Electric organ (biology)
An effector organ found in six different groups of fishes; output is an electric pulse. Voltages large enough to aid in prey capture or predator deterrence are produced by various strongly electric fishes. These include the electric eel (Electrophorus electricus) from South America; the electric catfish (Malapterurus electricus) from Africa; the family of electric rays, Torpedinidae, which are widely distributed in the world’s oceans; and possibly the stargazer genus, Astrosco pus, of the western Atlantic. Weakly electric fishes emit a lower voltage, the energy source for an active electrosensory system that monitors electrical impedance in the environment. These weak signals also serve in intra- and interspecific communica tion. There are three groups of weakly electric fishes. First, the South American knifefishes, the Gymnotiformes, are a large and diverse group of several families that also include Elect rophorus. Second, the electrically active African Mormyriformes are com posed of the numerous species of the family Mormyridae and the single species Gymnarchus niloticus in the family Gymnar chidae. Finally, many species of skates and rays of the family Rajidae occur in marine waters around the world.
The strongly electric fishes are remarkable for the high voltage and power of their discharges. For example, the electric eelgen erates pulses in excess of 500 V. In contrast, a large torpedo generates a smaller voltage, about 50 V in air, but the maximum current is larger, and the peak pulse power can exceed 1 kW (or about 1 hp). The electric organs may constitute a substantial fraction of the body mass. The weakly electric fishes have much smaller organs emitting pulses of a few tenths of a volt to sev eral volts; such amplitudes that are still large compared to those recorded outside ordinary, nonelectric fishes.
Electric organ discharge is explicable in terms of the properties of ordinary excitable cells, that is, those of nerve and muscle. The single generating cells of electric organs are called electrocytes. They are modified striated muscle fibers that have lost the abil ity to contract, although they retain muscle filaments to varying degrees, and they produce electric signals as muscle fibers do.
A question often asked about the strongly electric fishes is how they keep from electrocuting themselves. When immersed in water, they do not appear to be at all affected by their own discharges, although their electroreceptors are undoubtedly acti vated. In air, when the organ is not loaded down by the conduc tivity of the water, they often do twitch when they discharge their organs, but as would be expected, they are much more resistant than other fishes. One factor is that their nerves and central ner vous system are well insulated by many layers of connective and fatty tissue, Whether their brains are more resistant to stunning by a given current density has not been determined; certainly the excitability of individual cells is no different from that in animals in general.