How Do Electric Eels Make Electricity? Electrocytes Explained

What are electrocytes?

Electrocytes are flat, disk-shaped cells that evolved from ordinary muscle cells. They still contain some muscle proteins, such as actin and desmin, but these no longer form contracting fibers. Instead, each cell has become a tiny generator.

Like nerve and muscle cells, an electrocyte keeps a small electrical charge across its membrane by pumping sodium and potassium ions in opposite directions, using energy from the molecule ATP. At rest, both faces of the cell carry the same charge, so the cell as a whole produces nothing.

Only one face of each electrocyte, the side facing the tail, is connected to nerves. When the eel decides to fire, a command center in its brainstem sends a signal down the spinal cord. Nerve endings release the chemical acetylcholine onto the electrocytes, and ion channels open on the nerve side only. Sodium ions rush in, flipping the charge on that face. For an instant, the two faces of the cell have different voltages, about 0.15 volts apart, and the cell acts like a small battery. Potassium ions then flow out, and the cell resets in a few thousandths of a second.

How do tiny cells make hundreds of volts?

The secret is how the electrocytes are arranged. In the main electric organ, around 6,000 of them are lined up one after another along the length of the body, all facing the same way. Connected like this, in series, their voltages add together, just as batteries stacked end to end in a flashlight do: 6,000 cells multiplied by about 0.15 volts gives roughly 900 volts.

Dozens of these columns run side by side along each half of the body. Columns arranged in parallel do not raise the voltage further, but they let the organ deliver more current. During a strong discharge, the current can reach about 1 ampere for a moment. For this to work, the cells in a column must all fire within a fraction of a millisecond, and the eel’s nervous system is tuned to make that happen.

In the 2019 study that split electric eels into three species, the strongest discharge measured was 860 volts, in Volta’s electric eel (Electrophorus voltai). That is the highest voltage known from any animal. More on what such shocks can do is in how strong is an electric eel shock?

The three electric organs

The electric eel has three pairs of electric organs, which together fill about 80 percent of its body. The vital organs are crowded into the front fifth.

• Main organ: the largest, along the upper part of the body for most of its length. It produces the high-voltage shocks.
• Hunter’s organ: below the main organ, toward the belly. It helps with strong discharges and may coordinate them, though its exact role is still debated.
• Sachs’ organ: toward the tail. It produces weak pulses of about 10 volts used for sensing.

The organs grow in a fixed order as a young eel develops, starting with the main organ. By the time the fish is about 23 cm (9 in) long, all three are in place.

Low-voltage pulses: sensing with electricity

Electric eels live in muddy water and hunt mainly at night, and their eyes are small and weak. To find their way, they send out weak pulses from Sachs’ organ, creating an electric field around the body. Anything that conducts electricity better or worse than water, such as a fish, a rock or a plant stem, distorts this field.

Electroreceptors in the skin, scattered in patches over the body and concentrated on the head, detect these distortions. The result is a kind of electric picture of the surroundings, a sense called electrolocation. Electric eels also use weak pulses to communicate, and possibly to find mates.

High-voltage shocks for hunting and defense

Strong shocks come in volleys of very short pulses, each lasting about 2 milliseconds, fired at up to several hundred per second. Research by biologist Kenneth Catania showed that these pulses work through the prey’s own nervous system: they trigger the nerves that control the prey’s muscles, much like a police stun device.

A pair or trio of quick pulses makes a hidden fish twitch, giving away its location. A full volley then locks all of the prey’s muscles at once, freezing it for a moment while the eel sucks it in. When dealing with large or struggling prey, an electric eel can curl its body so that its positive head and negative tail are on opposite sides of the victim, concentrating the electric field.

For defense, an eel may leap partly out of the water and press its chin against a threat while discharging. Out of the water, the current cannot spread away, so more of it passes through the target.

How do electric eels avoid shocking themselves?

Scientists still do not fully understand this. Several features probably help. The brain, heart and other vital organs are packed into the front of the body, away from the electric organs. In water, most of the current flows out of the eel’s head end and back to its tail through the surrounding water, which offers an easier path than the eel’s own tissues. The large size of the fish also spreads the current over a wide area.

The eel’s discharges are not free, though. Producing long volleys uses a great deal of energy, and an eel that has fired many strong shocks needs rest and food to recover.

Other fish that make electricity

Electric organs have evolved independently at least six times in fish, a classic example of convergent evolution that Charles Darwin discussed in On the Origin of Species. Strongly electric fish include torpedo rays, African electric catfish and stargazers. Most electric fish, however, are weakly electric, including the other South American knifefishes and the African elephantfishes, which use low-voltage signals only to sense and communicate.

Saltwater conducts electricity far better than fresh water. Marine torpedo rays therefore deliver shocks at lower voltage but higher current, while freshwater electric eels need high voltage to push current through the water. For why the electric eel is grouped with knifefishes and not true eels, see is an electric eel an eel?