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An electric fish is any fish that can generate electric fields. A fish that can generate electric fields is called electrogenic while a fish that has the ability to detect electric fields is called electroreceptive. Most electrogenic fish are also electroreceptive. The only group of electrogenic fish who are not electroreceptive come from the family Uranoscopidae. Electric fish species can be found both in the ocean and in freshwater rivers of South America (Gymnotiformes) and Africa (Mormyridae). These two groups of weak electric fish diverged and independently evolved similar ways to communicate and localise objects through producing and receiving electric fields. Many fish such as sharks, rays and catfishes can detect electric fields and are thus electroreceptive, but they are not classified as electric fish because they cannot generate electricity. Most common bony fish (teleosts), including most fish kept in aquaria or caught for food, are neither electrogenic nor electroreceptive. There are approximately 350 species of fish that have the ability to create an electric field that can be used for communication, navigation or predation. These electric fishes are found within the bony fishes(Phylum Osteichthyes) and the cartilaginous fishes(Phylum Chondrichthyes).

Electric fish produce their electrical fields from a specialized structure called an electric organ, which as of 2019, approximately 1.5% of all fish species had. This is made up of modified muscle or nerve cells, which became specialized for producing bioelectric fields stronger than those that normal nerves or muscles produce, primarily for communication and predator defense or navigation. Evidence suggests that this organ evolved independently from muscular tissue. Typically this organ is located in the tail of the electric fish due to a possible need for rigid fixation in order for the electrodes within the organ to function. The electrical output of the organ is called the electric organ discharge, or EOD. EODs are typically functionally divided among electric fish. There are two types, pulse and wave, both originating from ancient evolutionary divergences in niche occupancy. EODs also have been observed to produce species-specific patterns or different electric signal/waveform patterns according to their functions.

Electric organ function
The function of the electric organ is most prominently self-defense and communication. Additional functions in some fish also include navigation and sexually dimorphic signaling. The organ in many electric fish can produce differing EODs depending on the function. Polarized arrangement of electrolyte cells in some eels (for example the electric eel, Electrophorus electricus) allow the generation of small voltages that can build or add up creating weak or strong currents. Electrogenic proteins trigger action potential-like sequences that result in the electric potential difference within the cells. Smaller, weaker currents like those produced by Sach’s organ in electric eels are more energy conservative and therefore utilized for navigation and communication while strong currents like those produced by the main organ in eels are used for predation/hunting and defense. Because these are such important factors in an individual’s life cycle, it is dangerous if other electric fish eavesdrop, and ends up causing selective pressures for individuals to make their signals less-detectable. To have successful use of the electric organ for communication, defense, and so on, Hypopomid electric fish have evolved a mechanism known as signal-cloaking to reduce detectability by predators and thus lower the likelihood they will experience predation.

Electric organs can be located in fish along the body or in the tail. These electric organs are made up of electrocytes which are large, flat cells that act like batteries. The anterior end of these cells react to stimuli from the nervous system and contain sodium channels. The posterior end of these cells contain sodium-potassium pumps. This arrangement can cause the electrocyte to become polar if triggered by stimuli. This occurs when electrocytes first receive a signal from the nervous system. Neurons release acetylcholine which triggers acetylcholine receptors to open and sodium ions flow into the electrocyte cell. This influx of positively charged sodium ions causes the cell membrane to slightly depolarize. Due to this depolarization, the gated sodium channels at the anterior end of the cell open and a large amount of positively charged sodium ions enter the cell. Consequently, the anterior end of the electrocyte becomes highly positive while the posterior end of the cell containing the sodium ion pump remains negative as it continues to pump out positively charged sodium ions. This difference in opposite charges creates a current and a voltage is released from each individual electrocyte, adding up to a strong overall voltage from the electric organ. After the voltage is released, the cell membranes go back to their resting potentials until they are triggered again.

