The Little Fish With A Mouth That Defies All Logic

Around 14 years ago at the Smithsonian, L. Patricia Hernandez, an evolutionary biologist and functional morphologist at George Washington University, saw a fish that gobsmacked her. The late ichthyologist Richard Vari had pulled out a specimen of a hingemouth that preserved the constellation of bones and cartilage that comprised the small brown fish's unconventional mouth. "I thought, this is just the strangest thing I have ever seen," Hernandez recalled. For nearly a decade and a half, Hernandez sat on the hingemouth, metaphorically speaking, waiting for a student who would be excited by the prospect of working with a fish that weird.

When Allyson Evans first visited for her PhD, those 14 years later, Hernandez knew that time had come. As an undergraduate, Evans studied male electric knifefish, some of which grow obscenely long jaws or balls of teeth that jut out of their faces. So when Hernandez showed Evans a video of the hingemouth—revealing how the fish's mouth erupts out of its forehead on a fleshy tube, almost like an elephant trunk tipped with a mouth—Evans knew she'd found her next muse. "I was smitten," Evans said.

Evans, Hernandez, and colleagues spent the next five years investigating how the hingemouth's trunk-like mouth fits inside its face and guzzles up food. The results were published this month in the Journal of Anatomy, and at least one of them won't be surprising to anyone who has read this far. "These jaws are really, really weird," said Katherine Corn, an evolutionary biologist at Virginia Tech and Washington State University, who was not involved in the research. "I would say that this fish is among the weirdest of the fish faces," said Kory Evans, a biologist at Rice University who was also not involved with the research.

To visualize the bizarre way the hingemouth's face is set up, Hernandez offered an analogy. "As you open your mouth, it's very much like you open a flip phone," she said. The two phone flaps are your jaws, and the hinge of the phone is your jaw joint at the very back of your chin, connecting to your head. This is how our jaws work. But now imagine, instead, that hinge at the back of your head is suddenly at the tip of your chin. "Now that top of the flip phone would be inside of your mouth, so your jaw joint would be at the tip of your chin, and your entire jaw would be folded up inside of your mouth," Hernandez said. "And then you would have an elephant trunk coming off that jaw," she added, almost as an afterthought. This is how the hingemouth feeds—flipping its lower jaw out of its mouth and then extending its trunk out of its forehead, ready to suck up food. "It's really the stuff of nightmares or dreams, depending on what your viewpoint is," Hernandez said.

The hingemouth's feeding apparatus is so unlike any other fish's that the researchers had to give the structure a new name: the proboscis. "Mouth didn't do it justice," Evans said. What we traditionally think of as a mouth—an opening in the face that lets food inside—would be where our jaws open. But for the hingemouth, that opening is inside of its face. So the hingemouth essentially evolved a second mouth at the tip of their proboscis, Hernandez said. "It's like the fish skull was dropped and randomly put together," she said. "Even though it looks superficially like what you might find in another fish, it is internally just like anatomical Twister," Evans said.

Hingemouths slink around the swampy forest pools of central west Africa, feeding on the detritus that sinks to the bottom. "Fish lineages often modify their jaws towards the direction of what they are eating," Corn said, adding that a fish sneaking insects from the water's surface would have an upward facing jaw, and a fish slurping sunken sludge would have a downward facing one. The hingemouth, a bottom-feeder with a mouth on its forehead, defies such easy logic. When the fish wants to feed, it merely sticks its proboscis out of its forehead and extends the tube all the way down to the ground. "It makes very little sense," