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Electric eel

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This article is about the fish genus. For other uses, seeElectric eel (disambiguation).

Electric eel
Electrophorus electricusspecimen at theNew England Aquarium
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Gymnotiformes
Family: Gymnotidae
Genus: Electrophorus
T. N. Gill,1864
Type species
Gymnotus electricus
Species[1]
Synonyms[2][a]
  • Gymnotus tremuliGronovius 1760
  • Gymnotus tremulusHouttuyn 1764
  • Gymnotus electricusLinnaeus 1766
  • Gymnotus RegiusDelle Chiaje 1847
  • Electrophorus multivalvulusNakashima 1941

Theelectric eelsare agenus,Electrophorus,ofneotropicalfreshwater fishfrom South America in the familyGymnotidae.They are known for theirability to stun their prey by generating electricity,delivering shocks at up to 860volts.Their electrical capabilities were first studied in 1775, contributing to the invention in 1800 of theelectric battery.

Despite their name, electric eels are not closely related to the true eels (Anguilliformes) but are members of theelectroreceptiveknifefishorder,Gymnotiformes.This order is more closely related tocatfish.In 2019, electric eels were split into three species: for more than two centuries before that, the genus was believed to bemonotypic,containing onlyElectrophorus electricus.

They are nocturnal, obligate air-breathing animals, with poor vision complemented by electrolocation; they mainly eat fish. Electric eels grow for as long as they live, adding more vertebrae to their spinal column. Males are larger than females. Some captive specimens have lived for over 20 years.

Evolution

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Taxonomy

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When the species now defined asElectrophorus electricuswas described byCarl Linnaeusin 1766, based on early field research by Europeans in South America and specimens sent back to Europe for study,[3][4][5]he used the nameGymnotus electricus,placing it in the same genus asGymnotus carapo(the banded knifefish).[6][7][8]He noted that the fish is from the rivers ofSurinam,that it causes painful shocks, and that it had small pits around the head.[6][b]

In 1864,Theodore Gillmoved the electric eel to its own genus,Electrophorus.[7] The name is from the Greekήλεκτρον( "ḗlektron",amber,a substance able to holdstatic electricity), andφέρω( "phérō",I carry), giving the meaning" electricity bearer ".[1][10]In 1872, Gill decided that the electric eel was sufficiently distinct to have its own family, Electrophoridae.[11]In 1998,Albertand Campos-da-Paz lumped theElectrophorusgenus with the familyGymnotidae,alongsideGymnotus,[12]as did Ferraris and colleagues in 2017.[8][2]

In 2019, C. David de Santana and colleagues dividedE. electricusinto three species based on DNA divergence, ecology and habitat, anatomy and physiology, and electrical ability. The three species areE. electricus(now in a narrower sense than before), and the two new speciesE. voltaiandE. varii.[13]

Phylogeny

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Electric eels form acladeof stronglyelectric fisheswithin the orderGymnotiformes,the South American knifefishes.[13]Electric eels are thus not closely related to the true eels (Anguilliformes).[14]The lineage of theElectrophorusgenus is estimated to have split from itssister taxonGymnotussometime in theCretaceous.[15]Most knifefishes are weakly electric, capable of activeelectrolocationbut not of delivering shocks.[16]Their relationships, as shown in the cladogram, were analysed by sequencing theirmitochondrial DNAin 2019.[17][18]Actively electrolocating fish are marked with a small yellow lightning flashsymbol for electrolocating fish.Fish able to deliver electric shocks are marked with a red lightning flashsymbol for strongly electric fish.[15][19][20]

Otophysi

Siluriformes(catfish) (somesymbol for electrolocating fishsymbol for strongly electric fish)image of catfish

Gymnotiformes

Apteronotidae(ghost knifefishes)symbol for electrolocating fishimage of ghost knifefish

Hypopomidae(bluntnose knifefishes)symbol for electrolocating fishimage of bluntnose knifefish

Rhamphichthyidae(sand knifefishes)symbol for electrolocating fishimage of sand knifefish

Gymnotidae

Gymnotus(banded knifefishes)symbol for electrolocating fishimage of banded knifefish

Electrophorus(electric eels)symbol for electrolocating fishsymbol for strongly electric fishimage of electric eel

Sternopygidae(glass knifefishes)symbol for electrolocating fishimage of glass knifefish

Characiformes

(piranhas,tetras,and allies)image of non-electric fish

Species

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There are three described species in the genus, not differing significantly in body shape or coloration:[13]

