of melatonin on the pacemaker nucleus (PN) in in vitro preparation. Melatonin treatment lowered EOD-BR in freely moving fish and PN basal rate, while increasing the variability of both. These results show that melatonin plays a key role in modulating the electric behavior of B. gauderio through its effect on rate and variability, both of which must be under a tight temporal regulation to prepare the animal for the challenging nocturnal environment.

The sensory effects of light on the electric organ discharge rate of Gymnotus omarorum

Gymnotiformes are nocturnal fishes inhabiting the root mats of floating plants. They use their electric organ discharge (EOD) to explore the environment and to communicate. Here, we show and describe tonic and phasic sensory-electromotor responses to light distinct from indirect effects depending on the light-induced endogenous circadian rhythm. In the dark, principally during the night, inter-EOD interval histograms are bimodal: the main peak corresponds to the basal rate and a secondary peak corresponds to high-frequency bouts. Light causes a twofold tonic but opposing effect on the EOD histogram: (i) decreasing the main mode and (ii) blocking the high-frequency bouts and consequently increasing the main peak at the expense of removal of the secondary one. Additionally, light evokes phasic responses whose amplitude increases with intensity but whose slow time course and poor adaptation differentiate from the so-called novelty responses evoked by abrupt changes in sensory stimuli of other modalities. We confirmed that Gymnotus omarorum tends to escape from light, suggesting that these phasic responses are probably part of a global 'light-avoidance response'. We interpret the data within an ecological context. Fish rest under the shade of aquatic plants during the day and light spots due to the sun's relative movement alert the fish to hide in shady zones to avoid macroptic predators and facilitate tracking the movement of floating plant islands by wind and/or water currents.

A Spark in the Dark: Uncovering Natural Activity Patterns of Mormyrid Weakly Electric Fish

To understand animal ecology, observation of wildlife in the natural habitat is essential, but particularly challenging in the underwater realm. Weakly electric fishes provide an excellent opportunity to overcome some of these challenges because they generate electric organ discharges (EODs) to sense their environment and to communicate, which can be detected non-invasively. We tracked the EOD and swimming activity of two species of mormyrid weakly electric fishes (Marcusenius victoriae and Petrocephalus degeni) over diel cycles in the laboratory, and we recorded EODs and environmental dissolved oxygen (DO) concentration and temperature over several months in a naturally hypoxic habitat in Uganda. Under laboratory conditions, both species showed increases of activity and exploration behavior that were closely synchronized to the onset of the dark phase. In the wild, fish preferred structurally complex habitats during the day, but dispersed toward open areas at night, presumably to forage and interact. Nocturnal increase of movement range coincided with diel declines in DO concentration to extremely low levels. The fact that fish showed pronounced nocturnal activity patterns in the laboratory and in the open areas of their habitat, but not under floating vegetation, indicates that light intensity exerts a direct effect on their activity. We hypothesize that being dark-active and tolerant to hypoxia increases the resistance of these fish against predators. This study establishes a new technology to record EODs in the field and provides a window into the largely unknown behavior of mormyrids in their natural habitat.

The diversity and evolution of electric organs in Neotropical knifefishes

The Gymnotiformes, also known as the South American or Neotropical knifefishes, include the strongly electric Electrophorus electricus and many other weakly electric species. These fish possess specialised electric organs that are able to release electric discharges into the water, for electrolocation and communication, and sometimes for predation and defence. All Gymnotiform species possess a myogenic electric organ (mEO) derived from the muscle tissue, and members of the Apteronotidae family uniquely possess a neurogenic electric organ (nEOs) derived from the nervous tissue. A mEO may consist of ‘Type A’ electrocytes that develop within the tail muscle (for example, in Apteronotus leptorhynchus), or ‘Type B’ electrocytes that develop below the tail muscle (for example, in Brachyhypopomus gauderio). In this review, we discuss the diversity in the anatomy, electric discharge and development of electric organs found in different Gymnotiform species, as well as the ecological and environmental factors that have likely contributed to this diversity. We then describe various hypotheses regarding the evolution of electric organs, and discuss the potential evolutionary origin of the nEO: a pair of nerve cords that are located on either side of the aorta in B. gauderio, and which may have expanded and developed into a nEO in the Apteronotidae family during its evolution from a common ancestral species. Finally, we compare potential Gymnotiform phylogenies and their supporting evidence.

