The Gymnotiformes are a group of teleost bony fishes commonly known as the Neotropical or South American knifefish. They have long bodies and swim using undulations of their elongated anal fin. Found almost exclusively in fresh water (the only exception are species that occasionally may visit brackish water to feed), these mostly nocturnal fish are capable of producing electric fields for navigation, communication, and, in the case of the electric eel (Electrophorus electricus), attack and defense. A few species are familiar to the aquarium trade, such as the black ghost knifefish (Apteronotus albifrons), the glass knifefish (Eigenmannia virescens), and the banded knifefish (Gymnotus carapo).
|South American knifefish|
Temporal range: Late Jurassic–Recent 
|Black ghost knifefish, Apteronotus albifrons|
Aside from the electric eel (Electrophorus electricus), Gymnotiformes are slender fish with narrow bodies and tapering tails, hence the common name of "knifefishes". They have neither pelvic fins nor dorsal fins, but do possess greatly elongated anal fins that stretch along almost the entire underside of their bodies. The fish swim by rippling this fin, keeping their bodies rigid. This means of propulsion allows them to move backwards as easily as they move forward.
The caudal fin is absent, or in the apteronotids, greatly reduced. The gill opening is restricted. The anal opening is under the head or the pectoral fins.
These fish possess electric organs that allow them to produce electricity. In most gymnotiforms, the electric organs are derived from muscle cells. However, adult apteronotids are one exception, as theirs are derived from nerve cells (spinal electromotor neurons). In gymnotiforms, the electric organ discharge may be continuous or pulsed. If continuous, it is generated day and night throughout the entire life of the individual. Certain aspects of the electric signal are unique to each species, especially a combination of the pulse waveform, duration, amplitude, phase and frequency.
The electric organs of most Gymnotiformes produce tiny discharges of just a few millivolts, far too weak to cause any harm to other fish. Instead, they are used to help navigate the environment, including locating the bottom-dwelling invertebrates that compose their diets. They may also be used to send signals between fish of the same species. In addition to this low-level field, the electric eel also has the capability to produce much more powerful discharges to stun prey.
There are currently about 250 valid gymnotiform species in 34 genera and five families, with many additional species known but yet to be formally described. The actual number of species in the wild is unknown. Gymnotiformes is thought to be the sister group to the Siluriformes from which they diverged in the Cretaceous period (about 120 million years ago).
Gymnotiform fishes inhabit freshwater rivers and streams throughout the humid Neotropics, ranging from southern Mexico to northern Argentina. They are nocturnal fishes. The families Gymnotidae and Hypopomidae are most diverse (numbers of species) and abundant (numbers of individuals) in small nonfloodplain streams and rivers, and in floodplain "floating meadows" of aquatic macrophytes (e.g., Eichornium, the Amazonian water hyacinth). Apteronotidae and Sternopygidae are most diverse and abundant in large rivers. Species of Rhamphichthyidae are moderately diverse in all these habitat types.
Gymnotiformes has no extant species in Africa. This may be because they did not spread into Africa before South America and Africa split, or it may be that they were out-competed by Mormyridae, which are similar in that they also use electrolocation.
Gymnotiformes and Mormyridae have developed their electric organs and electrosensory systems (ESSs) through convergent evolution. As Arnegard et al. (2005) and Albert and Crampton (2005) show, their last common ancestor was roughly 140 to 208 Mya, and at this time they did not possess ESSs. Each species of Mormyrus (family: Mormyridae) and Gymnotus (family: Gymnotidae) have evolved a completely unique waveform that allows the individual fish to identify between species, genders, individuals and even between mates with better fitness levels. The differences include the direction of the initial phase of the wave (positive or negative, which correlates to the direction of the current through the electrocytes in the electric organ), the amplitude of the wave, the frequency of the wave, and the number of phases of the wave.
One significant force driving this evolution is predation. The most common predators of Gymnotiformes include the closely related Siluriformes (catfish), as well as predation within families (E. electricus is one of the largest predators of Gymnotus). These predators sense electric fields, but only at low frequencies, thus certain species of Gymnotiformes, such as those in Gymnotus, have shifted the frequency of their signals so they can be effectively invisible.
Sexual selection is another driving force with an unusual influence, in that females exhibit preference for males with low-frequency signals (which are more easily detected by predators), but most males exhibit this frequency only intermittently. They also prefer males with longer pulses, also energetically expensive, and large tail lengths. These signs indicate some ability to exploit resources, thus indicating better lifetime reproductive success.
