Eels - complex hunters in the water

Electric shocks more complex than we have imagined

The eel's electrical sensors are shown in red and its water movement sensors are highlighted in green. (Catania Lab / Vanderbilt)

Electric eels are the most remarkable predators of the animal kingdom. They kill their prey with electric shocks as high as 600 volts… but that’s not all. A new insight into the voracious hunting behaviour has been uncovered by biologist Kenneth Catania of Vanderbilt University.

Having been in South America for the last three years, Catania has studied how the electric eel uses electric fields to navigate through the muddy waters of the Amazon and Orinoco basin, preying on ferrets.

Eels can grow up to 2.8 metres and weigh up to 200 kilogram’s. There are specialised cells that operate like small biological batteries in about two-thirds of its body. When hunting or in danger, these cells are simultaneously discharged, emitting at least 600 volts of electricity, five times the voltage of a standard US electrical outlet.

It had been long assumed that electric eels were primitive creatures that possessed the unique ability of using electricity of shocking their prey to death. Now, it appears that they are also able to manipulate the electric fields in a complex manner, thus giving them a number of remarkable abilities, said Catania.

One of these abilities involves the doubling the strength of their electric discharge at will. The electrical system of the eel is similar to a wireless taser used to stun prey. The eels produce three different types of electric discharges: low voltage pulses to detect their surroundings, short sequences of high-voltage pulses (two or three milliseconds) when hunting, and bursts of high-voltage high-frequency pulses when the eel captures prey or needs to defend itself.

The electric doesn't just use the electric impulses it generates to stun its prey, it also uses them like radar to track the location of their prey in dark or murky waters. (Kenneth Catania / Vanderbilt)

In a series of experiments, Catania showed that the eel’s electric discharge does not act on the prey’s muscles, but the nerves that control the muscles. This causes the prey to experience strong involuntary muscle contractions.

This ability to remotely control the prey’s muscles fascinated Catania. “I know of no other animal that literally can take over control over the body of another living being," he said.

Normally, the eel manipulates small foraging fish through rapprochement and emitting its electric discharges, causing whole-body muscle contractions and temporary paralysis in the prey. If, for some reason, the eel decides not to eat the fish, the victim will recover after some time and swim away without any visible damage.

Catania also discovered that the eel has an attack mode which it uses against larger prey. First, the eel bites it, then rolls its tail around the prey till the tail is directly opposite its head. At this time, the eel increases the rate of high-voltage electric discharges

This manoeuvre brings together the positive pole of the eel (located in its head) into the vicinity of the negative pole (located in the tail). The physical closeness of the two poles increases the amount of electric charge directed at the prey sandwiched in-between to as much as twice its usual intensity. This causes the muscles of the prey to contract so fast and hard that they are virtually paralysed. According to Catania, the effect is comparable to administering a dose of a nerve agent such as curare.

In addition, through his research, Catania also discovered that the eel uses its electric charges to navigate murky waters and track its prey.

"This dual use of the high-voltage system as a weapon and a sensory system shows that the hunting behaviour of eels is far more complex than we thought," Catania said.