Vocal and Electric Fish: Revisiting a Comparison of Two Teleost Models in the Neuroethology of Social Behavior

The communication behaviors of vocal fish and electric fish are among the vertebrate social behaviors best understood at the level of neural circuits. Both forms of signaling rely on midbrain inputs to hindbrain pattern generators that activate peripheral effectors (sonic muscles and electrocytes) to produce pulsatile signals that are modulated by frequency/repetition rate, amplitude and call duration. To generate signals that vary by sex, male phenotype, and social context, these circuits are responsive to a wide range of hormones and neuromodulators acting on different timescales at multiple loci. Bass and Zakon (2005) reviewed the behavioral neuroendocrinology of these two teleost groups, comparing how the regulation of their communication systems have both converged and diverged during their parallel evolution. Here, we revisit this comparison and review the complementary developments over the past 16 years. We (a) summarize recent work that expands our knowledge of the neural circuits underlying these two communication systems, (b) review parallel studies on the action of neuromodulators (e.g., serotonin, AVT, melatonin), brain steroidogenesis (via aromatase), and social stimuli on the output of these circuits, (c) highlight recent transcriptomic studies that illustrate how contemporary molecular methods have elucidated the genetic regulation of social behavior in these fish, and (d) describe recent studies of mochokid catfish, which use both vocal and electric communication, and that use both vocal and electric communication and consider how these two systems are spliced together in the same species. Finally, we offer avenues for future research to further probe how similarities and differences between these two communication systems emerge over ontogeny and evolution.

Diel Patterns of Pheromone Release By Male Sea Lamprey

Costs to producing sexual signals can create selective pressures on males to invest signaling effort in particular contexts. When the benefits of signaling vary consistently across time, males can optimize signal investment to specific temporal contexts using biological rhythms. Sea lamprey, Petromyzon marinus, have a semelparous life history, are primarily nocturnal, and rely on pheromone communication for reproduction; however, whether male investment in pheromone transport and release matches increases in spawning activity remains unknown. By measuring 1) 3keto-petromyzonol sulfate (3kPZS, a main pheromone component) and its biosynthetic precursor petromyzonol sulfate (PZS) in holding water and tissue samples at 6 points over the course of 24 hours, and 2) 3kPZS release over the course of several days, we demonstrate that 3kPZS release exhibits a consistent diel pattern across several days with elevated pheromone release just prior to sunset and at night. Trends in hepatic concentrations and circulatory transport of PZS and 3kPZS were consistent with patterns of 3kPZS release and suggest the possibility of direct upregulation in pheromone transport and release rather than observed release patterns being solely a byproduct of increased behavioral activity. Our results suggest males evolved a signaling strategy that synchronizes elevated pheromone release with nocturnal increases in sea lamprey behavior. This may be imperative to ensure that male signaling effort is not wasted in a species having a single, reproductive event.

Tracking activity patterns of a multispecies community of gymnotiform weakly electric fish in their neotropical habitat without tagging

Field studies on freely behaving animals commonly require tagging and often are focused on single species. Weakly electric fish generate a species- and individual-specific electric organ discharge (EOD) and therefore provide a unique opportunity for individual tracking without tagging. Here, we present and test tracking algorithms based on recordings with submerged electrode arrays. Harmonic structures extracted from power spectra provide fish identity. Localization of fish based on weighted averages of their EOD amplitudes is found to be more robust than fitting a dipole model. We apply these techniques to monitor a community of three species, Apteronotus rostratus, Eigenmannia humboldtii and Sternopygus dariensis, in their natural habitat in Darién, Panama. We found consistent upstream movements after sunset followed by downstream movements in the second half of the night. Extrapolations of these movements and estimates of fish density obtained from additional transect data suggest that some fish cover at least several hundreds of meters of the stream per night. Most fish, including E. humboldtii, were traversing the electrode array solitarily. From in situ measurements of the decay of the EOD amplitude with distance of individual animals, we estimated that fish can detect conspecifics at distances of up to 2 m. Our recordings also emphasize the complexity of natural electrosensory scenes resulting from the interactions of the EODs of different species. Electrode arrays thus provide an unprecedented window into the so-far hidden nocturnal activities of multispecies communities of weakly electric fish at an unmatched level of detail.

Convergent patterns of evolution of mitochondrial oxidative phosphorylation (OXPHOS) genes in electric fishes

The ability to generate and detect electric fields has evolved in several groups of fishes as a means of communication, navigation and, occasionally, predation. The energetic burden required can account for up to 20% of electric fishes' daily energy expenditure. Despite this, molecular adaptations that enable electric fishes to meet the metabolic demands of bioelectrogenesis remain unknown. Here, we investigate the molecular evolution of the mitochondrial oxidative phosphorylation (OXPHOS) complexes in the two most diverse clades of weakly electric fishes-South American Gymnotiformes and African Mormyroidea, using codon-based likelihood approaches. Our analyses reveal that although mitochondrial OXPHOS genes are generally subject to strong purifying selection, this constraint is significantly reduced in electric compared to non-electric fishes, particularly for complexes IV and V. Moreover, analyses of concatenated mitochondrial genes show strong evidence for positive selection in complex I genes on the two branches associated with the independent evolutionary origins of electrogenesis. These results suggest that adaptive evolution of proton translocation in the OXPHOS cellular machinery may be associated with the evolution of bioelectrogenesis. Overall, we find striking evidence for remarkably similar effects of electrogenesis on the molecular evolution of mitochondrial OXPHOS genes in two independently derived clades of electrogenic fishes. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

