Question of the Week

Self-Shocking Electric Eels?

Sun, 10th Feb 2008

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Robin, California asked:

I have a question about electric eels. I was wondering how they, themselves, do not get hurt by the electric shocks that they use to communicate or stun prey. And if they are in water how far does the current carry?


We put this question to Dr Mark Briffa,  Lecturer in Marine Biology at the University of Plymouth:

The Electric Eel - Electrophorus electricus First of all, an electric eel isnít really an eel.  Itís a member of a family of fish called gymnotids or knife fish and the scientific name is rather aptly, Electrophorus electricus.  In most animals electric currents are used in nerves to carry information and to control muscle movements.  What makes fish like electric eels special is that they can exploit this property of muscle tissue and discharge bursts of electric current into the surrounding water.  They do this using an electric organ made out of modified muscle tissue in the tail.

If electric eels can generate these strong discharges, what stops them electrocuting themselves?  First of all the electric organs are in the flank of their tail and this means that the discharge occurs straight into the water rather than the eels internal organs.  Since water is more conductive than body tissue the charge travels away from the eels.  The second possibility, and thatís that because the discharge is very quick the current doesnít flow for long enough to shock an animal as large as an electric eel.  But for its prey which are usually smaller fish the short burst is enough to shock and stun or sometimes even kill them.  So electric eels are unlikely to cause a damaging electric shock to humans but itís still not going to be a very pleasant experience to be on the receiving end of one and an impressive 2m long pet electric eel probably isnít a very good idea.  Itís quite difficult to work out how far the discharge will travel but prey being stunned or killed up to a distance of 2m away from the eel has been reported.

We also received this from Dr Carl Hopkins, Cornell University:

Electric eels generate electricity in much the same way that every nerve and muscle cell in an animal's body does, by establishing a weak voltage across the cell membrane.  In fact, the electric organ is believed to be derived from muscle cells, which over the course of evolution, have lost their ability to contract as muscle, but retain their ability to produce electric currents.  The big change during the evolution of the electric organs of fishes is the fact that the weak signals present in individual cells add up in series, like batteries put end to end in series in your flashlight.  In addition the time of the discharge is precisely synchronized so that the short pulses occur at exactly the same time - a necessity if the voltages are to add.

In order for an animal cell (or any cell, for that matter) to generate an electrical current, it must first use energy from food and metabolism to pump charged potassium and sodium ions across cell membranes so that potassium is high in concentration in the interior of the cell and sodium is concentrated outside the cell.  Once the concentration gradient is established, for both of these ion species, the next requirement is that there be a "pore" in the membrane that allows possitively charged ions to go through without letting negative ions through.  In animal cells, the membrane is permeable to potassium but not sodium and not negatively charged ions like chloride.   Potassium ions start to diffuse out through the membrane and this establishes a voltage across the cell membrane (about 80 millivolts negative on the inside).  This is the so-called resting potential.  Then, when the electric organ is activated by nerve impulses, sodium ion channels open in the membrane to allow sodium to flow into the cell, changing the membrane voltage from -70 millivolts (mV) to about + 50 mV, a  change of 120 mV.  This is a transient event lasting only 1 millisecond or so.  Since all of the cells in the electric organ discharge synchronously, their voltages add up to produce a substantial signal in the water.  This is the electric discharge.

To get the weak 130 millivolts discharge large enough to do anything outside the animal, the electric fish arranges the cells like a series of poker chips in a pile -- then the 130 mV signal across each cell adds to the next one. With the electric eel where there may be 5,000 cells arranged in series, the voltage will be 5000 x .130 Volts = 650 Volts.  The cells are also arranged in parallel, which increases the ability to generate current in the water.

Nobody knows exactly how the electric eel keeps from shocking itself, but the best working hypothesis is that the vital organs like the brain and the heart are located as far as possible from the electric organ (up near the head), surrounded by fatty tissue that acts as an insulator.  In cross section through the tail, the electric eel is nearly entirely electric organ.


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I'm sure we had a similar question here a while ago. DoctorBeaver, Wed, 6th Feb 2008

Yes, we did - but I hijacked it into a discussion about sharks  DoctorBeaver, Wed, 6th Feb 2008

Yes you did Doc.
Together with George, and Alandriel, last October. iko, Wed, 6th Feb 2008

DoctorBeaver, Wed, 6th Feb 2008

I imagine electric eels must have some equivalent circuit to what is used in Radar sets. Without taking precautions, the sensitive receiving circuit would get blown up by the enormous pulse from the transmitter (they share the same antenna). Despite the use of a duplexer which directs the worst of the transmitted pulse power into the antenna and ensures that a smaller pulse is presented at the receiver input there are still large voltages involved.
This problem is solved by having, effectively a fast acting diode across the input of the receiver. It behaves as if it were not there for low received signal voltages but behaves like a short circuit across the receiver input when the big pulse comes along and 'fires it'. I suspect that every nerve cell in the eel's body must operate in a similar way.
No doubt, there is some price which the eel has to pay for this in the form of poorer performance in some other respect. lyner, Thu, 7th Feb 2008

Well, that was sure the long way round to get to the point. The money quote: "Nobody knows exactly how the electric eel keeps from shocking itself..." Geez...if you don't know, just say so. The best guess is called a "working hypothesis". The question of how far the current travels in water was ignored. Here is a follow on question. How did such a system "evolve" since the Cambrian Explosion? What would the transitional form be? Skeptic, Sun, 8th Feb 2009

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