WHY THEY MAKE LIGHT
There are so many bioluminescent creatures in the ocean, because their ability to make light helps them to survive. Some use their light to help them find food, some use it to help them find mates, some use it to defend themselves against predators and some, like this Viperfish, use light for all these purposes.
Melanocetus johsonii(E. Widder)
Anglerfish (above) attract their prey to them with a glowing lure that dangles from the end of its fishing pole. When the unsuspecting prey tries to nibble at the tempting bait, it finds itself instantly engulfed in that mouthful of needle sharp teeth.
Finding A Mate
Bioluminescence can also be used to advertise for a mate. Sex signals which send messages like "Eligible female seeks compatible male" can be sent using a special flash code, as fireflies do on land, or light organs can have a very special shape or pattern that is specific to a particular species and allows a member of the opposite sex to recognize a potential mate. For example, this lovely lady (below) is an angler fish with a particularly fancy lure. Although at one time it was thought that each lure was designed to attract special prey, it now appears that the unique shape has evolved to attract a male of the same species who recognizes his future mate by her lure. Below left is a male angler fish.
Oneirodes sp. (E. Widder)