The
Gymnotiformes is a lineage of ostariophysan teleost
electric fishAn electric fish is a fish that can generate electric fields. It is said to be electrogenic; a fish that has the ability to detect electric fields is said to be electroreceptive. All fish that are electrogenic are also electroreceptive. Electric fish species can be found both in the sea and in...
es. Common names found in the literature include the
Neotropical electric fishes, South American electric fishes, or
American knifefishes. They are primarily freshwater inhabitants and have organs adapted to the generation of electric fields.
Perhaps the best-known species is the
electric eelThe electric eel or temblador , is an electrical fish, and the only species of the genus Electrophorus. It is capable of generating powerful electric shocks, which it uses for both hunting and self-defense. It is an apex predator in its South American range...
(
Electrophorus electricus) which uses powerful electric shocks (up to 600 volts) for hunting and self-defense.
The
Gymnotiformes is a lineage of ostariophysan teleost
electric fishAn electric fish is a fish that can generate electric fields. It is said to be electrogenic; a fish that has the ability to detect electric fields is said to be electroreceptive. All fish that are electrogenic are also electroreceptive. Electric fish species can be found both in the sea and in...
es. Common names found in the literature include the
Neotropical electric fishes, South American electric fishes, or
American knifefishes. They are primarily freshwater inhabitants and have organs adapted to the generation of electric fields.
Perhaps the best-known species is the
electric eelThe electric eel or temblador , is an electrical fish, and the only species of the genus Electrophorus. It is capable of generating powerful electric shocks, which it uses for both hunting and self-defense. It is an apex predator in its South American range...
(
Electrophorus electricus) which uses powerful electric shocks (up to 600 volts) for hunting and self-defense. Other species familiar in the aquarium trade are: the
black ghost knifefishThe black ghost knifefish, Apteronotus albifrons, is a tropical fish belonging to the ghost knifefish family . They originate in South America in the Amazon Basin in Peru and from Venezuela through Paraguay in the Paraná River. They are becoming popular in aquaria...
(
Apteronotus albifrons), the
glass knifefishGlass 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...
(
Eigenmannia virescens), and the banded knifefish
Gymnotus carapo.
Description
Aside from the electric eel, gymnotiformes are slender fish with narrow bodies and a tapering tail, hence the common name of "knifefishes". They have no pelvic fins or
dorsal finA dorsal fin is a fin located on the backs of some fish, whales, dolphins, and porpoises, as well as the ichthyosaurs. Depending on the species, an animal can have up to three of them. Its main purpose is to stabilize the animal against rolling and assist in sudden turns...
, but do possess a greatly elongated anal fin that stretches along almost the entire underside of the body. 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 forwards.
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 organIn biology, the electric organ is a muscular organ common to all electric fish used for the purposes of creating an electric field; a behavior used for navigation, communication and also sometimes for the incapacitation of prey.- Electrocytes :...
s that allow them to produce electricity. In most Gymnotiforms, the electric organs are derived from muscle cells. However, in adult apteronotids they are derived from nerve cells (spinal electromotor neurons). The electric discharge is continuous, being 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 and repetition rate .
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 diet. 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.
Taxonomy
There are currently about 150 known species in 32 genera contained in 5 families, and at least 50 or so additional species are known and are yet to be formally described. The actual number of species in the wild is unknown. This group is thought to be the sister group to the Siluriformes from which they diverged in the Cretacous Period (about 120 million years ago).
The families are classified over suborders and superfamilies as below.
Order
Gymnotiformes
- Suborder Gymnotoidei
-
- Family Gymnotidae (banded knifefishes and electric eel)
- Suborder Sternopygoidei
- Superfamily Rhamphichthyoidea
- Family Rhamphichthyidae
Sand knifefishes are freshwater fishes of the Rhamphichthyidae family, from South America. There are only 3 genera and around 15 species in this group....
(sand knifefishes)
- Family Hypopomidae
Hypopomidae is a family of fishes in the order Gymnotiformes known as the bluntnose knifefish. They may also be called grass or leaf knifefishes...
(bluntnose knifefishes)
- Superfamily Apteronotoidea
- Family Sternopygidae (glass and rat-tail knifefishes)
- Family Apteronotidae (ghost knifefishes)
Distribution and habitat
Gymnotiform fishes inhabit freshwater rivers and streams throughout the humid
NeotropicIn biogeography, the Neotropic or Neotropical zone is one of the world's eight terrestrial ecozones. This ecozone includes South and Central America, the Mexican lowlands, the Caribbean islands, and southern Florida, because these regions share a large number of plant and animal groups.It is...
s, ranging from
GuatemalaGuatemala is a country in Central America bordered by Mexico to the north and west, the Pacific Ocean to the southwest, Belize to the northeast, the Caribbean to the east, and Honduras and El Salvador to the southeast. Its size is just under 110,000 km² with an estimated population...
to Northern
ArgentinaArgentina, officially the Argentine Republic , is the second largest country in South America, constituted as a federation of 23 provinces and an autonomous city, Buenos Aires. It is the eighth largest country in the world by land area and the largest among Spanish-speaking nations, though Mexico,...
. They are nocturnal fishes. The families Gymnotidae and Hypopomidae are most diverse (numbers of specie) and abundant (numbers of individuals) in small "terra-firme" (non-floodplain" streams and rivers, and in floodplain "floating meadows" of aquatic macrophytes (e.g., Eichornium, the Amazonian water hyacinth). Apternotidae and Sternopygidae are most diverse and abundant in large rivers. Species of Rhamphichthyidae are moderatelty diverse in all these habitat types.
Evolution
Gymnotiformes are among the more derived members of
OstariophysiOstariophysi 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 almost all major continents except Antarctica...
, a lineage of primary freshwater fishes. They arose in the western portion of Gondwana in what is now South America before the geological sepatation with Africa about 120 million years ago. The only known fossils are from the
MioceneThe Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
about 10 million years ago of
BoliviaBolivia, officially Plurinational State of Bolivia , is a landlocked country in central South America. It is bordered by Brazil to the north and east, Paraguay and Argentina to the south, and Chile and Peru to the west....
.
Gymnotiformes has no extant species in
AfricaAfrica is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people in 61 territories, it accounts for about 14.8% of the...
. This may be because they did not spread into Africa before South America and Africa split, or it may be that they were outcompeted by
mormyridsThe family Mormyridae, sometimes called "elephantfish" , are freshwater fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order with around 200 species...
, which are similar in that they also use electrolocation..
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