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Fishes are the only animals known to possess specific electricity-producing organs. There are more than 400 species of fishes in total, representing nine families that have been determined to produce and detect electrical fields. In addition, other species that don't produce electrical currents can detect such phenomena from other living and non-living sources. This sense and capacity for electricity-related behavior is important to these fishes captive husbandry.
Known as electrogenic, or electricity-producing fishes, this evolutionarily diverse assemblage includes chondrichthyous (cartilaginous) fishes, such as electric skates and rays, as well as advanced bony fishes (teleosts) such as knifefishes and mormyrids. Among this latter group are some favourite "odd-ball" aquarium species: elephant noses, baby whales, black ghosts and other South American knifefishes. Some species- the electric "eel" and electric catfish for example- must be kept solitarily and are dangerous even to aquarists if mishandled.
Some Strongly Electrogenic Fishes:
Weakly Electrogenic Fish Species:
The vast majority of electrogenic fishes use their electrical power for social communication, prey detection, navigation and spatial orientation. These weakly electrogenic species can generate 1 to 4 volts, but usually generate much weaker fields. This is in sharp contrast to the electric "eel" which can produce more than 500 volts. Such high-powered discharges occur infrequently and are used to stun prey or deter enemies and predators.
Fishes that emit weak electrical signals usually do so continuously. Each species generates an electrical discharge that varies in frequency and that may be either a pulse or wave depending on the relative duration of the signal and the time between signals.
Weakly electrogenic fishes generate an electrical field about their bodies similar to that seen when iron filings are scattered around a magnet. Any object having conductivity different than the surrounding water, such as living tissues, will disturb the original configuration of the field. These distortions are sensed by special organs distributed in the skin or by sensors in the brain.
Diagram of an electric field:
The ability to detect objects and organisms appears to be limited to a distance of a few feet maximum. Nonetheless, this represents an ability to detect variations in an electric field of 0.03 microvolt per centimeter! For comparison, this is the equivalent of detecting a flashlight batter at several thousand meters. Sensitive indeed.
Observation of mormyrids and knifefishes in aquariums reveals that they often appear "rigid" from head to tail. It is believed that this posture reduces the complexity of the electrical field that the fish must interpret. Take a look and you will see that these fishes often approach an object. Tail first, and may wrap themselves around an object, probing for openings. Computer simulations demonstrate that this behaviour enhances the fishes' electrical "image" of the object.
Some Weakly Electrogenic Fish Species:
Electricity-producing organs are as diverse as the groups of fishes that possess them. These organs vary in location, physiology, size, and even evolutionary origin. The electrogenic organs are generally derived from muscle tissue, but may also be found in glandular or nervous system tissues. The organs are commonly located on the tail, fins or under the skin, but may be found in other places as well.
The basic functional unit of an electricity-producing organ is the electroplax (also spelled electroplaque), a box-like, multi-nucleated cell. The organ itself consists of thousands of these cells. In a resting state, each electroplax has a negative charge on the inside and a positive charge on the outside (see illustration'¦ to be done'¦ below). When the organ is innervated/fired by its nerves, the electric potential, which is the difference in the degree of charge between the inside and the outside, momentarily reverses.
How an electroplax works:
There are two measures of electricity strength: Volts and Amperes. Amperes represent the amount of current (number of electrons) flowing from point to point, whereas volts represent the (electro-motive) driving force behind the amperes. Multiplying amperage (the number of electrons) time's voltage (the power per group of electrons) is what we call wattage, or the watts produced. Electroplax cells may be arranged in series, in which the positive end of one cell is next to the negative end of the adjacent cell, or in parallel, in which the negative ends are adjacent and positive ends are adjacent.
When the cells are in series, they yield the maximum voltage, and when in parallel, they give the maximum amperage. As you might assume, when a fish grows larger, the amount of electricity generated increases in proportion. Another significant factor in the firing of cells is the water temperature. In colder water, the cells work slower and there is a longer period of time between discharges.
Active and Passive Electroceptors:
Fishes that produce weak fields of electricity are sometimes referred to as "active" electroreceptors because they can detect changes in the fields they themselves create. There is another form of electroreception that is termed "passive". Fishes that are passive electroreceptors don't generate electrical fields but can detect them from external sources, which include other organisms as well as non-living charged sources like water of ionic content in motion, and even atmospheric and geologic processes. The popular glass catfish, Kryptopterus bicirrhis is an example of a passive electroreceptor.
Electroreceptive pit organs, which detect electrical signals, are located in and associated with the lateral line, a series of openings, receptive cells and nerves one can often see running down horizontally on the sides and heads of fishes. These lateralis system is sensitive to pressure and sound waves, and in electroreceptive fishes, to electrical fields.
Functions of Electroreception:
Electroreception has numerous functions. At relatively short distances it provides detection of the size, distance and shape of other organisms (including sex, social status, temperament'¦) and conductive objects. At longer ranges in aids in navigation.
Electrocommunication, with the use of constant signals, yields information relating to location, species, gender and individual identity of other organisms, particularly conspecifics (members of the same species). When, as often occurs in the wild, a species is clustered in proximity, a small shift in the frequencies of signals occurs. This is known as a "jamming avoidance response", which allows all individuals to continue to use electrocommunication. Different species that occur in the same location have no signal overlap, and sue entirely different frequency ranges. Signals also change in response to food, threat submission, attack and mating.
Strongly electrogenic fishes, both freshwater (like the electric cat and eel) and marine (four families of rays), can use their large voltage production to stun prey to the point of immobility, or similarly zap unwary aquarists.
Many electric fishes make interesting pets. The mormyrids for example, which include the popular elephantnoses, have brain size to body weight rations similar to humans. They seem to display a rich repertoire of play behaviour and will amuse themselves for hours with a bit of aluminum foil. While electric knifefishes and mormyrids are worthy aquarium inhabitants, it is best to avoid the electric catfishes and eel, which are larger and capable of being dangerously shocking.
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