X-rays and photographs of the heads of the three species of electric eel
Differences between the three species of electric eel, namelyE. electricus,E. voltai,andE. varii[13]
Bodies (top to bottom) ofE. electricus,E. voltai,andE. varii[13]

E. variiappears to have diverged from the other species around 7.1 mya during thelate Miocene,whileE. electricusandE. voltaimay have split around 3.6 mya during thePliocene.[13]

Ecology

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The three species have largely non-overlapping distributions in the northern part of South America.E. electricusis northern, confined to theGuiana Shield,whileE. voltaiis southern, ranging from theBrazilian shieldnorthwards; both species live in upland waters.E. variiis central, largely in the lowlands.[13]The lowland region ofE. variiis a variable environment, with habitats ranging from streams through grassland and ravines to ponds, and large changes in water level between the wet anddry seasons.[21]All live on muddy river bottoms and sometimes swamps, favouring areas in deep shade. They can tolerate water low in oxygen as they swim to the surface to breathe air.[22]

Electric eels are mostlynocturnal.[23]E. voltaimainly eats fish, in particular the armoured catfishMegalechis thoracata.[24]A specimen ofE. voltaihad acaecilian(a legless amphibian),Typhlonectes compressicauda,in its stomach; it is possible that this means that the species is resistant to the caecilian'stoxicskin secretions.[25]E. voltaisometimes hunts in packs; and have been observed targeting a shoal oftetras,then herding them and launching joint strikes on the closely packed fish.[26]The other species,E. varii,is also a fishpredator;it preys especially onCallichthyidae(armoured catfishes) andCichlidae(cichlids).[27]

Map of South America showing distribution of the three species of electric eel
Map of the northern part of South America showing distribution of specimens of the three species ofElectrophorus:E. electricus(1, red);E. voltai(2, blue);E. varii(3, yellow).[13]

Biology

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General biology

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Electric eel skeleton, with the longvertebral columnat top, the row of bonyraysbelow

Electric eels have long, stout bodies, being somewhat cylindrical at the front but more flattened towards the tail end.E. electricuscan reach 2 m (6 ft 7 in) in length, and 20 kg (44 lb) in weight. The mouth is at the front of the snout, andopens upwards.They have smooth, thick, brown-to-black skin with a yellow or red underbelly and noscales.[13][28][29]The pectoral fins each possess eight tiny radial bones at the tip.[28] They have over 100 precaudal vertebrae (excluding the tail), whereas other gymnotids have up to 51 of these; there can be as many as 300 vertebrae in total.[12] There is no clear boundary between the tail fin and theanal fin,which extends much of the length of the body on the underside and has over 400 bonyrays.[13][30]Electric eels rely on the wave-like movements of their elongated anal fin topropelthemselves through the water.[31]

Electric eels get most of their oxygen by breathing air usingbuccal pumping.[29][32]This enables them to live in habitats with widely varying oxygen levels including streams, swamps, and pools.[32]: 719–720 Uniquely among the gymnotids, thebuccal cavityis lined with a frilledmucosawhich has a rich blood supply, enablinggas exchangebetween the air and the blood.[12][33]About every two minutes, the fish takes in air through the mouth, holds it in the buccal cavity, and expels it through theopercular openingsat the sides of the head.[33]Unlike in other air-breathing fish, the tiny gills of electric eels do not ventilate when taking in air. The majority ofcarbon dioxideproduced is expelled through the skin.[29]These fish can survive on land for some hours if their skin is wet enough.[34]

Electric eels have small eyes and poor vision.[29][35]They are capable of hearing via aWeberian apparatus,which consists of tiny bones connecting the inner ear to theswim bladder.[36]All of the vital organs are packed in near the front of the animal, taking up only 20% of space and sequestered from the electric organs.[37]

Electrophysiology

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Further information:Electric fishandElectroreception and electrogenesis
photograph of head of an electric eel
Lateral linepits in rows on the top and sides of the head and body. The pits contain bothelectroreceptorsandmechanoreceptors.[38]

Electric eels can locate their prey usingelectroreceptorsderived from thelateral line organin the head. The lateral line itself ismechanosensory,enabling them to sense water movements created by animals nearby. The lateral line canals are beneath the skin, but their position is visible as lines of pits on the head.[38]Electric eels use their high frequency-sensitivetuberous receptors,distributed in patches over the body, for hunting other knifefish.[1]

Electric eel anatomy: first detail shows stacks ofelectrocytesforming electric organs. Second detail shows an individual cell withion channelsandpumpsthrough thecell membrane;Anerve cell'sterminal buttons are releasingneurotransmittersto trigger electrical activity. Final detail shows coiledproteinchains of an ion channel.

