What’s this grey squirrel got to do with marine life? Stay with me for a second. Countershading is a form of camouflage used by many open water fish, although it is also commonly seen in many terrestrial creatures as well. Basically it is just a color pattern found on animals where the dorsal surface is dark and the ventral surface is light colored.
The most common explanation for the countershading coloration of a fish is that the dark back of the animal, when viewed from above by a predator, will blend in with the darkness of the ocean bottom. Meanwhile, for a predator looking up from below the white ventral surface of the prey will blend in with sunlight streaming down from above. This is all true, but there’s a little more to it than that.
When light strikes an animal that is a uniform color from above, it will create a uniform shadow which gives a predator a clue as to the shape and size of the prey. By using countershading an animal changes the way shadows are created as the light strikes the body, breaking up or distorting the shadow. This may confuse a predator for just enough time to interrupt its attack.
But prey species aren’t the only ones that benefit. Predators take advantage of countershading in order to sneak up on their prey without being seen. This explains the presence of this coloration on top predators such as hawks and sharks.
The countershading phenomenon found throughout the animal kingdom is also called Thayer’s Law, named after the New England naturalist Abbott Thayer. Also a painter, Thayer was the first to describe the protective advantages of countershading. Here are some of the works of Thayer. Thayer is known as the "father of camouflage", not only for his observations on countershading, but also for his contributions to the use of countershading and other camouflaging concepts by the military in the Spanish-American War to protect their soldiers and equipment. His designs for painting warships were based on the coloration of seagulls.
Here we see the countershading coloration of a dolphin.
And a beautiful example seen on this whale shark. Those two fish swimming beneath are symbionts called remoras. I wrote about them here. If you read that diary you'll see that one fascinating behavior of remoras is that they can exhibit reverse coloration. Normally a remora has a dark back and a white belly. The sucking plate of the this fish is on the top of the head, so when they attach onto the back of a host they swim upside down. The color reverses so that the belly turns dark and the dorsal surface turns white. These fish can actually control the countershading defense depending on the orientation of their bodies.
I believe kossack Lineatus will be doing an essay on how countershading helps birds, and I’ll link to that once it’s posted.
Other diaries in this series can be found here.