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What is countershading?

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Contemporary technology, especially military aircraft and naval vessels, have taken tips from nature’s camouflage when using countershading.

 

By Blaise Jones

While sharks come in all shades and colors, most people picture them with gray backs and white bellies. This is probably because some of the most iconic sharks have this color pattern, such as the infamous great white shark. However, while not all sharks share this exact coloration, many do share the same pattern.

Countershading

Most sharks rely on a concept called countershading. Countershading is when something displays two different colors that contrast with each other, in this case a dark coloration on the top and a lighter one on the bottom. Countershading is very common in marine animals, and can be found in dolphins, whales, and even crocodiles. Humans use countershading in many manmade creations, such as military aircraft that are painted differently on the top and bottom. The planes are painted blue-gray on the bottom to blend in the clouds when viewed from the bottom, but are colored differently on top to attempt to blend in with the Earth when viewed from above.

Due to countershading, sharks are nearly invisible in the ocean except when viewed in silhouette from below. Credit: NOAA.

Water column

The reason for this is because countershading is extremely effective at camouflaging animals in the water column. The reason for this is because of how sight and light behave. Sight occurs when light is reflected off of objects and absorbed by an organism’s eyes. Light in the water column either comes from directly above, known as downwelling light, or from below when it is reflected off the ground, known as upwelling light, which is much weaker than downwelling light.

Appear-less Predator

Countershading takes advantage of this. The two different colors of a countershaded animal either reflect or absorb light, making them functionally invisible. Darker colors absorb incoming light, while lighter colors reflect it. The darker side of a countershaded animal absorbs the strong downwelling light, while the bottom color reflects the weak upwelling light.

Getting a Good Angle

Countershading allows sharks to hide from almost any angle. When viewed from directly above, their darker back blends in with the darkness of the depths. However, most prey items would be looking at sharks from a horizontal angle, but sharks have that covered.

When seen from slightly above the horizontal angle, the darker portion of the shark absorbed the bright downwelling light and appears as a dark spot against the dark background of the upwelling light. When viewed from slightly below the horizontal angle the lighter underside reflects the light, creating a lighter shade that blends into the lighter background of the downwelling light that passes by the shark.

In fact, the only angle that a shark’s countershading does nothing is from directly below. While the light bottom does blend in with the light coming from the sun above, there is no way for the shark to hide in silhouette. Viewed from below, a shark has a very distinctive outline.

Several Shades of Gray

Combined with all of this is the gradual transition of darker to lighter colors that counter shaded sharks exhibit. The shark’s coloration gets lighter the further down its body until the white underbelly. This works with the already discussed principles to render the shark functionally invisible. The top of the shark is seen as dark against a dark background, the middle is seen as mildly light against a mildly light background, and the bottom is seen as bright against a bright background.

Dressing for the Occasion

Of course, not all sharks use shades of gray for their countershading. The color they display depends upon their habitat. Gray is commonly used because it blends just well enough in a wide variety of habitats, making it useful for sharks that are always on the move. Colored light doesn’t penetrate water well, so most marine animals are colorblind, making gray a good choice for camouflage.

Sharks that don’t move around too often, sticking mostly to muddy or sandy bottoms, typically have brown tops. The sluggish nurse shark and sand tiger shark are good examples of this, as are their cousins the sawfish and many species of stingray.

Darker sharks, such as the Greenland and salmon shark, are found in darker water where they blend in more effectively. Sharks that have a yellow/goldish coloration, such as the aptly named lemon shark, can be found in primarily sandy areas.

Finally, sharks that can be found in the open ocean tend to have a more blue coloration. This includes the thresher and the, again aptly named, blue shark.

-TSF-

#sharks #military #countershading #camo #camoflage #TSF

 

SOURCES:

“Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess

Moriah Moore, Marine Biologist from the Coastal Marine Education and Research Academy

“The Encyclopedia of Sharks” by Steve Parker

 
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