In the dark, deep sea food is very scarce. ~ American ichthyologist David Johnson
Besides the dark and crushing pressure, the deep sea is a dangerous place. Lacking light, many deep-sea animals independently evolved bioluminescence to help them see, and to communicate with conspecifics.
The risk of shedding (or reflecting) light is marking yourself as prey. Dragonfish cultivate this defect to their advantage by having a bioluminescent bulb at the end of a tether below their head that acts as a lure. These lie-and-wait ambush predators save energy by drawing dinner to them. The glowing lure is bait for a fish that is sublimely equipped to capture and eat more than its fair share.
Dragonfish have an array of traits that render them tiny sharks of the dark with their efficient design. (Dragonfish only grow at most to 46 cm; many are only pencil length.) Their skin is a nonreflective matt black, achieved by a film of microscopic melanin granules which soak up light. 2 noteworthy peculiarities – loose jaws and distensible stomachs – borrow from the playbook of snakes.
Loose dragonfish jaws are achieved by replacing neck vertebrate with flexible tendons. The rubber neck lets dragonfish open their mouths over 120°, allowing them to swallow prey as long as and larger than they are; hence the distensible stomachs.
To secure their catch, dragonfish have long saber-like teeth on jaws that snap shut with astonishing speed. The teeth are an engineering marvel. Dragonfish teeth are sharper than piranha teeth and as hard as the teeth on great white sharks. To avoid reflecting light, dragonfish teeth are transparent. The effect is achieved by miniscule nanocrystals spread throughout the enamel which pass light through without scattering, thereby eliminating reflection. The transparent teeth and non-reflective skin mean that dragonfishes’ bioluminescent lure doesn’t give them away to potential prey.