Electric fish

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prey
.

An electric fish is any

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. 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

convergently evolved
a nearly identical mechanism.

Evolution and phylogeny

Further information: Electroreception and electrogenesis

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

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

passively electrolocating species are not shown.[2][11][10]

Vertebrates
Chondrichthyes

Torpediniformes (electric rays) (69 spp)

Rajiformes (skates) (~200 spp)

430 
mya
Bony fishes
Osteogloss.
 
Mormyridae

elephantfishes (~200 spp)

knollenorgans  
 (pulses)
 
Gymnarchidae

African knifefish (1 sp)

knollenorgans  
 (waves)
110 mya
 
Gymnotiformes
S. Amer.

(>100 spp)

knifefishes
Electric eels

(3 spp)

119 mya
amp. recept.
Siluriformes
 
Uranoscopidae
  

Stargazers (50 spp)

no electro‑
location 
425 
mya
Amp. of Lorenzini 

Weakly electric fish

electroreceptors, and processes the detected signals in the brain to locate nearby objects.[12]

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

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.[13]
Further information: History of bioelectricity

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

electric organ and the water must be matched:[13]

Electric organ

Main article: Electric organ (fish)

Anatomy

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

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 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

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

Further information:
Antipredator adaptation

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.
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

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

References

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  12. ^ Von der Emde, G. (1999). "Active electrolocation of objects in weakly electric fish". Journal of Experimental Biology, 202 (10): 1205–1215. Full text
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  16. ^ Nelson, Mark. "What IS an electric fish?". Retrieved 10 August 2014.
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  32. ^ a b Bullock, Theodore H.; Hamstra, R. Jr.; Scheich, H. (1972). "The jamming avoidance response of high frequency electric fish". Journal of Comparative Physiology (77): 1–22.
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  35. ^ Kawasaki, M. (1975). "Independently evolved jamming avoidance responses in Gymnotid and Gymnarchid electric fish: a case of convergent evolution of behavior and its sensory basis". Journal of Comparative Physiology (103): 97–121.
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