Why Don't Electric Eels Kill Themselves?
One of the fun things about doing programs with children is that they will frequently give you a question that is so obvious that you wonder why you have not asked it yourself. Recently I had a little girl bring me a picture of an electric eel lighting up a florescent light bulb. "Why doesn't the eel shock itself?" she asked. I stammered for a while, and then finally told her I would have to study it and let her know--which I have done.
This turns out to be a very complex question. The eel generates electricity through thousands of disk shaped electrogenic cells. Each cell carries a negative charge of about 100 microvolts on the outside of the cell compared to the inside, and each cell is connected to a nerve terminal. When the signal comes to the nerve terminal, it emits a puff of acetylcholine, a neurotransmitter, which creates a path of low resistance between the inside and the outside of the cell, essentially turning each cell into a little battery. The cells are arranged in series so they act like a bunch of batteries put into a big flashlight, but there are enough cells to be equivalent to a 500 volt battery. This all happens in two milliseconds.
The reason the eel does not shock itself is that the electrical shock is distributed by its whole body, which is roughly the size of an adult man's arm. To make muscles in an arm to spasm you need 200 milliamps of current flowing for a minimum of 50 milliseconds. The animal's prey is smaller than the eel--a fish one tenth the eel's length would be one thousandth the eel's volume. The skin of the eel has a higher electrical resistance than a fish does, so the electrical flow will go through the prey much more readily than the eel's body. The duration of the electrical flow is too short to cause any discomfort or physical affect upon the eel, but it will stun most fish.
The eel has to have the capacity to send the signal to the nerve
terminal, it has to have specialized cells that can generate the
electricity, and it has to have a body that can distribute the
electricity to the water and prey around it without affecting the eel
itself. This is a highly specialized and incredibly complex system.
When we see incredibly complex designs like this in nature, which allow
balance and productivity in the biological world, we have to wonder at
the intelligence that has produced it all. Science continues to show us
complexity that defies chance explanations, and speaks eloquently of
the Creator's hand.
--Source: Scientific American, March 2006, page 104.
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