As for sexually dimorphic signaling, the organ can be used to produce distinct EOD or sinusoidal signals to be received across species and picked up by individuals. The electric organ will fire to produce an EOD with a certain frequency, an EODf, along with short modulations termed “chirps” and “gradual frequency rises”. Both EODfs and EOD modulations have high variability across species as well as between the sexes, producing species diversity. In one type of weakly electric fish (the brown ghost knifefish, Apteronotus leptorhynchus) two different kinds of chirps are emitted among the males and females depending on the conspecific EOD that is received. Sex differences in electrocommunication behavior can be seen in the family the brown ghost knifefish belongs to as well, where both the magnitude and direction of sexual dimorphism in EODfs, rate of chirping, duration of chirping, chirp frequency modulation (FM), and amount of frequency peaks can vary. In one study, researchers found that in some of the species belonging to the Apteronotidae, males have higher EODfs than females, but in others it is the other way around. Variation was also seen in chirp rate, which was found to be sexually dimorphic, as males in some species chirp at greater rates than females, but in others it does not differ at all between the sexes. The same applies to chirp structure, which was also found to be sexually dimorphic, with different aspects differing between species: some males were seen to produce bigger chirps with greater FM compared to females, and some with a longer duration, while in other species chirps were found to not be sexually dimorphic in neither FM nor duration. Finally, males in one species were seen to produce chirps with multiple frequency peaks while females did not.

The function of these fishes’ electric organs are expected to be greatly altered by global warming, since fish are ectotherms and will have increased metabolic rates with rising water temperature. Different studies have found that EOD duration will decrease, EOD repetition rate will increase, and what is known as the “thermal trap” hypothesis may occur. This hypothesis states that when temperature is rapidly changing, spectral mismatches between the electroreceptor array and the self-EOD will prevent wave-type fishes from electrolocating due to blinding them to their own EOD, further restricting them to thermally stable habitats. Additionally, species-specificity of the EOD could be affected, resulting in an interference in mate choice and species recognition.

Organ evolution
Fish with electric organs have evolved 8 separate times. Twice during the evolution of cartilaginous fishes, creating the electric skates and rays. As well as six times during the evolution of the bony fishes. These electric bony fishes include the African mormyrids, the electric eel and the knifefishes. Most organs evolved from myogenic tissue, however one group of gymnotiformes, the Apteronotidae, derived their electric organ from neurogenic tissue. In the electric fish Gymnarchus niloticus (AKA the African knifefish), the tail, trunk, hypobranchial, and eye muscles have been found to be incorporated into the organ, most likely to provide rigid fixation for the electrodes while swimming. This provides evidence for a convergent evolution. In some other species, complete loss or considerable reduction of the tail fin has occurred, also indicating a convergence. This evolution is hypothesized to provide support against lateral bending while swimming and to maintain symmetry in the electric field for object detection. If an electric fish lives in an environment with little to no obstructions, such as some bottom-living fish, their electric organ has been seen to have less prominent evolutionized convergences between the trunk and the organ.

I t is believed that fish have a predisposition to develop electrocytes due to the segmentation of the spinal chord and the organization of the axial muscles. Due to this predisposition, electric organs have developed independently by convergent evolution in unrelated and isolated species. These electric organs have evolved separately in both the cartilaginous fish (Phylum Chondrichthyes) and the bony fishes (Phylum Osteichthyes) which are sister clades.

Strongly electric fish
Strongly electric fish are fish with an electric organ discharge that is powerful enough to stun prey or be used for defense. Typical examples are the electric eel, the electric catfishes, and electric rays.

As an example of how powerful strongly electric fish can be, the electric eel, even very small in size, can impart substantial electric power to whomever it chooses to target, and is able to generate enough current to exceed the threshold in pain receptors (nociceptors) in a variety of different species. Additionally, studies have shown that electric eels actually will leap out of the water to electrify threats directly- including one published in 2017 in which researchers tested this with a human arm.

The amplitude of the signal from these fish can range from 10 to 860 volts with a current of up to 1 ampere, according to the surroundings, for example different conductances of salt and freshwater. To maximize the power delivered to the surroundings, the impedances of the electric organ and the water must be matched:
 * Strongly electric marine fish give low voltage, high current electric discharges. In salt water, a small voltage can drive a large current limited by the internal resistance of the electric organ. Hence, the electric organ consists of many electrocytes in parallel.
 * Freshwater fish have high voltage, low current discharges. In freshwater, the power is limited by the voltage needed to drive the current through the large resistance of the medium. Hence, these fish have numerous cells in series.

Weakly electric fish


Weakly electric fish generate a discharge that is typically less than one volt. These are too weak to stun prey and instead are used for navigation, object detection (electrolocation) and communication with other electric fish (electrocommunication). Two of the best-known and most-studied examples are Peters's elephantnose fish (Gnathonemus petersii) and the black ghost knifefish (Apteronotus albifrons). The males of the nocturnal Brachyhypopomus pinnicaudatus, a toothless knifefish native to the Amazon basin, give off big, long electric hums to attract a mate.