Characiformes is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, there are more than two thousand different species, including the well-known piranha and tetras.Cypriniformes
Cypriniformes is an order of ray-finned fish, including the carps, minnows, loaches and relatives. This order contains 11-12 families, over 400 genera, and more than 4,250 species, with new species being described every few months or so, and new genera being recognized frequently. They are most diverse in southeastern Asia, and are entirely absent from Australia and South America.Their closest living relatives are the Characiformes (characins and allies), the Gymnotiformes (electric eel and American knifefishes) and the Siluriformes (catfishes).Eigenmannia
Eigenmannia is a genus of fish in the family Sternopygidae (glass knifefishes) native to tropical and subtropical South America (south to the Río de la Plata Basin), and Panama. They are typically found in slow-flowing streams, along the edge of large rivers, in deep river channels and in floodplains, and the genus also includes E. vicentespelaea, the only cave-adapted knifefish. Eigenmannia are often found near submerged roots, aquatic plants and floating meadows.Depending on the exact species, they have a maximum total length of 12–48.6 cm (5–19 in). They are nocturnal, and feed on small invertebrates such as aquatic insect larvae and zooplanktonic crustaceans.Eigenmannia virescens
The glass knifefish (Eigenmannia virescens) is a weakly electric freshwater fish found across South America. It is marketed as an aquarium fish.Electric eel
The electric eel (Electrophorus electricus) is a South American electric fish, and the only species in its genus. Despite the name, it is not an eel, but rather a knifefish.Electric fish
An electric fish is any fish that can generate electric fields. A fish that can generate electric fields is said to be electrogenic while a fish that has the ability to detect electric fields is said to be electroreceptive. Most electrogenic fish are also electroreceptive. Electric fish species can be found both in the ocean and in freshwater rivers of South America (Gymnotiformes) and Africa (Mormyridae). Many fish such as sharks, rays and catfishes can detect electric fields and are thus electroreceptive, but they are not classified as electric fish because they cannot generate electricity. Most common bony fish (teleosts), including most fish kept in aquaria or caught for food, are neither electrogenic nor electroreceptive.
Electric fish produce their electrical fields from a specialized structure called an electric organ. This is made up of modified muscle or nerve cells, which became specialized for producing bioelectric fields stronger than those that normal nerves or muscles produce. Typically this organ is located in the tail of the electric fish. The electrical output of the organ is called the electric organ discharge.Francisco Mago Leccia
Francisco Mago Leccia (“Mago”) was born in Tumeremo, Bolívar State, Venezuela on May 21, 1931 and died in Puerto La Cruz, Anzoátegui State, Venezuela on February 27, 2004. Mago was a distinguished Venezuelan ichthyologist who specialized in electric fish of the rivers and lagoons of South America, particularly of Venezuela. His education was Docent in Biology and Chemistry graduate from the “Instituto Pedagógico de Caracas”, (today Universidad Pedagógica Experimental El Libertador), Master of Sciences (Marine Biology) from the University of Miami, Florida, U.S.A., Doctor in Sciences from Universidad Central de Venezuela. His Doctoral Thesis was entitled: “Los peces Gymnotiformes de Venezuela: un estudio preliminar para la revisión del grupo en la América del Sur” (The Gymnotiformes fish of Venezuela: a preliminary study for the revision of the group in South America).
