The Kick of an Electric Eel

by CHRISTOPHER W. COATES

1

THERE are so-called lower creatures on earth today which can accomplish electrical feats beyond those of our most advanced laboratories. These living dynamos are all fishes and they hail from Africa, South America, and the temperate and tropical seas. How long they have been electrocuting their enemies and prey is unknown, but fossil fishes from deposits laid down over a hundred million years ago have organs so similar to the electric ones of living forms that there is no doubt they, too, possessed electric powers.

Of all the electric fishes, the best known, both by scientists and by laymen, is the electric eel, and as self-appointed press agent to that fish I have extolled and demonstrated its powers to so many people that I have come to look upon it with paternalistic affection. Our first meeting was not auspicious, however, for in 1929, when I saw my first electric eel sluggishly swimming about at the old New York Aquarium and read on the label that such a four-foot animal could “knock down a horse,” I was more than a little dubious. How could one animal produce a current? Where did the current go in the water and why didn’t it electrocute the fish itself?

The scientific literature on the subject did not dispel my doubts either. Although such illustrious scientists as Galen, Lord Cavendish, Sir Humphry Davy, and Michael Faraday had all worked on electric fishes, none of them offered a satisfactory explanation of the fishes’ powers or even a measurement of their potency. Faraday did determine that externally the current from an electric eel flows from head to tail, but that was about all. I hoped I could do better.

My chance came a few years later when, as a member of the Aquarium staff and armed with assorted wires and lamps, I set out to trap the eel’s current. I learned the hard way, first by being literally knocked off my feet when I attempted to pick up the fish in a net. Maybe it could down a horse! I gingerly noosed a bit of wire around the front part of the fish’s elongate body, carefully holding on to the insulated portion, and did likewise with its tail. Now a lamp joining these two wires should light most brightly. But when I introduced the lamp into the circuit nothing at all happened, not even the slightest glow. With my first two fingers I touched the ends of the wires, but did not feel anything until I gently prodded the eel with a dry wooden handle of a net. The tingle in my hand proved that the fish was giving off electricity, but there was still not a sign of response from the lamp. What kind of electricity was this?

From here on my investigations took a most discouraging turn. I was convinced that until I could measure the eel’s electricity, or at least prove that it was electricity, I was in no position to approach some hard-working physicist or biochemist and request that he devote some of his time to my pet interest. Nothing worked. Voltmeters, ammeters, and galvanometers either burned out or failed to register any “juice” at all, while electric lights both in water and out reacted not one bit. It seemed as if the only way to gauge this strange power was to feel it—a nerve-racking process. To the detriment of my pocketbook and the chagrin of my friends (who had lent me some of the instruments) I continued this unsuccessful experimenting intermittently for over a year.

Everyone who came into contact with the eel swore that it was electric — in fact many simply swore. One time I was trying to unload a whole hogshead of eels stored ‘tween decks of a steamer. I had on a pair of heavy rubber gloves and we tried to move the hogshead towards the hatchway but made no progress at all, because the container was much too heavy for me to handle alone, and when the longshoremen tried to help, their bare hands were continually subject to shock. One of them, a giant of a man, said he would fix it and, hooking in his dog hook, gave a mighty heave. Over went the hogshead, out came the water and the eels in a slithery, slimy cascade, and up and down went all the people in a mad dance to get their feet off the steel deck. Imagine fourteen writhing giant eels — some of them nine feet long — all giving off several hundred volts of electricity at the same time. I am still not sure how we managed to get them back into the hogshead and finally into the Aquarium.

The turning point in my investigations came one Saturday afternoon. I was trying out a new kind of lamp, a neon one — not the kind employed for advertising, but a small bulb or pip such as is used for a night light. The laboratory floor was liberally splashed with water and was covered by a maze of wires. Things were both literally and figuratively at loose ends, for the electric eel that was the subject of the experiments had been behaving badly. At times like these I try to remember the “Harvard law of animal behavior” ¹ and laugh the trouble off, but this afternoon I was tired and discouraged and careless. Suddenly, I received the full shock of the eel, was thrown several feet into the air, and at the same time saw the neon pip burst into a dull orange glow. So intense was the jolt that I suspected the glow to be an illusion caused by the shock.

None too hopefully I traced out the circuit. Apparently, the pip was in simple direct circuit with the fish. I disturbed the eel with a rubbergloved hand and again the light flickered on and off. All at once it dawned on me why I had been unable to register the current before. Ordinary electrical apparatus has an appreciable warm-up time, or lag, as it is called, before it begins to operate; commercial incandescent lamps have a lag of about one fiftieth of a second, for example. The electric eel’s discharge was powerful enough, but of such short duration that it failed to excite the usual instruments and lights. I now also knew that the potential was at least eighty volts, because that was the minimum necessary to light such a neon pip.

