The bioluminescent firefly squids use an evolutionary phenomenon called counterillumination camouflage using which they light up the lower part of their bodies as they rise up underwater. The color of the light matches the light coming from the surface. The firefly squids disappear into deep waters during day time. But they return to the surface at night. They can also light up their entire bodies for mating purposes. This beautiful phenomenon can be witnessed at Toyama bay in Japan each year between March and June.
If you are interested in learning about camouflage in nature, you should check out the wrap-around spider.