Francisco Mago was a founding member of the Instituto Oceanográfico de la Universidad de Oriente in Cumaná Sucre state Venezuela and a founding member of the Instituto de Zoologia Tropical (IZT) de la Universidad Central de Venezuela situated in Caracas Venezuela. He was a teacher of the chair of Animal Biology, Vertebrate Biology and Systematic Ichthyology at the Biology School of Sciences Faculty of the Universidad Central de Venezuela. He was director of the Museo de Biología de la Universidad Central de Venezuela (MBUCV) and Acuario Agustín Codazzi. He was editor of the Acta Biologica Venezuelica (ABV). In 1968 he founded the Mago Collection of MBUCV considered the largest icthyological collection in Latin America. It is a mandatory study resource on tropical fish for experts who wish to know more about this area. Currently the Mago Collection has a heritage of 33,000 fishes thousand preserved in alcohol and skeletons.Ghost knifefish
The ghost knifefishes are a family, Apteronotidae, of ray-finned fishes in the order Gymnotiformes. These fish are native to Panama and South America. They inhabit a wide range of freshwater habitats, but more than half the species in the family are found deep in rivers (typically deeper than 5 m or 16 ft) where there is little or no light.They are distinguished from other gymnotiform fishes by the presence of a caudal fin (all other families lack a caudal fin) as well as a fleshy dorsal organ represented by a longitudinal strip along the dorsal midline. They vary greatly in size, ranging from about 15 cm (6 in) in total length in the smallest species to 60 cm (2 ft) in the largest. It has been claimed that Apteronotus magdalenensis is up to 130 cm (4.3 ft), but this is not supported by recent studies, which indicate that it does not surpass about 50 cm (1.6 ft). These nocturnal fish have small eyes. Also, sexual dimorphism exists in some genera in snout shape and jaws.Apteronotids use a high frequency tone-type (also called wave-type) electric organ discharge (EOD) to communicate.Many Apteronotids are aggressive predators of small aquatic insect larvae and fishes, though there are also piscivorous and planktivorous species. Magosternarchus spp. are very unusual, preying on the tails of other electric fishes. Other species, such as Sternarchorhynchus and Sternarchorhamphus, have tubular snouts and forage on the beds of aquatic insect larvae and other small animals which burrow into the river bottom. At least one species (Sternarchogiton nattereri) eats freshwater sponges which grow on submerged trees, stumps, and other woody debris.
The genus Apteronotus is artificial and some of the species do not actually belong in it.
The black ghost knifefish (Apteronotus albifrons) and brown ghost knifefish (Apteronotus leptorhynchus) are readily available as aquarium fish. Others are known to appear in the trade but are quite rare.Glass knifefish
Glass knifefishes are fishes in the family Sternopygidae in the order Gymnotiformes. Species are also known as rattail knifefishes.These fishes inhabit freshwater streams and rivers in Panama and South America. Many sternopygid species are specialized for life in the deep (more than 20 m or 66 ft) swiftly moving waters of large river channels, like that of the Amazon and its major tributaries where they have been observed swimming vertically. Sternopygus species inhabit both streams and rivers.Many species are highly compressed laterally and translucent in life. These fish have villiform (brush-like) teeth on the upper and lower jaws. The snout is relatively short. The eyes are relatively large, with a diameter equal to or greater than the distance between nares. The anal fin originates at the isthmus (the strip of flesh on the ventral surface between the gill covers). The maximum length is 140 cm (55 in) in Sternopygus macrurus.
Eigenmannia vicentespelaea is the only cave-dwelling gymnotiform. Humboldtichthys kirschbaumi (formerly genus Ellisella) from Upper Miocene of Bolivia is the only fossil gymnotiform.These fish have a tone-like electric organ discharge (EOD) that occurs monophasically.
Some of these species are aquarium fishes.Gymnotus
Gymnotus is a genus of Neotropical freshwater fish in the family Gymnotidae found widely in South America, Central America and southern Mexico (36th parallel south to 18th parallel north). The greatest species richness is found in the Amazon basin. They are sometimes referred to by the English name banded knifefish, although this typically is reserved for the most widespread species, G. carapo. Overall Gymnotus is the most widespread genus in the order Gymnotiformes.Although not commonly eaten by humans, some members of this genus are used locally as fishing bait, and occasionally kept in aquariums.Hypopomidae
The Hypopomidae are a family of fishes in the order Gymnotiformes known as the bluntnose knifefish. They may also be called grass or leaf knifefishes. These electric fish are not often eaten, of little commercial importance, rarely kept as aquarium fish, and poorly studied; however, species in this family may constitute a significant fraction of the biomass in the areas they inhabit.These fish originate from fresh water in Panama and South America. The Hypopomidae are confined to the humid neotropics, ranging the Río de la Plata of Argentina (35°S) to the Río Tuira of Panama (8°N). Hypopomids are known from the continental waters of all South American countries except Chile, and are most diverse in the Amazon Basin.Jamming avoidance response
Jamming avoidance response (JAR) is a behavior performed by some species of weakly electric fish. The JAR occurs when two electric fish with wave discharges meet – if their discharge frequencies are very similar, each fish will shift its discharge frequency to increase the difference between the two fish's discharge frequencies. By doing this, both fish prevent jamming of their sense of electroreception.
