Electric fish

thumb |upright=2|Among the electric fishes are [[electric eels, knifefish capable of generating an electric field, both at low voltage for electrolocation and at high voltage to stun their prey.]]

An electric fish is any fish that can generate electric fields, whether to sense things around them, for defence, or to stun prey. Most fish able to produce shocks are also electroreceptive, meaning that they can sense electric fields. The only exception is the stargazer family (Uranoscopidae). Electric fish, although a small minority of all fishes, include both oceanic and freshwater species, and both cartilaginous and bony fishes.

Electric fish produce their electrical fields from an electric organ. This is made up of electrocytes, modified muscle or nerve cells, specialized for producing strong electric fields, used to locate prey, for defence against predators, and for signalling, such as in courtship. Electric organ discharges are two types, pulse and wave, and vary both by species and by function.

Electric fish have evolved many specialised behaviours. The predatory African sharptooth catfish eavesdrops on its weakly electric mormyrid prey to locate it when hunting, driving the prey fish to develop electric signals that are harder to detect. Bluntnose knifefishes produce an electric discharge pattern similar to the electrolocation pattern of the dangerous electric eel, probably a form of Batesian mimicry to dissuade predators. Glass knifefish that are using similar frequencies move their frequencies up or down in a jamming avoidance response; African knifefish have convergently evolved a nearly identical mechanism.

Evolution and phylogeny

All fish, indeed all vertebrates, use electrical signals in their nerves and muscles. Cartilaginous fishes and some other basal groups use passive electrolocation with sensors that detect electric fields; the platypus and echidna have separately evolved this ability. The knifefishes and elephantfishes actively electrolocate, generating weak electric fields to find prey. Finally, fish in several groups have the ability to deliver electric shocks powerful enough to stun their prey or repel predators. Among these, only the stargazers, a group of marine bony fish, do not also use electrolocation.

In vertebrates, electroreception is an ancestral trait, meaning that it was present in their last common ancestor. Most common bony fish are non-electric. There are some 350 species of electric fish.

Electric organs have evolved eight times, four of these being organs powerful enough to deliver an electric shock. Each such group is a clade.

Actively electrolocating fish are marked on the phylogenetic tree with a small yellow lightning flash 15px. Fish able to deliver electric shocks are marked with a red lightning flash 12px. Non-electric and purely passively electrolocating species are not shown.]]

Weakly electric fish generate a discharge that is typically less than one volt. These are too weak to stun prey and instead are used for navigation, electrolocation in conjunction with electroreceptors in their skin, and electrocommunication with other electric fish. The major groups of weakly electric fish are the Osteoglossiformes, which include the Mormyridae (elephantfishes) and the African knifefish Gymnarchus, and the Gymnotiformes (South American knifefishes). These two groups have evolved convergently, with similar behaviour and abilities but different types of electroreceptors and differently sited electric organs. and navigation. The electric eel, even when very small in size, can deliver substantial electric power, and enough current to exceed many species' pain threshold. Electric eels sometimes leap out of the water to electrify possible predators directly, as has been tested with a human arm.

Electric organ

Anatomy

thumb|upright=2.5|Anatomy of a strongly electric freshwater fish: the electric eel's three [[Electric organ (biology)|electric organs. The main organ is long, with a stack of many electrocytes in series to provide a high voltage, matching the high impedance of freshwater.]]

Electric organs vary widely among electric fish groups. They evolved from excitable, electrically active tissues that make use of action potentials for their function: most derive from muscle tissue, but in some groups the organ derives from nerve tissue. The organ may lie along the body's axis, as in the electric eel and Gymnarchus; it may be in the tail, as in the elephantfishes; or it may be in the head, as in the electric rays and the stargazers.

Physiology

thumb|An [[electric ray (Torpediniformes) showing paired electric organs in the head, and electrocytes stacked vertically within it]]

Electric organs are made up of electrocytes, large, flat cells that create and store electrical energy, awaiting discharge. The anterior ends of these cells react to stimuli from the nervous system and contain sodium channels. The posterior ends contain sodium–potassium pumps. Electrocytes become polar when triggered by a signal from the nervous system. Neurons release the neurotransmitter acetylcholine; this triggers acetylcholine receptors to open and sodium ions to flow into the electrocytes. Many electric fishes also use EODs for communication, while strongly electric species use them for hunting or defence.

Sexual behaviour

In sexually dimorphic signalling, as in the brown ghost knifefish (Apteronotus leptorhynchus), the electric organ produces distinct signals to be received by individuals of the same or other species. The electric organ fires to produce a discharge with a certain frequency, along with short modulations termed "chirps" and "gradual frequency rises", both varying widely between species and differing between the sexes. For example, in the glass knifefish genus Eigenmannia, females produce a nearly pure sine wave with few harmonics, males produce a far sharper non-sinusoidal waveform with strong harmonics.

Antipredator behaviour

Electric catfish (Malapteruridae) frequently use their electric discharges to ward off other species from their shelter sites, whereas with their own species they have ritualized fights with open-mouth displays and sometimes bites, but rarely use electric organ discharges.

The electric discharge pattern of bluntnose knifefishes is similar to the low voltage electrolocative discharge of the electric eel. This is thought to be a form of bluffing, a Batesian mimicry of the powerfully protected electric eel.

Fish that prey on electrolocating fish may "eavesdrop" on the discharges of their prey to detect them. The electroreceptive African sharptooth catfish (Clarias gariepinus) may hunt the weakly electric mormyrid, Marcusenius macrolepidotus in this way. This has driven the prey, in an evolutionary arms race, to develop more complex or higher frequency signals that are harder to detect.

Jamming avoidance response

thumb|upright=1.9|When a [[glass knifefish encounters a neighbour with a closely similar frequency, one fish shifts its frequency upward and the other downward in the jamming avoidance response.]]

It had been theorized as early as the 1950s that electric fish near each other might experience some type of interference. In 1963, Akira Watanabe and Kimihisa Takeda discovered the jamming avoidance response in Eigenmannia. When two fish are approaching one another, their electric fields interfere. This sets up a beat with a frequency equal to the difference between the discharge frequencies of the two fish. A similar jamming avoidance response was discovered in the distantly related Gymnarchus niloticus, the African knifefish, by Walter Heiligenberg in 1975, in a further example of convergent evolution between the electric fishes of Africa and South America. Both the neural computational mechanisms and the behavioural responses are nearly identical in the two groups.