Electric fish

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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-eel (cropped).jpg
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.

An electric fish is any fish that can generate electric fields, whether to sense things around them, for defence, or to stun prey. Most fish able to produce shocks are also electroreceptive, meaning that they can sense electric fields. The only exception is the stargazer family (Uranoscopidae). Electric fish, although a small minority of all fishes, include both oceanic and freshwater species, and both cartilaginous and bony fishes.

Contents

Electric fish produce their electrical fields from an electric organ. This is made up of electrocytes, modified muscle or nerve cells, specialized for producing strong electric fields, used to locate prey, for defence against predators, and for signalling, such as in courtship. Electric organ discharges are two types, pulse and wave, and vary both by species and by function.

Electric fish have evolved many specialised behaviours. The predatory African sharptooth catfish eavesdrops on its weakly electric mormyrid prey to locate it when hunting, driving the prey fish to develop electric signals that are harder to detect. Bluntnose knifefishes produce an electric discharge pattern similar to the electrolocation pattern of the dangerous electric eel, probably a form of Batesian mimicry to dissuade predators. Glass knifefish that are using similar frequencies move their frequencies up or down in a jamming avoidance response; African knifefish have convergently evolved a nearly identical mechanism.

Evolution and phylogeny

All fish, indeed all vertebrates, use electrical signals in their nerves and muscles. [1] Cartilaginous fishes and some other basal groups use passive electrolocation with sensors that detect electric fields; [2] 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 deliver electric shocks powerful enough to stun their prey or repel predators. Among these, only the stargazers, a group of marine bony fish, do not also use electrolocation. [3] [4]

In vertebrates, electroreception is an ancestral trait, meaning that it was present in their last common ancestor. [2] 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. [2] [5] Most common bony fish are non-electric. There are some 350 species of electric fish. [6]

Electric organs have evolved eight times, four of these being organs powerful enough to deliver an electric shock. Each such group is a clade. [7] [2] Most electric organs evolved from myogenic tissue (which forms muscle), however, one group of Gymnotiformes, the Apteronotidae, derived their electric organ from neurogenic tissue (which forms nerves). [8] In Gymnarchus niloticus (the African knifefish), the tail, trunk, hypobranchial, and eye muscles are incorporated into the organ, most likely to provide rigid fixation for the electrodes while swimming. In some other species, the tail fin is lost or reduced. This may reduce lateral bending while swimming, allowing the electric field to remain stable for electrolocation. There has been convergent evolution in these features among the mormyrids and gymnotids. Electric fish species that live in habitats with few obstructions, such as some bottom-living fish, display these features less prominently. This implies that convergence for electrolocation is indeed what has driven the evolution of the electric organs in the two groups. [9] [10]

Actively electrolocating fish are marked on the phylogenetic tree with a small yellow lightning flash Farm-Fresh lightning.png . Fish able to deliver electric shocks are marked with a red lightning flash Lightning Symbol.svg . Non-electric and purely passively electrolocating species are not shown. [2] [11] [10]

Vertebrates
Chondrichthyes

Torpediniformes (electric rays) (69 spp) Farm-Fresh lightning.png Lightning Symbol.svg Fish4345 - Flickr - NOAA Photo Library (white background).jpg

Rajiformes (skates) (~200 spp) Farm-Fresh lightning.png Raja montagui2.jpg

430  mya
Bony fishes
Osteogloss.
 
Mormyridae

elephantfishes (~200 spp) Farm-Fresh lightning.png Gnathonemus petersii.jpg

knollenorgans   
 (pulses)
 
Gymnarchidae

African knifefish (1 sp) Farm-Fresh lightning.png Gymnarchus niloticus005 (white background).JPG

knollenorgans   
 (waves)
110 mya
 
Gymnotiformes
S. Amer.