The electric organ discharge waveform takes two general forms depending on the species. In some species the waveform is continuous and almost sinusoidal (for example the genera Apteronotus, Eigenmannia and Gymnarchus) and these are said to have a wave-type electric organ discharge. In other species, the electric organ discharge waveform consists of brief pulses separated by longer gaps (for example Gnathonemus, Gymnotus, Leucoraja) and these are said to have a pulse-type electric organ discharge.

EOD patterns
Electric organ discharges are generated from the animal’s electric organ. It emits pulse-like electric signals for a multitude of reasons, depending on the species. Many species use it for communication, while others use it for electrolocation, hunting, or defense. Their electric signals are often very simple as well as stereotyped, ie. always the same. A study from 2018 on two species of weakly electric African fish (Campylomormyrus compressirostris and the blunt jawed elephant nose, Campylomormyrus tamandua) looked at the communication aspect of their signals, specifically what information they are sending and receiving and how they are sending and receiving it.

Previous research has found that the two components of electrocommunication are EODs and sequence pulse interval, AKA SPI (ie. the temporal pattern EODs are released in). Using this as a starting point, the researchers conducted playback experiments to find the differences between EOD waveform and SPI between the two species, specifically how they relate to species recognition and discrimination, and what cues each species use to do this. They found that for SPI, C. compressirostris showed a tendency to burst when resting while C. tamandua presented a discharge pattern that was more heterogeneous. They also saw that the average EOD frequency and the average duration of SPI serial correlations were species specific which suggests that SPI may convey information to the receiver. In addition, the results showed evidence to support the idea that males mediate species recognition and discrimination in C. compressirostris as well as other mormyrid species. The researchers also noticed a significant relationship between EOD waveforms when they were paired with a natural SPI recording in C. compressirostris, however, this preference was not seen in all conditions. The males did not respond to artificial SPI recordings which researchers think suggests that there is some important information within the normal SPIs.

Another group of researchers studied the genetics of three species of the family Gymnotus (the naked back knifefishes G. arapaima, G. mamiraua, and G. jonasi of the Central Amazon Floodplain) and the diversity of their chromosomal and electrical signals. As of 2012, Gymnotus is the most diverse group out of the gymnotiform and mormyriform genera. They looked at their chromosomes and genes to find similarities, differences, and patterns to see how exactly they all evolved and are related to each other. The researchers used these species specifically because they are a model species for studying how postzygotic and prezygotic reproductive isolation events could lead to speciation and diversification. This paper is one of the first karyotypic analyses for these three species that also looks at EOD variation. They made a significant discovery for one species. They found that G. arapaima has a karyotypic formula that has never been seen before for the genus. They decided to place it within a small clade with a few other species, which all have more rows of scales and a larger body size. Their findings suggest G. arapaima to be similar to other species within this clade but also distinct because it has a smaller number of bi-armed chromosomes.

G. mamiraua also displayed a distinct karyotypic formula different from its closest relatives. They hypothesized that this could be from chromosomal rearrangements like translocation and pericentric inversion. This species also has a different NOR (nucleolar organizing region) composition compared to its closest relative, G. cf. mamiraua from the eastern Amazon. They also have evidence that these two populations have different interspacing sequences between ribosomal genes which could be why they may be reproductively isolated due to this karyotypic difference acting as a postzygotic barrier.

Lastly, when studying G. jonasi they found it has the highest number of st/a (subterminal acrocentric region) chromosomes within its group. They also found that it exhibits multiple NORs which only one other species has. Their study shows just how diverse- in terms of species, electric signals, karyotype, and overall genetics- the Gymnotus family is.

The findings of the previous study showed that Gymnotus has significant chromosomal diversity and this is strongly supported by another study. A 2013 study also researched the genus Gymnotus, specifically their electric diversity in the ultimate and proximate perspectives. With the proximate lens, the authors aimed to examine the diversity of EODs and with the ultimate lens they wanted to study signal diversity. They used 11 species, each representing a major clade. Their results integrated data from multiple fields and dozens of authors and researchers to create a big picture depiction of signal diversity, species diversity, and the evolution of the members of this genus.

