Neotropical electric fish

The Gymnotiformes are a group of teleost bony fishes commonly known as the Neotropical or South American knifefish. They have long bodies and swim using undulations of their elongated anal fin. Found exclusively in fresh water, these mostly nocturnal fish are capable of producing electric fields 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).

Aside from the electric eel (Electrophorus electricus), Gymnotiformes are slender fish with narrow bodies and tapering tails, hence the common name of "knifefishes". They have neither pelvic fins nor dorsal fins, but do possess greatly elongated anal fins that stretch along almost the entire underside of their bodies. 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 forward.

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 organs that allow them to produce electricity. In most gymnotiforms, the electric organs are derived from muscle cells. However, adult apteronotids are one exception, as theirs are derived from nerve cells (spinal electromotor neurons). In gymnotiforms, the electric organ discharge may be continuous or pulsed. 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 diets. 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.

Black ghost knifefish
The black ghost knifefish (Apteronotus albifrons) is a tropical fish belong to the ghost knifefish family (Apteronotidae). They originate in freshwater habitats in South America where ranging from Venezuela to the Paraguay–Paran√° River, including the Amazon Basin. They are popular in aquaria. The fish is all black except for two white rings on its tail, and a white blaze on its nose, which can occasionally extend into a stripe down its back. It moves mainly by undulating a long fin on its underside. It will grow to a maximum length of 50 cm (20 in). It does not have scales.

Black ghost knife fish are nocturnal. They are a weakly electric fish which use an electric organ and receptors distributed over the length of their body in order to locate insect larvae.
Black ghost knifefish with its underside pointing towards the camera

The black ghost knifefish natively lives in fast moving, sandy bottom creeks in a tropical climate. South American natives believe that the ghosts of the departed take up residence in these fish, hence the name.

The black ghost knifefish is a weakly electric fish as a result of the electromotor and electrosensory systems it possesses. While some fish can only receive electric signals, the black ghost knifefish can both produce and sense the electrical impulses. Electrogenesis occurs when a specialized electric organ found in the tail of the fish generates electrical signals, which are thus called electric organ discharges (EODs). Then, for these EODs to be sensed by the fish, electroreception occurs when groups of sensory cells embedded in the skin, known as electroreceptor organs, detect the electrical change. The EODs are used for two major purposes: electrolocation and communication.

The kind of EOD produced can be used to distinguish between two types of weakly electric fish: the pulse-type and the wave-type. The black ghost knifefish are considered to be the latter type, because they can continuously generate EODs in small intervals. Wave-type EODs have a narrow power spectra, and can be heard as a tonal sound, where the discharge rate establishes the fundamental frequency. By emitting its own continuous sinusoidal train of EODs, the fish can determine the presence of nearby objects by sensing perturbations in timing and amplitude of electric fields, an ability known as active electrolocation. The particular organs used to sense the self-generated high-frequency EODs are tuberous electroreceptor organs. On the other hand, when low-frequency electric fields are generated by external sources instead of the fish itself, a different class of electroreceptor organs is used for this passive electrolocation, called ampullary organs. Therefore, the black ghost knifefish uses an active and a passive electrosystem, each with its own corresponding receptor organs. The fish can also use a mechanosensory lateral line system, which detects water disturbances created by the motion of the fish's body. As nocturnal hunters, the fish can rely on all three systems to navigate through dark environments and detect their prey.

Each species has a characteristic EOD baseline frequency range, which varies with sex and age within the species, as well. The baseline frequency is maintained to be almost constant at stable temperature, but will usually be changed due to the presence of others of the same species. Such changes in frequency relevant to social interaction are called frequency modulations (FMs). The role these FMs have in communication is significant, as black ghost knifefish have developed jamming avoidance responses, which are behavioral responses that avoid the overlapping of EOD frequencies between conspecific individuals to prevent sensory confusion. Moreover, a study was conducted that focused on sexual dimorphism in electrocommunication signals. Female black ghost knifefish generate EODs at a higher frequency than the males, an FM which can be used for gender recognition. A study found the subdominant black ghost knifefish exhibited noticeable gradual frequency rises (GFRs) in their EODs whereas the dominant fish did not, supporting the researchers' hypothesis that GFRs during communication are indicative of submissive signals.

