The Gymnotiformes are an order of teleost bony fishes commonly known as Neotropical knifefish or South American knifefish, which inhabit fresh water. These mostly nocturnal fish have long bodies and swim using undulations of their elongated anal fin. They are electric fish, are capable of producing electric fields for various purposes, such as detecting prey, for navigation, communication, and, in the case of the electric eels, attack and defense. A few species are familiar to the aquarium trade, such as the black ghost knifefish, the glass knifefish, and the banded knifefish.