The ability of electric eels to shock their prey with a 600-volt blast is well known, but exactly how the fish orchestrate their attacks has remained a question as murky as the waters they hunt in.
Now it looks as if eels use a high-frequency barrage of shocks to disable fish by mimicking their prey’s nerve signals and making their muscles contract. In essence, they hijack the muscles and remote controlling the prey to near-certain death (see video above).
And if a fish is hiding behind a rock or algae, the eel has another shock pattern that makes the fish muscles twitch involuntarily, giving away their hiding place to the formidable predator.
The experiments that untangled these mechanisms were devised and run byKenneth Catania at Vanderbilt University in Nashville, Tennessee. In a natural environment, Catania watched an eel hunting and measured its electric discharges. As the eel was poised to strike, it emitted a barrage of high-voltage electric pulses. This stopped the fish in its tracks, allowing the eel to catch it easily.
To work out what was happening, Catania anaesthetised fish, removed their brains, and dangled them behind an electrically permeable agar barrier in an eel tank. Worms were then put into the tank for the eels to feed on, and the electric zaps sent out to catch the worms also reached the fish.
After about 3 milliseconds, the fish’s muscles completely contracted.
A chemical injected into another brainless fish to stop its motor neurons working, and another fish with its spine removed helped to complete the picture: the electric shock makes the motor neurons fire and contract the muscles, and it happens without the need for the central nervous system.
Catania also discovered that a different, high-voltage, two-pulse signal fired out by the eel makes a fish within range twitch uncontrollably, giving away its position and allowing the eel to go after it with its conventional attack of series of high-voltage pulses.
This two-pulse signal seems to tell the eel whether possible prey is living when information is limited, such as in murky or rocky environments, where the prey is hidden, the researchers say.
How and why the eel evolved this ability is still unknown, says Catania.
“It is possible that lower voltages had a similar effect in the course of evolution,” he says. “Smaller ‘blips’ induced in prey neurons by the ancestral eel discharge added together over a short time,” he suggests.
“This is an immensely interesting and important finding for electric fish biology,” says Jason Gallant at Michigan State University in East Lansing. “The discharge patterns have been described previously, but the mechanism by which the discharges act on their poor prey was only supposed.”
That eels evolved to not only disable prey, but to also flush them out is a surprise, Gallant says, and he wonders if this behaviour is eel-specific or is seen in other fish, even those that can’t produce such a forceful zap.
