If there's an undersea animal that deserves to be called a Jedi, it's the electric eel. Here is an animal that can bend the will of its prey to its own, controlling the helpless fish from a distance as if by remote control. Their powers are to be feared. At least if you're eel food.
The electric eel, which isn't actually an eel at all but a knifefish, is one of only a handful of animals that can generate their own electricity, using it to zap their prey, stun their predators and to communicate with each other. Researchers have known for some time that the electric eel (charmingly called Electrophorus electricus) is the most powerful animal Taser in the world, able to deliver shocks of up to 600-700 volts. In the right circumstances, that's enough to electrocute a human.
The eels have long been subject to investigation by researchers interested in neurotransmitters like acetylcholine, in the evolution of electric organs and even in electricity itself. Researchers have worked out that the critters can generate three different types of electric discharges. The first is a set of low-voltage pulses, which they use to sense the world around them. Then there are pairs and triplets of high-voltage pulses used while hunting in complex environments. Finally there are high-frequency volleys of high-voltage pulses emitted during the act of prey capture itself. It's their attack manoeuvre.
But until now, nobody really knew just how the eel's electricity affects its prey. That's where Vanderbilt University biologist Kenneth Catania comes in.
Catania discovered that electric eels use their electricity to hijack the musculature of their targets. That allows them to control the confused fish like a puppeteer controls a puppet – long enough to close in, open wide and swallow. He reported his findings this week in the journal Science.
“The electric eel is the most powerful animal Taser in the world, able to deliver shocks of up to 600-700 volts.”
Here's how Catania figured out the mechanism behind the eel's, er, stunning hunting strategy.
First he needed to characterise the eel's predatory strike, millisecond by millisecond. After finding a tasty fish, the eels reliably began their high-frequency (400 Hz) high-voltage discharges between 10 and 15 milliseconds before the final strike. Just 3-4 milliseconds after the first strong pulse, the prey became completely immobilised. In most cases, the eel was able to easily get the fish, though in a few cases the fish did manage to escape.
To see how that worked, Catania constructed a special tank divided in half by a barrier through which electricity – but not the eel – could pass. On one side went a pithed fish. Pithing is a humane process through which an organism's brain is removed but the organism itself remains alive. Essentially, Catania turned the fish into a zombie. He connected his zombie fish with an electrode to measure whatever electrical activity might be occurring inside it.
On the other side of the tank went an electric eel. Then, he dropped some earthworms into the tank on the eel's side, provoking it to hunt. Even though the eel was directing its electric discharges at the worms, its zaps would still affect the fish.
What Catania found was that just after the eel delivered its electric attack, the fish's muscles strongly contracted. Since the fish was a zombie, those contractions had to be the result of the eel's hunting tactics, rather than the fish's own movements.
This means that when eels immobilise their prey, it's thanks to massive, involuntary muscle contractions. A second experiment determined that the eel could do this by controlling the nerves of its prey, rather than by directly controlling its muscles.
And that's not all. The eels also rely on their electricity to get hiding fish to give up their locations.
Eels give off paired discharges – 'doublets' – while they're hunting in complex environments. To see what their function was, Catania did another experiment.
This time, he put a live fish on one side of the tank and an eel on the other. As the eels explored the tank, they sometimes gave off that high-voltage doublet. Invariably, this would cause a brief twitch in the fish swimming around behind the barrier. And just 40 milliseconds after the fish twitched, the eel went into full attack mode. Lucky for the fish, it was safe behind the barrier.
This experiment verified that the doublet caused the prey to twitch. It seemed as if that twitch was then exploited by the electric eel, as if it was waiting for the echo to return from the fish. But since the attack always followed the doublet, Catania couldn’t be sure of that. So he did one final experiment.
Into the tank went an eel, like before, and into the other side of the tank went a zombie fish, also like before. This time, the fish was enclosed inside a plastic coating, to isolate it from responding to the eel's electricity. It was also outfitted with two electric leads, allowing the researchers to artificially induce a twitch.
Half the time, after the eel emitted its hunting doublet, the researchers flipped a switch, causing their zombie fish to twitch. As expected, the eel tried to attack. But then the researchers switched tactics and did nothing in response to the eel's discharge. And with no twitch to detect, the eel didn't proceed into its full attack.
The doublet discharge works like an exploratory nudge – the eel's way of asking the question "are you living prey?" If the hiding fish responds by twitching, then the eel moves on to the next stage of the hunt. If the eel gets no response, it keeps looking.
Combined, the results of these experiments explain, for the first time, the complex hunting strategies of the electric eel. And it comes down to one basic rule: you can't run and you can't hide.
Top header image: Christine Schmidt, Flickr