Dominance in Habitat Preference and Diurnal Explorative Behavior of the Weakly Electric Fish Apteronotus leptorhynchus

Electrocommunication and -localization behaviors of weakly electric fish have been studied extensively in the lab, mostly by means of short-term observations on constrained fish. Far less is known about their behaviors in more natural-like settings, where fish are less constrained in space and time. We tracked individual fish in a population of fourteen brown ghost knifefish (Apteronotus leptorhynchus) housed in a large 2 m³ indoor tank based on their electric organ discharges (EOD). The tank contained four different natural-like microhabitats (gravel, plants, isolated stones, stacked stones). In particular during the day individual fish showed preferences for specific habitats which provided appropriate shelter. Male fish with higher EOD frequencies spent more time in their preferred habitat during the day, moved more often between habitats during the night, and less often during the day in comparison to low-frequency males. Our data thus revealed a link between dominance indicated by higher EOD frequency, territoriality, and a more explorative personality in male A. leptorhynchus. In females, movement activity during both day and night correlated positively with EOD frequency. In the night, fish of either sex moved to another habitat after about 6 s on average. During the day, the average transition time was also very short at about 20 s. However, these activity phases were interrupted by phases of inactivity that lasted on average about 20 min during the day, but only 3 min in the night. The individual preference for daytime retreat sites did not reflect the frequent explorative movements at night.

Electroreception, electrogenesis and electric signal evolution

Electroreception, the capacity to detect external underwater electric fields with specialised receptors, is a phylogenetically widespread sensory modality in fishes and amphibians. In passive electroreception, a capacity possessed by c. 16% of fish species, an animal uses low-frequency-tuned ampullary electroreceptors to detect microvolt-range bioelectric fields from prey, without the need to generate its own electric field. In active electroreception (electrolocation), which occurs only in the teleost lineages Mormyroidea and Gymnotiformes, an animal senses its surroundings by generating a weak (< 1 V) electric-organ discharge (EOD) and detecting distortions in the EOD-associated field using high-frequency-tuned tuberous electroreceptors. Tuberous electroreceptors also detect the EODs of neighbouring fishes, facilitating electrocommunication. Several other groups of elasmobranchs and teleosts generate weak (< 10 V) or strong (> 50 V) EODs that facilitate communication or predation, but not electrolocation. Approximately 1.5% of fish species possess electric organs. This review has two aims. First, to synthesise our knowledge of the functional biology and phylogenetic distribution of electroreception and electrogenesis in fishes, with a focus on freshwater taxa and with emphasis on the proximate (morphological, physiological and genetic) bases of EOD and electroreceptor diversity. Second, to describe the diversity, biogeography, ecology and electric signal diversity of the mormyroids and gymnotiforms and to explore the ultimate (evolutionary) bases of signal and receptor diversity in their convergent electrogenic-electrosensory systems. Four sets of potential drivers or moderators of signal diversity are discussed. First, selective forces of an abiotic (environmental) nature for optimal electrolocation and communication performance of the EOD. Second, selective forces of a biotic nature targeting the communication function of the EOD, including sexual selection, reproductive interference from syntopic heterospecifics and selection from eavesdropping predators. Third, non-adaptive drift and, finally, phylogenetic inertia, which may arise from stabilising selection for optimal signal-receptor matching.

Daily changes in the electric behavior of weakly electric fish naturally persist in constant darkness and are socially synchronized

Daily rhythms allow anticipation of changes and allocation of energy to better cope with predictable events. Rhythms in behavior result from a complex combination of physiological processes timed by the nervous system and synchronized with external information. We aimed to understand how rhythmic behaviors arise in nature, when weakly electric fish are exposed to cyclic environmental influences and social context. Gymnotus omarorum is a South American nocturnal pulse-type gymnotiform. Its electric behavior encodes information about species, sex and physiological state. The rate of emission of the electric organ discharge (EOD-BR) is modulated by exploratory activity and by physical and social environmental stimuli. We show that the EOD-BR increases during the night in the natural habitat even in individuals maintained in constant dark conditions. Locomotor activity is higher at night, however the nocturnal increase of EOD-BR still occurs in motionless fish, demonstrating an independent origin for the locomotor and electric components of exploratory behavior. When fish are observed in nature, social context exerts a synchronizing role on electric behavior. G. omarorum emerges as an exciting wild model for the study of daily rhythms arising in the complexity of the real world, integrating environmental, physical and social cues in the modulation of rhythmic behavior.

Summary: The nocturnal increase of electric behavior in Gymnotus omarorum is analyzed in the wild, in constant darkness and social isolation. This daily trait is independent of locomotor activity and modulated by social context.

