Electric fish

An electric fish is any fish that can generate electric fields. Most electric fish are also electroreceptive, meaning that they can sense electric fields.[1] The only exceptions are the stargazers.[2] Electric fish include both oceanic and freshwater species.[3] Many cartilaginous fish such as sharks, rays and catfishes are electroreceptive but not electrogenic. Most common bony fish are non-electric. There are some 350 species of electric fish.[4]

Among the electric fishes are electric eels, knifefish capable of generating an electric field, both at low voltage for electrolocation and at high voltage to stun their prey.

Electric fish produce their electrical fields from a specialized electric organ in the tail.[5] This is made up of modified muscle or nerve cells, specialized for producing strong electric fields,[6] primarily for defense against predators[7] and navigation.[8][9][10] Electric organ discharges are two types, pulse and wave,[11] and vary both by species[12] and by function.[8]

Evolution and phylogeny
Further information: Electroreception
Electric fish clades
Vertebrates
Cartilaginous fishes
 

Torpediniformes (electric rays)

 
 

Rajiformes (skates)

 
430 mya
Bony fishes
 
Osteoglossiformes
Mormyridae

elephantfishes

knollenorgans  
 (pulses)
Gymnarchidae

African knifefish

knollenorgans  
 (waves)
 
 
 
Gymnotiformes  
S. Amer. knifefishes

 
Electric eel

 
ampullary 
receptors 
Siluriformes
Electric catfish

 
 
 
Uranoscopidae  

Stargazers

no electro‑
location 
 
 
425 mya
Ampullae of Lorenzini 
Actively electrolocating fish are marked with a small yellow lightning flash . Fish able to deliver electric shocks are marked with a red lightning flash . Non-electric and purely passively-electrolocating species are not shown.[13]

All fish, indeed all vertebrates, use electrical signals in their nerves and muscles.[14] Cartilaginous fishes and some other basal groups use passive electrolocation with sensors that detect electric fields;[15] the platypus and echidna have separately evolved this ability. The knifefishes and elephantfishes actively electrolocate, generating weak electric fields to find prey. Finally, fish in several groups have the ability to stun their prey. Among these, only the stargazers do not also use electrolocation.[1]

In vertebrates, electroreception is an ancestral trait, meaning that it was present in their last common ancestor.[15] This form of ancestral electroreception is called ampullary electroreception, from the name of the receptive organs involved, ampullae of Lorenzini. These evolved from the mechanical sensors of the lateral line, and exist in cartilaginous fishes (sharks, rays, and chimaeras), lungfishes, bichirs, coelacanths, sturgeons, paddlefish, aquatic salamanders, and caecilians. Ampullae of Lorenzini were lost early in the evolution of bony fishes and tetrapods. Where electroreception does occur in these groups, it has secondarily been acquired in evolution, using organs other than and not homologous with ampullae of Lorenzini.[15][16]

Fish with electric organs able to deliver a shock have evolved four separate times, each one forming a clade: once during the evolution of cartilaginous fishes, creating the electric skates and rays, and three times during the evolution of the bony fishes.[17][15]

Electric organ function
Further information: Electric organ (biology)

The function of the electric organ is most prominently self-defense and communication.[7] Additional functions in some fish also include navigation[8] and sexually dimorphic signaling.[18] 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.[8] Electrogenic proteins trigger action potential-like sequences that result in the electric potential difference within the cells.[8] Smaller, weaker currents like those produced by Sachs’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.[8] 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.[19] 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.[19]

Electric organs can be located in fish along the body or in the tail.[4] 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.[20] 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.[20]

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.[18] 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”.[21] Both EODfs and EOD modulations have high variability across species as well as between the sexes, producing species diversity.[21] 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.[18] 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.[21] 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.[21]

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.[5]

Organ evolution

Fish with electric organs have evolved eight separate times: twice during the evolution of cartilaginous fishes, creating the electric skates and rays, and six times during the evolution of the bony fishes. These electric bony fishes include the African mormyrids, the electric eel and the knifefishes.[22] Most organs evolved from myogenic tissue, however one group of gymnotiformes, the Apteronotidae, derived their electric organ from neurogenic tissue.[2] 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 evolutionary convergences between the trunk and the organ.[9]

It 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.[22] 

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. The four groups of strongly electric fish are the electric eel, the electric catfishes, the electric rays, and the stargazers.

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.[23] 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.[23]

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.[24] 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.[25]

Weakly electric fish
The elephantnose fish is a weakly electric fish which generates an electric field with its electric organ and then processes the returns from its electroreceptors to locate nearby objects.[26]

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.[27]

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.

