Electric Eel (Electrophorus electricus)
The Electric Eel is the only known species of freshwater fish that has been known to literally electrocute its prey and enemies. In fact, humans and alligators have been known to succumb to these evolutionary marvels.
Electric eels are in fact not eels at all. They are a scaleless freshwater fish. They also need to breathe air every ten minutes or so by gulping air at the surface. One can imagine that this ability may have come about due to its ancestors evolving in oxygen-poor waters. Those ancestors that just happened to have an ability to extract some oxygen from the air, was more fit in these waters. The survivors were then able to reproduce and passed that trait on to their offspring. Like a new superpower!
Electric eels produce enough watts of energy to stun any sized organism. Most encounters with these fish by humans rarely end in death as the goal to simply remove the threat. Prolonged exposure however can be fatal and some reports have seen them emitting sustained discharges if cornered. See link here.
Since nothing evolves in nature from nothing, the evolution of this mechanism was thought to have evolved from special organs that sense electrical signals from prey. Many organisms in aquatic environments have this ability. The electric eels however use these same organs in obviously very specialized ways. They actually have specialized cells that are stacked on top of each other and they act like batteries.
As mentioned earlier, most encounters with these organisms rarely end in death, typically, what is seen is what you see at the video below. Drowning is the biggest risk by humans that have come in contact with Electric eels. A discharge in water can be felt many meters away and this why it is not recommended to have these as pets. Rarely do you see Electric eels in aquariums. Do not try this at home:
Electric eels are fairly large getting up to 6ft in length and can weigh over 40 pounds. They primarily live in the fresh waters of South America are eaten by some natives. Their only natural enemies as adults are jaguar, caiman, and humans.