An electric fish is any
fish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
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.
Electric fish produce their electrical fields from an
electric organ
An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since develop ...
. This is made up of electrocytes, modified
muscle
Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
or
nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the Electrochemistry, electrochemical nerv ...
cells, specialized for producing strong electric fields, used to locate prey, for
defence against predators, and for
signalling
A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology.
In ...
, 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
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, such ...
pattern of the dangerous electric eel, probably a form of
Batesian mimicry
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 butt ...
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
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s, use
electrical signals in their nerves and muscles.
Cartilaginous fishes and some other basal groups use passive electrolocation with sensors that detect electric fields;
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.
In
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s, electroreception is an
ancestral trait, meaning that it was present in their last common ancestor.
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
The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelia ...
, and exist in
cartilaginous fish
Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...
es (
shark
Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s,
rays, and
chimaeras),
lungfish
Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...
es,
bichir
Bichirs and the reedfish comprise Polypteridae , a family (biology), family of archaic Actinopterygii, ray-finned fishes and the only family in the order (biology), order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The D ...
s,
coelacanth
Coelacanths ( ) are an ancient group of lobe-finned fish (Sarcopterygii) in the class Actinistia. As sarcopterygians, they are more closely related to lungfish and tetrapods (the terrestrial vertebrates including living amphibians, reptiles, bi ...
s,
sturgeon
Sturgeon (from Old English ultimately from Proto-Indo-European language, Proto-Indo-European *''str̥(Hx)yón''-) is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the ...
s,
paddlefish
Paddlefish (family Polyodontidae) are a family of ray-finned fish belonging to order Acipenseriformes, and one of two living groups of the order alongside sturgeons (Acipenseridae). They are distinguished from other fish by their elongated rost ...
, aquatic
salamander
Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s, and
caecilian
Caecilians (; ) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) amphibians with small or sometimes nonexistent eyes. They mostly live hidden in soil or in streambeds, and this cryptic lifestyle renders caecilians ...
s. Ampullae of Lorenzini were lost early in the
evolution
Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
of bony fishes and
tetrapod
A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
s. 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.
Most common
bony fish
Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...
are non-electric. There are some 350 species of electric fish.
Electric organ
An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since develop ...
s have evolved eight times, four of these being organs powerful enough to deliver an electric shock. Each such group is a
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
.
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).
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
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
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.
Actively electrolocating fish are marked on the
phylogenetic tree
A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...
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.
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
Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...
,
electrolocation
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, such ...
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.
Strongly electric fish

Strongly electric fish, namely the
electric eel
The electric eels are a genus, ''Electrophorus'', of neotropical freshwater fish from South America in the family Gymnotidae, of which they are the only members of the subfamily Electrophorinae. They are known for their electric fish, ability ...
s, the
electric catfish
Electric catfish or Malapteruridae is a family of catfishes (order Siluriformes). This family includes two genera, '' Malapterurus'' and '' Paradoxoglanis'', with 21 species. Several species of this family have the ability to generate electricit ...
es, the
electric rays, and the
stargazers, have an electric organ discharge powerful enough
to stun prey or be used
for defence,
and
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...
.
The electric eel, even when very small in size, can deliver substantial electric power, and enough current to exceed many species'
pain threshold.
Electric eels sometimes leap out of the water to electrify possible predators directly, as has been tested with a human arm.
The
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of the electrical output from these fish can range from 10 to 860
volt
The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
s with a current of up to 1
ampere
The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
, 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
An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since develop ...
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.
Electric organ
Anatomy
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. 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.
Physiology
file: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 channel
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell (biology), cell's cell membrane, membrane. They belong to the Cation channel superfamily, superfamily of cation channels.
Classific ...
s. The posterior ends contain
sodium–potassium pump
The sodium–potassium pump (sodium–potassium adenosine triphosphatase, also known as -ATPase, pump, or sodium–potassium ATPase) is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal cells. It performs s ...
s. Electrocytes become polar when triggered by a signal from the nervous system. Neurons release the neurotransmitter
acetylcholine
Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...
; this triggers
acetylcholine receptor
An acetylcholine receptor (abbreviated AChR) or a cholinergic receptor is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter.
Classification
Like other transmembrane receptors, acetylcholine receptor ...
s to open and
sodium ion
Sodium is a chemical element; it has symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable isotope ...
s to flow into the electrocytes.
The influx of positively charged sodium ions causes the
cell membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
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
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
) between the ends of the cell. After the voltage is released, the cell membranes go back to their
resting potential
The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential. The re ...
s until they are triggered again.
Discharge patterns
Electric organ discharges (EODs) need to vary with time for
electrolocation
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, such ...
, whether with pulses, as in the Mormyridae, or with waves, as in the Torpediniformes and ''
Gymnarchus'', the African knifefish.
Many electric fishes also use EODs for communication, while strongly electric species use them for hunting or defence.
Their electric signals are often simple and stereotyped, the same on every occasion.
Electrocommunication
Weakly electric fish can communicate by modulating the electrical
waveform
In electronics, acoustics, and related fields, the waveform of a signal is the shape of its Graph of a function, graph as a function of time, independent of its time and Magnitude (mathematics), magnitude Scale (ratio), scales and of any dis ...
they generate. They may use this to attract mates and in territorial displays.
Sexual behaviour
In
sexually dimorphic
Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...
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.
The electric organ fires to produce a discharge with a certain
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
, along with short modulations termed "chirps" and "gradual frequency rises", both varying widely between species and differing between the sexes.
For example, in the
glass knifefish genus ''
Eigenmannia'', females produce a nearly pure
sine wave
A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic function, periodic wave whose waveform (shape) is the trigonometric function, trigonometric sine, sine function. In mechanics, as a linear motion over time, this is ''simple ...
with few harmonics, males produce a far sharper non-sinusoidal waveform with strong
harmonics
In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st harm ...
.
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
A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...
, with more activity coinciding with
night-time courtship and spawning, and less at other times.
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.
The electric discharge pattern of bluntnose knifefishes is similar to the low voltage electrolocative discharge of the
electric eel
The electric eels are a genus, ''Electrophorus'', of neotropical freshwater fish from South America in the family Gymnotidae, of which they are the only members of the subfamily Electrophorinae. They are known for their electric fish, ability ...
. This is thought to be a form of bluffing
Batesian mimicry
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 butt ...
of the powerfully protected electric eel.
Fish that prey on electrolocating fish may "eavesdrop"
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. This has driven the prey, in an
evolutionary arms race, to develop more complex or higher frequency signals that are harder to detect.
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''.
When two fish are approaching one another, their electric fields interfere.
This sets up a
beat with a frequency equal to the difference between the discharge frequencies of the two fish.
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.
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.
Both the neural computational mechanisms and the behavioural responses are nearly identical in the two groups.
See also
*
Feature detection (nervous system) Feature detection is a process by which the nervous system sorts or filters complex natural stimuli in order to extract behaviorally relevant sensory cue, cues that have a high probability of being associated with important objects or organisms in t ...
References
{{Neuroethology
Electroreceptive animals
Neuroethology
Articles containing video clips
Bioelectricity