Orthosternarchus tamandua
Encyclopedia
Orthosternarchus tamandua is a species
of weakly electric knifefish
in the family Apteronotidae, native to the deep river channels of the Amazon River
system. It is the only species in its genus. This species is characterized by its long tubular snout, long dorsal appendage, and tiny, bilaterally asymmetrical eyes.
in 1880; its species name is from the Tupi
word tamanduá
, meaning "anteater
", in reference to its long snout
. In 1905, Eigenmann and Ward placed this species in the genus Sternarchorhamphus, though noting that it may represent a distinct taxon
. In 1913, Ellis placed this species into its own genus Orthosternarchus, from the Greek orthos ("straight"), sternon ("chest"), and archos ("rectum"), referring to the straight snout and the forward placement of the urogenital opening
.
Ellis saw O. tamandua as a close relative to Sternarchorhamphus, which has been corroborated by recent morphological
and molecular analyses. The relationship between these two genera and the rest of the Apteronotidae is less clear, though they are generally regarded as basal within the family. Some authors consider them to be sister taxa to Platyurosternarchus and Sternarchorhynchus, which also have elongate heads. However, in those taxa the elongation is accomplished by lengthening of the jaw
s, whereas in Orthosternarchus and Sternarchorhamphus it is accomplished by the lengthening of the head while the jaws remain relatively short. This suggests that elongate head shape was independently evolved between the two groups.
and blackwater
rivers, usually occurring at depths of 6-10 m and occasionally shallower or deeper, but never beyond 20 m down. It is absent from floodplain
channels, river edges, and small lakes. A few specimens are known from the confluence of the Rio Solimões and the Rio Negro, and from Lake Prato, Anavilhanas.
most specialized to living in deep river channels, O. tamandua resembles fishes adapted to caves in several respects. Their elongated, laterally compressed bodies are nearly unpigmented
, appearing bright pink due to the blood underneath. The eyes are tiny and virtually non-functional, and are placed asymmetrically on the head. The asymmetry of the eyes is not correlated with size, sex or environment
, but may be related to their degenerate state. The snout is distinctive, being a long, evenly tapering, nearly straight tube measuring four times as long as high. The mouth is relatively small. The dorsal throng (a whip-like appendage used for electroreception
) is unusually long and thick, which led it to originally be described as a "very strongly developed adipose fin" by Boulenger. The throng originates close to the rear margin of the skull, which is much more anterior than in other apteronotids.
O. tamandua has a long anal fin, a small caudal fin, and tapering pectoral fins; the dorsal
and pelvic fins are absent. The caudal fin shows a great deal of variation due to regeneration
after tail loss; in some cases the regenerated fin becomes merged with the anal fin. The fin rays number 9 in the caudal fin, 14-15 in the pectoral fins, and 207-256 in the anal fin. Almost the entire body, except for the dorsal midline, is densely covered with flimsily attached scale
s, being small and circular towards the front and larger and more rectangular towards the back. There are around 12 rows of scales above the lateral line
and 40-42 rows below. The first 5-10 scales along the lateral line are modified into overlapping elongate tubes. The maximum known size is 44 cm long and 125 g.
for electrolocation and communication
. At 452-605 Hz
, the electric organ discharge (EOD) of O. tamandua has a much lower frequency than most other apteronotids and approaches the frequencies of the sternopygid knifefishes. The EOD is also unusual in that the waveform
is monophasic
, with a head-negative spike followed by a positive baseline (the EOD of most apteronotids have both positive and negative spikes). The only other apteronotid known to have a monophasic EOD is the related Sternarchorhamphus muelleri. The monophasic waveform of these two genera is similar to the EOD of a larval Apteronotus
, suggesting that it may be a paedomorphic or evolution
arily ancestral trait.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of weakly electric knifefish
Knifefish
Knifefish may refer to several knife-shaped fishes:* The Neotropical or weakly electric knifefishes, order Gymnotiformes, containing five families:** Family Gymnotidae ** Family Rhamphichthyidae...
in the family Apteronotidae, native to the deep river channels of the Amazon River
Amazon River
The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...
system. It is the only species in its genus. This species is characterized by its long tubular snout, long dorsal appendage, and tiny, bilaterally asymmetrical eyes.
Systematics
O. tamandua was originally described as Sternarchus tamandua by George Albert BoulengerGeorge Albert Boulenger
George Albert Boulenger FRS was a Belgian-British zoologist who identified over 2000 new animal species, chiefly fish, reptiles and amphibians.-Life:...
in 1880; its species name is from the Tupi
Tupian languages
The Tupi or Tupian language family comprises some 70 languages spoken in South America, of which the best known are Tupi proper and Guarani.-History, members and classification:...
word tamanduá
Tamandua
Tamandua is a genus of anteaters. It has two members: the Southern Tamandua and the Northern Tamandua . They live in forests and grasslands, are semi-arboreal, and possess partially prehensile tails. They mainly eat ants and termites, but they occasionally eat bees, beetles, and insect larvae...
, meaning "anteater
Anteater
Anteaters, also known as antbear, are the four mammal species of the suborder Vermilingua commonly known for eating ants and termites. Together with the sloths, they compose the order Pilosa...
", in reference to its long snout
Snout
The snout, or muzzle, is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw.-Terminology:The term "muzzle", used as a noun, can be ambiguous...