2

Electrophorus electricus, as ichthyologists call it, looks like a caricature of an eel, being eel-shaped but completely lacking the sinuous grace and streamlined contours of the common fish-market variety. It is a much stouter creature, a specimen forty-eight inches long having a girth of over a foot. From its bluntly rounded head to the tip of its clumsy tail it is a more or less uniform dull gray in color, with the exception of an expanse of salmon red on its throat and chin. This area is yellowish green in eels from Venezuela. The Amazonian variety, which I am here describing, belongs to the same species. Its skin is naked and wrinkled, with small flattened papillae irregularly distributed over it. Two absurd little round fins stick out on either side of its head like the ears on a cartoon character. No fins at all are to be found on its back, but one long continuous fin stretches along the posterior four fifths of its underside. By undulating this the eel moves about, seemingly as easily backwards as forwards.

Practically nothing is known about the life history of the electric cel. For example, we are not absolutely positive whether the fish lays eggs or gives birth to live young, although our anatomical investigations strongly point to the former method of reproduction. No one has yet seen an electric eel’s egg with the embryo eel developing in it.

Very small eels have never been brought to this country, but collectors in South America have reported finding specimens an inch or so long at the ebb of flood waters towards the end of the rainy season. They are reported as accompanying an adult fish, sex unknown, swimming in a more or less compact group of from fifty to five hundred about the head of the presumed parent. At this size they can produce a discernible amount of electricity, although no more than enough to tingle the cupped hands in which they are held. The collectors are quite certain that the adults swim over the inundated lowlands during the wet season to spawn, not returning to the streams and rivers until the receding floods force them back. Fish six or eight inches long apparently have left the parental brood and seem to be on their own. At this stage of development the amount of electricity per unit length of eel is higher than at any other, for while the average voltage per centimeter of a large eel may be eight or ten, that of a small one is nearer thirty for each centimeter of electric tissue. Since any voltage smaller than this would be inadequate to discourage enemies or to immobilize food, it seems reasonable to assume that the young eels remain under the care of their parent until they are large enough to protect themselves with their own discharges.

My first device for actually measuring the electricity of the eel was a simple series of neon pips connected with different resistances. With these I was able to make good estimates of the fish’s voltage and found that it ranged to over three hundred and that Faraday had correctly determined the direction of the flow of current — from head to tail outside the body of the fish. Now I felt I had sufficient tangible evidence to approach some specialist for aid in further investigations. But biophysicists or physicists interested in animate things are few and far between, and I “peddled” my eel at a number of different institutions before I discovered one sufficiently interested in “eel-ectricity ” to work with me on it. Dr. Richard T. Cox, at that time Professor of Physics at New York University, was already up to his ears in nuclear physics, but he agreed to come down to the old Aquarium and try out on the eel the new portable model Cathode Ray Oscillograph he had recently received.

Like a neon light, the oscillograph has no appreciable lag, and this made it suitable for picking up the eel’s discharges; best of all, it gave a graphic picture of them, providing the means whereby the electricity drew a picture of itself on a fluorescent screen. We came up with some amazing results. We recorded voltages up to 550 and found that the average discharge in water amounted to about forty watts — more than enough to stun a man or a horse. Each discharge lasted only two onethousandths of a second, but the eel could send out four hundred or more per second!

3

THAT a creature of flesh and blood could energize so much seemed fantastic. However, we soon learned that the eel was largely composed of a very special kind of flesh: electric tissue. All its vital organs — stomach, intestine, liver, and so forth — are confined to the front fifth of its body, and even its vent is located under the chin; the remainder of its elongate body is principally occupied by three pairs of electric organs. Measured by volume, nearly half of the fish is electric tissue. These organs are, in turn, made up of smaller units, which are separated by thin walls of electrically resistant tissue and act very similarly to cells in a storage battery. They are the producers of the electricity, each one creating about one tenth of a volt. It is by hooking these tiny batteries together in series, so to speak, that the eel builds up its powerful discharge. Just how it does this, throwing thousands of “switches” on and off hundreds of times a second, we do not know. That is another of the mysteries of the electric eel.

The electric organs of the eel are in three pairs: the Large Organs, the Bundles of Sachs, and the Organs of Hunter. The Large Organs are apparently so called for want of a better name; the Bundles of Sachs were named for Dr. Carl Sachs, the naturalist and Amazon explorer, who devoted much time to the study of the eel in the 1870’s; the Organs of Hunter were first described by Dr. John Hunter, the eighteenth-century anatomist. The first of these is functionally the most important, and begins at about one fifth of the length of the fish behind the snout, continuing unchanged to a point about twothirds the length of the fish behind the snout. From this place on, it tapers off and the resulting space is taken up by the Bundles of Sachs, which grow in size as the Large Organs diminish. The Bundles are responsible for the small discharges apparently used for locating food. The third pair of organs, those of Hunter, start at the same level as do the Large Organs and run to the end of the tail. In cross-section these are very small and their discharge is irregular and appears to be a function of that of the Large Organs. In relation to the area of the whole crosssection of the fish at a point midway from head to tail the electric organs occupy about 55 per cent. The appearance of electric tissue is different from any other. It is a flaccid whitish jelly and, by analysis, is composed of 92 per cent water.