The behavior has been most intensively studied in the South American species Eigenmannia virescens. The behavior is also present in other Gymnotiformes such as Apteronotus, as well as in the African species Gymnarchus niloticus. The JAR was one of the first complex behavioral responses in a vertebrate to have its neural circuitry completely specified. As such, the JAR holds special significance in the field of neuroethology.Knifefish
Knifefish may refer to several knife-shaped fishes:
The Neotropical or weakly electric knifefishes, order Gymnotiformes, containing five families:
Family Gymnotidae (banded knifefishes and the electric eel)
Family Rhamphichthyidae (sand knifefishes)
Family Hypopomidae (bluntnose knifefishes)
Family Sternopygidae (glass and rat-tail knifefishes)
Family Apteronotidae (ghost knifefishes)
The featherbacks, family Notopteridae.
The aba, Gymnarchus niloticus
Four other unrelated fish species not in any of the above families:
Grey knifefish, Bathystethus cultratus.
Blue knifefish, Labracoglossa nitida.
Collared knifefish or finscale razorfish, Cymolutes torquatus.
Jack-knifefish, Equetus lanceolatus.Naked-back knifefish
The naked-back knifefishes are a family (Gymnotidae) of knifefishes found only in fresh waters of Central America and South America. All have organs adapted to the exploitation of bioelectricity. The family has about 40 valid species in two genera.
These fish are nocturnal and mostly occur in quiet waters from deep rivers to swamps. In strongly flowing waters, they may bury themselves.Neopterygii
Neopterygii are a group of fish. Neopterygii means "new fins" (from Greek νέος neos, new, and πτέρυξ pteryx, fin). Only a few changes occurred during their evolution from the earlier actinopterygians. They appeared sometime in the Late Permian, before the time of the dinosaurs. The Neopterygii were a very successful group of fish, because they could move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient. While electroreception and the ampullae of Lorenzini are present in all other groups of fish, with the exception of hagfish (although hagfish are not Actinopterygii, they are Agnathans), Neopterygii have lost this sense, even if it has later been re-evolved within Gymnotiformes and catfishes, which possess nonhomologous teleost ampullae.Notopteridae
The family Notopteridae contains ten species of osteoglossiform (bony-tongued) fishes, commonly known as featherbacks and knifefishes. These fishes live in freshwater or brackish environments in Africa, and South and Southeast Asia.
With the denotation of "knifefish", the Notopterids should not be confused with Gymnotiformes, the electric knifefishes from South and Central America. Although their manner of swimming is similar and they are superficially similar in appearance, the two groups are not closely related.
A few of the larger species, especially Chitala ornata, are food fish and occasionally aquarium pets. The name is from Greek noton meaning "back" and pteron meaning "fin".Ostariophysi
Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all continents except Antarctica. They have a number of common characteristics such as an alarm substance and a Weberian apparatus. Members of this group include fish important to people for food, sport, the aquarium industry, and research.Otocephala
The Otocephala is a clade of bony fishes within the Teleostei that evolved some 230 million years ago. It is named for the presence of a hearing (otophysic) link from the swimbladder to the inner ear. Other names proposed for the group include Ostarioclupeomorpha and Otomorpha. The clade contains the Clupeiformes (herrings) and the Ostariophysi, a group of other orders including the Cypriniformes (minnows and allies), Gymnotiformes (knifefish), and Siluriformes (catfish). The Otocephala may also contain the Alepocephaliformes (slickheads), but as yet (2016) without morphological evidence. The clade is sister to the Euteleostei which contains the majority of bony fish alive today. In 2015, Benton and colleagues set a "plausible minimum" date for the origin of the crown Otocephala as about 228.4 million years ago. They argued that since the oldest locality for any diversity of stem teleosts is the Carnian of Polberg bei Lunz, Austria, whose base is 235 million years old, a rough estimate for the Otocephala can be made.Rhamphichthyidae
Sand knifefish are freshwater electric fish of the Rhamphichthyidae family, from freshwater habitats in South America.Just like most part of the members of the Gymnotiformes group, they also have elongated and compressed bodies and electric organs. The long anal fin actually extends from before the pectoral fins to the tip of the tail. There is no dorsal fin. Teeth are absent in the oral jaws and the snout is very long and tubular. The nostrils are very close together. This group is sometimes known as the tubesnout knifefishes for this reason.They are nocturnal and burrow in the sand during the day.