(>100 spp) Farm-Fresh lightning.png Johann Natterer - Itui-cavalo (Apteronotus albifrons).jpg

knifefishes
Electric eels

(3 spp) Farm-Fresh lightning.png Lightning Symbol.svg Lateral view of Electrophorus electricus.png

119 mya
amp. recept.
Siluriformes
 
Uranoscopidae   

Stargazers (50 spp) Lightning Symbol.svg Uranoscopus sulphureus (white background).jpg

no electro‑
location 
425  mya
Amp. of Lorenzini  

Weakly electric fish

The elephantnose fish is a weakly electric fish which generates an electric field with its electric organ, detects small variations in the field with its electroreceptors, and processes the detected signals in the brain to locate nearby objects. Electroreception system in Elephantfish.svg
The elephantnose fish is a weakly electric fish which generates an electric field with its electric organ, detects small variations in the field with its electroreceptors, and processes the detected signals in the brain to locate nearby objects.

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, electrolocation in conjunction with electroreceptors in their skin, and electrocommunication with other electric fish. The major groups of weakly electric fish are the Osteoglossiformes, which include the Mormyridae (elephantfishes) and the African knifefish Gymnarchus , and the Gymnotiformes (South American knifefishes). These two groups have evolved convergently, with similar behaviour and abilities but different types of electroreceptors and differently sited electric organs. [2] [11]

Strongly electric fish

Impedance matching in strongly electric fishes. Since seawater conducts far better than freshwater, marine fish operate at much higher currents but lower voltages. Impedance matching in electric fishes.svg
Impedance matching in strongly electric fishes. Since seawater conducts far better than freshwater, marine fish operate at much higher currents but lower voltages.

Strongly electric fish, namely the electric eels, the electric catfishes, the electric rays, and the stargazers, have an electric organ discharge powerful enough to stun prey or be used for defence, [14] and navigation. [15] [9] [16] The electric eel, even when very small in size, can deliver substantial electric power, and enough current to exceed many species' pain threshold. [17] Electric eels sometimes leap out of the water to electrify possible predators directly, as has been tested with a human arm. [17]

The amplitude of the electrical output 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: [13]

Electric organ

Anatomy

Anatomy of a strongly electric freshwater fish: the electric eel's three electric organs. The main organ is long, with a stack of many electrocytes in series to provide a high voltage, matching the high impedance of freshwater. Electric eel's electric organs.svg
Anatomy of a strongly electric freshwater fish: the electric eel's three electric organs. The main organ is long, with a stack of many electrocytes in series to provide a high voltage, matching the high impedance of freshwater.

Electric organs vary widely among electric fish groups. They evolved from excitable, electrically active tissues that make use of action potentials for their function: most derive from muscle tissue, but in some groups the organ derives from nerve tissue. [18] The organ may lie along the body's axis, as in the electric eel and Gymnarchus; it may be in the tail, as in the elephantfishes; or it may be in the head, as in the electric rays and the stargazers. [3] [8] [19]

Physiology

An electric ray (Torpediniformes) showing paired electric organs in the head, and electrocytes stacked vertically within it Elektroplax Rochen.png
An electric ray (Torpediniformes) showing paired electric organs in the head, and electrocytes stacked vertically within it

Electric organs are made up of electrocytes, large, flat cells that create and store electrical energy, awaiting discharge. The anterior ends of these cells react to stimuli from the nervous system and contain sodium channels. The posterior ends contain sodium–potassium pumps. Electrocytes become polar when triggered by a signal from the nervous system. Neurons release the neurotransmitter acetylcholine; this triggers acetylcholine receptors to open and sodium ions to flow into the electrocytes. [15] The influx of positively charged sodium ions causes the cell membrane to depolarize slightly. This in turn causes the gated sodium channels at the anterior end of the cell to open, and a flood of sodium ions enters the cell. Consequently, the anterior end of the electrocyte becomes highly positive, while the posterior end, which continues to pump out sodium ions, remains negative. This sets up a potential difference (a voltage) between the ends of the cell. After the voltage is released, the cell membranes go back to their resting potentials until they are triggered again. [15]

