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Electric Eels and Knife Fish

The Amazon river is very murky, making it very difficult to see underwater. Amazingly, some fish have managed to get around this problem by sensing things around them – using electricity. All creatures generate electricity naturally, though only at very small levels. Electrical signals are used to send messages back and forth along our our nervous systems. When a muscle contracts it generates an electrical charge, firing an electrical signal back to the brain – telling the brain that the muscle has contracted. But electric eels and electric knife fish generate electrical in a similar way, but on a far greater scale.

The purpose of these electrical signals is to help the fish sense the objects, and other fish, around it. The electrical signals aren't very strong, they range from three to ten volts. By the signals are generated at high frequency, roughly three hundred times per second. Each electric pulse generates an electric field around the fish – and the fish can sense disruptions in the field. Disruptions are caused by objects, such as rocks, plants, and other fish. Although the technique works, it has interesting side-effects. One is that the electrical signals generated by one fish can interfere with another. When two electric fish encounter each other, both stop generating, then choose different frequencies. There are two types of electric fish in the Amazon – the electric eel and the knife fish.

The knife fish has its name because its shape resembles a knife or dagger. It is about 20 centimetres (8 inches) long, with a body which tapers down from the head to the tail, which forms a point. The knife fish doesn't have tail fins or a dorsal fin. Its only fin is long ribbon-like fin which runs along its underside. It swims by undulating the fin while keeping its body rigid – a method which it has been forced to adopt since bending its body like other fish would cause disruptions in the electric field, rendering its sensing system useless. This swimming method means that the knife fish can move just as well backwards as forwards.

The electric eel isn't really an eel at all. Its just a fish than looks like an eel. It's much bigger than the knife fish, and reaches 2.5 metres (8 ft) in length. They have thick round bodies and a stubby round head. Like the knife fish, the electric eel uses electricity for sensing, and keeps its body straight while swimming. The eel is also capable of generating much stonger charges – using large discharges to stun and kill its prey. It can reportedly discharge 500 volts at one amp – which is strong enough to knock over a horse standing in shallow water, and severely shock a human. Repeated discharges can kill. People wading in muddy channels in the Amazon region must take great care since this is where electric eels are most commonly found. Lack of oxygen in these waters means that electric eels can sometimes be seen gasping for air at the water's surface.

European scientists first became aware of electric fish from the reports of Frenchman, Charles Marie de la Condamine, who descended the Amazon in 1743. He reported that mild electrical charges were received from the flesh of a dead fish brought in by Indians. European scientists  were so sceptical of these reports that many refused to believe these reports even though de la Condamine was, himself, a respected scientist honoured by the Academie Francaise.