Why do deep-sea fish often glow in the dark?

Below the sunlit upper ocean, light fades quickly and disappears entirely in the deep sea. In that dark environment, many fish, squid, and other animals produce their own light through chemical reactions involving luciferin and luciferase or through symbiotic bacteria. Some use flashes to lure prey, while others use light patterns to recognize their own species. The lanternfish is one of the most abundant deep-sea groups and is famous for this trait. Bioluminescence has evolved many times independently in marine animals because it is so useful in the deep ocean.