The New York Times, December 4, 2014
For thousands of years, fishermen knew that certain fish could deliver a painful shock, even though they had no idea how it happened. Only in the late 1700s did naturalists contemplate a bizarre possibility: These fish might release jolts of electricity — the same mysterious substance as in lightning.
That possibility led an Italian physicist named Alessandro Volta in 1800 to build an artificial electric fish. He observed that electric stingrays had dense stacks of muscles, and he wondered if they allowed the animals to store electric charges. To mimic the muscles, he built a stack of metal disks, alternating between copper and zinc.
Volta found that his model could store a huge amount of electricity, which he could unleash as shocks and sparks. Today, much of society runs on updated versions of Volta’s artificial electric fish. We call them batteries.
Now a new study suggests that electric fish have anticipated other kinds of technology. The research, by Kenneth C. Catania, a biologist at Vanderbilt University, reveals a remarkable sophistication in the way electric eels deploy their shocks.
Dr. Catania, who published the study on Thursday in the journal Science, found that the eels use short shocks like a remote control on their victims, flushing their prey out of hiding. And then they can deliver longer shocks that paralyze their prey at a distance, in precisely the same way that a Taser stops a person cold.
“It shows how finely adapted eels are to attack prey,” said Harold H. Zakon, a biologist at the University of Texas at Austin, who was not involved in the study. He considered Dr. Catania’s findings especially impressive since scientists have studied electric eels for more than 200 years.
“He sees things that just go unnoticed,” said Dr. Zakon.
Scientists have long known that when electric eels attack their prey, they unleash a volley of electric pulses — as many as 400 a second. But it has been a struggle to tease apart the fine details of how the eels strike.
“They can basically attack something and swallow it fast enough that you can’t even really see what happened,” Dr. Catania said.
Last year, he decided to take a closer look by filming electric eels in a tank, using a high-speed video camera that captures 1,000 frames per second.
“I was imagining a fish sort of jerking around and maybe dying and being eaten,” he said. But the movies revealed something else entirely.
When an eel delivered its electric pulses, its victim stopped swimming and became perfectly still within four-hundredths of a second. “They’re stuck in whatever position they were in,” Dr. Catania said. “They just stay that way.”
As the fish floated, statue-like, the eel could then easily scoop it up. In some cases, the shock wore off before the eel could reach its prey, and the fish swam off unharmed.
“It’s extraordinary to me,” Dr. Catania said. “How do you stop all voluntary movement in three milliseconds?”
So he set up a new experiment to find out. He removed the brain from a fish and placed its body in a recess in the eel tank. To record the activity of its muscles, he attached it to a force-measuring device. Then he sealed the recess with a barrier that blocked the eels, but not their electric pulses.
Next he put an eel in the tank and fed it live earthworms. The eel zapped the worms, and the electric pulses also reached the brainless fish hidden nearby.
Every pulse, Dr. Catania found, caused all of the fish’s trunk muscles to contract. As the pulses came faster, the contractions fused together, and the muscles simply locked up, freezing the fish.
Because the fish was brainless, Dr. Catania knew that the shocks must be affecting some other part of its body. He ran the experiment again on fish from which he removed both the brain and the spinal cord. The eels could make these fish freeze, too.
It was possible that the electric pulses caused contractions in the muscles themselves. To test that possibility, Dr. Catania injected the fish with the drug curare, which blocks the connection from nerves to muscles. The drugged fish remained relaxed when they were shocked.
These experiments leave only one possibility, Dr. Catania said: Electric eels manipulate the nerves that run from a fish’s spinal cord to its muscles. They set off precise spikes of voltage that travel down the nerves, causing the muscles to contract all at once. The pulses are spaced at just the right frequency to cause the fish to quickly become immobilized.
The fish behave as if they were hit with a Taser, a device that fires darts through which it can send pulses of current. When Dr. Catania looked closer at studies on Tasers, he was stunned by the similarity.
“It is the exact mechanism of a Taser,” Dr. Catania said.
As he was working out the effects of these bursts, he stumbled across an equally impressive way in which electric eels stalk their prey.
Back in the 1970s, scientists noticed that electric eels exploring murky river bottoms sometimes release two quick pulses in a row — called a doublet — in less than a hundredth of a second. But it was unclear why.
When Dr. Catania put electric eels in a tank with brainless fish, some would swim up to the barrier and deliver a doublet. “And then they’d go crazy trying to break through the barrier — I mean literally trying to chew through the barrier,” he said. “It was clear they gave off the doublet and they knew something was behind there.”
Dr. Catania discovered that doublets cause fish to contract all their muscles in a huge twitch. Using a biological form of remote control, electric eels can force their victims to flail, producing waves that the eels can detect with motion-sensitive hairs in their skin.
Combined, these two strategies make electric eels effective hunters, Dr. Catania said. They search river bottoms for potential prey. They can sense the odors of fish and even detect electric signals from their prey’s muscles.
Their victims try to hide from the eels by holding as still as possible. If an eel suspects a fish is nearby, it will unleash a doublet, forcing the fish to flail and reveal its hiding place.
As the fish tries to escape, the eel delivers a full blast, freezing the fish in place and making it an easy catch.
Electric eels are just one of six lineages of fish that separately evolved the ability to produce blasts of electricity. In every case, a set of muscles turned into a special electric organ. In a study published in Science in June, Dr. Zakon and his colleagues showed that many of the same genes changed in all six lineages to produce this transformation. Evolutionary biologists refer to this parallel change as convergent evolution.
Dr. Zakon suspects that at least some other electric fish have also evolved the same remote control and Taser maneuvers that Dr. Catania has now discovered in electric eels. In the 1970s, some scientists proposed that other species might use electric pulses to startle their prey. But no one has tested those ideas.
“So this is potentially a much wider, broader example of convergent evolution,” Dr. Zakon said.
The new research also raises some big questions. How is it, for example, that electric eels can manipulate the nerves of other fish without making themselves flail or freeze? “Nobody knows how this is happening,” Dr. Catania said.
That mystery only deepens his fascination with electric fish. “It’s the pure beauty of seeing an animal that has evolved to this degree of almost having superpowers,” he said. “If there were no electric fish and I told you this could happen, you’d probably say I was crazy.”
Copyright 2014 The New York Times Company. Reprinted with permission.