Slime Reduces Drag

Biological Strategy

Fish

AskNature Team

Image: Jenny Huang / Wikimedia commons /

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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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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Skin of fish reduces drag by being covered by a slime layer of complex proteins, polysaccharides, and bacteria.

“The external layer of mucus on fish was investigated as a drag reducing . Comparing velocity profiles for water flow over rainbow trout (Salmo gairdneri) and wax models of trout with and without hydrodynamically smooth surfaces revealed that the integumental mucous secretion can significantly reduce the rate of momentum transfer through the boundary layer. The difference in momentum transfer is expressed as a reduction in friction drag…” (Daniel 1981:376)

“Several conditions must be satisfied for mucus to act as a drag reducer and for my treatment of velocity gradients to be valid: (1) Mucus must consist, in part, of polymers soluble in water and with molecular weights exceeding 50,000 (White and Hemmings, 1976). (2) There must be turbulent or pulsed laminar flow about the fish (Driels and Ayyash, 1976). And (3) the density and viscosity of the fluid from the surface of the fish outwards must be constant.” (Daniel 1981:380)

“Mechanisms involved in utilizing the slime to achieve good swimming efficiency include transition delay, and turbulent flow drag reduction, in addition to the recently discovered drag reduction in pulsed laminar flow of polymer solutions…Both theory and experiment indicate the transient laminar shear flows show reduced wall shear stress in polymer solutions. Hence, the possibility exists that small fish with low length s can utilize polymers to reduce drag, as well as the larger fish with high Reynolds numbers and turbulent flow.” (Hoyt 1975:653)