Sunday, December 24, 2000		Nature

The charge of the fish brigade
By Nutan Shukla

IN some species of fish, locating prey, killing it, avoiding obstacles and talking to the neighbours can all involve electricity. Skates and rays, like their near relatives, the sharks, have electroreceptors on their snouts with which they can accurately detect buried food. But some fish can generate their own electricity.

A group of fresh water fish, known as the weak-electric fish, surround themselves with an electric field which can be turned off and on at will. In muddy waters, where the visibility is very poor, these fish use their electrical field to find their way, but they can also communicate with it and investigate potential food items.

In the process of locomotion, if the field is interrupted, they know there is an obstacle to be avoided or prey to be caught. To obtain more detailed information about the object, some species, such as South American and African knife fish, can focus their electrical sensing systems. By bending their tails round an object in the water, they can concentrate the self-generated electric field on the object and ‘interrogate’ it to find out more about it.

These fish can also ‘talk’ to each other using electrical signals. Each species has its own signal pattern that contains information about its identity, sex and age, as well as about its readiness to mate or to defend a territory. During ‘electro-arguments’, the electrical pulse rates of the fish rise sharply in an attack pattern. Friendly fish are known to change their broadcast wavelengths in order to avoid jamming the signals of another fish.

The electrical field is generated by modified muscles or nerves, depending on the species. The eight-inch long, elephant trunk fish and the South African and South American knife fish have modified axial and tail muscles.

At the other end of the scale are the ‘strong’ electric fish — the stunning electric eel of the Amazon, the electric catfish of Africa, and the formidable electric torpedo ray of the shallow sea. These fish not only locate their prey with electricity, but also store such an enormous charge that they can knock out an adult person in one mighty discharge.

The electric ray was known by the ancient Greeks, and its Greek name ‘narke’ or numbfish gave rise to our currently used word ‘narcotic’. The numbfish was thought of as a magical beast, bewitching its prey and any unfortunate fisherman who happened to tangle with it. In the olden times it was believed that a pregnant woman was assured an easy birth if a numbfish was placed in the delivery room.

Today, the electric ray is as revered as it was in the time of Socrates. Growing to over 6ft (2m) and weighing over 2001b (90 kg), the torpedo ray produces and stores its electrical charge in two, large blocks of modified muscle on either side of the flattened body. Discharges of up to 220 volts have been recorded. The fish can make an electric light bulb glow, but only on the first few attacks. The electric organ is quickly spent and the fish must retire, quite literally, to recharge its batteries.

The freshwater electric catfish, which can grow to 3.3ft (1m) in length and weigh up to 441b (20 kg), delivers a 350-450 volt blow, and the 9ft long electric eel — not an eel at all, but a relative of minnow — is capable of delivering a staggering 550 volts, not just once or twice, but several times a minute for as long as it wants.