Don't try to eat anything bigger than your own head. That's good advice for most living things, but a group of creatures called the dragonfishes found an evolutionary way to break that rule.

A pair of scientists have discovered how the dragonfishes are able to swallow prey that is almost as big as they are. They found the first known hinged cranium in a fish and described it recently in the journal Plos One.

These dozens of dragonfish species live in extremely deep areas of the ocean in almost total darkness. With a long, glowing barbel hanging beneath their face, the creatures uses bioluminescence to attract prey. But in the deep, dark ocean it could be months between one fish sighting and another. So these dragonfishes must be able to take advantage of any opportunity to eat—even something nearly their own size.

“Probably close to 100 years ago someone looked at the anatomy of dragonfishes and noticed that there was a gap by the brain case,” says Dave Johnson, a curator in the division of fishes at Smithsonian's National Museum of Natural History and co-author of the study along with Nalani Schnell of the Muséum national d'Histore naturelle at the Sorbonne in Paris. “But at that time they didn't have X-rays.”

Being gape-limited is a problem for many types of animals. The dragonfish's solution is a unique one, which invites the question of why no other fish or other animals are known to have a similar cranial design.

“That's the $64,000 question,” says Johnson. “Just the way evolutionary paths work. It's more likely you are going to find that in the deep sea,” where resources are scarce and missed opportunities to eat could easily mean death. “There are other ways of getting at larger food, I can't answer why other species haven't gone that way. . . There's three ways to expand your gape. Dorsally, vertically and laterally.”

In the 1960s, scientists pioneered a technique to dissolve the flesh from a whole fish and leave behind an assembled, articulated skeleton to learn from. Johnson and his co-authors used this method to see exactly how some species of dragonfish's bones are put together and were able to demonstrate the hinged movement of the cranium.

Part of the cranium tips back as the dragonfish opens its mouth, allowing large meals to go head-first into the stomach.

Among snakes, the two sides of the lower jaw are not fused the way that they are in most animals. Flexible ligaments connect them. The lower jaw also has an especially flexible joint on the sides where it meets the cranium. These and other adaptations allow a python with a head the size of a grapefruit to swallow a deer.

Flathead catfish are among the least gape-limited of all freshwater fish and are able to swallow even the biggest largemouth bass with their extraordinarily wide mouths.

Another way that some predators deal with large prey is by ripping it apart into smaller chunks, like lions.

“There was a study published recently on moray eels,” Johnson says, “they are able to take those pharyngeal jaws and throw them out into the oral cavity and grab things and suck them back.” This arrangement is similar to the mouths—yes, plural—of the Xenomorph queen in the science fiction movie Alien.

Not all species of dragonfish have the fully developed hinged cranium. Some basal species that are believed to be representative of earlier forms of dragonfish have unusual attachments between the vertebrae and the cranium but not a hinge, per se.

Living at depths beyond the reach of SCUBA gear, dragonfish have never been observed in the act of feeding. Some have been recovered from nets with full bellies and were X-rayed or dissected, but scientists can only infer as to how those large prey items were swallowed.

“We're never going to be able to bring these things alive into a lab,” Johnson says. “They are coming into a lower pressure environment. . . but they have no idea what a boundary or a wall is. You put them into a contained structure and they have no idea what to do. We aren't going to be able to observe them feeding in captivity. Doing it in the wild is expensive. You can't watch interactions between the animals down there.”

The adaptations of the dragonfish are odd, but it isn't odd that it is odd. Strange adaptations are very common among creatures that live at extreme depths.

“Looking at deep sea fishes, the most successful ones have these radical adaptations,” Johnson says.