The Amazon River is very muddy, so muddy that fish that swim in its water have developed special abilities to live there.
The South American electric eel is an example. It evolved over 100 million years ago to change its muscles into electric-producing organs.
All muscle cells can produce small amounts of electricity. But electric fish evolved to create more electricity by creating different types of cells that can generate more voltage. The electric eel has gotten so good at this that it can create about 100 volts per foot of fish. The standard electrical outlet that you plug a lamp into is 120 volts.
The eels don’t use electricity just to kill prey. They also use it to communicate with other eels and to navigate in the muddy water, kind of like bats use sonar.
Only fish with backbones have developed this ability to create electrical charges and scientists studying them have now discovered that they all evolved to do this in much the same way, each on its own. The reason that fish can do this, and not other animals, is because the water they live in carries the electricity so well.
Scientists hope that if they can figure out how the fish adapted to create electricity, they might some day be able to power bionic devices for humans without batteries.