''Electrophorus electricus'' is the best-known species of

electric eel 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 ...

. It is a South American

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. The only exception is the stargazer family. Electric fish, although a small minority, in ...

. Until the discovery of two additional species in 2019, the genus was classified as the

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

, with this species the only one in the genus. Despite the name, it is not an

eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...

, but rather a knifefish. It is considered as a freshwater

teleost Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tel ...

which contains an electrogenic tissue that produces electric discharges.

Taxonomic history

The species has been reclassified several times. When originally described by

Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...

in 1766, he used the name ''Gymnotus electricus'', placing it in the same genus as ''

Gymnotus carapo The banded knifefish (''Gymnotus carapo'') is a species of gymniform knifefish native to a wide range of freshwater habitats in South America. It is the most widespread species of ''Gymnotus'', but it has frequently been confused with several r ...

'' (banded knifefish) which he had described several years earlier. It was only about a century later, in 1864, that the electric eel was moved to its own genus ''Electrophorus'' by

Theodore Gill Theodore Nicholas Gill (March 21, 1837 – September 25, 1914) was an American ichthyologist, mammalogist, malacologist and librarian. Career Born and educated in New York City under private tutors, Gill early showed interest in natural histor ...

. In September 2019, David de Santana et al. suggested the division of the genus into three species based on DNA divergence, ecology and habitat, anatomy and physiology, and electrical ability: ''E. electricus'', '' E. voltai'' sp. nov., and '' E. varii'' sp. nov. The study found ''E. electricus'' to be the

sister species In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

to ''E. voltai'', with both species diverging during the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 ''E. electricus'' has an elongated,

cylindrical A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infini ...

body, typically growing to about in length, and in weight, making them the largest of the

Gymnotiformes 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 (the ...

. Their coloration is dark gray-brown on the back and yellow or orange on the belly. Mature females have a darker abdomen. They have no scales. The mouth is square and positioned at the end of the snout. The anal fin extends the length of the body to the tip of the tail. As in other

ostariophysan Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains 10,758 species, about 28% of known fish species in the world and 68% of freshwater species, and are present ...

fishes, the swim bladder has two chambers. The anterior chamber is connected to the inner ear by a series of small bones derived from neck vertebrae called the

Weberian apparatus The Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system in fishes belonging to the superorder Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes col ...

, which greatly enhances its hearing capability. The posterior chamber extends along the whole length of the body and maintains the fish's buoyancy. ''E. electricus'' has a vascularized respiratory system with gas exchange occurring through

epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

tissue in its

buccal cavity The buccal space (also termed the buccinator space) is a fascial space of the head and neck (sometimes also termed fascial tissue spaces or tissue spaces). It is a potential space in the cheek, and is paired on each side. The buccal space is super ...

. As obligate air-breathers, ''E. electricus'' must rise to the surface every ten minutes or so to inhale before returning to the bottom. Nearly eighty percent of the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

used by the fish is obtained in this way.

Physiology

''E. electricus'' has three pairs of abdominal organs that produce electricity: the main organ, Hunter's organ, and Sachs' organ. These organs occupy a large part of its body, and give the electric eel the ability to generate two types of

electric organ discharge An electric ray (Torpediniformes) showing location of paired electric organs in the head, and electrocytes stacked within it">Torpediniformes.html" ;"title="electric ray (Torpediniformes">electric ray (Torpediniformes) showing location of paired ...

s: low voltage and high voltage. These organs are made of electrocytes, lined up so a current of ions can flow through them and stacked so each one adds to a potential difference. The three electrical organs are developed from muscle and exhibit several biochemical properties and morphological features of the muscle sarcolemma; they are found symmetrically along both sides of the eel. When the eel finds its prey, the brain sends a signal through the

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...

to the electrocytes. This opens the

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

channels, allowing

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') 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 iso ...

to flow through, reversing the polarity momentarily. By causing a sudden difference in

electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...

, it generates an

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...

in a manner similar to a

battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...

, in which stacked plates each produce an electric potential difference. Electric eels are also capable of controlling their prey's nervous systems with their electrical abilities; by controlling their victim's nervous system and muscles via electrical pulses, they can keep prey from escaping or force it to move so they can locate its position. Electric eels use electricity in multiple ways. Low voltages are used to sense the surrounding environment. High voltages are used to detect prey and, separately, stun them, at which point the electric eel applies a suction-feeding bite. Electric organs

Sachs' organ is associated with

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 to stu ...

