Found almost exclusively in fresh water (the only exceptions are species that occasionally may visit brackish water to feed), these mostly nocturnal fish are capable of producing electric fields to detect prey, for navigation, communication, and, in the case of the electric eel (Electrophorus electricus), attack and defense.

Aside from the electric eel (Electrophorus electricus), Gymnotiformes are slender fish with narrow bodies and tapering tails, hence the common name of "knifefishes".

[4] Different wave patterns produced along the length of the elongated anal fin allow for various forms of thrust.

[7] The speed at which the fish moved through the water had no correlation to the amplitude of its undulations, however it was directly related to the frequency of the waves generated.

[8] Studies have shown that the natural angle between the body of the knifefish and its fin is essential for efficient forward motion, for if the anal fin was located directly underneath, then an upwards force would be generated with forward thrust, which would require an additional downwards force in order to maintain neutral buoyancy.

[7] A combination of forward and reverse wave patterns, which meet towards the center of the anal fin, produce a heave force allowing for hovering, or upwards movement.

[5] The ghost knifefish can vary the undulation of the waves, as well as the angle of attack of the fin to achieve various directional changes.

Certain aspects of the electric signal are unique to each species, especially a combination of the pulse waveform, duration, amplitude, phase and frequency.

[12] The electric organs of most Gymnotiformes produce tiny discharges of just a few millivolts, far too weak to cause any harm to other fish.

[14] In addition to this low-level field, the electric eel also has the capability to produce much more powerful discharges to stun prey.

[18] Gymnotiformes is thought to be the sister group to the Siluriformes[19][20] from which they diverged in the Cretaceous period (about 120 million years ago).

This shows that contrary to earlier ideas, the Apteronotidae and Sternopygidae are not sister taxa, and that the Gymnotidae are deeply nested among the other families.

[15] Approximately 150 Mya, the ancestor to modern-day Gymnotiformes and Siluriformes were estimated to have convergently evolved ampullary receptors, allowing for passive electroreceptive capabilities.

[27] As this characteristic occurred after the prior loss of electroreception among the subclass Neopterygii[28] after having been present in the common ancestor of vertebrates, the ampullary receptors of Gymnotiformes are not homologous with those of other jawed non-teleost species, such as chondricthyans.

[30] As Arnegard et al. (2005) and Albert and Crampton (2005) show,[31][32] their last common ancestor was roughly 140 to 208 Mya, and at this time they did not possess ESSs.