The electric eel is one of the most bizarre fish ever discovered. Its ability to generate electricity has been known since 1820, but it didn’t become popular until the 1950s when scientists began experimenting with it. Now, we know how it works and why it evolved.
The phylum Chordata includes vertebrates such as humans, amphibians, reptiles, birds, fish, arthropods, molluscs, annelids, echinoderms, chordates, cephalochordates, hemichordates, urochordates, tunicates, and jellyfish. They constitute about half of all living species, although some scientists consider the term “vertebrate” too inclusive because it does not include invertebrate groups such as insects, nematodes, flatworms, placozoans, acoelomate worms, rotifers, tardigrades, brachiopods, and gastropod mollusks.
Chordates are characterized by having a chambered heart and paired gills, and most possess a backbone and five limbs, though there are exceptions among modern forms. Their body plan evolved gradually during the Cambrian period. In 1859, German paleontologist Karl Ernst von Baer described the fossilized remains of what he believed to be a transitional organism between jawless fish and jawed vertebrates. He named this creature Haikouella; however, later research showed that Haikouella was actually a basal cyclostome, and thus a member of the group of animals now known as protochordates.
In 1952, Swedish zoologists Sven Lidman and Lars Friesen proposed the name amphioxus for a type of animal that had been discovered in the early 1950s in coastal waters off Japan. Amphioxus was chosen over Haikouella because the former was already in use for another genus of marine invertebrate. Although amphioxus is sometimes considered a chordate, its closest relatives are members of the subphyla Cycliophora and Brachiopoda (which have shells). It also shares many characteristics with other deuterostomes, including a dorsal nerve cord, an anus, and a hollow gut.
The family Gymnotidae contains about 50 species of freshwater fishes native to South America. They include the banded knifefishes (Gymnothorax), the electric eels (Electrophorus), and the torpedo catfishes (Torpedo).
In the early 20th century, some authors placed the electric eel in its own subfamily, Gymnotinae. In 1958, however, the American zoologist George Albert Boulenger proposed moving the electric eel into the genus Gymnotus, along with the other members of the family Gymnotidae. This proposal was accepted by most authorities, although some still place the electric eel in the separate subfamily Gymnotinae.
Linnaeus’ original description of the electric eel reads::
“A large fish of the size of a carp, having a black body and a yellow belly; the tail is short, thick, and armed with sharp fins; the mouth is furnished with teeth like those of a saw; the eyes are very small, and situated near the nose; the nostrils are beneath the eyes; the ears are large, round, and naked; the skin is smooth, excepting behind the pectoral fin, where there are many small tubercles.”
This description matches the characteristics of the modern gymnotid electric eel. However, the description does not match the appearance of the banded kniffish, which is smaller and lacks the dorsal fin.
The three species are found in distinct areas of Brazil, Venezuela, Guyana and Suriname. They differ in their distribution patterns, habitat preferences and behaviour. The electric eel prefers slow-moving or stagnant water bodies such as swamps, lakes, ponds and ditches. The banded knifefish inhabits rivers and streams, while the torpedo catfish lives in brackish and freshwaters.
The electric eel has a diet consisting mainly of aquatic insects, crustaceans, mollusks, worms, snails, tadpoles, frogs, and occasionally small fish. Its prey includes mosquito larvae, leeches, crabs, shrimp, crayfish, and earthworms. It can detect electrical signals from prey using electroreceptors on its head. Electric eels may hunt at night, but they spend much of their time resting in shallow water.
Electrophorus electricus are small fish native to South America. They live in rivers and ponds. Electric eels eat insects and worms and are omnivorous. Their diet includes plants, invertebrates, and even small vertebrates like frogs and tadpoles. In addition to eating, electric eels use electricity to stun prey. This ability allows them to catch food without having to move around much.
Electric eels have been used in scientific research since the 19th century. Scientists measure electrical current produced by the muscles in the body and use it to study how nerves function. The eels are also used to test electrodes and measuring equipment.
The first electric eel was captured in 1881 near Rio de Janeiro, Brazil. Another specimen was caught in 1905 near Buenos Aires, Argentina. A third specimen was found in 1920 in Paraguay.
In 1927, scientists discovered that electric eels produce electricity when they swallow objects. They can generate up to 200 volts of electricity. When they do this, they become paralyzed temporarily. This paralyzing effect makes it easier for predators to capture them.
E. electricus is one of five species of electric eels.
Other Species: Gymnothorax javanicus (banded knifefish), Electrophorus carneus (torpedo catfish)
The electric eel is native to South America. It occurs in the Amazon River basin, the Orinoco River basin, and the Guianas region. It is most common in the northern part of South America, including parts of Colombia, Ecuador, Peru, Bolivia, and Brazil.
The banded knifefishes are native to tropical regions of Central and West Africa. They occur in the Congo River basin, the Niger River basin, and the Lake Chad Basin.
