Mucus Reduces Turbulence

a dark gray barracuda swims over sea grass and around coral

Biological Strategy

Barracuda

AskNature Team

Image: Wouter Naert / Unsplash / Some rights reserved

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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Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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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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Mucus on barracuda reduces turbulence by increasing the viscosity of the boundary layer on the skin.

“We keep discovering refinements of fluid mechanics in the animal kingdom. Recently Rosen and Cornford investigated the effect of the mucous secretions of fish on turbulent friction drag in seawater and found that 5 percent of barracuda slime reduced the turbulent friction of seawater by 66 percent. The slime of halibut was found to have a similar effect. These results indicated that slime in the flow boundary layer of rapid swimmers can effectively subdue turbulence and thus prevent energy loss…Assuming that mucous secretions (such as castor oil) are about a thousand times more viscous than water, it is simple to estimate that a few percent of the stuff in the boundary layer can easily lower the Reynolds number to a tenth or less. In this way the danger of turbulence can be considerably diminished.” (Tributsch 1984: 48-49)