The Swimming Battery
The ability for animals to generate, store and release electricity is more common than many people realise. All living animals produce electrical impulses on an infinitesimal scale; they are the medium by which messages are sent along nerves and are discharged whenever muscles contract. But some fish have developed banks of modified tissue, which can produce electricity on a much greater scale. In fact, the ability to generate a sizeable electrical discharge has evolved in fish on at least six different, independent occasions – a remarkable example of convergent evolution.
Electric rays, for example, can produce electricity that ranges from as little as 8 volts up to 220 volts, depending on the species. Some can even temporarily paralyse a human, giving them the alternate name of ‘numbfish’. The Ancient Greeks used electric rays to numb the pain of childbirth and operations, while some Roman physicians recommended that chronic headaches and gout be treated by the application of a live electric ray to the affected area.
But the most famous electric fish of all has got to be the electric eel. Found in the murky rivers, swamps and floodplains of South America, this impressive fish can reach 2.5 m in length, grow to be as thick as a man’s arm, and produce the most powerful electric shocks ever measured in an animal.
Despite its name, the electric eel isn’t a true eel at all – it’s actually a giant member of the knifefish family – but there’s certainly no disputing the ‘electric’ part. Only the front 20% of this animal’s long, cylindrical body contains vital organs; the rest of its internal volume is dedicated to producing electricity. Three different oblong-shaped groups of modified muscles, which have long since given up contracting to provide movement, now act as the fish’s electrical organs. These organs consist of specialised flat, disc-shaped cells called electrolytes, which are stacked very close together. The rapid transfer of sodium ions along the length of these electrolytes generates an electrical current. Each electrolyte produces only a tiny electrical impulse, but if they are all fired together at the same time, the result can be truly shocking.
Normally, the electricity produced by the eel is steady but quite weak – only around 10 volts. These low-level pulses aid communication, navigation, and prey location in murky, stagnant rivers. In uncluttered water, these discharges create a symmetrical electrical field around the fish, which it can sense with a series of receptors in its skin. A solid object with a conductivity different from that of water – whether that be a rock, a plant, or another fish – distorts the field, and the electric eel can immediately sense the change. This ability, known as electroreception, enables the fish to tell the shape and disposition of objects around it, even if visibility in the water has been reduced to zero.
In addition to this minor semi-continuous discharge, the electric eel is also capable of delivering a high voltage shock by using the batteries in another organ. These larger doses of bioelectricity are used to deter predators or stun prey. Bursts of sharp, sudden discharges act like a Taser gun, overstimulating the victim’s nerves, contracting the muscles and temporarily paralysing them. Even fish hiding under the sand at the bottom of the river are not safe. These pulses of electricity can hijack the prey’s nervous system and cause it to twitch involuntarily, thereby revealing its precise location. The electric eel can, in effect, remotely control its prey.
So just how strong can these electric shocks be? For a time, it was thought that the maximum strength was about 650 volts – a little under three times the power of a standard UK wall socket. But recent research suggests that instead of there being just a single species of the electric eel as previously assumed, there might actually be three. And one of the new proposed species, Electrophorus voltai, is capable of producing 860 volts, making it the strongest bioelectricity generator known.
In the past, the electric eel was sought-after by many scientists and zoo collectors due to its unusual abilities. But catching one of these fish was potentially dangerous because it is the only animal that can produce voltage high enough to kill a person. The odds of this actually happening are microscopically small, since even a shock at full power is unlikely to be deadly to a human due to the very short duration of the discharge. However, multiple shocks can cause respiratory or heart failure, and people have been known to drown in shallow water after a paralysing jolt.
The only safe way to obtain an electric eel was to tire it out beforehand by making it continuously release electricity. Some people did this by chasing horses and mules into the water where the fish lived. The surprised eels used their electricity to ward off the invading equines, but the humans kept driving the animals back into the water. Eventually, the horses may have received so many shocks that they were knocked out or even killed, by which time the electric eels’ batteries would have been drained. This allowed the collectors to go in with nets and catch the fish without being electrocuted themselves.
The first man-made battery, created by the Italian scientist Alessandro Volta (after which the unit of electrical potential was named), was partly inspired by the biology of the electric eel. In his experiments, Volta used two discs (one zinc, one copper) to sandwich a piece of paper soaked in salty water, but the amount of electricity generated was tiny – less than a single volt. How could Volta make them more powerful?
Dissections of electric eels provided a breakthrough. Volta speculated that it was significant that the muscles producing the electrical power in these animals were arranged in stacks. By connecting the discs together in a series on top of one another, just like the electrolytes in the electric eel’s body, Volta was able to increase the voltage and produce a continuous stream of electricity. He had created the first electric battery, the voltaic pile.
This very early design, admittedly, didn’t last very long (the paper kept drying out), but Volta and other scientists continued to make improvements. Today, it is almost impossible to imagine a world without batteries. The remarkable electric eel had advanced our understanding of electricity forever.
In a later article, we’ll be looking at another animal with ties to electricity, one that happens to be probably my favourite animal – the platypus. It can’t produce electricity like the electric eel, but it can certainly perceive electrical stimuli. And that’s very unusual for a mammal. But then again, there’s very little about the platypus that isn’t unusual…