Electric fish genomics: Progress, prospects, and new tools for neuroethology

Electric fish have served as a model system in biology since the 18th century, providing deep insight into the nature of bioelectrogenesis, the molecular structure of the synapse, and brain circuitry underlying complex behavior. Neuroethologists have collected extensive phenotypic data that span biological levels of analysis from molecules to ecosystems. This phenotypic data, together with genomic resources obtained over the past decades, have motivated new and exciting hypotheses that position the weakly electric fish model to address fundamental 21^st century biological questions. This review article considers the molecular data collected for weakly electric fish over the past three decades, and the insights that data of this nature has motivated. For readers relatively new to molecular genetics techniques, we also provide a table of terminology aimed at clarifying the numerous acronyms and techniques that accompany this field. Next, we pose a research agenda for expanding genomic resources for electric fish research over the next 10years. We conclude by considering some of the exciting research prospects for neuroethology that electric fish genomics may offer over the coming decades, if the electric fish community is successful in these endeavors.

Energetics of Sensing and Communication in Electric Fish: A Blessing and a Curse in the Anthropocene?

Weakly electric freshwater fish use self-generated electric fields to image their worlds and communicate in the darkness of night and turbid waters. This active sensory/communication modality evolved independently in the freshwaters of South America and Africa, where hundreds of electric fish species are broadly and abundantly distributed. The adaptive advantages of the sensory capacity to forage and communicate in visually-unfavorable environments and outside the detection of visually-guided predators likely contributed to the broad success of these clades across a variety of Afrotropical and neotropical habitats. Here we consider the potentially high and limiting metabolic costs of the active sensory and communication signals that define the gymnotiform weakly electric fish of South America. Recent evidence from two well-studied species suggests that the metabolic costs of electrogenesis can be quite high, sometimes exceeding one-fourth of these fishes' daily energy budget. Supporting such an energetically expensive system has shaped a number of cellular, endocrine, and behavioral adaptations to restrain the metabolic costs of electrogenesis in general or in response to metabolic stress. Despite a suite of adaptations supporting electrogenesis, these weakly electric fish are vulnerable to metabolic stresses such as hypoxia and food restriction. In these conditions, fish reduce signal amplitude presumably as a function of absolute energy shortfall or as a proactive means to conserve energy. In either case, reducing signal amplitude compromises both sensory and communication performance. Such outcomes suggest that the higher metabolic cost of active sensing and communication in weakly electric fish compared with the sensory and communication systems in other neotropical fish might mean that weakly electric fish are disproportionately susceptible to harm from anthropogenic disturbances of neotropical aquatic habitats. Fully evaluating this possibility, however, will require broad comparative studies of metabolic energetics across the diverse clades of gymnotiform electric fish and in comparison to other nonelectric neotropical fishes.

Melatonin Regulates Daily Variations in Electric Behavior Arousal in Two Species of Weakly Electric Fish with Different Social Structures

Timing is crucial for social interactions. Animal behavior is synchronized with biotic and abiotic environment variables ensuring that the activity phase of conspecifics occurs during the same period of the day. As biological rhythms are embedded in the complex integrative control of the brain, it is fundamental to explore its interaction with environmental and social factors. This approach will unravel the link between external stimuli carrying information on environmental cycles and the neural commands for behavior, including social behavior, associated with precise phases of those cycles. Arousal in the solitary Gymnotus omarorum and in the gregarious Brachyhypopomus gauderio is characterized by a nocturnal increase in the basal discharge rate of electric behavior, which is mild and transient in G. omarorum and large and persistent in B. gauderio. In this study, we show that the major integrator of social behavior, AVT (arginine vasotocin), is not involved in the nocturnal increase of electric behavior basal rate in isolated animals of either species. On the other hand, endogenous melatonin, the major modulator of the circadian system, is responsible for the nocturnal increase in electric behavior in isolated individuals of both species.

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 200-600Hz 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.

Long-term behavioral tracking of freely swimming weakly electric fish

Long-term behavioral tracking can capture and quantify natural animal behaviors, including those occurring infrequently. Behaviors such as exploration and social interactions can be best studied by observing unrestrained, freely behaving animals. Weakly electric fish (WEF) display readily observable exploratory and social behaviors by emitting electric organ discharge (EOD). Here, we describe three effective techniques to synchronously measure the EOD, body position, and posture of a free-swimming WEF for an extended period of time. First, we describe the construction of an experimental tank inside of an isolation chamber designed to block external sources of sensory stimuli such as light, sound, and vibration. The aquarium was partitioned to accommodate four test specimens, and automated gates remotely control the animals' access to the central arena. Second, we describe a precise and reliable real-time EOD timing measurement method from freely swimming WEF. Signal distortions caused by the animal's body movements are corrected by spatial averaging and temporal processing stages. Third, we describe an underwater near-infrared imaging setup to observe unperturbed nocturnal animal behaviors. Infrared light pulses were used to synchronize the timing between the video and the physiological signal over a long recording duration. Our automated tracking software measures the animal's body position and posture reliably in an aquatic scene. In combination, these techniques enable long term observation of spontaneous behavior of freely swimming weakly electric fish in a reliable and precise manner. We believe our method can be similarly applied to the study of other aquatic animals by relating their physiological signals with exploratory or social behaviors.

