Fin Movements Aid in Swimming Backward and Forward

Biological Strategy

African knifefish

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Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Navigate Through Liquid

Living systems that move in liquids must navigate around physical obstacles and find their way from place to place to locate resources or suitable climates. Many liquids are denser than air, which means it’s a thicker medium through which signals must pass. On the other hand, liquids tend to conduct some signals better than air or solids. Liquid dwellers must use strategies that enable them to detect and follow signals in this dense medium. For example, electricity transmits well in water and several organisms, such as Amazon electric eels, have organs that detect and use electric signals to navigate.

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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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

With its slick, streamlined body and long belly fin, moving around underwater may seem simple for an African knifefish (Gymnarchus niloticus). 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.

Last Updated November 3, 2020

References

“The fish 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)

Journal article

Aquatic manoeuvering with counter-propagating waves: a novel locomotive strategy

Journal of The Royal Society Interface | 23/12/2010 | O. M. Curet, N. A. Patankar, G. V. Lauder, M. A. MacIver

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