How Does a Fish Make Electricity? Muscles That Gave Up Moving

A muscle cell generates a small voltage across its membrane when stimulated, which normally produces contraction. In these fish, cells derived from muscle have lost the contractile machinery and retained the electrical part, becoming flattened discs stacked in columns like cells in a battery. Each disc produces a small voltage, and because they are stacked and fire together the voltages add, so a column of several thousand produces a substantial total. Nerve impulses trigger the whole stack simultaneously, which requires precise timing since the nerves reaching different parts differ in length, and the fish achieves that by varying nerve thickness so that signals arrive together.

Most of these species produce weak fields and use them for information rather than force:

• •Sensing objects, since anything nearby distorts the field measurably
• •Navigating in muddy water and at night where vision is useless
• •Recognising species and individuals from the pattern of discharge
• •Courtship and aggression signals between individuals
• •Detecting prey by the distortion a living body produces
• •In a small number of species, stunning prey or deterring predators outright

Three groups produce discharges powerful enough to be used as weapons. The electric eel of South America, which is a knifefish rather than an eel, produces the strongest known, with recorded discharges exceeding eight hundred volts in one recently described species, and it uses them both to stun prey and to cause involuntary muscle contraction that makes hidden prey reveal itself. Electric rays produce lower voltages at higher current and were known to ancient Mediterranean physicians, who applied them deliberately to patients for headache and gout, which is a documented early use of electricity in medicine. The electric catfish of Africa is the third. All three also produce weak discharges for sensing, so the strong capability is an addition rather than a replacement.

These animals have had a disproportionate role in the history of science and medicine. Rays were used therapeutically in the classical Mediterranean, applied living to the head or the foot to numb pain, which is described in Roman medical texts. The stacked structure of the electric organ directly inspired Alessandro Volta, who named his stack of metal discs after the animal in 1800 and produced the first battery. Nineteenth century physiologists used them to establish that nerve signals are electrical. Modern neuroscience uses the electric organ of rays as a source of the molecular machinery of nerve transmission in usable quantity, since the tissue is packed with exactly the components that are scarce elsewhere, and several fundamental results came from that material.

Independent evolution of the same capability at least six times in unrelated lineages indicates that the raw materials are readily available and the advantage is substantial. The materials are there because every animal already has muscle cells producing voltages and nerves controlling them, so the modification required is a loss of contractile function rather than an invention. Genetic work comparing the lineages has found that the same small set of genes was recruited in each case, which explains how the transition happened repeatedly. The advantage is largest in murky tropical fresh water, where vision is nearly useless and where most of these fish live, and the sensing capability works in complete darkness at short range where nothing else does.

Cells derived from muscle lost the contractile machinery and kept the electrical part, stacking into columns whose voltages add, fired simultaneously by nerves tuned so signals arrive together. Most species produce weak fields for navigation, prey detection and signalling in muddy water. Electric eels exceed eight hundred volts, and rays were applied to patients by ancient physicians. The capability arose independently at least six times from the same set of genes.