Fish Using Counterillumination

This fish is using counterillumination to disappear. At left it stands out against the light above it. At right, with bioluminescent structures lit, it blends in.
Smithsonian Institution

The fish at left stands out against the lighter waters above. At right the same fish—now with bioluminescent structures on its underside lit—blends in. Many deep sea creatures have evolved this adaptation (known as counterillumination), which enables them to hide in an environment with few hiding places. More about deep ocean exploration can be found in the Deep Sea section.