The energetics of electric organ discharge generation in gymnotiform weakly electric fish

Gymnotiform weakly electric fish produce an electric signal to sense their environment and communicate with conspecifics. Although the generation of such relatively large electric signals over an entire lifetime is expected to be energetically costly, supporting evidence to date is equivocal. In this article, we first provide a theoretical analysis of the energy budget underlying signal production. Our analysis suggests that wave-type and pulse-type species invest a similar fraction of metabolic resources into electric signal generation, supporting previous evidence of a trade-off between signal amplitude and frequency. We then consider a comparative and evolutionary framework in which to interpret and guide future studies. We suggest that species differences in signal generation and plasticity, when considered in an energetics context, will not only help to evaluate the role of energetic constraints in the evolution of signal diversity but also lead to important general insights into the energetics of bioelectric signal generation.

Behavioral ecology, endocrinology and signal reliability of electric communication

The balance between the costs and benefits of conspicuous animal communication signals ensures that signal expression relates to the quality of the bearer. Signal plasticity enables males to enhance conspicuous signals to impress mates and competitors and to reduce signal expression to lower energetic and predation-related signaling costs when competition is low. While signal plasticity may benefit the signaler, it can compromise the reliability of the information conveyed by the signals. In this paper we review the effect of signal plasticity on the reliability of the electrocommunication signal of the gymnotiform fish Brachyhypopomus gauderio. We (1) summarize the endocrine regulation of signal plasticity, (2) explore the regulation of signal plasticity in females, (3) examine the information conveyed by the signal, (4) show how that information changes when the signal changes, and (5) consider the energetic strategies used to sustain expensive signaling. The electric organ discharge (EOD) of B. gauderio changes in response to social environment on two time scales. Two hormone classes, melanocortins and androgens, underlie the short-term and long-term modulation of signal amplitude and duration observed during social interaction. Population density drives signal amplitude enhancement, unexpectedly improving the reliability with which the signal predicts the signaler's size. The signal's second phase elongation predicts androgen levels and male reproductive condition. Males sustain signal enhancement with dietary intake, but when food is limited, they 'go for broke' and put extra energy into electric signals. Cortisol diminishes EOD parameters, but energy-limited males offset cortisol effects by boosting androgen levels. While physiological constraints are sufficient to maintain signal amplitude reliability, phenotypic integration and signaling costs maintain reliability of signal duration, consistent with theory of honest signaling.

Cellular mechanisms of developmental and sex differences in the rapid hormonal modulation of a social communication signal

Some gymnotiform electric fish species rapidly modify their electric signal waveforms by altering the action potential (AP) waveforms of their electrocytes, the excitable cells that produce the electric organ discharge (EOD). The fish Brachyhypopomus gauderio alters EOD amplitude and pulse duration as a social signal in accordance with the prevailing social conditions, under the dual regulation of melanocortins and androgens. We show here that B. gauderio uses two distinct cellular mechanisms to change signal amplitude, and its use of these two mechanisms varies with age and sex of the signaler. EOD amplitude and waveform are regulated by the coordinated timing and shaping of two APs generated from two opposing excitable membranes in each electrocyte. The two membranes fire in sequence within 100 μs of each other with the second AP being broader than the first. We have shown previously that mature males increase EOD amplitude and duration when melanocortin peptide hormones act directly on electrocytes to selectively broaden the second AP and increase the delay between the two APs by approximately 25 μs. Here we show that females selectively broaden only the second AP as males do, but increase amplitude of both APs with no change in delay between them, a previously unreported second mechanism of EOD amplitude change in B. gauderio. Juvenile fish broaden both APs and increase the delay between the APs. Cellular mechanisms of EOD plasticity are therefore shaped during development, presumably by sex steroids, becoming sexually dimorphic at maturity.

Weakly electric fish for biomonitoring water quality

Environmental pollution is a major issue that calls for suitable monitoring systems. The number of possible pollutants of municipal and industrial water grows annually as new chemicals are developed. Technical devices for pollutant detection are constructed in a way to detect a specific and known array of pollutants. Biological systems react to lethal or non-lethal environmental changes without pre-adjustment, and a wide variety have been employed as broad-range monitors for water quality. Weakly electric fish have proven particularly useful for the purpose of biomonitoring municipal and industrial waters. The frequency of their electric organ discharges directly correlates with the quality of the surrounding water and, in this way, concentrations of toxicants down to the nanomolar range have been successfully detected by these organisms. We have reviewed the literature on biomonitoring studies to date, comparing advantages and disadvantages of this test system and summarizing the lowest concentrations of various toxicants tested. Eighteen publications were identified investigating 35 different chemical substances and using six different species of weakly electric fish.

