The Spark of Life: Electricity in the Human Body (17 page)

Sharks are not alone in being able to detect an electric field – many other fish are able to do so, including catfish, rays, lampreys, lungfish and coelacanths. It is thought that some can even detect the change in the Earth’s electric field that precedes an earthquake. This may be the origin of the Japanese legend that earthquakes are caused by a giant catfish, known as namazu. This fish features in numerous beautiful ukioy-e woodblock prints and, more prosaically, on modern Japanese earthquake early warning devices.

‘Electroreception’ has evolved independently on more than one occasion because the sense organs used to detect electric currents differ between different types of fish. The electroreceptor cells that give the sharks and rays their exquisite sensitivity to electric fields lie in specialized sense organs known as the ampullae of Lorenzini,
⁴
which are concentrated in the head of the shark, around its nose and mouth. We still do not understand how these cells manage to achieve their extraordinary sensitivity. In contrast, the electroreceptors of bony fish are modified from the lateral line receptors that are used to detect movement. Next time you prepare a whole fish for dinner, take a careful look at its flanks. You will see a fine line running along the centre of its side that stretches from head to tail. This is the ‘lateral line’. In most fish, the sense organs that form part of the lateral line perceive changes in water pressure. In a few fish species, however, the lateral line receptors have been modified to detect an electric field.

Adrianus Kalmijn’s classic experiment that showed how sharks use electroreception to locate their prey. The sharks were collected from the English Channel and the North Sea and studied in captivity. (a) A flatfish introduced into a tank immediately buries itself beneath the sand but is instantly found by a hungry shark. (b) The shark still finds its prey even when the flatfish is placed in an agar chamber and covered with sand to prevent visual, mechanical or chemical clues giving away its location. As the agar has the same conductivity as seawater, it does not act as a barrier to electric signals. (c) The feeding response is also abolished by covering the agar chamber with a thin film of plastic, whose electrical resistance is high enough to block out the electric field of the flatfish. This suggests the shark may be detecting the tiny electric currents produced by muscles of the flatfish as it breathes. (d) Crucially, when the flatfish is removed and replaced by a pair of electrodes that emit an electrical signal similar to that of the flatfish, the shark attacks the electrodes and tries to eat them. (e) In fact, it is even more interested in the electrodes than a piece of fish, indicating that at short range the electric field is a much stronger directive than visual or chemical stimuli.

Hunting in the Dark

Some amphibians, such as the axolotl and the giant salamander, as well as primitive egg-laying mammals (monotremes) like the duck-billed platypus, are also electrosensitive. It is no coincidence that all of these animals live in aquatic environments as electroreception needs a conductive medium.

The platypus is a most remarkable mammal that lives in streams in Australia. It is covered in fur, has webbed feet, spurs filled with poison on its hind legs, a flexible, rubbery beak shaped like that of a duck, and it lays eggs. It also has a highly developed electroreceptive sense that enables it to catch prey in muddy streams at night even though it closes its eyes, ears and nostrils during a dive. The skin of the bill is rich in electrosensory cells, as many as 40,000 of them, that are arranged in long rows that run from the base of the bill to its tip. The electroreceptive system is highly directional, and as it hunts, the platypus sweeps its head from side to side. This may help it locate its prey by enabling it to compare inputs from electroreceptors on the left and right side of the bill, in the same way that you swing your head from side to side when trying to locate a sound source. Unusually, the platypus is also able to determine the distance to its prey. It does so by integrating its electrical and mechanical senses, using the delay between the arrival of electrical signals and pressure changes in the water produced by movements of the prey to gauge distance.

The Western echidna or spiny anteater, a terrestrial monotreme, has a similar but less complex electrosensory system. It looks a bit like a long-nosed hedgehog and uses its snout to probe damp leaf litter for the earthworms and other invertebrates on which it feeds. Electroreceptors concentrated in the skin covering the tip of the snout help detect its prey. The short-beaked echidna, which has far fewer electroreceptors, feeds on ants. It is thought it may only use its electrosense after rain, when it feeds particularly actively.

The electroreceptors in monotremes are quite different from those of fish and appear to have evolved from mucous glands. This is helpful for an animal that is not fully aquatic as it ensures the sensory cells remain moist, increasing their ability to detect an electrical signal. It is the bare nerve endings that serve as electrodetectors – there is no specialized sense organ. Although an individual sensory nerve fibre ending has a detection threshold of just one to two millivolts per centimetre, the platypus can detect field strengths almost a hundredfold smaller. The remarkable sensitivity of the platypus probably originates from its ability to integrate information from many thousands of receptors, thus markedly increasing its ability to detect a signal.

The Guiana dolphin lives in the coastal regions and estuaries of the north-east coast of South America, where suspended silt and sediment can cloud the water. It uses electrosensors located in pits in its ‘beak’ to detect the weak electric fields emitted by small fish. It seems likely its electrosense functions as a supplementary means of detecting prey at close quarters.

