This topic is over a year old, but it looks like the OP was a drive-by with no follow-up, so I'd like to take his place, since I've been researching this topic and coming across old forum posts like this without any real discussion.
I too have come across
the source mentioned by Coyote, but it leaves me with more questions than answers:
We do not know the crucial features that evolved to alter the adequate stimulus from a mechanical event to an electrical one, but the change in sensitivity was many orders of magnitude and, in each class of electroreceptor, was tuned to a best frequency that could be many octaves apart.
The lack of explanation, I think, makes this even more difficult to believe:
It seems likely that evolution has independently discovered how to make and make use of electric organs at least five times, and possibly more — an outstanding case of parallel evolution.
Now, what steps do we need for a non-electric eel to evolve into an electric one. Based on the above and other links I have followed, it seems we have something like this:
1. A change in sensitivity of many orders of magnitude.
2. A change in the organization of the muscles so that the voltage increases by lining them up
3. Tuning the electroreceptors to a "best frequency"
4. The ability of all the muscles to fire at the same time
5. (Possibly) The loss of the muscles' ability to contract (Articles mention this as happening, and the electric function replacing them, but I'm not sure yet if this function would have to be lost for the electric function to work, or if it could have just happened later to no disadvantage)
6. (Possibly) An increase in fat around the head / vital organs to protect it from shocking itself (This seems to depend on whether or not this is what actually keeps it from shocking itself. Alternately, #6 could be whatever the actual method is of preventing shock, unless it is really something that was present all along.)
Now, if I have my list right (and I certainly may not), the question then is whether or not these things needed to arise simultaneously, or if they could have been advantageous individually. If they are advantageous individually, might we expect to see some of these changes individually existent in other creatures, or did natural selection just happen to always put all of them together each of the 5+ times it started down this path? (or do all the cases of "parallel evolution" of "mechanical" to "electric" organs not need all these same steps?)