Signal modulation as a mechanism for handicap disposal

Signal honesty may be compromised when heightened competition provides incentive for signal exaggeration. Some degree of honesty might be maintained by intrinsic handicap costs on signalling or through imposition of extrinsic costs, such as social punishment of low quality cheaters. Thus, theory predicts a delicate balance between signal enhancement and signal reliability that varies with degree of social competition, handicap cost, and social cost. We investigated whether male sexual signals of the electric fish Brachyhypopomus gauderio would become less reliable predictors of body length when competition provides incentives for males to boost electric signal amplitude. As expected, social competition under natural field conditions and in controlled lab experiments drove males to enhance their signals. However, signal enhancement improved the reliability of the information conveyed by the signal, as revealed in the tightening of the relationship between signal amplitude and body length. Signal augmentation in male B. gauderio was independent of body length, and thus appeared not to be curtailed through punishment of low quality (small) individuals. Rather, all individuals boosted their signals under high competition, but those whose signals were farthest from the predicted value under low competition boosted signal amplitude the most. By elimination, intrinsic handicap cost of signal production, rather than extrinsic social cost, appears to be the basis for the unexpected reinforcement of electric signal honesty under social competition. Signal modulation may provide its greatest advantage to the signaller as a mechanism for handicap disposal under low competition rather than as a mechanism for exaggeration of quality under high competition.

Tight hormonal phenotypic integration ensures honesty of the electric signal of male and female Brachyhypopomus gauderio

Hormones mediate sexually selected traits including advertisement signals. Hormonal co-regulation links the signal to other hormonally-mediated traits such that the tighter the integration, the more reliable the signal is as a predictor of those other traits. Androgen administration increases the duration of the communication signal pulse in both sexes of the electric fish Brachyhypopomus gauderio. To determine whether the duration of the signal pulse could function as an honest indicator of androgen levels and other androgen-mediated traits, we measured the variation in sex steroids, signal pulse duration, and sexual development throughout the breeding season of B. gauderio in marshes in Uruguay. Although the sexes had different hormone titres and signal characteristics, in both sexes circulating levels of the androgens testosterone (T) and 11-ketotestosterone (11-KT) were strongly related to signal pulse duration. Consequently, signal pulse duration can serve as an honest indicator of circulating androgens in males and females alike. Additionally, through phenotypic integration, signal pulse duration also predicts other sexual traits directly related to androgen production: gonad size in males and estradiol (E2) levels in females. Our findings show that tight hormonal phenotypic integration between advertisement signal and other sex steroid-mediated traits renders the advertisement signal an honest indicator of a suite of reproductive traits.

Glucocorticoid receptor blockade inhibits brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus

When animals are under stress, glucocorticoids commonly inhibit adult neurogenesis by acting through glucocorticoid receptors (GRs). However, in some cases, conditions that elevate glucocorticoids promote adult neurogenesis, and the role of glucocorticoid receptors in these circumstances is not well understood. We examined the involvement of GRs in social enhancement of brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus. In this species, long-term social interaction simultaneously elevates plasma cortisol, enhances brain cell addition and increases production of aggressive electrocommunication signals ("chirps"). We implanted isolated and paired fish with capsules containing nothing (controls) or the GR antagonist, RU486, recorded chirp production and locomotion for 7d, and measured the density of newborn cells in the periventricular zone. Compared to isolated controls, paired controls showed elevated chirping in two phases: much higher chirp rates in the first 5h and moderately higher nocturnal rates thereafter. Treating paired fish with RU486 reduced chirp rates in both phases to those of isolated fish, demonstrating that GR activation is crucial for socially induced chirping. Neither RU486 nor social interaction affected locomotion. RU486 treatment to paired fish had a partial effect on cell addition: paired RU486 fish had less cell addition than paired control fish but more than isolated fish. This suggests that cortisol activation of GRs contributes to social enhancement of cell addition but works in parallel with another GR-independent mechanism. RU486 also reduced cell addition in isolated fish, indicating that GRs participate in the regulation of cell addition even when cortisol levels are low.

