Sternarchogiton nattereri is a species

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

Gymnotiformes

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...

 in the family Apteronotidae. It is native to 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 and feeds on sponges. Unlike other members of the genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Sternarchogiton

Sternarchogiton

Sternarchogiton is a genus of weakly electric knifefish in the family Apteronotidae, with four known species. They are distinguished from other apteronotid genera by details of the skull and the broad descending blades of the pterygiophores in the anal fin...

, there is pronounced sexual dimorphism

Sexual dimorphism

Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

 in S. nattereri, with reproductively mature males developing strong external teeth on tips of their 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. These male

Male

Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

s are so different from the female

Female

Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

s and juveniles

Juvenile (organism)

A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...

 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

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 channels in the lowlands of the Amazon basin

Amazon Basin

The Amazon Basin is the part of South America drained by the Amazon River and its tributaries that drains an area of about , or roughly 40 percent of South America. The basin is located in the countries of Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, and Venezuela...

. Its habitat

Habitat

* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

 ranges from high-conductivity, sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

-rich whitewater

Whitewater

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...

s such as the Rio Solimões, to low-conductivity, sediment-poor blackwaters

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...

 such as the Rio Negro. It is one of the most common gymnotiforms in Rio Solimões.

Description

S. nattereri has a laterally compressed, knife

Knife

A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

-shaped body

Body

With regard to living things, a body is the physical body of an individual. "Body" often is used in connection with appearance, health issues and death...

. The dorsal profile of the head

Head

In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....

 is sharply curved from the nape

Nape

The nape is the back of the neck. In technical anatomical/medical terminology, the nape is referred to by the word nucha, which also gives the adjective corresponding to "nape" in English, "nuchal"....

 to the 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...

. The mouth

Mouth

The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

 is terminal, with the upper jaw longer than the lower. The dentary 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

Pharyngeal teeth

Pharyngeal teeth are teeth in the pharyngeal arch of the throat of cyprinids, suckers, and a number of other fish species lacking teeth.Popular aquarium fish such as goldfish and loaches have these structures. Members of the Botia genus such as clown loaches are known to make distinctive clicking...

 bearing 9–11 and 7–9 teeth respectively. The eye

Eye

Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

s are small and covered by translucent skin

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

. The long anal fin contains 180–198 rays. The pectoral fins are broad and pointed with 13–15 rays each, and the lanceolate caudal fin contains 11–15 rays and is almost completely covered with scales

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...

. There is a whip

Whip

A whip is a tool traditionally used by humans to exert control over animals or other people, through pain compliance or fear of pain, although in some activities whips can be used without use of pain, such as an additional pressure aid in dressage...

-like dorsal appendage anchored into a groove along the back by many fine threads of tendon

Tendon

A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...

.

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

Gonad

The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s than those without. The coloration is a uniform pale white with a pinkish hue caused by subdermal capillaries, especially over anal fin pterygiophores (fin support bones). The operculum

Operculum (fish)

The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....

 is pink due to the presence of the gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s underneath. The dorsal surface is gray, with tuberous electroreceptors

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...

 visible as white dots. All the fins are hyaline

Hyaline

The term hyaline denotes a substance with a glass-like appearance.-Histopathology:In histopathological medical usage, a hyaline substance appears glassy and pink after being stained with haematoxylin and eosin — usually it is an acellular, proteinaceous material...

, without dark chromatophore

Chromatophore

Chromatophores are pigment-containing and light-reflecting cells found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye colour in cold-blooded animals and are generated in the neural crest during embryonic development...

s. The maximum known length is 25 cm, with males tending to be larger than females.

Biology and ecology

The diet

Diet (nutrition)

In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...

 of S. nattereri consists of freshwater

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 sponges growing on submerged tree

Tree

A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...

s, stumps, and other wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

y debris. Before they were recognized to be the males of S. nattereri, "Oedemognathus exodon" was speculated to use its dentition

Dentition

Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...

 to feed on fish scales

Lepidophagy

Lepidophagy is a specialised feeding behaviour in fish that involves eating of scales of other fish. Lepidophagy is widespread, having been independently evolved in at least five freshwater families and seven marine families...

. The external teeth of male S. nattereri are now thought to be used in combat for territory

Territory (animal)

In ethology the term territory refers to any sociographical area that an animal of a particular species consistently defends against conspecifics...

 or mates

Mating

In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...

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

In the Tefé

Tefé

Tefé is a city and a municipality in the state of Amazonas in Brazil. Its population was 70,809 as of 2005 and its area is 23,704 km². It is located about 500 km to the west of Manaus on the south bank of the Rio Solimões....

 region of Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

, breeding

Breeding in the wild

Breeding in the wild is the natural process of animal reproduction occurring in the natural habitat of a given species. This terminology is distinct from animal husbandry or breeding of species in captivity...

 occurs during the rising water period, with adults spawning

Spawn (biology)

Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...

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

s). As water levels rise further and dissolved oxygen levels drop, the growing juveniles

Juvenile (organism)

A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...

 eventually disperse out of these floodplains within floating rafts of vegetation

Vegetation

Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...

.

Like other apteronotids, S. nattereri produces a continuous, weak 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...

 for the purposes of 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...

; this electric organ discharge (EOD) has a fundamental frequency

Fundamental frequency

The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the lowest frequency of a periodic waveform. In terms of a superposition of sinusoids The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the...

 between 732–1465 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....

, one of the largest ranges in the Apteronotidae. 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...

 of this discharge has 2–4 phases

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:...

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

Amplitude

Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

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.