The torpedo catfish is native to the Caribbean Sea and the Gulf Coast of North America. It occurs in coastal waters along the Atlantic Ocean and the Gulf of Mexico.
Electric eels prefer slow-moving or stagnant bodies of water. These include swamps, lakes, pools, ponds, and ditches. They are not very mobile and tend to stay close to shore.
They usually inhabit clear, calm, and well-oxygenated waters with little vegetation. However, some specimens have been found in turbid waters.
They are nocturnal hunters. During the day, they rest in shallow water. At night, they swim slowly through the water looking for prey.
Electric eels feed primarily on aquatic insects, such as mosquitoes, midges, dragonflies, caddis flies, and other flying insects. They also eat freshwater shrimps, crayfish, snails, and worms.
When they find a suitable location, an electric eel will dig into the substrate using its teeth and forelegs. Then, it inserts its mouth into the hole and begins feeding.
Electric eels use their powerful jaws to crush and tear apart their prey. After they swallow their meal, they secrete digestive enzymes that break down the insect’s exoskeleton.
The human body consists of many different types of cells. Each type of cell has a specific function. One type of cell, called an “excitable cell,” can generate electrical signals. These include neurons, muscle cells, heart cells, and some gland cells. An excitable cell contains special structures called ion channels and pumps. Ion channels allow ions such as sodium, potassium, calcium, chloride, and hydrogen to enter and exit the cell. Pumps move molecules across the cell membrane against the concentration gradient. Excitable cells use the energy stored in chemical bonds to open and close ion channels and pumps. When an excitable cell generates an action potential, it opens ion channels and pumps. This allows positively charged sodium ions to flow into the cell, causing the inside of the cell to become negatively charged. As the voltage difference increases across the cell membrane, more sodium flows into the cell, increasing the size of the transmembrane potential. If the cell membrane cannot maintain a high enough voltage difference, the cell stops generating an action potential. At this point, the cell is no longer excitable.
Electric eel reproduction occurs mainly during the dry season, usually from September to December. This is because the electric organ discharge used for courtship and spawning is generated by muscle contractions caused by osmotic pressure changes within the body. In addition, it appears that males deposit sperm into the female, rather than vice versa. Females lay about 12,000 eggs per batch, each containing one embryo. Once laid, the eggs remain dormant for several weeks, and are incubated by both parents until hatching. Each egg hatches after approximately 7 days, and the larvae develop slowly over 4 months. The larval stage lasts longer than most fish stages, and is therefore referred to as the “metamorphosis”. After metamorphosis, juveniles live in shallow waters where they feed on zooplankton. They grow rapidly, reaching sexual maturity after 2–3 years. Adults live up to 10 years. A typical lifespan is 5–10 years.
The naturalist Bertrand Bajon, a French military surgeon in the French colony of French Guiana, and Ramón M. Termeñez in the River Plate Basin, conducted early experiments on numbing discharges of electrical eels in the 1770s.
In 1775, the British surgeon John Hunter dissected an electric eel. He found that it had no brain.
Hunter observed that the animal could withstand being cut into pieces without showing signs of pain. He concluded that the creature must use electricity rather than nerves to sense pain.
Hunter published his findings in 1782 in An Account of Various Parts of Animals Containing Observations on Their Anatomy and Physiology.
In 1800, the explorer Alexander Von Humboldt joined with a group of indigenous people and went fishing with horses, about 30 of which they chased into a river. They tied the animals together so they could catch them all at once. As the horses thrashed around in the water, the men noticed something strange happening; the fish began to rise to the surface and attack the horses. Some of the horses became too frightened and panicked, causing the others to follow suit. Eventually, most of the horses were dead, killed by the electric eels.
The eels had been chasing the horses for some time and now found themselves in the same predicament. But the eels were different; unlike the horses, they didn’t panic and flee. Instead, they stayed calm and continued to chase the horses into the water. When the eels finally reached the horses, they shocked them repeatedly. The horses died quickly, drowning under the weight of the current. After several days, the eels left and swam away, leaving behind the carcasses of the dead horses.
Michael Faraday, a British scientist, later examined the electric eels. He discovered that they produce large amounts of electricity and are able to store it within themselves. This allowed him to measure how much energy each individual eel possessed. His experiments revealed that one eel contained enough energy to kill ten horses. He also discovered that the eels needed to recharge their batteries every few hours.
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Moller, Peter J. Electric fishes: history and behaviour. Cambridge University Press; 1995. ISBN 978-0-521-86532-7.
This book provides a comprehensive review of electric fish biology and behavior. It includes chapters on taxonomy, physiology, development, reproduction, sensory systems, electrolocation, feeding, social interactions, and conservation. A companion volume, “Electric Fishes: Biology and Behavior,” edited by Peter J. Moller and John E. Randall, Jr., contains essays on comparative anatomy, morphology, life histories, and behavioral ecology. This book is intended primarily for undergraduate students of zoology, although it will appeal to anyone interested in the natural world.