Electric eels curl up to deliver double strength shocks

The electric eel boasts what must surely be one of the most sophisticated and versatile weapon systems in the whole animal kingdom. This large Amazonian fish is well known for its ability to discharge pulses of electricity that are powerful enough to incapacitate a human, or even a horse. But its electric organ also acts as a tracking system, enabling the eel to locate fast-moving prey rapidly and precisely, and as a wireless Taser, with which it can stun prey and control their movements from a distance.

Research published today in the journal Current Biology reveals yet another of the electric organ’s remarkable features. When struggling to capture large prey, the electric eel curls its long, flexible body to bring its tail round towards its head. This simple manoeuvre not only traps the prey, but also superimposes the electrical fields generated by the opposite ends of the eel’s body, concentrating the fields and doubling their strength so that the eel can subdue its unruly victim.

Electric eels are not eels at all, but are actually members of the knifefish family, about half of which are weakly electric. The electric eel is unique in both the power of its electrical organ and its versatility, however. Adults can grow to more than 2 meters in length, and their bodies are packed from head to tail with about 6,000 specialised cells called electrocytes. Each of these produces a tiny voltage, but they have a combined discharge of up to 600 volts. Eels can discharge up to 500 per second, and generate several distinct discharge patterns, each of which serves a specific purpose.

The new findings are the latest of a series of remarkable discoveries by neurobiologist Kenneth Catania of Vanderbilt University in Nashville, Tennessee. Earlier this month, Catania reported that electric eels use their high voltage discharges to track fast-moving prey, and before that, he described the eel’s shocking predatory strike: When prey are hiding, they emit periodic bursts of two or three high voltage discharges, which remotely activate the motor neurons in nearby animals, causing massive involuntary twitching that reveals their exact location by sending pressure waves through the water. This is followed by a volley of high voltage discharges, which causes whole-body muscle contractions that temporarily immobilizes the prey and prevents them from escaping.

This behaviour is usually studied in eels bred in captivity, where they are fed crayfish. But the electric eels live in the Amazon, one of the most biodiverse parts of the world, and so, although we know very little about their natural diet, it is likely to include species of various shapes and sizes, including larger ones that are likely to put up a fight.

During a typical predatory attack, eels discharge electricity from their head to incapacitate prey, then reposition them until they can be swallowed whole. Catania noticed, however, that juvenile eels curl their bodies up when struggling with large prey. Unlike other electric fish, the eel’s electric organ spans much of the length of its long body, so that the positive and negative poles of its electrical discharges are normally kept separate. Catania realised that curling could, in theory, double the strength of their discharges by superimposing the electric fields from the positive and negative poles.

To test this possibility, Catania set up an experiment to measure how the curling affects the electrical field experienced by the eel’s prey. Using moldable plastic and insulated wires, he placed electrodes into dead fish and then dangled the bait into the eels’ aquarium, enabling him to film their strikes while simultaneously recording the voltage from the fish. This was done so that the fish could not be immediately swallowed, to mimic, as closely as possible, an attack on struggling prey.

This revealed that the ‘curling’ attacks involve a stereotyped sequence of events. The initial high voltage discharges partially incapacitate the prey, allowing the eel to approach and seize it. At this point, a captive fish would normally struggle to break free, but the eel curls its body to sandwich its prey between its head and tail. It then gives off a long volley of high voltage discharges, before releasing the immobilised fish, repositioning it, and then swallowing it.

Analysing his film footage, Catania noted that the eels curl their bodies in such a way as to align the positive and negative poles of its electrical fields in parallel with each other. And the electrode measurements further showed that curling significantly amplifies the eel’s electrical discharge, so that the average voltage produced during a curling attack is more than double that generated in uncurled attacks.

Catania then tested the effects of the electrical discharges produced during curling attacks, by subjecting dead fish first to high voltages produced by a stimulator, and then to high frequency pulses generated by the eels themselves, while measuring the contractile forces in their muscles. This confirmed his earlier findings that the high voltage volleys rapidly block preys’ muscle activity. Thus, curling amplifies the eels’ discharges, ensuring maximal stimulation of the prey’s motor neurons, and prolonged high frequency causes temporary muscle fatigue, giving the eel time to capture it and reposition it for swallowing.

Catania has made a career out of single-handedly studying the sensory abilities of weird and wonderful creatures, such as the star-nosed mole, which has an unparalleled sense of touch, and uses air bubbles to sniff out food underwater. But he believes electric eels are among the most remarkable predators in the world.

“Historically, electric eels have been viewed as unsophisticated, primitive creatures that... shock their prey to death, but it turns out that they can manipulate their electric fields in an intricate fashion,” he says. “The dual use of the high voltage system as both a weapon and a sensory system indicates that eels’ hunting behavior is far more sophisticated than we have thought. I don’t know of any other animal that can literally take control of the body of another animal like this.”

References

Catania, K. (2015). Electric Eels Concentrate Their Electric Field to Induce Involuntary Fatigue in Struggling Prey. Curr. Biol., 25, 1–10. [Full text]

Catania, K. (2015). Electric eels use high-voltage to track fast-moving prey. Nat. Commun., 6: 8638. DOI: 10.1038/ncomms9638. [Full text]

Catania, K. (2014). The shocking predatory strike of the electric eel. Science, 346: 1231-34. [PDF]