Eels master electricians

Faced with a challenging meal, electric eels curl up to double the damage dealt by their electric organs and immobilise their prospective prey.

These large Amazon-dwellers, known scientifically as Electrophorus electricus, can grow up to two metres in length and are not actually eels at all but members of a family known as knife-fish.

These animals are endowed with some 6000 specialised "electrocytes", which are modified muscle cells capable of generating electricity.

They are arranged in series along the length of the animal so that the eel's head becomes positive and the tail negative.

A potential difference between the two ends of the fish is up to 600 volts. Usually, electric eels hunt by releasing brief electrical pulses into the water every second or two.

Anything lurking nearby goes into involuntary muscle spasms, revealing its position to the eel, which then unleashes a salvo of shocks at a rate of more than 100 per second.

The effect is like a human hit with a taser. The animal's muscles cannot keep up with the train of  stimulating electrical pulses, resulting in temporary paralysis, and the eel snaps up a meal.

However, some prey items can be harder to shock into submission, hold on to and swallow than others.

And releasing a captured prey item to manoeuvre it for swallowing risks letting a meal escape. Under these circumstances, though, the eel has a crafty sting in its tail.

Writing in Current Biology, University of Vanderbilt scientist Kenneth Catania has found, using high-speed photography, that the animals curl up to bring their tails around to the opposite side of the prey they have grabbed.

Using a dead fish rigged up with electrodes, and dangling this in front of an electric eel, he found that this curling behaviour has the effect of doubling the intensity of the eel's electric field and intensifying the crippling effect on the prey.

This is because when the opposite "poles" of the fish are brought close together the intensity of the electric field between them becomes much greater and with it the ability to hijack the nervous system of the prey animal.

Subjecting test fish to equivalent electric fields, and then real shocks delivered by the eels themselves, while measuring the effect on muscle constraction in the bait confirmed the disabling effect of the eel's curling manoeuvre.

According to Catania, "we know from basic physics that bringing two electrical poles together concentrates the electric field, and we know from basic muscle physiology that running a muscle too fast for too long causes exhaustion. But I would never have imagined an electric eel could produce the same results..."