Electric eel

Taxonomy When electric eels were described by Carl Linnaeus in 1766, based on early field research by Europeans in South America and specimens sent back to Europe for study, The name is from the Greek ( 'amber, a substance able to hold static electricity'), and ( 'I carry'), giving the meaning 'electricity bearer'. In 1872, Gill decided that the electric eel was sufficiently distinct to have its own family, Electrophoridae. In 1998, Albert and Campos-da-Paz lumped the Electrophorus genus with the family Gymnotidae, alongside Gymnotus, 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. Therefore, E. varii (described from the same region) may be a junior synonym of E. multivalvulus and has been regarded as such by some biologists. Phylogeny Electric eels form a clade of strongly electric fishes within the order Gymnotiformes, the South American knifefishes. The lineage of the Electrophorus genus is estimated to have split from its sister taxon Gymnotus sometime in the Cretaceous. Actively electrolocating fish are marked with a small yellow lightning flash . Fish able to deliver electric shocks are marked with a red lightning flash . }} Species There are three described species in the genus, not differing significantly in body shape or coloration: • Electrophorus electricus This, the type species, has a U-shaped head, with a flattened skull and cleithrum. • Electrophorus voltai This species is the strongest bioelectricity generator in nature, capable of generating 860 V. Like E. electricus, this species has a flattened skull and cleithrum but the head is more egg-shaped. • Electrophorus varii Compared to the other two species, this one has a thicker skull and cleithrum but the head shape is more variable. , E. voltai, and E. varii''|alt=X-rays and photographs of the heads of the three species of electric eel E. varii appears to have diverged from the other species around 7.1 mya during the late Miocene, while E. electricus and E. voltai may have split around 3.6 mya during the Pliocene. == Ecology ==

The three species have largely non-overlapping 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. Electric eels are mostly nocturnal. E. voltai mainly eats fish, in particular the armoured catfish Megalechis thoracata. 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. E. voltai sometimes hunts in packs; and have been observed targeting a shoal of tetras, then herding them and launching joint strikes on the closely packed fish. The other species, E. varii, is also a fish predator; it preys especially on Callichthyidae (armoured catfishes) and Cichlidae (cichlids). (1, red); E. voltai (2, blue); E. varii'' (3, yellow).|alt=Map of South America showing distribution of the three species of electric eel == Biology ==

General biology at top, the row of bony rays below Electric eels have long, stout bodies, being somewhat cylindrical at the front but more flattened towards the tail end. E. electricus can reach in length, and 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. The pectoral fins each possess eight tiny radial bones at the tip. 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. Electric eels rely on the wave-like movements of their elongated anal fin to propel themselves through the water. Electric eels get most of their oxygen by breathing air using buccal pumping. Uniquely among the gymnotids, the buccal cavity is lined with a frilled mucosa which has a rich blood supply, enabling gas exchange between the air and the blood. 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. They are capable of hearing via a Weberian apparatus, which consists of tiny bones connecting the inner ear to the swim bladder. All of the vital organs are packed near the front of the animal, taking up only 20% of space and sequestered from the electric organs. Electrophysiology pits in rows on the top and sides of the head and body. The pits contain both electroreceptors and mechanoreceptors. Electric eels use their high frequency-sensitive tuberous receptors, distributed in patches over the body, for hunting other knifefish. Like muscle cells, the electric eel's electrocytes contain the proteins actin and desmin, but where muscle cell proteins form a dense structure of parallel fibrils, in electrocytes they form a loose network. Five different forms of desmin occur in electrocytes, compared to two or three in muscle, but its function in electrocytes remained unknown as of 2017. Potassium channel proteins involved in electric organ discharge, including KCNA1, KCNH6, and KCNJ12, 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. These organs are also rich in sodium potassium ATPase, an ion pump used to create a potential difference across cell membranes. The maximum discharge from the main organ is at least 600 volts, making electric eels the most powerful of all electric fishes. 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. The maximum electric current delivered during each pulse can reach about 1 ampere at 500V. in strongly electric fishes. Since freshwater is a poor conductor, limiting the electric current, electric eels need to operate at high voltage to deliver a stunning shock. They achieve this by stacking a large number of electrocytes, each producing a small voltage, in series. 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. but this has been disputed. The shocks from leaping electric eels are powerful enough to drive away animals as large as horses. == Life cycle ==

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. When they reach , the hatched larvae consume any leftover eggs, and after they reach they begin to eat other foods. Electric eels are sexually dimorphic, males becoming reproductively active at in length and growing larger than females; females start to reproduce at a body length of around . 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. The male provides protection for both the young and the nest. Captive specimens have sometimes lived for over 20 years. 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 . Electric eels are able to produce electrical discharges when they are as small as . == Interactions with humans ==

Early research The first written mention of the electric eel or ('the one that numbs' in Tupi) is in records by the Jesuit priest Fernão Cardim in 1583. The naturalists Bertrand Bajon, a French military surgeon in French Guiana, and the Jesuit in the River Plate basin, conducted early experiments on the numbing discharges of electric eels in the 1760s. In 1775, the "torpedo" (the electric ray) was studied by John Walsh; 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." 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." The studies by Williamson, Walsh, and Hunter appear to have influenced the thinking of Luigi Galvani and Alessandro Volta. Galvani founded electrophysiology, with research into how electricity makes a frog's leg twitch; Volta began electrochemistry, with his invention of the electric battery. In 1800, the explorer Alexander von Humboldt joined 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 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 small harpoons on 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. In 1839, the chemist Michael Faraday extensively 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 a galvanometer. 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 of electric charge released by the fish to "the electricity of a Leyden battery of fifteen jars, containing of glass coated on both sides, charged to its highest degree". The German zoologist Carl Sachs was sent to Latin America by the physiologist Emil du Bois-Reymond, to study the electric eel; he took with him a galvanometer and electrodes to measure the fish's electric organ discharge,--> 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. File:Gymnoten-Humboldt battle with horses.jpg|Artist's impression of Alexander von Humboldt's 1800 experience of hunting electric eels using a herd of horses, as told in his 1859 Journey to the Equinoctial Regions of the New Continent. 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. == Notes ==