Another mystery of the electric eel is its sensitivity to electric currents. When I finally got the fourteen eels — the ones that had been spilled out of their hogshead on the steamer deck — into a tank at the Aquarium, I noticed that the fish were aware of each other’s discharges. (This was apparently the first time anyone had made any observations on more than one eel at a time — at least anyone who cared to talk about it.) The natural food of electric eels consists of fishes and other small aquatic animals which they stun before swallowing whole. Although they can be taught in captivity to eat cut-up raw fish and strips of beef, when first caught they will eat only live fish. While feeding these fourteen eels I discovered that when one fish discharged, stunning its prey, all the others in the tank came over, apparently to see what was going on, and they always went to the spot where the feeding eel had discharged, even if it had subsequently moved away. Apparently, eels were not only aware of one another’s discharges but could nicely judge whence the current came.

Later on, Professor Cox and I “electrolyzed” a tankful of eels by dropping an electrode at each end of their tank and passing a strong current through the water. All the eels then gathered at the anode or positive pole. This was reasonable, because the head of a discharging eel is always positive in respect to the body behind it, and the head would be exactly where a hungry eel would want to go, when he sensed his brother eel shocking some prey.

We now wanted to find out whether captive electric eels behaved like wild ones, since our researches were more or less based on the proposition that what the fish did in the Aquarium’s tanks and laboratory was “normal.” For this purpose Professor Cox established headquarters at the Goeldi Museum in Para at the mouth of the Amazon.² What he found confirmed our supposition: eels in Brazil behaved no differently from those in New York. However, he also discovered some electrical activity we hadn’t noticed before. When lying quietly on the bottom, only moving occasionally to come to the surface for a gulp of air, — the fish is an air-breather and drowns if kept under water, — electric eels give off no electricity at all, but while “cruising about” they emit a series of weak discharges having a voltage of the order of fifty and at a rate of about fifty per second.

We first believed that these were warning discharges by which the eel kept potential enemies at a distance, and we were supported in this view by some observations on the eel’s closest relatives. These are fishes of the family of gymnotid eels of which the electric eel is a member — elongate fishes, not at all related to the true, edible eel, but rather to the infamous piranhas of South American rivers and the radiant neon tetras of tropical-fish fame. Their popular name is knife fish, and a triangular carving knife, minus its handle, gives a good approximation of their body shape. None of these “cousins” to the electric eel has any electric powers, and all of them are more or less likely to have their long tails nipped off in the course of growing up. In fact, in some species it is almost impossible to find an intact specimen. Electric eels, on the other hand, almost never show such mutilations.

Undoubtedly the electric eel’s discharges protect it, but the steady repetitions of minor discharges which it emits while swimming serve another purpose, quite as utilitarian. We discovered this use by carefully observing the behavior of a lone eel. Adult electric eels are virtually blind, since they develop cataracts when quite young, undoubtedly as a consequence of either their own electric shocks or those of their fellows. Nevertheless, when a food fish was put into its tank, the lone eel unerringly made for it. That the eel could see through the clouded lenses of its eyes seemed unbelievable; yet to be positive that sight was not being utilized, we repeated the test in nearly total darkness. Not only did the eel easily find the food fish in the dark, but it could even distinguish between a dead floating food fish and a piece of floating wood of approximately the same size.

Was the eel using its weak discharges to locate the fish? We couldn’t stop the eel from broadcasting its current, so perhaps we could stop it from receiving that current back, and this we did. Arranged in definite patterns on both sides and the top of an electric eel’s head are series of prominent pits. No such development is found in other gymnotids; therefore we thought these might in some way be associated with electric powers. We painted the head of our eel with an insulating lacquer, thus sealing off the pits from any possible electric impulses. Sure enough, that fish now failed to find any prey, living or dead, dropped into its tank, and simply swam aimlessly around. If the food fish was placed on its lips, however, it would gulp the meal down; and when the lacquer was removed, the eel once again easily located its food.

