Electric eel
Electric eel | |
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Electrophorus electricus specimen at the New England Aquarium | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Actinopterygii |
Order: | Gymnotiformes |
Family: | Gymnotidae |
Genus: | Electrophorus T. N. Gill, 1864 |
Type species | |
Gymnotus electricus Linnaeus, 1766
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Species[1] | |
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Synonyms[2][a] | |
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The electric eels are a
Despite their name, electric eels are not closely related to the true eels (
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
Taxonomy
When the species now defined as Electrophorus electricus was described by
In 1864, Theodore Gill moved the electric eel to its own genus, Electrophorus.[7] The name is from the Greek ήλεκτρον ("ḗlektron",
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
Electric eels form a
Otophysi
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Species
There are three described species in the genus, not differing significantly in body shape or coloration:[13]
- Linnaeus, 1766) This, the type species, has a U-shaped head, with a flattened skull and cleithrum.[13]
- 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.[13]
- Electrophorus varii de Santana, Wosiacki, Crampton, Mark H. Sabaj, Dillman, Mendes-Júnior and Castro e Castro, 2019 Compared to the other two species, this one has a thicker skull and cleithrum but the head shape is more variable.[13]
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.[13]
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.[13] 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.[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 mostly
Biology
General biology
Electric eels have long, stout bodies, being somewhat cylindrical at the front but more flattened towards the tail 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.[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 the
Electric eels get most of their oxygen by breathing air using
Electric eels have small eyes and poor vision.[29][35] They are capable of hearing via a Weberian apparatus, which consists of tiny bones connecting the inner ear to the swim 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
Electric eels can locate their prey using
Electric eels have three pairs of
Potassium channel
The maximum discharge from the main organ is at least 600 volts, 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 has high resistance; powerful marine electric fishes like the torpedo ray give 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 500 Hertz, 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 1 ampere.[48]
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]
When an electric eel identifies prey, its brain sends a nerve signal to the electric organ;
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
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
Early research
The naturalists Bertrand Bajon, a French military surgeon in
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The surgeon John Hunter dissected an electric eel in 1775.
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Hunter's "Gymnotus Electricus", underside and upperside, 1775.The figure occupied four pages of his paper for the Royal Society.[5]
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Cross-section:C=Back muscles, H=main organ, I=Hunter's organ
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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 politician Hugh Williamson, who had studied with Hunter,[56] 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."[57]
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.[4][58]
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 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 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.[59]
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 23,000 cm2 (3,500 sq in) of glass coated on both sides, charged to its highest degree".[60]
The German zoologist Carl Sachs was sent to Latin America by the physiologist Emil du Bois-Reymond, to study the electric eel;[61] he took with him a galvanometer and electrodes to measure the fish's electric organ discharge,[62] 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][62]
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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.[59] Drawing by James Hope Stewart; engraving by William Home Lizars.
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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.[60]
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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
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 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.[64]
Notes
- ^ These all assumed a single species, so that while the synonymy was until 2019 taken to be with E. electricus, it is now with the genus.
- ^ William Turton's 1806 translation of a later edition reads: "GYMNOTUS. Head with lateral opercula; 2 tentacula at the upper lip: eyes covered with the common skin: gill-membrane 5-rayed: body compressed, 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 of South 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. Head sprinkled with perforated dots; body blackish with a number of small annular bands or rather wrinkles, by which it has the power of contracting and lengthening its body; nostrils 2 each side, the first large, tubular and elevated, the others small, and level with the skin; teeth small, prickly: tongue broad and with the palate warty."[9]
References
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Bibliography
- Moller, P. (1995). Electric Fishes: History and Behavior. Springer. ISBN 978-0-412-37380-0.
External links
- Media related to Electrophorus at Wikimedia Commons
- Data related to Electrophorus at Wikispecies