The fish can open their mouths at least 120 degrees (a straight line measures 180 degrees) allowing them to eat extremely large prey relative to the size of these predators. Dragonfishes, described in the journal PLOS ONE, grow up to about one feet, six inches long.
"In the dark, deep sea food is very scarce," co-author G. David Johnson of the Smithsonian's National Museum of Natural History told Seeker.
"The barbeled dragonfishes are ambush lie-and-wait predators, advantageous because less energy is used searching for food. Because prey items are scarce, it is also advantageous to be able to swallow a wider size range of encountered prey and the head joint facilitates engulfing larger items," explained lead author Nalani Schnell of the French Muséum national d'Histoire naturelle.
The researchers, who have studied a myriad of fish anatomy over the years, have never seen anything like the head joint of the deep-sea dragonfishes.
The joint comprises the back of the head and a flexible rod running to the top of the fish's vertebral column. The researchers determined that, when the fish is in a resting position and not elevating its head, the rod has an additional sheath that embraces the back of the head like a socket. When the fish opens its mouth, the sheath gets stretched out and the bottom part of this rod is exposed.
The process is illustrated in the below video:
Barbeled dragonfish "swallow their prey whole," Schnell explained. "The teeth are not for chewing; they instead hold onto prey and pass it into the stomach. Some dagger-like teeth even act as a cage, so that swallowed prey won't be able to escape anymore."
Once the fish has swallowed prey, such as another fish, its stomach can expand, causing the predator to look somewhat like a snake that has just swallowed a big victim.
|X-ray of a barbeled dragonfish that has eaten a whole lanternfish.|
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