Electric eels have three pairs ofelectric organs,arranged longitudinally: the main organ, Hunter's organ, and Sachs' organ. These organs give electric eels the ability to generate two types ofelectric organ discharges:low voltage and high voltage.[13]The organs are made ofelectrocytes,modified frommuscle cells.[39][40]Like muscle cells, the electric eel's electrocytes contain the proteinsactinanddesmin,but where muscle cell proteins form a dense structure of parallelfibrils,in electrocytes they form a loose network. Five different forms of desmin occur in electrocytes, compared to two or three in muscle,[41]but its function in electrocytes remained unknown as of 2017.[42]

Potassium channelproteinsinvolved in electric organ discharge, includingKCNA1,KCNH6,andKCNJ12,are distributed differently among the three electric organs: most such proteins are most abundant in the main organ and least abundant in Sachs's organ, but KCNH6 is most abundant in Sachs's organ.[42]The main organ and Hunter's organ are rich in the proteincalmodulin,involved in controlling calcium ion levels. Calmodulin and calcium help to regulate thevoltage-gated sodium channelsthat create the electrical discharge.[42][43]These organs are also rich insodium potassium ATPase,anion pumpused to create a potential difference across cell membranes.[42][44]

The maximum discharge from the main organ is at least 600volts,making electric eels the most powerful of all electric fishes.[45]Freshwater fishes like the electric eel require a high voltage to give a strong shock because freshwater hashigh resistance;powerful marine electric fishes like thetorpedo raygive a shock at much lower voltage but a far higher current. The electric eel produces its strong discharge extremely rapidly, at a rate of as much as 500Hertz,meaning that each shock lasts only about two milliseconds.[46]To generate a high voltage, an electric eel stacks some 6,000 electrocytes in series (longitudinally) in its main organ; the organ contains some 35 such stacks in parallel, on each side of the body.[46]The ability to produce high-voltage, high-frequency pulses in addition enables the electric eel to electrolocate rapidly moving prey.[47]The total electric current delivered during each pulse can reach about 1ampere.[48]

diagram showing why freshwater electric fish need to produce a high voltage
Impedance matchingin strongly electric fishes. Since freshwater is a poor conductor, limiting theelectric current,electric eels need to operate at highvoltageto deliver a stunning shock. They achieve this by stacking a large number ofelectrocytes,each producing a small voltage,in series.[46]

It remains unclear why electric eels have three electric organs but basically produce two types of discharge, to electrolocate or to stun. In 2021, Jun Xu and colleagues stated that Hunter's organ produces a third type of discharge at a middle voltage of 38.5 to 56.5 volts. Their measurements indicate that this is produced just once, for less than 2 milliseconds, after the low-voltage discharge of Sachs's organ and before the high-voltage discharge of the main organ. They believed that this is insufficient to stimulate a response from the prey, so they suggested it may have the function of co-ordination within the electric eel's body, perhaps by balancing the electrical charge, but state that more research is needed.[49]

Electric eel shocking and eating prey

When an electric eel identifies prey, its brain sends a nerve signal to the electric organ;[46]the nerve cells involved release theneurotransmitterchemicalacetylcholineto trigger an electric organ discharge.[42]This opension channels,allowingsodiumto flow into the electrocytes, reversing the polarity momentarily.[42]The discharge is terminated by an outflow ofpotassiumions through a separate set of ion channels.[42]By causing a sudden difference inelectric potential,it generates anelectric currentin a manner similar to abattery,in which cells are stacked to produce a desired total voltage output.[39]It has been suggested that Sachs' organ is used for electrolocation; its discharge is of nearly 10 volts at a frequency of around 25 Hz. The main organ, supported by Hunter's organ in some way, is used to stun prey or to deter predators; it can emit signals at rates of several hundred hertz.[1][45]Electric eels can concentrate the discharge to stun prey more effectively by curling up and making contact with the prey at two points along the body.[45]It has also been suggested that electric eels can control their prey's nervous systems and muscles via electrical pulses, keeping prey from escaping, or forcing it to move so they can locate it,[50]but this has been disputed.[49]Inself-defence,electric eels have been observed to leap from the water to deliver electric shocks to animals that might pose a threat.[51]The shocks from leaping electric eels are powerful enough to drive away animals as large as horses.[52]

