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Narrator: Listen to part of a lecture in a biology class. Professor: Today we're going to talk about a fascinating adaptation in some deep-sea fish: bioluminescence. Now, you might think that bioluminescence is just for attracting prey or mates, but it also serves a defensive purpose. Take the hatchetfish, for example. It has light-producing organs on its belly that emit light matching the downwelling light from the surface. This makes the fish nearly invisible to predators swimming below, a technique called counter-illumination. Another example is the vampire squid, which, when threatened, releases a cloud of bioluminescent particles to confuse predators. But bioluminescence isn't just about hiding; it can also be used to lure prey. The anglerfish uses a glowing lure on its head to attract small fish within striking distance. Interestingly, these light-producing reactions involve a chemical called luciferin, which reacts with oxygen to produce light. The key point is that bioluminescence has evolved multiple times across different species, each with its own unique application for survival in the dark ocean.