The details of this wonderfully acute eel “radar” have yet to be worked out; we believe it operates something like this. The minor discharges of the eel radiate out in all directions from the rear portion of its tail, the area in which they are produced. Whenever they come into contact with some solid object in the water, they are reflected back towards the sensory pits on the eel’s head. By turning its head and discharging again, the eel can so orient itself that both left and right sides receive the reflected pulses simultaneously. Then it is pointed towards the reflecting object. When we realize that this locating of an object is done through a complex “background” of reflected impulses (from the walls of the tank, for example); that although the impulses travel at the speed of light, the eel can differentiate between differences of a few inches; and that the eel can make allowance for its own movement as well as that of living prey, the truly amazing character of this behavior is apparent. Nothing I have ever learned about this incredible creature has astounded me more.

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ELECTRIC fishes in general have played a long role in medical history, if not always an honorable one. No one knows how long the Indians of Surinam have employed the discharge of the electric eel in the treatment of disease. Even today in Brazil its flesh is considered by some to be a cure for rheumatism. One learned rheumatic doctor, however, told Professor Cox that he had eaten eel meat and, of the two, he considered rheumatism the more bearable. Dr. Peter Kellaway of McGill University has made a thorough study of the use of electric fishes in medicine, and the first record he found of the therapeutic use of these creatures’ electric powers was in A.D. 46 when one Seribonius Largus, a Roman physician, claimed the shock of the torpedo to be a cure for headache and gout.

For any type of gout, a live black torpedo should, when the pain begins, be placed under the feet. The patient must stand on a moist shore washed by the sea and he should stay like this until his whole foot and leg up to the knee is numb. This takes away present pain and prevents pain from coming on if it has not already arisen.

The discharge of electric catfishes, too, has been utilized since ancient times by African tribes as a cure for various ailments, and one Moslem physician of the eleventh century thought that to place a live electric catfish on the brow of a person suffering an epileptic fit was beneficial. Electrotherapy was all the rage in Europe during the eighteenth century, and electric fishes vied successfully with Leyden jars and other shocking machines. When many electric eels were imported for this purpose, people flocked to them for relief from gout, rheumatism, and other ills. One London advertisement of 1777 gives a price of two shillings and sixpence per treatment of “natural” electricity from a “torporific eel.”

The importance of the electric eel in present-day medical science rests not with its use as a shocking machine, but with its physiology. The physicochemical reactions that take place in a nerve whenever an impulse passes over it are of prime importance to physiologists, and books have been written on this subject alone. A nerve is so small, however, that to determine quantitatively just what chemical reactions are taking place inside it is next to impossible. But the discharge of an electric eel is identical in nature with the passage of an impulse over a nerve, and since thousands of times more tissue is involved, the chemical changes can be much more easily measured.

Briefly, this is how such an investigation is made. A tiny piece of electric tissue is removed from a resting eel; then the fish is made to discharge and the amount of electricity produced is carefully recorded. Another small piece of electric tissue is now taken out. By chemically analyzing these two pieces, the changes which took place in the electric organ during the electric discharges are determined. This amount of change is then correlated with the amount of electricity produced. Studies like this have enabled Dr. David Nachmansohn of the College of Physicians and Surgeons to confirm certain basic theories on the nature of nervous activity — a matter of fundamental importance to biologists, physicians, neurologists, and psychologists. And lest it be thought that such treatment of an eel injures it, let me point out that not only is the fish apparently unaware of the operation, since it does not discharge during the process, but so great are its regenerative capabilities that in less than a month not even a scar remains where the pieces of tissue were removed.

One of the key substances in the production of electric energy in electric fishes, and in the production of a nervous impulse too, is cholinesterase. During the war, it became necessary to obtain large amounts of this rare chemical, which cannot yet be synthetically produced but must be extracted from living tissue. Ounce for ounce, the electric organs of the eel are far richer in cholinesterase than any other known tissue; so when the Chemical Warfare Service called for large amounts of it in order to study the effects of a new, deadly nerve gas they were investigating, scores of eels were sacrificed to provide them with the precious substance.

Not only is the electric eel well known to scientists these days; it is familiar to laymen as well. Thousands of people witnessed a demonstration of its powers in the New York Zoological Society’s building at the World’s Fair, and even more now come to the Lion House in the Zoological Park in the Bronx to see it. But tall tales die hard: the one Humboldt told about the electric eel still circulates around, even in some high-class encyclopedias and natural history books. Humboldt reported that South American Indians capture the eels by driving horses into water containing the fish. When the eels have exhausted themselves shocking the horses, the Indians harpoon them and remove them from the water with impunity. Our investigations have proved that this notion is tommyrot, because an electric eel can’t be exhausted so easily. If kept moist, so that it can breathe properly, it will discharge intermittently all day long without showing appreciable fatigue.

Even after giving off electric shocks at its greatest rate, for twenty minutes, a five-minute rest is all that is needed to bring its activity back to normal. In fact, one of our leading battery manufacturers has been studying the eel, trying to pick up some pointers on current-production. One of the company’s experts wistfully remarked to me the other day: “If we only could make batteries that would operate as efficiently as that eel!”