Life cycle

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Electric eels reproduce during the dry season, from September to December. During this time, male-female pairs are seen in small pools left behind after water levels drop. The male makes a nest using his saliva and the female deposits around 1,200 eggs forfertilisation.Spawn hatch seven days later and mothers keep depositing eggs periodically throughout the breeding season, making them fractional spawners.[53]When they reach 15 mm (0.59 in), the hatched larvae consume any leftover eggs, and after they reach 9 cm (3.5 in) they begin to eat other foods.[54]Electric eels aresexually dimorphic,males becoming reproductively active at 1.2 m (3 ft 11 in) in length and growing larger than females; females start to reproduce at a body length of around 70 cm (2 ft 4 in). The adults provide prolonged parental care lasting four months.E. electricusandE. voltai,the two upland species which live in fast-flowing rivers, appear to make less use of parental care.[21]The male provides protection for both the young and the nest.[55]Captive specimens have sometimes lived for over 20 years.[28]

As the fish grow, they continually add more vertebrae to their spinal column.[28]The main organ is the first electric organ to develop, followed by Sachs' organ and then Hunter's organ. All the electric organs are differentiated by the time the body reaches a length of 23 cm (9.1 in). Electric eels are able to produce electrical discharges when they are as small as 7 cm (2.8 in).[54]

Interactions with humans

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Early research

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The first written mention of the electric eel orpuraké('the one that numbs' inTupi) is in records by theJesuitpriest Fernão Cardim in 1583.[56] The naturalists Bertrand Bajon, a French military surgeon inFrench Guiana,and the JesuitRamón M. Termeyer[pl]in theRiver Plate basin,conducted early experiments on the numbing discharges of electric eels in the 1760s.[3]In 1775, the "torpedo" (the electric ray) was studied byJohn Walsh;[4]both fish were dissected by the surgeon and anatomistJohn Hunter.[4][5]Hunter informed theRoyal Societythat "Gymnotus Electricus[...] appears very much like an eel[...] but it has none of the specific properties of that fish. "[5]He observed that there were "two pair of these [electric] organs, a larger [the main organ] and a smaller [Hunter's organ]; one being placed on each side", and that they occupied "perhaps[...] more than one-third of the whole animal [by volume] ".[5]He described the structure of the organs (stacks of electrocytes) as "extremely simple and regular, consisting of two parts;viz.flat partitions orsepta,and cross divisions between them. "He measured the electrocytes as1⁄17inch (1.5 mm) thick in the main organ, and1⁄56inch (0.45 mm) thick in Hunter's organ.[5]

  • Painting of John Hunter, 18th century anatomist
    The surgeonJohn Hunterdissected an electric eel in 1775.
  • Engraving of an electric eel, 1775
    Hunter's "Gymnotus Electricus", underside and upperside, 1775.
    The figure occupied four pages of his paper for theRoyal Society.[5]
  • Engraving of cross-section of electric eel
    Cross-section:
    C=Back muscles, H=main organ, I=Hunter's organ
  • Engraving of dissected part of an electric eel
    Dissection, showing the electric organs inside the body. At right, the skin is folded back to reveal the main organ above Hunter's organ.

Also in 1775, the American physician and politicianHugh Williamson,who had studied with Hunter,[57]presented a paper "Experiments and observations on the Gymnotus Electricus, or electric eel" at the Royal Society. He reported a series of experiments, such as "7. In order to discover whether the eel killed those fish by an emission of the same [electrical] fluid with which he affected my hand when I had touched him, I put my hand into the water, at some distance from the eel; another cat-fish was thrown into the water; the eel swam up to it... [and] gave it a shock, by which it instantly turned up its belly, and continued motionless; at that very instant I felt such a sensation in the joints of my fingers as in experiment 4." and "12. Instead of putting my hand into the water, at a distance from the eel, as in the last experiment, I touched its tail, so as not to offend it, while my assistant touched its head more roughly; we both received a severe shock."[58]

The studies by Williamson, Walsh, and Hunter appear to have influenced the thinking ofLuigi GalvaniandAlessandro Volta.Galvani foundedelectrophysiology,with research into how electricity makes a frog's leg twitch; Volta beganelectrochemistry,with his invention of theelectric battery.[4][59]

In 1800, the explorerAlexander von Humboldtjoined a group of indigenous people who went fishing with horses, some thirty of which they chased into the water. The pounding of the horses' hooves, he noted, drove the fish, up to 5 feet (1.5 m) long out of the mud and prompted them to attack, rising out of the water and using their electricity to shock the horses. He saw two horses stunned by the shocks and then drowned. The electric eels, having given many shocks, "now require long rest and plenty of nourishment to replace the loss of galvanic power they have suffered", "swam timidly to the bank of the pond", and were easily caught using smallharpoonson ropes. Humboldt recorded that the people did not eat the electric organs, and that they feared the fish so much that they would not fish for them in the usual way.[60]

