This video says about itself:
1 February 2017
A previously undescribed joint probably allows dragonfish and other deep sea fishes to gorge on more than guppies.
From Science News:
Dragonfish opens wide with flex neck joint
Soft tissue at base of skull helps deep sea fish swallow big
By Cassie Martin
2:34pm, February 1, 2017
Dragonfish are the stuff of nightmares with their oversized jaws and rows of fanglike teeth. The deep sea creatures may be only several centimeters long, but they can trap and swallow sizeable prey. How these tiny terrors manage to open their mouths so wide has puzzled scientists, until now.
In most fish, the skull is fused to the backbone, limiting their gape. But a barbeled dragonfish can pop open its jaw like a Pez dispenser — up to 120 degrees — thanks to a soft tissue joint that connects the fish’s head and spine, researchers report February 1 in PLOS ONE.
Nalani Schnell of the National Museum of Natural History in Paris and Dave Johnson of the National Museum of Natural History in Washington, D.C., examined preserved specimens of nine barbeled dragonfish genera. Five had a flexible rod, called a notochord, covered by special connective tissue that bridged their vertebrae and skulls. When Schnell and Johnson opened the mouths of the fish, the connective tissue stretched out. The joint may provide just enough give for dragonfish to swallow whole crustaceans and lanternfish almost as long as they are.