Many Old Earth fish, particularly members of the chondricthyes (sharks and rays) and the teleosts (bony fish), had an electrical sense, usually for navigation or the detection of prey or predators. A minority of unrelated groups, 11 families in all, with fewer than 400 species, developed the ability to generate an electrical field as well. This field enabled them to detect other objects or organisms in the darkness, or in muddy water, or beneath a cover of sand and mud, within a radius of a metre or more. They did this either by picking up the natural emissions of a life form, or by detecting small changes in their own field produced by the varying conductivity of different materials.
The organs of detection and the organs of electrical generation were various, since this trait evolved separately more than once. Detection organs were usually derived from the lateral line, a system that detects sounds and currents in the water. The electrogenic tissue involved stacks of flattened cells, called electroplaques or electrocytes, derived from muscle or nerve tissue. All electrogenic fish held the body rigid, apparently to simplify interpretation of their electrical field. They moved about with small motions of their fins, or by using ripples of a single long fin along the dorsal or ventral surface, or (as in rays) by ripples of two long fins on either side of the body. This meant that they were very maneuverable but usually not very quick. Usually the electrogenic organ was quite weak, generating less than 4 volts: just sufficient for use as a detection and communication device. Weakly electrogenic fish included mormyrids, African "knife" fishes, the unrelated South American "knife" fishes, and a number of other more obscure groups. In a few cases the basic electrogenic ability was greatly enhanced (sometimes the organs involved accounted for more than a third of body mass), and was used to stun prey or predators. In freshwater such fish (notably the electric "eel" of South America and the electric catfish of Africa) used very long stacks of electrocytes to generate the extremely high voltages required to achieve significant currents; their marine equivalents (stargazers, and various species of torpedo ray) achieved similar amperage by linking larger numbers of shorter stacks of electrocytes in parallel.
Most species were exterminated directly or indirectly by humans in the terminal decades of the Information Age, and survived only in artificial environments or genetic banks. It is believed that GAIA has restored most of these to their original habitats in the time since the Great Expulsion.
Subsophont splices of the original species are fairly common in the Terragen sphere, partly because of their novelty value and partly because their ability to detect and generate electrical fields has applications for those who prefer biotech to drytech. In a few cases versions of the strongly electrogenic species are used as a sentient element in security systems.
A very few Terragen provolves have been produced from electrogenic fish. Often they are clades adapted to conditions of extreme darkness, such as deep sea habitats on Gaian worlds or the seas of Europan worlds. Some terragen rianth and splice clades, especially aquatic clades, carry grafts of genes from electrogenic fish either as part of their sensory equipment, but it does place constraints on body design. Strongly electrogenic traits have been incorporated by a minority of clades, including a few human-derived clades, but the size and physiological cost of such a weapon is prohibitive. Still, fads for "shocker" gene grafts do arise from time to time.