Testosterone and 11-ketotestosterone have different regulatory effects on electric communication signals of male Brachyhypopomus gauderio

The communication signals of electric fish can be dynamic, varying between the sexes on a circadian rhythm and in response to social and environmental cues. In the gymnotiform fish Brachyhypopomus gauderio waveform shape of the electric organ discharge (EOD) is regulated by steroid and peptide hormones. Furthermore, EOD amplitude and duration change on different timescales and in response to different social stimuli, suggesting that they are regulated by different mechanisms. Little is known about how androgen and peptide hormone systems interact to regulate signal waveform. We investigated the relationship between the androgens testosterone (T) and 11-ketotestosterone (11-KT), the melanocortin peptide hormone α-MSH, and their roles in regulating EOD waveform of male B. gauderio. Males were implanted with androgen (T, 11-KT, or blank), and injected with α-MSH before and at the peak of androgen effect. We compared the effects of androgen implants and social interactions by giving males a size-matched male stimulus with which they could interact electrically. Social stimuli and both androgens increased EOD duration, but only social stimuli and 11-KT elevated amplitude. However, no androgen enhanced EOD amplitude to the extent of a social stimulus, suggesting that a yet unidentified hormonal pathway regulates this signal parameter. Additionally, both androgens increased response of EOD duration to α-MSH, but only 11-KT increased response of EOD amplitude to α-MSH. Social stimuli had no effect on EOD response to α-MSH. The finding that EOD amplitude is preferentially regulated by 11-KT in B. gauderio may provide the basis for independent control of amplitude and duration.

Reproductive and diurnal rhythms regulate vocal motor plasticity in a teleost fish

Seasonal and circadian rhythms control fundamental physiological processes including neural excitability and synaptic plasticity that can lead to the periodic modulation of motor behaviors like social vocalizations. Parental male midshipman fish produce three call types during the breeding season: long duration (min to >1 h) advertisement 'hums', frequency and amplitude modulated agonistic 'growls' (s), and very brief (ms) agonistic 'grunts' produced either singly or repetitively as ;grunt trains' for up to several minutes. Fictive grunts that establish the temporal properties of natural grunts are readily evoked and recorded in vivo from vocal occipital nerve roots at any time of day or year by electrical microstimulation in either the midbrain periaqueductal gray or a hindbrain vocal pre-pacemaker nucleus. Now, as shown here, the longer duration fictive growls and hums can also be elicited, but are restricted to the nocturnal reproductive season. A significant drop in call threshold accompanies the fictive growls and hums that are distinguished by their much longer duration and lower and more regular firing frequency. Lastly, the long duration fictive calls are dependent upon increased stimulation time and intensity and hence may result from activity-dependent changes in the vocal motor circuit that are themselves modulated by seasonal and circadian rhythms.

Social competition affects electric signal plasticity and steroid levels in the gymnotiform fish Brachyhypopomus gauderio

Sexually-selected communication signals can be used by competing males to settle contests without incurring the costs of fighting. Steroid regulation of these signals can render them as reliable indicators of a male's physiological state. We investigated how plasticity in electrocommunication signals is driven by social competition for mates, mediated by steroid hormones, and subject to the effects of past social experience. We measured the electric waveform's amplitude and duration and steroid hormone levels of male gymnotiform electric fish (Brachyhypopomus gauderio) following week-long periods of social isolation, and low or high social competition. To quantify the effect of social history on the modulation of the electric signal, six groups of six males experienced all three social conditions but in different order. We found that males differentially modulate their electric signals depending on the order they experienced these conditions. Thus, past social interactions affect both present and future social electric signals. Cortisol levels and the amplitude of the electric signal appeared to track the intensity of competition, while androgen levels and the duration of the electric signal only responded to the presence (low and high competition) or absence (isolation) of a social environment (low and high androgens respectively). In addition, cortisol levels were related to the body size of the males at high social competition. Taken together, these findings suggest that the capacity of males to modulate their signals in response to social competition is regulated by steroids.

Circadian and social cues regulate ion channel trafficking

Electric fish strengthen their communication signals nightly and during social encounters by rapidly trafficking ion channels into cell membranes, demonstrating a direct relationship between environmental stimuli, channel trafficking, and behavior.

Electric fish generate and sense electric fields for navigation and communication. These signals can be energetically costly to produce and can attract electroreceptive predators. To minimize costs, some nocturnally active electric fish rapidly boost the power of their signals only at times of high social activity, either as night approaches or in response to social encounters. Here we show that the gymnotiform electric fish Sternopygus macrurus rapidly boosts signal amplitude by 40% at night and during social encounters. S. macrurus increases signal magnitude through the rapid and selective trafficking of voltage-gated sodium channels into the excitable membranes of its electrogenic cells, a process under the control of pituitary peptide hormones and intracellular second-messenger pathways. S. macrurus thus maintains a circadian rhythm in signal amplitude and adapts within minutes to environmental events by increasing signal amplitude through the rapid trafficking of ion channels, a process that directly modifies an ongoing behavior in real time.

