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.
he genus has been reclassified several times. When the species now defined as Electrophorus electricus was originally described by Carl Linnaeus in 1766, he used the name Gymnotus electricus, placing it in the same genus as Gymnotus carapo (the banded knifefish). 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.[2][a]
In 1864, Theodore Gill moved the electric eel to its own genus, Electrophorus.[3] The name is from the Greek ήλεκτρον ("elektron", amber, a substance able to hold static electricity), and ϕέρω ("phero", I carry), giving the meaning "electricity bearer". In 1872, Gill decided that the electric eel was sufficiently distinct to have its own family, Electrophoridae,[8] but Ferraris and colleagues (2017) placed the genus into the family Gymnotidae, alongside Gymnotus.[4][9]
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In 2019, C. David de Santana et al. 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.
The three species have different distributions in the northern part of South America. E. electricus is northern, confined to the Guiana Shield, while E. voltai is southern, ranging from the Brazilian shield northwards; both species live in upland waters. E. varii is central, largely in the lowlands.[10] The lowland region of E. varii is a variable environment, with habitats ranging from streams through grassland and ravines to ponds, and large changes in water level between the wet and dry seasons.[16] 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.
Electric eels are mostly nocturnal.[18] It had been thought that they were solitary, but E. voltai sometimes hunts in packs. Groups of the animals were observed to coordinate their activities after targeting a shoal of tetras, then herding them and launching joint strikes on the closely-packed fish.[19] The species mainly eats fish, in particular the armoured catfish Megalechis thoracata.[20] A specimen of E. voltai had a caecilian (a legless amphibian), Typhlonectes compressicauda in its stomach; it is possible that this means that the species is resistant to the caecilian's toxic skin secretions.[21] The other species, E. varii, is also a fish predator; it preys especially on Callichthyidae (armoured catfishes) and Cichlidae (cichlids).
Electric eels have long, stout eel-like bodies, being somewhat cylindrical at the front but more flattened towards the end. E. electricus can 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, and opens upwards. They have smooth, thick brown to black skin with a yellow or red underbelly and no scales.[10][23][24] The pectoral fins each possess eight radials.[23] There is no clear boundary between the tail fin and the anal fin, which extends much of the length of the body on the underside and has over 400 bony rays.[10][25] Electric eels rely on the wave-like movements of their elongated anal fin to propel themselves through the water.[26] The body cavity reaches into the tip of the tail.[10]
Electric eels get most of their oxygen by breathing air using buccal pumping.[24][27] This enables them to live in habitats subject to widely varying oxygen levels including streams, swamps, and pools.[27] The buccal cavity is lined with a mucosa which has a rich blood supply, enabling gas exchange between the air and the blood. About every two minutes, the fish takes in air through the mouth, holds it in the buccal cavity, and expels it through the opercular openings at the sides of the head.[28] Unlike in other air-breathing fish, the tiny gills of electric eels do not ventilate when taking in air. The majority of carbon dioxide produced is expelled through the skin.[24] These fish can survive on land for some hours if their skin is wet enough.[29]
Electric eels have small eyes and poor vision.[24][30] They are capable of hearing via a Weberian apparatus, which consists of tiny bones connecting the inner ear to the swim bladder.[31] 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.
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.[49] 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. 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). Captive specimens have sometimes lived for over 20 years.[23] 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.[16] The male provides protection for both the young and the nest.[51]
As the fish grow, they continually add more vertebrae to their spinal column. The main organ is the first electric organ to develop, followed by Sachs' organ and them Hunter's organ. All the electric organs are differentiated by 23 cm (9.1 in). Electric eels are able to produce electrical discharges as early as 7 cm (2.8 in). 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). Captive specimens have sometimes lived for over 20 years.
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. 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 of conductors at nanoscopic scale. Such cells would use ion transport as electrocytes do, with a greater output power density, and converting energy more efficiently. They suggest that such artificial electrocytes could be developed as a power source for medical implants such as retinal prostheses and 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.[35] In 2009, they made synthetic protocells which can provide about a twentieth of the energy density of a lead–acid battery, and an energy conversion efficiency of 10%.[65]
In 2016, Hao Sun and colleagues described a family of electric eel-mimicking devices that serve as high output voltage electrochemical capacitors. 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 as electric watches or light-emitting diodes.
