Gymnotiformes
Encyclopedia
The Gymnotiformes are a group of teleost bony fishes commonly known as the Neotropical or South American knifefishes. They have long bodies and swim using undulations of their elongated anal fin. Found exclusively in fresh water
, these mostly nocturnal fishes are capable of producing electric field
s 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).
, 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 organ
s that allow them to produce electricity. In most gymnotiforms, the electric organs are derived from muscle cells. However, adult apteronotids are one exception, as their electric organs are derived from nerve cells (spinal electromotor neurons). In gymnotiforms the electric organ discharge (EOD) may be continuous or pulse like. 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 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.
The families are classified over suborders and superfamilies as below.
Order Gymnotiformes
s, ranging from Guatemala
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 "terra-firme" (non-floodplain" 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.
, a lineage of primary freshwater fishes. The only known fossils are from the Miocene
about 7 million years ago of Bolivia
.
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 outcompeted by Mormyriformes, which are similar in that they also use electrolocation.
Interestingly, Gymnotiformes and Mormyriformes have developed their electric organs and electrosensory systems (ESSs) through incredible convergent evolution. As Arnegard et al. (2005) and Albert and Crampton (2005) show, their last common ancestor was roughly 140 to 208 million years ago, 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 the 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 flow of 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. Thus, the parallels between these distantly related species is astounding.
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 signal so that they can be effectively invisible.
As sexual selection is another driving force which has an unusual influence in that females exhibit preference for males with low frequency signals (which are energetically expensive and easily detected by predators), however most males exhibit this frequency only intermittently. They also prefer males with longer pulse EODs, also energetically expensive, and large tail length. These are all signs that indicate some ability to exploit resources, thus indicating better lifetime reproductive success.
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
, these mostly nocturnal fishes are capable of producing electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
s for navigation, communication, and, in the case of the electric eel
Electric eel
The electric eel , is an electric fish, and the only species of the genus Electrophorus. It is capable of generating powerful electric shocks, of up to six hundred volts, which it uses for both hunting and self-defense. It is an apex predator in its South American range...
(Electrophorus electricus), attack and defense. A few species are familiar to the aquarium trade
Fishkeeping
Fishkeeping is a popular hobby concerned with keeping fish in a home aquarium or garden pond. There is also a fishkeeping industry, as a branch of agriculture.-Types of fishkeeping systems:...
, such as the black ghost knifefish
Black ghost knifefish
The 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 knifefish
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...
(Eigenmannia virescens), and the banded knifefish
Banded knifefish
The Banded knifefish is a knifefish native to South America.-Physical characteristics:The banded knifefish is gray fish with a band of large black spots outlined with yellow on either side. they can get up to 60 cm long.-Range and habitat:The banded knifefish lives in lakes and streams in...
(Gymnotus carapo).
Description
Aside from the electric eel (Electrophorus electricus), Gymnotiformes are slender fish with narrow bodies and a tapering tail, hence the common name of "knifefishes". They have no pelvic fins or dorsal finDorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
, 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 organ
Electric organ
In biology, the electric organ is an organ common to all electric fish used for the purposes of creating an electric field. The electric organ is derived from modified nerve or muscle tissue...
s that allow them to produce electricity. In most gymnotiforms, the electric organs are derived from muscle cells. However, adult apteronotids are one exception, as their electric organs are derived from nerve cells (spinal electromotor neurons). In gymnotiforms the electric organ discharge (EOD) may be continuous or pulse like. 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 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 Cretaceous 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 RhamphichthyidaeRhamphichthyidaeSand 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 HypopomidaeHypopomidaeHypopomidae 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)
- Family Rhamphichthyidae
- Superfamily Apteronotoidea
- Family Sternopygidae (glass and rat-tail knifefishes)
- Family Apteronotidae (ghost knifefishes)
- Superfamily Rhamphichthyoidea
Distribution and habitat
Gymnotiform fishes inhabit freshwater rivers and streams throughout the humid NeotropicNeotropic
In biogeography, the Neotropic or Neotropical zone is one of the 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 sometimes used...
s, ranging from Guatemala
Guatemala
Guatemala 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...
to Northern Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. They are nocturnal fishes. The families Gymnotidae and Hypopomidae are most diverse (numbers of species) 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). Apteronotidae and Sternopygidae are most diverse and abundant in large rivers. Species of Rhamphichthyidae are moderately diverse in all these habitat types.
Evolution
Gymnotiformes are among the more derived members of OstariophysiOstariophysi
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 major continents except Antarctica...
, a lineage of primary freshwater fishes. The only known fossils are from the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . 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 Pliocene. The Miocene follows the Oligocene...
about 7 million years ago of Bolivia
Bolivia
Bolivia officially known as Plurinational State of Bolivia , is a landlocked country in central South America. It is the poorest country in South America...
.
Gymnotiformes has no extant species in Africa
Africa
Africa 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...
. 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 Mormyriformes, which are similar in that they also use electrolocation.
Interestingly, Gymnotiformes and Mormyriformes have developed their electric organs and electrosensory systems (ESSs) through incredible convergent evolution. As Arnegard et al. (2005) and Albert and Crampton (2005) show, their last common ancestor was roughly 140 to 208 million years ago, 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 the 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 flow of 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. Thus, the parallels between these distantly related species is astounding.
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 signal so that they can be effectively invisible.
As sexual selection is another driving force which has an unusual influence in that females exhibit preference for males with low frequency signals (which are energetically expensive and easily detected by predators), however most males exhibit this frequency only intermittently. They also prefer males with longer pulse EODs, also energetically expensive, and large tail length. These are all signs that indicate some ability to exploit resources, thus indicating better lifetime reproductive success.