Bioluminescence

Use your mouse to magnify this krill, which is producing light through the process of bioluminescence. The krill has small light producing organs (photophores) on its ventral surface (its belly) to cast a light shadow. Without bioluminescence, the krill would stand out against the background light to a predator looking up from below. But, its bioluminescent light shadow hides the krill. Some terrestrial animals are bioluminescent (including fireflies), but many more marine animals are bioluminescence, particularly in the deep sea and in the winter in polar waters.