Electric eels can wield their zapping power in subtle and surprising ways. A new study finds that when dealing with struggling or hard-to-subdue prey, these eels bend their bodies into a horseshoe-like shape to more than double the voltage they deliver to their almost-meal.
The findings, published in the journal Current Biology, shed light on the stunning abilities of these remarkable hunters.
"In my mind, it's sort of a testament to how even an animal that's been studied for centuries may be doing amazing things that we were unaware of [until] we look closely," said study author Kenneth Catania, a neurobiologist at Vanderbilt University in Tennessee.
Catania has studied these eels for a while, discovering that they can "remote control" their prey, forcing them to twitch (and thus reveal their location). Once the eels shock their quarry, they reposition it in the water so they can swallow it head-first. In that brief moment, the fish, if it can withstand the electric attack, has its only chance to swim off and avoid becoming an eel meal.
But during his research, Catania started to notice something odd: Many of the eels would curl up their bodies when zapping a meal, bringing their head toward their tail with the fish in between. Then they would release the fish to reposition it and then gobble it down.
The scientist scribbled a note to himself about the observation and later set up an experiment to figure out what was happening. He first noticed that younger, juvenile eels — often just three to 10 inches long, and with less shocking power — curled up frequently during their electric attack. More massive, adult eels almost never bothered to do so — unless they were trying to quell particularly difficult prey, such as a large crayfish, which could take extra time to properly reposition.
The eels' electricity-producing organ has both a positive and negative end, with the positive end at the head and the negative end in the tail. So it seemed that by curling their bodies, the eels might be bringing the opposing ends of its organ closer together to concentrate the stunning effect, just as the magnetic field of a horseshoe magnet is concentrated by bringing the positive and negative poles together.
To find out if this was what the eel's prey was feeling, Catania took brain-dead fish equipped with electrodes, put it near an eel as bait, and measured the voltage applied when the eel attacked. Sure enough, the fish received more than double the voltage when the eel curled its body, bringing the opposite ends of its electricity-generating organ together.
The eel then would batter the prey's body with a series of shocks that forced the unfortunate animal's muscles to contract until they reached exhaustion, effectively paralyzing the animal temporarily and making it safe for the eel to release and reposition it.
The findings are part of a growing body of research showing that eels do not use their electric powers as a blunt tool, but in a myriad of subtle and sophisticated ways.
But Catania still wants to explore a basic and still-perplexing question: How does the eel avoid shocking itself?
"How they protect their nervous system and their own brain and their own muscles from being activated — as far as I know, it's pretty much an open question," Catania said.