. Inside the organ are many muscle-like cells, called electrocytes. Each cell produces 0.15 V, the cells being stacked in series to enable the organ to generate nearly 10 V at around 25 Hz in frequency. These signals are emitted by the main organ; Hunter's organ can emit signals at rates of several hundred hertz. There are several physiological differences among the three electric organs, which allow them to have very different functions. The main electrical organ and the strong-voltage section of Hunter's organ are rich in

calmodulin Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bind ...

, a protein that is involved in high-voltage production. Additionally, the three organs have varying amounts of

Na+/K+-ATPase NA, N.A., Na, nA or n/a may refer to: Chemistry and physics * Sodium, symbol Na, a chemical element * Avogadro constant (''N''A) * Nucleophilic addition, a type of reaction in organic chemistry * Numerical aperture, a number that characterizes ...

, which is a Na+/K+ ion pump that is crucial in the formation of voltage. The main and Hunter’s organs have a high expression of this protein, giving it a high sensitivity to changes in ion concentration, whereas Sachs' organ has a low expression of this protein. The typical output is sufficient to stun or deter virtually any animal. The eels can vary the intensity of the electric discharge, using lower discharges for hunting and higher intensities for stunning prey or defending themselves. They can also concentrate the discharge by curling up and making contact at two points along its body. When agitated, they can produce these intermittent electric shocks over at least an hour without tiring. ''E. electricus'' also possesses high frequency–sensitive tuberous receptors, which are distributed in patches over its body. This feature is apparently useful for hunting other Gymnotiformes. ''E. electricus'' has been prominent in the study of bioelectricity since the 18th century. The species is of some interest to researchers, who make use of its

acetylcholinesterase Acetylcholinesterase (HGNC symbol ACHE; EC 3.1.1.7; systematic name acetylcholine acetylhydrolase), also known as AChE, AChase or acetylhydrolase, is the primary cholinesterase in the body. It is an enzyme Enzymes () are proteins that a ...

and

adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...

. Despite being the first described species in the genus and thus the most famous example, ''E. electricus'' actually has the weakest maximum voltage of the three species in the genus, at only 480 volts (as opposed to 572 volts in ''E. varii'' and 860 volts in ''E. voltai'').

Ecology and life history

Habitat

''E. electricus'' is restricted to freshwater habitats in the

Guiana Shield The Guiana Shield (french: Plateau des Guyanes, Bouclier guyanais; nl, Hoogland van Guyana, Guianaschild; pt, Planalto das Guianas, Escudo das Guianas; es, Escudo guayanés) is one of the three cratons of the South American Plate. It is a ...

. Populations in the

Amazon basin The Amazon basin is the part of South America drained by the Amazon River and its tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries of Bolivi ...

, Brazilian Shield, and other parts of the Guiana Shield are now thought to belong to ''E. varii'' and ''E. voltai''.

Feeding ecology

''E. electricus'' feeds on

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s, although adult eels may also consume

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

and small

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, such as rats. First-born hatchlings eat other eggs and embryos from later

clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...

es. The juveniles eat invertebrates, such as

shrimp Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are refer ...

and

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

Reproduction

''E. electricus'' is known for its unusual breeding behavior. In the dry season, a male eel makes a nest from his saliva into which the female lays her eggs. As many as 3,000 young hatch from the eggs in one nest. Males grow to be larger than females Piper, Ross (2007), ''Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals'',

Greenwood Press Greenwood Publishing Group, Inc. (GPG), also known as ABC-Clio/Greenwood (stylized ABC-CLIO/Greenwood), is an educational and academic publisher (middle school through university level) which is today part of ABC-Clio. Established in 1967 as Gr ...

. by about .

References

External links

* *
1954 educational film about the electric eel
from the Moody Institute of Science {{Taxonbar , from=Q201235 Electrophoridae Strongly electric fish Freshwater fish of Brazil Freshwater fish of Colombia Vertebrates of Guyana Fish of Peru Fish described in 1766 Taxa named by Carl Linnaeus