A new study carried out by Dr Kenneth Catania from Vanderbilt University in Nashville, TN, reveals that an electric fish known the electric eel (Electrophorus electricus) has evolved a precise remote control mechanism for prey capture, one that takes advantage of an organisms’ own nervous system.
Electric eel (Electrophorus electricus) at the New England Aquarium, Boston. Image credit: Steven G. Johnson / CC BY-SA 3.0.
Electric fish have long fascinated humans. Ancient Egyptians used the torpedo, an electric marine ray, in an early form of electrotherapy to treat epilepsy.
Much of what Benjamin Franklin and other pioneering scientists learned about electricity came from studies of electric fish.
In Victorian times, parties were organized where guests would form a chain to experience the shock of an electric fish.
The most powerful electric fish is the electric eel, with most of its body composed of electrocytes – muscle-derived biological batteries. It produces a jolting electric field of up to 600 volts, about 100 volts per foot of fish.
Until now, no one had figured out how the electric eel’s electroshock system actually worked.
In order to do so, Dr Catania equipped a large aquarium with a system that can detect the eel’s electric signals and obtained several eels, ranging up to four feet in length.
As he began observing the eels’ behavior, he discovered that their movements are incredibly fast.
They can strike and swallow a worm or small fish in about a tenth of a second. So the scientist rigged up a high-speed video system that ran at a thousand frames per second so he could study the electric eel‘s actions in slow motion.
He recorded three different kinds of electrical discharges from the eels: low-voltage pulses for sensing their environment; short sequences of two or three high-voltage millisecond pulses (called doublets or triplets) given off while hunting; and volleys of high-voltage, high-frequency pulses when capturing prey or defending themselves from attack.
Dr Catania found that the eel begins its attack on free-swimming prey with a high-frequency volley of high-voltage pulses about 10 to 15 milliseconds before it strikes.
In the high-speed video, it became apparent that the fish were completely immobilized within three to four milliseconds after the volley hit them.
The paralysis was temporary: if the eel didn’t immediately capture a fish, it normally regained its mobility after a short period and swam away.
The observations raised an obvious question: how do the eels do it?
“I have some friends in law enforcement, so I was familiar with how a taser works. And I was struck by the similarity between the eel’s volley and a taser discharge. A taser delivers 19 high-voltage pulses per second while the electric eel produces 400 pulses per second,” said Dr Catania, who is an author of the paper published in the journal Science.
The taser works by overwhelming the nerves that control the muscles in the target’s body, causing the muscles to involuntarily contract.
To determine if the eel’s electrical discharge had the same effect, Dr Catania walled off part of the aquarium with an electrically permeable barrier. He placed a fish on other side of the barrier from the eel and then fed the eel some earthworms, which triggered its electrical volleys. The volleys that passed through the barrier and struck the fish produced strong muscle contractions.
To determine whether the discharges were acting on the prey’s motor neurons – the nerves that control the muscles – or on the muscles themselves, he placed two fish behind the barrier: one injected with saline solution and other injected with curare, a paralytic agent that targets the nervous system.
The muscles of the fish with the saline continued to contract in response to the eel’s electrical discharges but the muscle contractions in the fish given the curare disappeared as the drug took effect.
This demonstrated that the eel’s electrical discharges were acting through the motor neurons just like taser discharges.
Next Dr Catania turned his attention to the way in which the eel uses electrical signals for hunting. The eel is nocturnal and doesn’t have very good eyesight. So it needs other ways to detect hidden prey.
He determined that the closely space doublets and triplets that the eel emits correspond to the electric signal that motor neurons send to muscles to produce an extremely rapid contraction.
Putting together the fact that the eels are extremely sensitive to water movements with the fact that the whole-body muscle contraction causes the prey’s body to twitch, creating water movements that the eel can sense, Dr Catania concluded that the eel is using these signals to locate hidden prey.
To test this hypothesis, Dr Catania connected a fish to a stimulator. He put the fish in a clear plastic bag to protect it from the eel’s emissions. He found that when he stimulated the fish to twitch right after the eel emitted one of its signals, the eel would attack. But, when the fish failed to respond to its signal, the eel did not attack.
The result supports the idea that the eel uses its electroshock system to force its prey to reveal their location.
“If you take a step back and think about it, what the eel can do is extremely remarkable. It can use its electrical system to take remote control of its prey’s body,” Dr Catania said.
“If a fish is hiding nearby, the eel can force it to twitch, giving away its location, and if the eel is ready to capture a fish, it can paralyze it so it can’t escape.”
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Kenneth Catania. 2014. The shocking predatory strike of the electric eel. Science, vol. 346, no. 6214, pp. 1231-1234; doi: 10.1126/science.1260807
