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. Their electrical capabilities were first studied in 1775, contributing to the invention in 1800 of the electric battery.
This article is about the fish genus. For other uses, see Electric eel (disambiguation).
Despite their name, electric eels are not closely related to the true eels (Anguilliformes) but are members of the electroreceptive knifefish order, Gymnotiformes. This order is more closely related to catfish. In 2019, electric eels were split into three species: for more than two centuries before that, the genus was believed to be monotypic, containing only Electrophorus 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[edit]
Taxonomy[edit]
When the species now defined as Electrophorus electricus was described by Carl Linnaeus in 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 name Gymnotus electricus, placing it in the same genus as Gymnotus carapo (the banded knifefish).[6][7][8] He noted that the fish is from the rivers of Surinam, that it causes painful shocks, and that it had small pits around the head.[6][b]
In 1864, Theodore Gill moved the electric eel to its own genus, Electrophorus.[7]
The name is from the Greek ήλεκτρον ("ḗlektron", amber, a substance able to hold static electricity), and φέρω ("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, Albert and Campos-da-Paz lumped the Electrophorus genus with the family Gymnotidae, alongside Gymnotus,[12] as did Ferraris and colleagues in 2017.[8][2]
In 2019, C. David de Santana and colleagues divided E. electricus into three species based on DNA divergence, ecology and habitat, anatomy and physiology, and electrical ability. The three species are E. electricus (now in a narrower sense than before), and the two new species E. voltai and E. varii.[13]
Phylogeny[edit]
Electric eels form a clade of strongly electric fishes within the order Gymnotiformes, the South American knifefishes.[13] Electric eels are thus not closely related to the true eels (Anguilliformes).[14] The lineage of the Electrophorus genus is estimated to have split from its sister taxon Gymnotus sometime in the Cretaceous.[15] Most knifefishes are weakly electric, capable of active electrolocation but not of delivering shocks.[16] Their relationships, as shown in the cladogram, were analysed by sequencing their mitochondrial DNA in 2019.[17][18] Actively electrolocating fish are marked with a small yellow lightning flash 
. Fish able to deliver electric shocks are marked with a red lightning flash 
.[15][19][20]
Life cycle[edit]
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 for fertilisation. 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 are sexually 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. electricus and E. 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[edit]
Early research[edit]
The first written mention of the electric eel or puraké ('the one that numbs' in Tupi) is in records by the Jesuit priest Fernão Cardim in 1583.[56] The naturalists Bertrand Bajon, a French military surgeon in French Guiana, and the Jesuit Ramón M. Termeyer in the River 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 by John Walsh;[4] both fish were dissected by the surgeon and anatomist John Hunter.[4][5] Hunter informed the Royal Society that "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 or septa, and cross divisions between them." He measured the electrocytes as 1⁄17 inch (1.5 mm) thick in the main organ, and 1⁄56 inch (0.45 mm) thick in Hunter's organ.[5]