Discharge patterns

Electric organ discharges (EODs) need to vary with time for electrolocation, whether with pulses, as in the Mormyridae, or with waves, as in the Torpediniformes and Gymnarchus , the African knifefish. [19] [20] [21] Many electric fishes also use EODs for communication, while strongly electric species use them for hunting or defence. [20] Their electric signals are often simple and stereotyped, the same on every occasion. [19]

Electrocommunication

Weakly electric fish can communicate by modulating the electrical waveform they generate. They may use this to attract mates and in territorial displays. [22]

Sexual behaviour

In sexually dimorphic signalling, as in the brown ghost knifefish (Apteronotus leptorhynchus), the electric organ produces distinct signals to be received by individuals of the same or other species. [23] The electric organ fires to produce a discharge with a certain frequency, along with short modulations termed "chirps" and "gradual frequency rises", both varying widely between species and differing between the sexes. [24] [20] For example, in the glass knifefish genus Eigenmannia , females produce a nearly pure sine wave with few harmonics, males produce a far sharper non-sinusoidal waveform with strong harmonics. [25]

Male bluntnose knifefishes (Brachyhypopomus) produce a continuous electric "hum" to attract females; this consumes 11–22% of their total energy budget, whereas female electrocommunication consumes only 3%. Large males produced signals of larger amplitude, and these are preferred by the females. The cost to males is reduced by a circadian rhythm, with more activity coinciding with night-time courtship and spawning, and less at other times. [26]

Antipredator behaviour

Electric catfish (Malapteruridae) frequently use their electric discharges to ward off other species from their shelter sites, whereas with their own species they have ritualized fights with open-mouth displays and sometimes bites, but rarely use electric organ discharges. [27]

The electric discharge pattern of bluntnose knifefishes is similar to the low voltage electrolocative discharge of the electric eel. This is thought to be a form of bluffing Batesian mimicry of the powerfully protected electric eel. [28]

Fish that prey on electrolocating fish may "eavesdrop" [29] on the discharges of their prey to detect them. The electroreceptive African sharptooth catfish ( Clarias gariepinus ) may hunt the weakly electric mormyrid, Marcusenius macrolepidotus in this way. [30] This has driven the prey, in an evolutionary arms race, to develop more complex or higher frequency signals that are harder to detect. [31]

Jamming avoidance response

When a glass knifefish encounters a neighbour with a closely similar frequency, one fish shifts its frequency upward and the other downward in the jamming avoidance response. Jamming avoidance response in glass knifefish.svg
When a glass knifefish encounters a neighbour with a closely similar frequency, one fish shifts its frequency upward and the other downward in the 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 Eigenmannia. [32] When two fish are approaching one another, their electric fields interfere. [33] This sets up a beat with a frequency equal to the difference between the discharge frequencies of the two fish. [33] The jamming avoidance response comes into play when fish are exposed to a slow beat. If the neighbour's frequency is higher, the fish lowers its frequency, and vice versa. [32] [25] A similar jamming avoidance response was discovered in the distantly related Gymnarchus niloticus , the African knifefish, by Walter Heiligenberg in 1975, in a further example of convergent evolution between the electric fishes of Africa and South America. [34] Both the neural computational mechanisms and the behavioural responses are nearly identical in the two groups. [35]

See also

Related Research Articles

<span class="mw-page-title-main">Gymnotiformes</span> Order of bony fishes

The Gymnotiformes are an order of teleost bony fishes commonly known as Neotropical knifefish or South American knifefish. They have long bodies and swim using undulations of their elongated anal fin. Found almost exclusively in fresh water, these mostly nocturnal fish are capable of producing electric fields to detect prey, for navigation, communication, and, in the case of the electric eel, attack and defense. A few species are familiar to the aquarium trade, such as the black ghost knifefish, the glass knifefish, and the banded knifefish.

<span class="mw-page-title-main">Naked-back knifefish</span> Family of freshwater fishes

The naked-back knifefishes are a family (Gymnotidae) of knifefishes found only in fresh waters of Central America and South America. All have organs adapted to electroreception. The family has about 43 valid species in two genera. These fish are nocturnal and mostly occur in quiet waters from deep rivers to swamps. In strongly flowing waters, they may bury themselves.