This paper compared three studies about the phylogeny of this genus and concluded that the genus is stable and will not likely change, even with new species that could be discovered in the future with emerging molecular and morphological tests and data. They found a basal dichotomy between the clade with all remaining Gymnotus species and the G. coatesi clade. The researchers observed several evolutionary ht-EOD structure transitions, too. They also concluded that the ht-EOD differences between closely related Gymnotus species were mostly differences in EOD duration, amplitude, and relative size, all of which oftentimes varied. In the proximate perspective, this variation correlates to the “diversity in innervation patterns of the electrocytes, auto-excitability, electrocyte density and distribution, and the expression of neurogenic components in the EOD”. These components show a strong correlation to the ht-EOD structure and phylogenetic signal. In comparison, with the ultimate perspective they found several extrinsic biotic selective pressures that seem to have an effect on the shaping of the communication aspect of EODs.

EOD evolution
A paper on the three different electric organs of Electrophorus electricus, the electric eel, details what each organ’s function is. Overall, all three are used for navigation, communication, hunting, and defense but each purpose happens in different levels of electric organ discharge. E. electricus is the only species that has evolved three separate organs for discharge and one of the only ones that can generate strong discharges. Their goal was to find the differences between the three electric organs and what proteins, phosphosites, and phosphorylation events occurred that may have caused the changes.

The researchers found that the electrolyte cells in the electric organs of E. electricus have a polarized cell arrangement, which is how they are able to generate small voltages that can add up. The cells are triggered by acetylcholine, which causes an action potential-like sequence of events that results in an electric potential difference inside each electrolyte cell, which, combined with every other cell, sums up to create a large voltage. They also found several new phosphorylation sites in the electrogenic proteins, including one that has only been seen elsewhere in the electric ray species Tetronarce californica. These new sites are not seen in nonelectrogenic species which means they may be unique and important to the ability to generate EODs. These sites very likely evolved independent of one another, because the two species are very distantly related. Finally, they also found a consistent and special abundance pattern in a handful of the electrogenic proteins within each of the three electric organs (the main organ, Sach’s organ, and Hunter’s organ). Having different abundances likely reflects the energy each organ requires to emit EOD’s, whether weak or strong.

A recent study from 2020 looked at the evolution of sensorimotor integration in terms of communication signal diversification in Mormyrid fishes, AKA African weakly electric fishes. Corollary discharge is one of the ways motor control can influence sensory processing which is done by filtering out the individual’s own signals from being processed. Because corollary discharge is responsible for cancelling its own signals, it would make evolutionary sense if signal diversification was selected for. To answer this question the researchers looked at 7 different mormyrid species with varying corollary discharge inhibitions (CDI) and EOD durations. The researchers did find a correlation between signal diversification and CDI, as well as between EOD duration and the onset of CDI (but not duration of CDI). With these findings, they were able to conclude that, in response to an individual’s own EODs, SCDIs evolved to shift their time window in order to impede spikes in KO.

Jamming avoidance response
It had been theorized as early as the 1950s that electric fish near each other might experience some type of interference or inability to segregate their own signal from those of neighbors. This issue does not arise, however, because the electric fish adjust to avoid frequency interference. In 1963, two scientists, Akira Watanabe and Kimihisa Takeda, discovered the behavior of the jamming avoidance response in the knifefish Eigenmannia sp. In collaboration with T.H. Bullock and colleagues, the behavior was further developed. Finally, the work of Walter Heiligenberg expanded it into a full neuroethology study by examining the series of neural connections that led to the behavior. It is now known that when two fish are approaching one another, their EODs will end up interfering, which can result in amplitude modulations (AM). This will have a frequency equal to the difference frequency (DF), or the difference between the two electric organ discharge frequencies (EODf) of the two fish that are interacting. Unfortunately, when the DF is small, the AMs can copy those that are used in echolocation, which leads to confusion and electrosensory blurring. The jamming avoidance response comes into play when fish are exposed to a small DF. Eigenmannia is a weakly electric fish that can self-generate electric discharges through electrocytes in its tail. Furthermore, it has the ability to electrolocate by analyzing the perturbations in its electric field. However, when the frequency of a neighboring fish's current is very close (less than 20 Hz difference) to that of its own, the fish will avoid having their signals interfere through a behavior known as jamming avoidance response. If the neighbor's frequency is higher than the fish's discharge frequency, the fish will lower its frequency, and vice versa. The sign of the frequency difference is determined by analyzing the "beat" pattern of the incoming interference which consists of the combination of the two fish's discharge patterns.

