Predators can force a hidden fish to twitch and betray its hiding place
Electric eels evolved hacking long before humans did. Zapping other fish with high-voltage bursts lets eels remotely control their prey’s nervous system to make muscles twitch and clench.
That takeover is how electric eels (Electrophorus electricus) immobilize their prey, Kenneth Catania of Vanderbilt University in Nashville reports in the Dec. 5 Science. And in a series of experiments to explore just what the eels’ high-voltage discharges — delivering up to 600 volts — do to prey physiology, Catania uncovered another spooky effect: Isolated electrical discharges that the eels occasionally release make nearby prey fish twitch involuntarily, giving away any hiding places.
Discovering that those electrical volleys —pairs or triplets of quick discharges — apparently probe for prey is “a remarkable finding,” says Mark Nelson, a sensory neurobiologist at the University of Illinois at Urbana-Champaign who has also studied the animals. “This elevates the eel’s strategy from a simple, reflexive stun-and-strike behavior to a more deliberative process.”
Only a few fish species have beaten humankind to development of the Taser, evolving specialized muscle tissue that emits strong electrical discharges that stun prey. A champion at this kind of attack, the electric eel has fascinated scientists for centuries. Less attention has gone into exactly what the stun attack does to eels’ prey. Catania got curious about it when he turned a high-speed video camera on the attacks and was intrigued at their speed. An eel can stop all motion in another fish within just 3 milliseconds.
Unpleasant as this attack would be for the target, Catania doesn’t want electric eels demonized. He doesn’t know of any serious reports of an electric eel killing a human being.
To see whether the jolt of eel voltage acts directly on prey’s muscles or on the nervous system, Catania suspended a pair of anesthetized fishes in an eel tank. One had been treated with curare, a substance that keeps nerves from transmitting signals to muscles. When the eel sent out a volley of electric shocks, muscles in the curare-treated fish didn’t clench but the other fish’s muscles did. The prey fish’s own nervous system must relay the contraction signals to the muscles, Catania concluded.
The eels also remote-control quick muscle twitches in prey, Catania found. But he struggled to find a way to test whether those quick volleys actually helped the eel reveal some hapless fish in range or were merely a routine prelude to an attack. Finally, he placed anesthetized prey fish in a plastic bag to prevent the eel’s zaps from reaching them. Then he used laboratory electrical stimulators inside the bag to create — or not — a fish twitch.
When the eel sent out an electrical volley and no fish twitch answered it, the eel didn’t fire its full stun attack. But the eel did proceed to an attack when Catania triggered the twitching response. That, plus some other experiments, convinced Catania that the discharges really are a nocturnal predator’s search tricks.
Electric organs, either as strong as the eels’ or much weaker, have evolved independently at least six times among the world’s fishes, notes Jason Gallant of Michigan State University in East Lansing. Now he’d like to know whether other electric predators give little search zaps, too.
TASED AND CONFUSED Watch an electric eel hunt and catch small fish (shown in speeds slower than life). In full-color sequences, knocklike sounds indicate electrical discharges from the eels. Black-and-white sequences have been colorized (red) to indicate when the eel is releasing electrical charges. Images, video and illustration: K. Catania/Vanderbilt University; Produced and narrated by Ashley Yeager
K. Catania. The shocking predatory strike of the electric eel. Science. Vol. 346, December 5, 2014, p. 1231. doi: 10.1126/science.1260807.
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J.R. Gallant et al. Genomic basis for convergent evolution. Science. Vol. 344, June 27, 2014, p. 1522. doi: 10.1126/science.1254432.
M.E. Nelson. Electric fish. Current Biology. Vol. 21, July 26, 2011, p. R528. doi: 10.1016/j.cub.2011.03.045.