<i>Electrophorus electricus</i> South American electric fish

Electrophorus electricus is the best-known species of electric eel. It is a South American electric fish. Until the discovery of two additional species in 2019, the genus was classified as the monotypic, with this species the only one in the genus. Despite the name, it is not an eel, but rather a knifefish. It is considered as a freshwater teleost which contains an electrogenic tissue that produces electric discharges.

<span class="mw-page-title-main">Batesian mimicry</span> Bluffing imitation of a strongly defended species

Batesian mimicry is a form of mimicry where a harmless species has evolved to imitate the warning signals of a harmful species directed at a predator of them both. It is named after the English naturalist Henry Walter Bates, who worked on butterflies in the rainforests of Brazil.

<span class="mw-page-title-main">Black ghost knifefish</span> Species of fish

The black ghost knifefish is a tropical fish belonging to the ghost knifefish family (Apteronotidae). They originate in freshwater habitats in South America where they range from Venezuela to the Paraguay–Paraná River, including the Amazon Basin. They are popular in aquaria. The fish is all black except for two white rings on its tail, and a white blaze on its nose, which can occasionally extend into a stripe down its back. It moves mainly by undulating a long fin on its underside. It will grow to a length of 18"-20". Only a fish for those with large aquariums, minimum 100 gallons.

<span class="mw-page-title-main">Electroreception and electrogenesis</span> Biological electricity-related abilities

Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes to stun prey. The capabilities are found almost exclusively in aquatic or amphibious animals, since water is a much better conductor of electricity than air. In passive electrolocation, objects such as prey are detected by sensing the electric fields they create. In active electrolocation, fish generate a weak electric field and sense the different distortions of that field created by objects that conduct or resist electricity. Active electrolocation is practised by two groups of weakly electric fish, the Gymnotiformes (knifefishes) and the Mormyridae (elephantfishes), and by Gymnarchus niloticus, the African knifefish. An electric fish generates an electric field using an electric organ, modified from muscles in its tail. The field is called weak if it is only enough to detect prey, and strong if it is powerful enough to stun or kill. The field may be in brief pulses, as in the elephantfishes, or a continuous wave, as in the knifefishes. Some strongly electric fish, such as the electric eel, locate prey by generating a weak electric field, and then discharge their electric organs strongly to stun the prey; other strongly electric fish, such as the electric ray, electrolocate passively. The stargazers are unique in being strongly electric but not using electrolocation.

<span class="mw-page-title-main">Mormyridae</span> Family of fishes

The Mormyridae, sometimes called "elephantfish", are a superfamily of weakly electric fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order, with around 200 species. Members of the family can be popular, if challenging, aquarium species. These fish have a large brain size and unusually high intelligence.

Knifefish may refer to several knife-shaped fishes:

<span class="mw-page-title-main">Hypopomidae</span> Family of knifefishes in the order Gymnotiformes

The Hypopomidae are a family of fishes in the order Gymnotiformes known as the bluntnose knifefish. They may also be called grass or leaf knifefishes. These electric fish are not often eaten, of little commercial importance, rarely kept as aquarium fish, and poorly studied; however, species in this family may constitute a significant fraction of the biomass in the areas they inhabit.

<span class="mw-page-title-main">Electric organ (fish)</span> Organ in electric fish

In biology, the electric organ is an organ that an electric fish uses to create an electric field. Electric organs are derived from modified muscle or in some cases nerve tissue, called electrocytes, and have evolved at least six times among the elasmobranchs and teleosts. These fish use their electric discharges for navigation, communication, mating, defence, and in strongly electric fish also for the incapacitation of prey.

