Mark Bowler / Electric eel (Electrophorus electricus), Peru

Norbert Wu / Electric Eel (Electrophorus electricus) can generate up to 220 volts, freshwater, Steinhart Aquarium, San Francisco, California

Michael Pitts / Glass knifefish (Eigenmannia sp) Amazon, Brazil

Michael Pitts / Knife fish (Porotergus nattereri) known locally as 'Vampiro' in tributary of the Amazon, Brazil.

Jane Burton / South American electric knifefish {Sternarchus albifrons}, captive, from Surinam and Amazon basin

Luiz Claudio Marigo / Barred / elegant knifefish {Steatogenys elegens} an electric fish, Brazil, Amazonas

Joel Sartore / Photo Ark / Glass knifefish (Eigenmannia virescens) swimming into frame, Fin-Tastic Fish, Nebraska. Captive.

Doug Perrine / Male Hawaiian razorfish / Knifefish (Cymolutes lecluse) swimming over seabed, North Kona, Hawaii, Pacific Ocean.