Facts Explained: Electric Fish

Posted on 19th Sep 2020

Post Category: Animal Science

Why some fishes are live electric batteries?

• Electric fish are unique creatures that differ from their brethren in that they carry living galvanic cells with them.
• What purpose the electric organs of these unusual fishes serve is still a matter of debate, However, on theory is that in the distant past, some evolutionary advantage was provided by this trait, which is now a leftover.  The electricity they generate serves as a form of defence and attack.
• It is interesting to note that among fossil fishes there were more electric species than presently exist. Apparently, the use of electromagnetic forces was not so effective as perfecting other systems that are not so obvious, first of all, the muscular systems.

The most brilliant representative of this kind is the torpedo fish, which lives in warm seas, weighs about 100 kilograms and grows to about two metres in length. Its electrical organs, which are located laterally along the head, weigh over 16 pounds. A torpedo fish is able to produce an electric shock of 8 amperes at a voltage of 300 volts. This is definitely dangerous to human beings. 

• Roman doctors used to treat gout by making their patients stand on a Torpedo until they were numbed to the knees, and they- even tried to cure chronic headaches by wrapping a live Torpedo round the forehead. South American Indians are said to use electric eels in a similar way, to treat cases of paralysis.
• The natural thing is to suppose that electric fish are not very sensitive to electricity. Which is actually the case, for the torpedo fish can easily stand up to voltages that kill other fishes.
• The electric organs of the torpedo fish are remarkably similar to a battery of galvanic cells. They consist of numerous plates arranged in piles (the cells are connected in series) which are situated one after the other in a number of rows (this is a parallel connection). One side of each plate is smooth and carries a negative charge, the other side has projections and is charged positively. The whole set-up is contained in an insulating tissue, as one would suppose. 
• We shall not try to delve deep into the mechanism for developing electromotive force in the organs of the torpedo fish. There are many obscure points here. We are sure about one thing, however, and that is that (like in the galvanic cell) chemical forces lie at the heart of the operation of these electric organs.

• We shall not take up more electric fishes but just must dwell for a moment on a remarkable inhabitant of the Nile River-the mormyrus.(See Diagram) This fish is equipped with a marvellous radar system. At the base of the tail it has a generator of alternating electric current that sends out impulses with a frequency of several hundred oscillations per second. The surrounding objects distort the electromagnetic field about the fish, and this is immediately recorded by a receiving device on the back. This radar system is extremely sensitive. The mormyrus cannot be caught in a net. In an aquarium, it will become agitated even when you draw a comb through your hair.
• We still don't know exactly how the radar works, but there is hope that a detailed study of the problem will help perfect submarine electromagnetic communications, which has long been a stumbling block due to the rapid attenuation of electromagnetic waves in water.

The power-packing electric eel is quite a stunner. In 1941 two men who fell into a research pool containing electric eels owned by the US Army were killed instantly 

For many of us, the only fish we associate with electricity are electric eels, and we assume that they would zap anybody who gets too close. However, electric eels are just one of many different species of fish capable of electrogenesis. The various types of these live-wire water-dwellers fall into two distinct categories - weakly electric and strongly electric fish. In both groups, electric transmission, as in visual systems, is nearly instantaneous and is little affected by 'noise' in a system, but doesn't go far and thus is effective only over short distances. Like chemical and sound communication, an electric signal can also pass around objects.

Strongly electric fish produce much higher voltage pulses than their weakly electric cousins. The strongly electric fish category includes not only the electric eel, but the electric catfish and the electric ray or torpedo. Fish with electrogenesis usually produce electric charges through electrocytes or electro-plaques - typically flat, disc-like modified muscle or nerve cells all stacked together. As mentioned above, in marine fish, these are connected like batteries in a parallel circuit, whereas in freshwater fish these are connected like batteries in series. These latter are capable of producing discharges of higher voltage, necessary as freshwater doesn't conduct electricity as well as saltwater. Each cell in the battery can produce nearly 0.15 volts by pumping out positive sodium and potassium ions.