<i>Magosternarchus</i> Genus of fishes

Magosternarchus is a genus of weakly electric knifefish in the family Apteronotidae, containing two species. They are endemic to Brazil, occurring in large river channels in the Amazon River basin. Both species are unusual benthic predators that specialize in biting off the tails of other knifefishes, and are characterized by their greatly enlarged jaws and teeth. Recent systematic studies indicate that both species should be included in Sternarchella instead of being placed in their own genus.

Sternarchogiton nattereri is a species of weakly electric knifefish in the family Apteronotidae. It is native to the Amazon River system and feeds on sponges. Unlike other members of the genus Sternarchogiton, there is pronounced sexual dimorphism in S. nattereri, with reproductively mature males developing strong external teeth on tips of their jaws. These males are so different from the females and juveniles that they were thought to be a different genus and species, the "tooth-lip knifefish" Oedemognathus exodon, for over 40 years.

<i>Orthosternarchus tamandua</i> Species of fish

Orthosternarchus tamandua, the tamandua knifefish, is a species of weakly electric knifefish in the family Apteronotidae, native to the deep river channels of the Amazon basin. This species is characterized by its whitish-pink color, long tubular snout, long dorsal appendage, and tiny, bilaterally asymmetrical eyes.

Sternarchogiton labiatus is a species of weakly electric knifefish in the family Apteronotidae. Its species name labiatus comes from the Latin labium, meaning "lip", referring to a distinctive three-lobed structure on its lower lips. S. labiatus is only known from the Tefé River, at a depth of 6–14 m (20–46 ft), and from the lower Rio Negro, in the Amazon River basin. They have been captured from both whitewater and blackwater habitats.

Sternarchogiton porcinum is a species of weakly electric knifefish in the family Apteronotidae. It is native to deep river channels in the Río Huallaga, Río Napo, and Río Amazonas in Peru, and in the Río Orinoco in Venezuela. Many specimens once identified as S. porcinum from the Brazilian Amazon Basin and the Venezuelan Orinoco are now known to be a different species, S. preto.

<span class="mw-page-title-main">Jamming avoidance response</span> Behavior performed by weakly electric fish to prevent jamming of their sense of electroreception

The jamming avoidance response is a behavior of some species of weakly electric fish. It occurs when two electric fish with wave discharges meet – if their discharge frequencies are very similar, each fish shifts its discharge frequency to increase the difference between the two. By doing this, both fish prevent jamming of their sense of electroreception.

<i>Rhamphichthys</i> Genus of fishes

Rhamphichthys(Rhamphos = Greek for beak and Ichthys = Greek for fish) is a genus of fish that includes the South American sand knifefish. These fish are eel shaped with a distinct beak like snout which gave them their name. Like most other knifefish Rhamphichthys species have electrical organs that help them live in the murky waters of South America. Currently there are 10 recognized species of Rhamphichthys, although many changes have been made in their taxonomy since their original discovery.

Distocyclus conirostris is a species of glass knifefishes found in the deep waters of the Amazon basin, lower portions of the Potaro River and in major parts of the Rio Orinoco. They are typically relegated to flood basins, flooded forests and main river channels. They have often been found gathering in small groups around vegetation, indicating a social nature. The fish has semi-translucent, glass-like pectoral and anal fins. The main body is a ground-like color with a lighter head. The largest currently recorded specimen is 34.5 cm.

<span class="mw-page-title-main">Electric eel</span> Genus of fishes in South America

The electric eels are a genus, Electrophorus, of neotropical freshwater fish from South America in the family Gymnotidae. They are known for their ability to stun their prey by generating electricity, delivering shocks at up to 860 volts. Their electrical capabilities were first studied in 1775, contributing to the invention in 1800 of the electric battery.

<span class="mw-page-title-main">Mormyroidea</span> Superfamily of fishes

The Mormyroidea are a superfamily of fresh water fishes endemic to Africa that, together with the families Hiodontidae, Osteoglossidae, Pantodontidae and Notopteridae, represents one of the main groups of living Osteoglossiformes. They stand out for their use of weak electric fields, which they use to orient themselves, reproduce, feed, and communicate.

References

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