Finding One’s Way

The shocks produced by the electric eel were a concern to Charles Darwin, who found it difficult to see how they could have evolved since the organ was of no defensive or offensive use until fully formed. What possible advantage could the ability to produce a small electric shock be to an animal, he wondered. But, as we now know, a weak electric discharge is actually of considerable value.

Fish that produce weak electric pulses, just a few volts in magnitude, were discovered in the late nineteenth and early twentieth century. These fish have a sophisticated electrosensory system that combines generation of weak electric shocks with electroperception. It is used for detection of predators and prey and is also invaluable for finding their way around in the murky waters in which they live, where it is too dark to see. Passive electroreception such as that of the shark is like hearing: it simply detects electric fields in the environment. Active electroreception is more like radar: the fish produces an electric field and detects objects by the way in which they distort the field.

The crucial experiments that showed the function of these weak electric discharges were carried out by Hans Lissmann and Ken Machin in the 1950s. Lissman was fascinated by his discovery that the knifefish
Gymnarchus
frequently swam backwards without bumping into anything, that it could navigate easily around obstacles, and that it could locate its prey from some distance away, despite its vision being very poor. There is a story, possibly apocryphal, that the ability of
Gymnarchus
to detect an electric current was revealed when a student combed her hair near its tank and the fish promptly went wild. This may be a myth, but Lissman did report that combing his own hair had this effect (the electrostatic charge generated may have stimulated the fish). By placing electrodes in the fish tank, he found that the fish generated a constant stream of electrical pulses and that it was very sensitive to any changes in the electric field that it set up. His paper concludes forlornly on a poignant note: ‘Unfortunately, while the investigation was still in progress my
Gymnarchus
died, and it appears very difficult to replace this animal [. . .] I should be grateful if anyone who could suggest a possible source of supply would write to me.’

Apparently no one did, because shortly after, in 1951, Lissman set off to Africa to obtain some more specimens. His destination was the Black Volta River in the northern territories of Ghana. During the rainy season the river is extremely muddy as it contains very high levels of suspended particulates. Thus not only is it impossible for the fish to see their prey, it was also difficult for the scientists to see the fish. They detected their presence using a pair of electrodes that they suspended from a long pole on the riverbank (or the boat) and wired up to an amplifier that converted the electrical signal into an audible one. It is possible to ‘hear’ electric fish in this way, and Lissman commonly picked up a uniform high frequency hum of around 300 cycles per second. This enabled him to capture several fish, three of which he succeeded in bringing back alive to Cambridge, where he was able to study them in more detail.

Lissman and Machin set out to test the idea that
Gymnarchus
could detect objects in the water by sensing the distortions they cause in the electric field the fish itself produces. They used porous earthenware pots of different electrical conductivity: some pots were filled with distilled water so that they had low conductivity, whereas others were filled with a concentrated salt solution to simulate the higher conductivity expected of a fish. What they found was that
Gymnarchus
was easily able to discriminate between pots of different conductivity.

The electrosensory apparatus of
Gymnarchus
consists of an electric organ that generates a weak electric field and a detector system that picks up distortions in this field produced by objects in the environment. In effect, the fish is creating an electrical image of their environment analogous to the visual one we use to navigate. The electrical impulses such fish generate are relatively weak – less than a volt. They are produced by an electric organ which works in a similar way to that of the electric eel, but because there are fewer electroplaques the charge they produce is correspondingly smaller. The electric field generated by an electric fish looks somewhat similar to the pattern of iron filings placed around a bar magnet. Lines of force (at the same potential) run from the head to the tail, becoming increasingly weaker the further away they are from the fish. The current flows at right angles to the isopotential lines and thus leaves the fish at right angles to the body and enters it at the tail.

If an object is placed in this electric field it will distort it. For example, if its resistance is greater than that of the water (as in the case of a rock) electrical current will be shunted around the object, causing a local decrease in current density and an ‘electrical shadow’ on the surface of the fish. On the other hand, if the object has a lower resistance – another fish, for example – then the current will preferentially flow through it increasing the current density and creating an ‘electrical spotlight’ on the skin. The closer the object, the larger the spot. By detecting these changes in current strength the fish can not only sense the presence and size of the object but also what it is made of, enabling it to decide whether to attack, run away from, or simply ignore it. Of course, if the object is exactly the same resistance as the water, it is not detected.

The electric field around
Gymnarchus
is distorted both by an object of higher conductivity than the water, like a fish (
left)
, or of lower conductivity, such as a rock (
right
). The lines represent the flow of electric current.

Electroreceptors in the skin of the fish monitor its own electric field and the distortions produced by objects in the environment. In knifefish like
Gymnarchus
, there are about 15,000 of them, concentrated in the head, but also found at lesser density along the top of its back. There is an especially sensitive ‘hot spot’ of receptors located on the jaw. These tuberous electroreceptor organs consist of a small pit, lined at the bottom with sensory cells that act like miniature voltmeters and detect the voltage drop across the skin. They are extremely sensitive: when Machin built an electrical model to try to simulate the fish’s electrical sense the fish triumphed over it every time.