Excitable cells, such as neurons and muscle cells, control behavior by generating action potentials, electrical signals that propagate along the cell membrane. Action potentials are generated when the cell allows charged molecules (ions) such as sodium and potassium to move across the membrane through specialized proteins called ion channels. By changing the number of ion channels in the plasma membrane, excitable cells can rapidly remodel their functional characteristics, potentially causing changes in behavior. To gain an understanding of how environmental events cause the remodeling of excitable cell membranes and the resulting behavioral adaptations, we studied the electric communication/navigation signals of an electric fish, Sternopygus macrurus. High amplitude signals facilitate communication and electrolocation, but are energetically costly and more detectable by those predators that can detect electrical signals. We found that Sternopygus increase signal amplitude at night, when they are active, and increase signal amplitude rapidly during social encounters. Electrocytes, the cells that produce the signal, rapidly boost the signal amplitude when they allow more sodium to cross the cell membrane, thereby generating larger action potentials. To increase sodium currents during the action potential, electrocytes rapidly insert additional sodium channels into the cell membrane in response to hormones released into circulation by the pituitary. By adding new ion channels to the electrocyte membrane only during periods of activity or social encounters and removing these channels during inactive periods, these animals can save energy and reduce predation risks associated with communication.

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.

Melanocortins regulate the electric waveforms of gymnotiform electric fish

The hypothalamic-pituitary-adrenal/interrenal axis couples serotonergic activity in the brain to the peripheral regulators of energy balance and response to stress. The regulation of peripheral systems occurs largely through the release of peptide hormones, especially the melanocortins (adrenocorticotropic hormone [ACTH] and alpha melanocyte stimulating hormone [alpha-MSH]), and beta-endorphin. Once in circulation, these peptides regulate a wide range of processes; alpha-MSH in particular regulates behaviors and physiologies with sexual and social functions. We investigated the role of the HPI and melanocortin peptides in regulation of electric social signals in the gymnotiform electric fish, Brachyhypopomus pinnicaudatus. We found that corticotropin releasing factor, thyrotropin-releasing hormone, and alpha-MSH, three peptide hormones of the HPI/HPA, increased electric signal waveform amplitude and duration when injected into free-swimming fish. A fourth peptide, a synthetic cyclic-alpha-MSH analog attenuated the normal circadian and socially-induced EOD enhancements in vivo. When applied to the electrogenic cells (electrocytes) in vitro, only alpha-MSH increased the amplitude and duration of the electrocyte discharge similar to the waveform enhancements seen in vivo. The cyclic-alpha-MSH analog had no effect on its own, but blocked or attenuated alpha-MSH-induced enhancements in the single-cell discharge parameters, demonstrating that this compound functions as a silent antagonist at the electrocyte. Overall, these results strongly suggest that the HPI regulates the EOD communication signal, and demonstrate that circulating melanocortin peptides enhance the electrocyte discharge waveform.

Sex differences in energetic costs explain sexual dimorphism in the circadian rhythm modulation of the electrocommunication signal of the gymnotiform fish Brachyhypopomus pinnicaudatus

To understand the evolution of sexually dimorphic communication signals, we must quantify their costs, including their energetic costs, the regulation of these costs, and the difference between the costs for the sexes. Here, we provide the first direct measurements of the relative energy expended on electric signals and show for the focal species Brachyhypopomus pinnicaudatus that males spend a significantly greater proportion of their total energy budget on signal generation (11-22%) compared with females (3%). Both sexes significantly reduce the energy spent on electric signals during daylight hours through circadian modulation of the amplitude, duration and repetition rate of the electric signal, but this effect is more marked in males. Male body condition predicted the energy spent on electric signals (R(2)=0.75). The oxygen consumed by males for signal production closely paralleled the product of the electric signal's waveform area (R(2)=0.99) and the discharge rate (R(2)=0.59), two signal parameters that can be assessed directly by conspecifics. Thus the electric communication signal of males carries the information to reveal their body condition to prospective mates and competing males. Because the electric signal constitutes a significant fraction of the energy budget, energy savings, along with predation avoidance, provides an adaptive basis for the production of circadian rhythms in electric signals.

Signal Cloaking by Electric Fish

Electric fish produce weak electric fields to image their world in darkness and to communicate with potential mates and rivals. Eavesdropping by electroreceptive predators exerts selective pressure on electric fish to shift their signals into less-detectable high-frequency spectral ranges. Hypopomid electric fish evolved a signal-cloaking strategy that reduces their detectability by predators in the lab (and thus presumably their risk of predation in the field). These fish produce broad-frequency electric fields close to the body, but the heterogeneous local fields merge over space to cancel the low-frequency spectrum at a distance. Mature males dynamically regulate this cloaking mechanism to enhance or suppress low-frequency energy. The mechanism underlying electric-field cloaking involves electrogenic cells that produce two independent action potentials. In a unique twist, these cells orient sodium and potassium currents in the same direction, potentially boosting their capabilities for current generation. Exploration of such evolutionary inventions could aid the design of biogenerators to power implantable medical devices, an ambition that would benefit from the complete genome sequence of a gymnotiform fish.