Elephant nose
Peters' elephant-nose fish (Gnathonemus petersii; syn. Gnathonemus brevicaudatus Pellegrin, 1919, Mormyrus petersii G√ľnther, 1862) is an African freshwater elephantfish in the genus Gnathonemus. Other names in English include elephantnose fish, long-nosed elephant fish, and Ubangi mormyrid, after the Ubangi River. As the Latin name petersii confirms it is named after someone called "Peters" (probably Wilhelm Peters), although the apostrophe is often misplaced and the common name given as "Peter's elephantnose fish". It uses electrolocation to find prey, and has the largest brain-to-body oxygen use ratio of all known vertebrates (around 0.6).

Peters' elephantnose fish are native to the rivers of West and Central Africa, in particular the lower Niger River basin, the Ogun River basin and in the upper Chari River. It prefers muddy, slowly moving rivers and pools with cover such as submerged branches. It is a dark brown to black in colour, laterally compressed (averaging 23–25 cm), with a rear dorsal fin and anal fin of the same length. Its caudal or tail fin is forked. It has two stripes on its lower pendicular. Its most striking feature, as its names suggest, is a trunk-like protrusion on the head. This is not actually a nose, but a sensitive extension of the mouth, that it uses for self-defense, communication, navigation, and finding worms and insects to eat. This organ is covered in electroreceptors, as is much of the rest of its body. The elephantnose fish has poor eyesight and uses a weak electric field, which it generates with specialized cells called electrocytes, which evolved from muscle cells, to find food, to navigate in dark or turbid waters, and to find a mate. Peters' elephantnose fish live to about 6–10 years, but there are reports of them living even longer.

Electric eel
The electric eel (Electrophorus electricus) is an electric fish, and the only species in that genus. Despite the name, it is not an eel, but rather a knifefish.

The electric eel has three pairs of abdominal organs that produce electricity: the main organ, the Hunter's organ, and the Sach's organ. These organs make up four-fifths of its body, and give the electric eel the ability to generate two types of electric organ discharges: low voltage and high voltage. These organs are made of electrocytes, lined up so a current of ions can flow through them and stacked so each one adds to a potential difference.

When the eel locates its prey, the brain sends a signal through the nervous system to the electrocytes. This opens the ion channels, allowing sodium to flow through, reversing the polarity momentarily. By causing a sudden difference in electric potential, it generates an electric current in a manner similar to a battery, in which stacked plates each produce an electric potential difference.

In the electric eel, some 5,000 to 6,000 stacked electroplaques are capable of producing a shock up to 860 volts and 1 ampere of current (860 watts) for a duration of two milliseconds. Such a shock is extremely unlikely to be deadly for an adult human, due to the very short duration of the discharge. Atrial fibrillation requires that roughly 700 mA be delivered across the heart muscle for 30 ms or more, far longer than the eel is able to produce. Still, this level of current is reportedly enough to produce a brief and painful numbing shock likened to a stun gun discharge, which due to the voltage can be felt for some distance from the fish; this is a common risk for aquarium caretakers and biologists attempting to handle or examine electric eels.

The Sach's organ is associated with electrolocation. Inside the organ are many muscle-like cells, called electrocytes. Each cell can only produce 0.15 V, though the organ can transmit a signal of nearly 10 V overall in amplitude at around 25 Hz in frequency. These signals are emitted by the main organ; the Hunter's organ can emit signals at rates of several hundred hertz.

The electric eel is unique among the Gymnotiformes in having large electric organs capable of producing potentially lethal discharges that allow them to stun prey. Larger voltages have been reported, but the typical output is sufficient to stun or deter virtually any animal. Juveniles produce smaller voltages (about 100 V). They are capable of varying the intensity of the electric discharge, using lower discharges for hunting and higher intensities for stunning prey or defending themselves. They can also concentrate the discharge by curling up and making contact at two points along its body. When agitated, they are capable of producing these intermittent electric shocks over a period of at least an hour without tiring.

The electric eel also possesses high frequency-sensitive tuberous receptors, which are distributed in patches over its body. This feature is apparently useful for hunting other Gymnotiformes.

Electric eels have been used as a model in the study of bioelectrogenesis. The species is of some interest to researchers, who make use of its acetylcholinesterase and adenosine triphosphate.

Glass knifefish
The Glass Knifefish (Eigenmannia virescens) is a weakly electric freshwater fish found across South America. It is marketed as an aquarium fish.

The appendix on the body has black lines running through the bottom of the sides, with the most intense line running on the anal fin.

Like all members of its order, it is distinguished by its ability to produce electric fields. This is achieved by discharging an electric organ in the tail.