In 1839, the chemistMichael Faradayextensively tested the electrical properties of an electric eel imported from Surinam. For a span of four months, he measured the electrical impulses produced by the animal by pressing shaped copper paddles and saddles against the specimen. Through this method, he determined and quantified the direction and magnitude of electric current, and proved that the animal's impulses were electrical by observing sparks and deflections on agalvanometer.He observed the electric eel increasing the shock by coiling about its prey, the prey fish "representing a diameter" across the coil. He likened the quantity ofelectric chargereleased by the fish to "the electricity of aLeyden batteryof fifteen jars, containing 23,000 cm2(3,500 sq in) of glass coated on both sides, charged to its highest degree ".[61]

The German zoologistCarl Sachswas sent to Latin America by the physiologistEmil du Bois-Reymond,to study the electric eel;[62]he took with him a galvanometer and electrodes to measure the fish's electric organ discharge,[63]and used rubber gloves to enable him to catch the fish without being shocked, to the surprise of the local people. He published his research on the fish, including his discovery of what is now called Sachs' organ, in 1877.[49][63]

  • Engraving of hunting electric eels using horses
    Artist's impression ofAlexander von Humboldt's 1800 experience of hunting electric eels using a herd of horses, as told in his 1859Journey to the Equinoctial Regions of the New Continent.[60]Drawing byJames Hope Stewart;engraving byWilliam Home Lizars.
  • Diagram of experimental setup showing fish in tank
    Michael Faraday's diagram of the setup for his "Experimental Researches in Electricity" on the electric eel, 1838. The fish is in a circular wooden tub in shallow water. He noted that the strongest shock was obtained when both hands or a pair of copper paddles were placed in the water, at positions 1 and 8, i.e. by the head and tail of the fish.[61]
  • historic graphs and anatomical drawings
    Carl Sachs's illustration of his discovery of Sachs's organ (shown in black at 6) with electric discharge patterns (4, 5, 8), 1877

Artificial electrocytes

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The large quantity of electrocytes available in the electric eel enabled biologists to study the voltage-gated sodium channel in molecular detail. The channel is an important mechanism, as it serves to trigger muscle contraction in many species, but it is hard to study in muscle as it is found in extremely small amounts.[40]In 2008, Jian Xu and David Lavan designed artificial cells that would be able to replicate the electrical behaviour of electric eel electrocytes. The artificial electrocytes would use a calculated selection ofconductorsatnanoscopic scale.Such cells would use ion transport as electrocytes do, with a greater outputpower density,andconverting energy more efficiently.They suggest that such artificial electrocytes could be developed as a power source formedical implantssuch asretinal prosthesesand other microscopic devices. They comment that the work "has mapped out changes in the system level design of the electrocyte" that could increase both energy density and energy conversion efficiency.[39]In 2009, they made syntheticprotocellswhich can provide about a twentieth of the energy density of alead–acid battery,and an energy conversion efficiency of 10%.[64]

In 2016, Hao Sun and colleagues described a family of electric eel-mimicking devices that serve as high output voltage electrochemicalcapacitors.These are fabricated as flexible fibres that can be woven into textiles. Sun and colleagues suggest that the storage devices could serve as power sources for products such aselectric watchesorlight-emitting diodes.[65]

Notes

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  1. ^These all assumed a single species, so that while the synonymy was until 2019 taken to be withE. electricus,it is now with the genus.
  2. ^William Turton's 1806 translation of a later edition reads: "GYMNOTUS.Headwith lateral opercula; 2tentaculaat the upper lip:eyescovered with the common skin:gill-membrane5-rayed:bodycompressed, carinate beneath with a fin. Electricus.Blackish, without dorsal fin; caudal fin very obtuse and joined to the anal [fin].Electrical G[ymnotus].Inhabits various rivers ofSouth America;3–4 feet long; has a remarkable power of inflicting an electrical shock whenever it is touched. This may be conveyed through a stick to the person that holds it, and is so severe as to benumb the limbs of such as are exposed to it. By this power it stupifies and then seizes such smaller fish and animals as have ventured to approach it. Headsprinkled with perforated dots;bodyblackish with a number of small annular bands or rather wrinkles, by which it has the power of contracting and lengthening its body;nostrils2 each side, the first large, tubular and elevated, the others small, and level with the skin;teethsmall, prickly:tonguebroad and with thepalatewarty. "[9]

References

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