''Sternarchogiton nattereri'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of weakly electric knifefish in the family

Apteronotidae The ghost knifefishes consist of the family Apteronotidae, which are 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 specie ...

. It is native to the Amazon River system and feeds on

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

s. Unlike other members of the

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

Sternarchogiton ''Sternarchogiton'' is a genus of weakly electric knifefish in the family Apteronotidae, all living in the main channel of large rivers in the Amazon and Orinoco basins in South America. They have a blunt snout and depending on the exact speci ...

'', there is pronounced

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

in ''S. nattereri'', with reproductively mature males developing strong external teeth on tips of their jaws. These males are so different from the females and juveniles that they were thought to be a different genus and species, the "tooth-lip knifefish" ''Oedemognathus exodon'', for over 40 years.

Distribution and habitat

''S. nattereri'' is restricted to large river channels in the lowlands of the

Amazon basin The Amazon basin is the part of South America drained by the Amazon River and its tributary, tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries ...

. Its habitat ranges from high- conductivity,

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

-rich whitewater rivers such as the Rio Solimões, to low-conductivity, sediment-poor blackwaters such as the Rio Negro. It is one of the most common gymnotiforms in Rio Solimões.

Description

''S. nattereri'' has a laterally compressed, knife-shaped body. The dorsal profile of the head is sharply curved from the

nape The nape is the back of the neck. In technical anatomical/medical terminology, the nape is also called the nucha (from the Medieval Latin rendering of the Arabic , ). The corresponding adjective is ''nuchal'', as in the term ''nuchal rigidity'' ...

to the snout. The mouth is terminal, with the upper jaw longer than the lower. The

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

is as long as deep, bearing a row of 7–8 conical teeth on the posterior portion, and there are upper and lower pharyngeal tooth plates bearing 9–11 and 7–9 teeth respectively. The eyes are small and covered by translucent skin. The long

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

contains 180–198

rays Ray or RAY may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), the bony or horny spine on ray-finned fish Science and mathematics * Half-line (geometry) or ray, half of a line split at an ...

. The

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

s are broad and pointed with 13–15 rays each, and the lanceolate

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

contains 11–15 rays and is almost completely covered with

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...

. There is a whip-like dorsal appendage anchored into a groove along the back by many fine threads of

tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...

. In reproductively mature males, the head becomes elongated and prominent external teeth develop on the upper and lower jaws. Males with teeth tend to have larger

gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

s than those without. The coloration is a uniform pale white with a pinkish hue caused by subdermal

capillaries A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the in ...

, especially over anal fin pterygiophores (fin support bones). The operculum is pink due to the presence of the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s underneath. The dorsal surface is gray, with tuberous

electroreceptors Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes, such ...

visible as white dots. All the fins are

hyaline A hyaline substance is one with a glassy appearance. The word is derived from , and . Histopathology Hyaline cartilage is named after its glassy appearance on fresh gross pathology. On light microscopy of H&E stained slides, the extracellula ...

, without dark

chromatophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopod A cephalopod is any member o ...

s. The maximum known length is , with males tending to be larger than females.

Etymology

The fish is named in honor of

Johann Natterer Johann Natterer (9 November 1787 – 17 June 1843) was an Austrian natural history, naturalist and List of explorers, explorer. He was the son of royal falconer Joseph Natterer and along with his brother Joseph Natterer (1786–1852) took a keen i ...

Biology and ecology

The diet of ''S. nattereri'' consists of freshwater

s growing on submerged trees, stumps, and other woody debris. Before they were recognized to be the males of ''S. nattereri'', "''Oedemognathus exodon''" was speculated to use its

dentition Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology ...

to feed on fish scales. The external teeth of male ''S. nattereri'' are now thought to be used in combat for

territory A territory is an area of land, sea, or space, belonging or connected to a particular country, person, or animal. In international politics, a territory is usually a geographic area which has not been granted the powers of self-government, ...

or mates; scars and scratches have been found on the napes of several specimens. In the

Tefé Tefé, known in early accounts as Teffé, is a Municipalities of Brazil, municipality in the state of Amazonas (Brazilian state), Amazonas, northern Brazil. History As early as 1620 the Portuguese Carmelites could already boast of converts amo ...

region of Brazil, breeding occurs during the rising water period, with adults

spawning Spawn is the Egg cell, eggs and Spermatozoa, sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of freely releasing eggs and sperm into a body of water (fresh or marine); the physical act is ...

in floating meadows along the edges of paraná channels (side branches of whitewater rivers that run through adjacent várzea

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...

s). As water levels rise further and

dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can ...

levels drop, the growing juveniles eventually disperse out of these floodplains within floating rafts of vegetation. Like other apteronotids, ''S. nattereri'' produces a continuous, weak

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

for the purposes of

electrolocation Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes, such ...

and

communication Communication is commonly defined as the transmission of information. Its precise definition is disputed and there are disagreements about whether Intention, unintentional or failed transmissions are included and whether communication not onl ...

; this

electric organ discharge An Torpediniformes.html" ;"title="electric ray (Torpediniformes">electric ray (Torpediniformes) showing location of paired electric organs in the head, and electrocytes stacked within it In biology, the electric organ is an organ (biology), or ...

(EOD) has a

fundamental frequency The fundamental frequency, often referred to simply as the ''fundamental'' (abbreviated as 0 or 1 ), is defined as the lowest frequency of a Periodic signal, periodic waveform. In music, the fundamental is the musical pitch (music), pitch of a n ...

between 732–1465 Hz, one of the largest ranges in the Apteronotidae. The

waveform In electronics, acoustics, and related fields, the waveform of a signal is the shape of its Graph of a function, graph as a function of time, independent of its time and Magnitude (mathematics), magnitude Scale (ratio), scales and of any dis ...

of this discharge has 2–4

phases Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...

and exhibits significant variation amongst individuals, but there is no obvious sexual dimorphism. ''S. nattereri'' is additionally able to modulate the frequency and

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

of its EOD to produce communication signals called "chirps" (short-duration, high-frequency signals) and "gradual frequency rises" (long-duration, low-frequency signals). The frequency elevation of ''S. nattereri'' chirps ranges between 50 and 300 Hz above baseline, with a maximum of 500 Hz.

References

{{Taxonbar, from=Q6406160 nattereri Weakly electric fish Fish of the Amazon basin Knifefish of Brazil Freshwater fish of Peru Freshwater fish of Venezuela Taxa named by Franz Steindachner Fish described in 1868