How electric eels double their zapping power

Slippery, slimy and beady-eyed, the electric eel looks like a remnant from the age of the dinosaurs. But its hunting tactics are far from primitive. Kenneth Catania, a biologist at Tennessee’s Vanderbilt University, has uncovered how the eel exploits simple physics to track and subdue its prey. He published his findings in two papers in October.

Native to South America, electric eels have tiny eyes, but barely use them – they live in murky waters and usually hunt at night. They navigate by emitting low-voltage electrical clicks, a little like the sonar chirrups emitted by bats.

But these clicks are slight compared to the short, high-voltage shocks they use to stun or kill their prey. Up to 80% of the electric eel’s body is made of electricity-generating cells called electrocytes. These cells are amped-up versions of the muscle cells in mammals’ bodies, which transmit the electric signals that cause the muscles to contract. In the eel, electrocytes are stacked, like batteries in a torch, and can produce shocks of up to 600 volts – enough to electrocute an adult human.

Last year Catania set up an aquarium in his lab to study electric eels while writing a book on animal sensory systems and predator-prey interactions. He was entranced by the eels’ hunting skills – an eel in total darkness could immobilise its prey within milliseconds.

When electric eels hunt they send out a high-voltage pulse in all directions. The pulse makes the muscles of any prey in the vicinity twitch, then freeze. When an eel senses the ripple of the twitch through the water they know they have hit a target. But how does an eel find its immobilised prey, Catania wondered – especially in muddy waters? It turns out they use their high-voltage pulse not only as a weapon, but also as a tracking system.

Catania showed this by placing a twitching fish in an electrically insulating bag, then dropping it in an eel’s tank along with a carbon rod that conducts electrical current. Sensing movement in the water, the eel sent out electric pulses to stun the source. The eel then homed in on the carbon rod, ignoring the fish twitching in the bag.

The experiment shows an eel is guided not by the prey’s movement, but by its conductivity. Electricity travels through the most conductive material available – lightning, for instance, prefers metal rods to trees. And fish flesh is more conductive than water. So the eel somehow knows the prey’s location is where its electrical pulses are conducted best. Catania published this finding in an October study in Nature Communications.

But what if the stunned prey wakes up and starts to struggle before the eel can devour it? After further observation, Catania realised the eel has other tricks. When an electric eel captures a wriggly fish, for instance, it curls its body around it, a little like a snake wraps around a rat. The prey ends up between the eel’s head and tail.

The electric eel uses a curling maneuver to deliver concentrated zaps of electricity to its prey. CREDIT: KENNETH CATANIA, VANDERBILT UNIVERSITY.

Catania suspected this behaviour was connected to how the eel channels its electric field. An eel’s head is positively charged and its tail is negatively charged. When its positive and negative ends move closer together, Catania thought, the current between them should strengthen, disabling the fish.

The problem was how to prove the physics experimentally. So Catania came up with a measuring device he calls his “eel chew toy” – a plastic rod attached to wires and electrodes. Catania impaled a dead fish on it. He then attached an instrument called an ammeter to measure electric current, submerged the toy in the eel’s tank and lightly tugged at the wires to simulate struggling prey.

“It turned out to work perfectly!” Catania says. The eel took the bait and zapped away, over and over, curling and unfurling all the while. The amount of electric current that shot through the fish while the eel was curled was more than double the shock it received when the eel straightened out. Catania thinks eels use this technique to deliver the final blow to their victims, and perhaps allows them to tackle tougher prey such as crayfish. He published the finding in the journal Current Biology.

“The experiments are very innovative,” says Gerhard von der Emde, a neuroscientist at the University of Bonn in Germany, who studies electric fish. “It’s incredible how he measured all this.”

How did these observations evade scientists for so long? Catania believes biologists have been too kind to their eels in labs by feeding them fish that don’t put up much of a fight. Nobody really knows what eels eat in the wild, he says, and how difficult its prey is to subdue.

So what’s next in the electric eel’s story? Catania wants to investigate how this primitive-looking but remarkably advanced fish remains unharmed by its own zaps. “People go to the movies to see something that’s amazing,” he says. “But when we look closely, we see those things are already here.”