The electric eel has around 5,000 or 6,000 electro-plaques in its abdomen, allowing it to generate shocks as strong as 600 volts to stun its prey. It can also lower its voltage pulses to around 10 volts to navigate and to detect prey. As a result of its electro-generative abilities, the electric eel is found only in freshwater habitats, as saltwater can have the unhelpful effect of causing the fish to naturally short-circuit.

• The electric ray, which has no such problem, has a pair of special kidney-shaped organs at the base of the pectoral fins that generate and store electricity and send out charges from 8 to 220 volts to electrocute prey or to stun a possible predator.
• Electric catfish, common in African freshwaters, generate their electrical discharges from their skin rather than from electric organs that consist of individual electro-plaques.
• When a foraging torpedo ray detects prey it swims forward and upward, exposing its ventral surface towards the fish while emitting low-frequency voltage pulses. The currents passing through the victim's body excite its nerves and muscles, stunning it and immobilising it, whereupon the torpedo descends over it and consumes it while continuing to emit pulses. Large Atlantic torpedo rays can generate enough power to produce a shock of up to 220 volts; that's enough voltage to run your everyday appliances, such as a mixer or clothes dryer. Of course, animals can't deliver such voltages in a sustained way, as the purpose is simply to stun the prey. So it's more like carrying a stun gun inside your body. Smaller rays, like the lesser electric ray (Narcine brasiliensis) can only muster a shock of about 37 volts because their prey are smaller in size.

Weakly electric fish, like the elephantnose fish, use their electro-generative ability either to navigate or locate prey or to communicate.

Instead of the electro-plaques as in the electric eel, they have electric organs consisting of columns of electrocytes that generate relatively feeble electric fields. This type of active electrolocation relies on the ability of the elephantnose fish and other species that use it to detect any distortions in an electric field of less than 1 volt.

Although it's not well understood how the brain extracts all of the sensory information for active electroreception, the sensors (also called electroreceptors) in the skin are sensitive to the rate of change of voltage across their cell membrane.

In contrast, sharks and rays, as well as most species of catfish, use passive electroreception where the animal senses the weak bioelectric fields generated by other animals. Sharks are the most electrically sensitive animals known; they respond to DC (non-alternating current) fields as low as 5 nano volts per centimetre and can use this ability to detect a small fish buried in the sand.

• In nearly all-electric fish the discharges are produced by discrete electric organs, which consist of modified muscle. These organs have been carefully studied in the electric eel, which can produce discharges of over 500 V. The large electric organs of the electric eel run along most of the body, one large mass on each side, and make up about 40% of the animal's volume.
• When the organ is at rest, the two surfaces of the electro-plaque are positively charged on the outside, each being +84 mV relative to the inside. The overall potential from the outside of one membrane to the outside of the other therefore is zero. During a discharge, however, the potential on the innervated surface is reversed, and the total voltage across the single electro-plaque becomes about 150 mV.
• With the serial arrangement of the electro-plaques, the voltage adds up as when we connect a number of batteries in series. With several thousand electro-plaques arranged in this way, the electric eel can reach several hundred volts. (Because of internal losses the voltage does not reach the full magnitude of an ideal serial connection of the electro-plaques.)
• The largest and most powerful of these electric organs are those of the torpedoes, dwellers of shallow tropical and temperate waters. Some are very small, others reach up to five feet from wingtip to wingtip. A large torpedo ray may have as many as 1,050 electroplates linked in series on each side of its body, and shocks of 220 volts have been measured-twice the voltage of ordinary household current and more than enough to knock a man down. The discharge goes from the belly to the back, the blood-vessel side of the electroplates being on the upper side.
• Just what purpose these electric organs serve is still a matter of debate among scientists. Most authorities agree, however, that they are used defensively in almost all cases and offensively on occasion, according to the fish's feeding habits. It is possible that they may be used also for identification in areas where visual recognition is difficult-setting up lines of electric force in the water around it, the fish may be able, for example at breeding time, to keep other species at a distance while seeking its mate.