Electric Organs

in some fishes, paired organs capable of generating electric discharges for purposes of defense, attack, signaling within species, and orientation in space. Electric organs evolved in unrelated groups of freshwater and marine fishes. Common in fossil fishes and invertebrates, they are now found in more than 300 recent species. The location, shape, and structure of the organs vary among different species. They may lie symmetrically along the sides of the body in the form of kidney-shaped structures (electric rays, electric eels), a thin subcutaneous layer (electric catfish), or cylindrical filamentous structures (mormyrids and gymnotids). Sometimes the organs occur in the infraorbital space (American stargazer).

The electric organs may constitute about one-sixth (electric rays) to one-fourth (electric eels and electric catfish) of the total body weight. Each organ consists of numerous columns of electric plates, or electroplaxes, which are flattened muscle, nerve, or glandular cells whose membranes are electric generators. The number of electroplaxes and columns varies from species to species. The electric ray has about 600 columns, which resemble beehives and bear 400 electroplaxes each. The electric eel has 70 horizontally arranged columns, each with 6,000 electroplaxes. The approximately 2 million electroplaxes of the electric catfish are scattered about in no discernible pattern. The electroplaxes in each column are successively joined together, whereas the electric columns are parallel to each other.

Electric organs are innervated by branches of the vagus, facial, and glossopharyngeal nerves that approach the electronegative side of the electroplaxes. The difference in potential at the tips of the organs may be as much as 1,200 volts (v) in the electric eel, with the power of a discharge in an impulse ranging from 1 to 6 kilowatts (Torpedo occidentalis). The discharges are emitted as a series of volleys whose shape, duration, and order depend on the intensity of excitation and on the species of fish. The frequency of the pulses is related to their purpose: For example, the electric ray emits ten to 12 defensive pulses per second and 14 to 562 “hunting” pulses per second (depending on the size of the prey). The voltage varies from 20 v in the electric ray to 600 v in an electric eel. The current varies from 0.1 amperes in an electric catfish to 50 amperes in an electric ray.

Fishes equipped with electric organs can withstand voltages that kill fishes lacking the organs. The electric eel, for example, can survive a discharge of 220 v. The electric discharges of large fishes are dangerous to man.