The undulating fin of the knifefish enables it to swim forward and backward, as well as keep it afloat, by creating propulsive water jets.

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With their slick, streamlined bodies and long belly fin, moving around underwater may seem simple for the knifefish. It is, however, a lot more complicated than how it appears at first glance. The knifefish must heavily rely on its long stomach fin to keep it both moving and afloat, as its undersized side fins don’t provide the lift needed in the water. Fortunately, this fin is a marvel in the animal world when it comes to movement, allowing the fish to quickly move in all directions--including backward.

These movements are all achieved with different types of undulations (wave-like movements) that the fin is able to undergo, as in this video. Similar to a human arm or leg, the fins of a fish are controlled by muscles, and it is thanks to these muscles that the stomach fin of the knifefish is able to move in such a way. By sending undulations (picture sports fans doing the wave during a sports game) along the fin from the front to the back of the fish, the water is churned up, creating a backward directed jet that pushes the fish forward. In reverse, undulating the fin from the back to the front churns up the water toward the front of the fish, creating a forward-directed jet, which pushes the fish backward.

Staying in place, however, is a bit more complicated, as two equal waveforms must be initiated. These two waves must be created along the fin, starting from the opposite sides of the fish, and meeting in the middle. Once the two waves meet, both the forward and backward jets cancel out, and water is pushed downward, creating a new downward jet that keeps the fish from sinking by pushing it upward. Using these different undulation patterns in combination, the knifefish is able to move quickly and smoothly forward or backward.

This summary was contributed by Thomas McAuley-Biasi.

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“Propulsion is accomplished by means of undulating an elongated ?n on the belly that runs nearly the entire length of the ?sh.” (Curet et al., 2011:1042)
“Typically, during forward or backward swimming, the undulations mostly proceed from one end of the ?n to the other.” (Curet et al., 2011: 1042)
“The ?sh exhibits inward counter-propagating waves [two separate waves that travel from opposite ends of the body, meeting in the middle] typically when it is hovering – staying stationary with active stabilization against perturbations.” (Curet et al., 2011: 1042)
“This mode of ?n movement, in addition to facilitating station-keeping along the longitudinal axis, provides an upward force to the ?sh.” (Curet et al., 2011: 1048)
“Our previous work on ribbon ?n propulsion and black ghost simulations shows that the ribbon ?n generates a streamwise central jet with associated vortex rings [6,13]. In this case, the inward counter-propagating wave along the ribbon ?n generates two opposite ?uid jets that collide at an angle. After the jets collide, they are de?ected downward.” (Curet et al., 2011: 1048)

Journal article
Aquatic manoeuvering with counter-propagating waves: a novel locomotive strategyJournal of The Royal Society InterfaceDecember 23, 2010
O. M. Curet, N. A. Patankar, G. V. Lauder, M. A. MacIver

Robotic fish aids understanding of how animals move

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