Neuroethologists performed several experiments under Eigenmannia's natural conditions to study how it determined the sign of the frequency difference. They manipulated the fish's discharge by injecting it with curare which prevented its natural electric organ from discharging. Then, an electrode was placed in its mouth and another was placed at the tip of its tail. Likewise, the neighboring fish's electric field was mimicked using another set of electrodes. This experiment allowed neuroethologists to manipulate different discharge frequencies and observe the fish's behavior. From the results, they were able to conclude that the electric field frequency, rather than an internal frequency measure, was used as a reference. This experiment is significant in that not only does it reveal a crucial neural mechanism underlying the behavior but also demonstrates the value neuroethologists place on studying animals in their natural habitats.

Species
The following is a table of electric fish species listed by family. Most families inhabit fresh water. Two groups of marine fish, the electric rays (Torpediniformes: Narcinidae and Torpedinidae) and the stargazers (Perciformes: Uranoscopidae), are capable of generating strong electric pulses.

 Electric Eel 

The electric eel is the first known electric fish and also one of the strongest, able to produce up to 600 volts. It is also the only known species to have three different electric organs present. These three electric organs are: the main electric organ, the Hunter’s organ, and the Sach’s organ. The main electric organ runs along the length of the eel’s body. The sach’s organ is located near the tail and is about three times smaller than the main electric organ. The Hunter’s organ also runs along the length of the eel's body but below the main electric organ. These electric organs make up nearly 80% of the eel’s elongated body and contain about 6000 electrocytes in total. The electric organs are able to emit both strong and weak signals based on function. For communication and navigation, they discharge weak and continuous signals that are one volt or less in strength. For predation and defense they emit strong signals that are able to stun and even kill prey. The weak signals come from the Sach’s electric organ or the posterior part of the Hunter’s electric organ. The strong signals come from the anterior part of the Hunter’s electric organ.

Additionally, the strength of a shock received from an electric eel depends on where the prey is relative to the eel. Greater strength will be received if the prey is touching both ends of the eel as it has positive and negative poles. The head being positive and the tail being negative. If the prey only touches a location in the middle of the eel's body, it will receive much less of a shock. Oftentimes, when an eel hunts it will wrap its long body around the prey so both ends of the body touch it. This generates the most electricity and the prey will receive a more intense shock. This stuns the prey and the eel is then able to release it and feed.

Another hunting method of electric eels is shocking prey that are hidden under sediment at the ocean floor. Shocking these prey causes muscle contractions, making them move out of their hiding place where they can then be attacked.

An additional advantage of electric organs in eels is their use for navigation. Electric eels are found in the fresh waters of South America that often consist of muddy and murky water with decreased visibility.

Knifefishes

In the same group as electric eels (the Gymnotiformes), there are also many species of weakly electric knifefishes. Those belonging to the knifefish family Apteronotidae, are particularly unique because they are the only electric fishes with electric organs not derived from muscle tissue. Instead, these fishes have electric organs derived from neural tissue called neurogenic electric organs. This again, shows how electric fishes have evolved from independent origins. These neurogenic electric organs are located along the length of the spinal cord. However, during the larval stage there are often electric organs derived from muscle tissue present. These later disappear as the organism develops.

Stargazers

Another unique species of electric fish are the stargazers. They are actinopterygians(ray-finned fishes) that belong to the genus Astroscopus. They have electric organs located behind the eyes that are derived from eye muscles.

Elephant Fish

A species of elephant fish within the family Mormyridae is Mormyrus rume proboscirostris. They are weak electric fish that emit electric charges in pulses for purposes of electrolocation and electrocommunication. Electrolocation is the perception and sensing of electric fields for purposes of object identification and navigation. Electrocommunication is when electrical signals are sent and received between electric fish. Mormyrus rume proboscirostris have two different electric organs: the mormyromast electroreceptor organ and the knolleorgans. The mormyromast electroreceptor organ is used for electrolocation and the knolleorgans is used for electrocommunication.

Elephant fish have developed advanced methods of perceiving electrostatic fields, resulting in improved communication and sensory systems. These fish are able to echo back signals to each other to communicate. Generally, they use the same pulsing signals for both electrolocation and electrocommunication. However, different behaviours in elephant fish have been found to have modified electric signals. During aggression, the signals accelerate and during territorial behaviour, double pulse signals may be emitted. Additionally, low frequency signals may be seen during courtship.

Other species of Mormyridae fish belonging to the genus Campylomormyrus also use similar methods of communication. They use species-specific electric organ discharges in pulsing patterns. They have a specialized electric organ located in the caudal peduncle near the tail. Their electrical discharges are able to carry information about their identity to the receiving fish. This information can include their species, sex and dominance status. This plays a role in discrimination of non-species fish and also in mate selection.