Read an Academic Passage

[1]In the deep ocean, where sunlight never reaches, a surprising number of animals make their own light. This ability, called bioluminescence, comes from a chemical reaction in which an enzyme causes a molecule called luciferin to give off light. Surveys suggest that roughly three quarters of the animals living in deep water can produce light in some way.

[2]One common use of this light is camouflage. Seen from below, a fish in dimly lit water appears as a dark shape against the faint light coming down from the surface. The hatchetfish avoids this problem by switching on rows of light organs along its belly and adjusting their brightness to match the light from above, so that its outline almost disappears. This technique is known as counter-illumination.

[3]Light is also used to attract other animals. Some anglerfish dangle a glowing lure in front of their jaws to draw prey close, and certain tiny crustaceans flash patterns of light to find mates. Such signals carry a risk, however, because a light that attracts a partner can also reveal its owner to a predator. Some species seem to reduce this danger by using colors of light that their main predators cannot easily see.