Electric organ discharge patterns

Electric organ discharges are generated from the animal’s electric organ.[12] 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.[12] Their electric signals are often very simple as well as stereotyped, ie. always the same.[11] 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 (SPI) (ie. the temporal pattern in which EODs are released).[12] 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.[12]

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.[28] As of 2012, Gymnotus is the most diverse group out of the gymnotiform and mormyriform genera.[11] 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.[28] 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.[28] 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.[28]

Jamming avoidance response
Main article: Jamming avoidance response

It had been theorized as early as the 1950s that electric fish near each other might experience some type of interference. In 1963, Akira Watanabe and Kimihisa Takeda discovered the jamming avoidance response in the weakly electric (electrolocating) knifefish genus Eigenmannia.[29][30] When two fish are approaching one another, their electric fields interfere.[31] This sets up a beat with a frequency equal to the difference between the discharge frequencies of the two fish.[31] The jamming avoidance response comes into play when fish are exposed to a slow beat. If the neighbor's frequency is higher, the fish will lower its frequency, and vice versa.[30]

Taxonomic distribution

Most electric fish are in freshwater families. Two groups of marine fish, the electric rays (Torpediniformes: Narcinidae and Torpedinidae) and the stargazers (Perciformes: Uranoscopidae), are capable of generating strong electric pulses.

Mormyrids and Gymnotiformes

The Mormyrids and Gymnotiformes are freshwater fishes. They are the only two groups which actively electrolocate, generating weak electric fields and sensing their prey by the small differences in the electric field distortions from the capacitance and resistance of the objects around them. The Gymnotiformes include the electric eels, the only fish which are both electric and actively electrolocating.[1]

In the Gymnotiformes are many species of weakly electric knifefishes.[22] 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, and thus not homologous with other electric organs. Instead, these fishes have electric organs derived from neural tissue called neurogenic electric organs. These are located along the length of the spinal cord. However, during the larval stage there are often electric organs derived from muscle tissue; these disappear during development.[22]

Among the Gymnotiformes, the electric eel (originally thought to exist as a single species) is the first electric fish to have been discovered and is one of the strongest, able to produce up to 600 volts. It is the only known species to have three different electric organs, namely the main electric organ, Hunter's organ, and Sachs's organ.[20] The main electric organ runs the length of the eel's body. Sachs's organ is near the tail and is about three times smaller than the main electric organ. Hunter's organ also runs the length of the eel's body but lies below the main electric organ.[22] These electric organs make up nearly 80% of the eel's body and contain about 6000 electrocytes in total. The electric organs are able to emit both strong and weak signals. For communication and navigation, they discharge weak and continuous signals one volt or less in strength. For predation and defence they emit strong signals to stun and even kill prey. The weak signals come from the Sachs's electric organ or the posterior part of the Hunter's electric organ.[20] The strong signals come from the anterior part of the Hunter's electric organ.[20] The strength of a shock received from an electric eel depends on where the prey is relative to the eel.[32] Greater strength is received if the prey is touching both ends of the eel as it has positive and negative poles: the head is positive and the tail is negative. If the prey only touches the middle of the eel's body, it receives much less of a shock. Often, when an eel hunts it wraps its long body around the prey so that both ends of the body touch it. This generates the most electricity and the prey receive a more intense shock.[32] This stuns the prey, and the eel is then able to release it and feed. Another hunting method of electric eels is to shock prey hidden under sediment at the ocean floor. Shocking these prey causes muscle contractions, making them move out of their hiding places so that they can 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.[20]

Among the Mormyridae, the elephantfish are weak electric fish that emit electric charges in pulses for purposes of electrolocation and electrocommunication. They have two different electric organs: the mormyromast electroreceptor organ and the Knollenorgan. The mormyromast electroreceptor organ is used for electrolocation and the knollenorgan is used for electrocommunication.[33] 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. Some behaviours in elephantfish involve modified electric signals. During aggression, the signals accelerate; during territorial behaviour, double pulse signals may be emitted. Additionally, low frequency signals are used during courtship.[33] Other species of Mormyrid, in the genus Campylomormyrus, 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 carry information about their identity to the receiving fish. This can include their species, sex and dominance status, used in discrimination of non-conspecific fish and in mate selection.[34]

Electric rays and electric catfish

The electric rays are the only electric fishes among the Chondrichthyes, the cartilaginous fishes. They combine their ability to stun prey with passive electrolocation.[1] Electric catfish, which are bony fish, are similarly both electric and passively electrolocating.[1]

Stargazers

The stargazers (Uranoscopidae) are the only electric fishes which do not electrolocate.[1] They are ray-finned fishes of the genus Astroscopus. They have electric organs behind the eyes, derived from eye muscles.[22]

See also

References
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