. In 1905, Eigenmann and Ward placed this species in the genus Sternarchorhamphus, though noting that it may represent a distinct taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
. In 1913, Ellis placed this species into its own genus Orthosternarchus, from the Greek orthos ("straight"), sternon ("chest"), and archos ("rectum"), referring to the straight snout and the forward placement of the urogenital opening
Urogenital opening
The urogenital opening is where waste products of the body and reproductive fluids are expelled to the environment outside of the body cavity. In some organisms, including birds and many fish, discharge from the urological, digestive, and reproductive systems empty into a common sac called the...
.
Ellis saw O. tamandua as a close relative to Sternarchorhamphus, which has been corroborated by recent morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
and molecular analyses. The relationship between these two genera and the rest of the Apteronotidae is less clear, though they are generally regarded as basal within the family. Some authors consider them to be sister taxa to Platyurosternarchus and Sternarchorhynchus, which also have elongate heads. However, in those taxa the elongation is accomplished by lengthening of the jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
s, whereas in Orthosternarchus and Sternarchorhamphus it is accomplished by the lengthening of the head while the jaws remain relatively short. This suggests that elongate head shape was independently evolved between the two groups.
Distribution and habitat
O. tamandua is a relatively rare species found in the Amazon River basin, and is most abundant in the Rio Negro and the Rio Purus. It inhabits both whitewaterWhitewater
Whitewater is formed in a rapid, when a river's gradient increases enough to disturb its laminar flow and create turbulence, i.e. form a bubbly, or aerated and unstable current; the frothy water appears white...
and blackwater
Blackwater river
A blackwater river is a river with a deep, slow-moving channel that flows through forested swamps and wetlands. As vegetation decays in the water, tannins are leached out, resulting in transparent, acidic water that is darkly stained, resembling tea or coffee. Most major blackwater rivers are in...
rivers, usually occurring at depths of 6-10 m and occasionally shallower or deeper, but never beyond 20 m down. It is absent from floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
channels, river edges, and small lakes. A few specimens are known from the confluence of the Rio Solimões and the Rio Negro, and from Lake Prato, Anavilhanas.
Description
One of the gymnotiformsGymnotiformes
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...
most specialized to living in deep river channels, O. tamandua resembles fishes adapted to caves in several respects. Their elongated, laterally compressed bodies are nearly unpigmented
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
, appearing bright pink due to the blood underneath. The eyes are tiny and virtually non-functional, and are placed asymmetrically on the head. The asymmetry of the eyes is not correlated with size, sex or environment
Environment (biophysical)
The biophysical environment is the combined modeling of the physical environment and the biological life forms within the environment, and includes all variables, parameters as well as conditions and modes inside the Earth's biosphere. The biophysical environment can be divided into two categories:...
, but may be related to their degenerate state. The snout is distinctive, being a long, evenly tapering, nearly straight tube measuring four times as long as high. The mouth is relatively small. The dorsal throng (a whip-like appendage used for electroreception
Electroreception
Electroreception is the biological ability to perceive natural electrical stimuli. It has been observed only in aquatic or amphibious animals, since water is a much better conductor than air. Electroreception is used in electrolocation and for electrocommunication.- Overview :Electroreception is...
) is unusually long and thick, which led it to originally be described as a "very strongly developed adipose fin" by Boulenger. The throng originates close to the rear margin of the skull, which is much more anterior than in other apteronotids.
O. tamandua has a long anal fin, a small caudal fin, and tapering pectoral fins; the dorsal
Dorsal 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...
and pelvic fins are absent. The caudal fin shows a great deal of variation due to regeneration
Regeneration (biology)
In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...
after tail loss; in some cases the regenerated fin becomes merged with the anal fin. The fin rays number 9 in the caudal fin, 14-15 in the pectoral fins, and 207-256 in the anal fin. Almost the entire body, except for the dorsal midline, is densely covered with flimsily attached scale
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
s, being small and circular towards the front and larger and more rectangular towards the back. There are around 12 rows of scales above the lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...
and 40-42 rows below. The first 5-10 scales along the lateral line are modified into overlapping elongate tubes. The maximum known size is 44 cm long and 125 g.
Biology and ecology
As in other apteronotids, O. tamandua generates a continuous weak electric fieldElectric 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...
for electrolocation and communication
Animal communication
Animal communication is any behavior on the part of one animal that has an effect on the current or future behaviour of another animal. The study of animal communication, is sometimes called Zoosemiotics has played an important part in the...
. At 452-605 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, the electric organ discharge (EOD) of O. tamandua has a much lower frequency than most other apteronotids and approaches the frequencies of the sternopygid knifefishes. The EOD is also unusual in that the waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
is monophasic
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
, with a head-negative spike followed by a positive baseline (the EOD of most apteronotids have both positive and negative spikes). The only other apteronotid known to have a monophasic EOD is the related Sternarchorhamphus muelleri. The monophasic waveform of these two genera is similar to the EOD of a larval Apteronotus
Apteronotus
Apteronotus is a genus of weakly electric knifefish in the family Apteronotidae, distinguished by the presence of a tiny caudal fin.-Species:* Black ghost knifefish, Apteronotus albifrons ....
, suggesting that it may be a paedomorphic or evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
arily ancestral trait.