A fish swims in the Amazon, amid murky water and overgrown vegetation. It is concealed, but it’s not safe. Suddenly, two rapid bursts of electricity course through the water, activating the neurons that control the fish’s muscles. It twitches, giving away its position, and dooming itself. Now, it gets zapped by a continuous volley of electric pulses. All its muscles contract and its body stiffens. It can’t escape; it can’t even move. Its attacker—an electric eel—moves in for the kill.
The electric eel can (in)famously create its own electricity. More than four-fifths of its two-metre-long body consists of special battery-like cells, which can collectively deliver a jolt of up to 600 volts. But the way the eel uses that ability is even more shocking. Kenneth Catania from Vanderbilt University has found that this astonishing predator can use its electricity like a remote control, activating its prey’s muscles from afar. It effectively has a button that says “Reveal Yourself” and another that says “Freeze”.
“This is one of the most amazing things I’ve encountered in studying animals, and I’ve seen a lot of unusual things” says Catania. He’s not exaggerating. This is a man who showed that crocodile faces are more sensitive than our fingertips, that tentacled snakes can persuade prey to swim into their mouths, and that star-nosed moles blow bubbles to smell underwater. Guy knows amazing and unusual.
The eel tops them all, not least because its hunting tactics seems unbeatable. It should work on any prey with nerves and muscles, which is most of them.
“I don’t see any defence against it,” says Catania.
Although scientists have studied the electric eel’s anatomy and even sequenced its genome, hardly anyone had looked at how it hunts. “The sense, and I had the same reaction, was that they zap their prey with electricity and eat it; what more is there to know?” says Catania. As it happens: a lot!
Once Catania got his (rubber-gloved) hands on some eels, he realised that they are surprisingly fast. “I thought they might lazily shock their prey and then deal with it afterwards, but they combine the shock with a really rapid strike,” he says. He filmed them with a high-speed camera and noticed something remarkable. When the eels approached their prey, they released an intense volley of high-voltage pulses—around 400 a second. These pulses completely freeze the prey, and that’s when the eel lunges. If the fish isn’t paralysed, the strike would miss. “At that point, I was hooked,” says Catania. “I just had to know more.”
To work out what the pulses were doing, Catania presented eels with zombie prey—lobotomised and anaesthetised fish that were hooked up to a device for measuring forces. An agar barrier prevented the eel from reaching the morsels but allowed its discharges to pass. This macabre set-up confirmed that the eels’ high-voltage pulses force the fish’s muscles to involuntarily contract.
The pulses could either act on the muscles themselves, or on the neurons that control them. To distinguish between these possibilities, Catania injected the fish with curare, a poison that blocks the junctions connecting nerves and muscles. These individuals were immune to the eels’ shocks—a clear sign that the pulses act on the neurons, rather than directly on the muscles.
“That’s pretty much how a taser works,” says Catania. Indeed, getting shot with a taser is probably the closest anyone might come to being on the receiving end of an electric eel. Failing that, “the next most common experience might be to accidentally touch an electric fence at a horse or cattle farm”.
Catania also found that the eels precede their lengthy volleys with a quick pair of pulses. Doublets like these are known to trigger disproportionately strong muscle contractions. The eel, it seems, produces exactly the right kinds of discharge to freeze its prey as efficiently as possible.
But it also produces doublets when there’s no prey in sight. Since the 1970s, scientists have known that hungry eels will prowl around their cages, giving off electric doublets as they explore. And in Catania’s experiments, the eels often released a doublet, and then tried to break through the agar barrier and snatch the fish. That was a big clue: Catania wondered if they might be using the pulses to find their prey in the first place.
“Transport yourself to the Amazon and imagine these nocturnal animals hunting for diverse prey, which are hidden in a complex environment,” he says. So, they release doublets. Any fish or crab that gets hit would twitch and give itself away.
Catania tested this idea by placing the zombie fish in a thin plastic bag to isolate them from any electric pulses. When the eels released their doublets, the fish didn’t react, and the eels never attacked. When Catania deliberately jostled the fish to mimic a twitch, the eels struck. They are extraordinarily sensitive to ripples in the water, and the slightest movement sets them off. And they’re so fast that they can hit the source of the twitch within 20 thousandths of a second. “If you saw this in real-time, you wouldn’t even know that the doublet had elicited a movement in the fish because it would happen so fast,” says Catania.
Many fish can produce weak electric fields, including the elephantfishes of Africa and the knifefish of South America. But only a few can produce strong potentially lethal fields. These include the electric eel (which is actually a knifefish and not an eel at all), the electric catfishes, and the torpedo ray. The eel is the most powerful of these shockers, but Catania suspects that the others might use the same hunting technique. “I wish I had some in the lab,” he says.
Reference: Catania, 2014. The shocking predatory strike of the electric eel. Science http://dx.doi.org/10.1126/science.1260807
PS: Yes, this really is a single-author paper, published in Science in 2014. That’s arguably as shocking as the eel.