High-Tech Fish

By Jack Myers, Senior Science Editor

Of the world’s thousands of kinds of fish, a few hundred are electric fish. I call them high-tech because just studying them requires some of our most advanced technology. These fish are found in the river systems of Africa and South America.

Electric fish have a wide range of electrical abilities. There is an electric eel that can stun its prey with a shock of five hundred volts. It is called a strongly electric fish. More common are the many kinds of weakly electric fish, which produce low voltages but can use them to sense their surroundings.

I got into electric fish by a happy accident. In a laboratory next to mine at the University of Texas in Austin, I saw a man counting out earthworms. I asked if he was going fishing. “No,” he said, “I am getting ready to feed my fish.” His name is Troy Smith. He had come for advanced studies with my friend Harold Zakon, who is an old hand with electric fish. They are studying neurobiology—how nerves work—and are using electric fish in their experiments.

Home of the electric fish Sternopygus. The fish were taken from this stream in Venezuela. It’s a tributary of the Orinoco River.

Troy’s aquarium holds about a dozen of his pets—long skinny knife-shaped fish of various sizes up to a foot long. Their official name is Sternopygus (ster-NOP-ih-gus). They swim slowly—sometimes forward, sometimes backward—by the rippling motion of feathery fins on their undersides.

Troy had a little gadget on a long plastic rod. It was connected to an amplifier and loudspeaker. When it was held in the water near a fish, we could hear the electrical signal as a steady hum at seventy-three beats per second.

“That’s a male,” Troy told me. “The females have a hum with higher frequency, usually over a hundred.”

With that introduction I began studying up on electric fish, first by reading so I could talk to Troy and Harold about them. Here’s the story: first the big idea and then how the electric sense really works. The idea is that a fish creates an electric field, two clouds of electric charges around it. Then by sensing the density of the cloud, the fish can tell about anything in its way. How a fish can do all that takes us into many different parts of science.

Sternopygus has an electric organ at the base of its tail. The organ is made of neat and regular stacks of cells that look like muscle cells when seen under
a microscope. Instead of doing mechanical work as cells in muscle do, all their energy goes into making electricity.

Each cell in the electric organ makes only a tiny voltage, but when they are stacked together they give voltages high enough to be useful. The electricity does not come out into the water as a steady current. Instead it has a very regular on-off pattern. When recorded, as you can see below, each fish has its own different pattern or “signature.”

The water around a fish can carry electricity only because the water contains some salts dissolved from rocks and soil. And in water, salts always split apart to form oppositely charged particles or ions.

	The fish’s electric field is in two clouds of electric charges around it. The fish can feel anything that gets in the way or changes the clouds of electric charges.

In chemical language, ordinary table salt is sodium chloride. It’s written in shorthand as NaCl. But in water it occurs only as positive sodium ions (Na+) and negative chloride ions (Cl-).

The fish’s electric organ works just like an electric battery. It pulls positive ions toward one end and negative ions toward the other. That creates two clouds of electric charges around the fish.

An Electric Sense
To sense those electric clouds around it, the fish has a regular pattern of special nerve endings called electroreceptors in its skin. Each one works as a tiny volt-meter to tell about the density of the electric cloud right outside.

All together, the nerve mes-sages tell the brain about the shape of the electric cloud and about anything—like a rock or a plant or another fish—that changes the cloud’s shape. Its electric sense gives Sternopygus a way to “feel” things nearby even without touching them.

Electric Language
Some interesting things happen when a number of electric fish get together. When two fish are making electric patterns that are close together, one fish will change its timing so the two patterns do not interfere with each other.

	The image above shows the regular on-off electric patterns recorded for four diferent kinds of weakly electric fish.

Since the fish can sense one another’s electrical patterns, they also have a special way to communicate. Scientists have “listened in.” And it is possible to record the electrical signatures and then play them back to other fish. One kind of message was easy for the scientists to read. Even at a distance and in the dark an electric fish can tell the difference between a friendly neighbor and a threatening stranger.

Electric fish are studied partly out of curiosity to under-stand their special sense and how it works. A more practical reason is that their electrical signature is very closely tied to the nervous system that controls it. Usually nerves and the muscles they control are buried deep down inside skin, muscles, or other tissues. But nerve control of the electric organ is easy to study because the nerve endings are right at the surface. So Sternopygus provides a good place to study how nerves work.

I think it’s a cool part of science that fish from a muddy tropical river should help us learn about how nerves work.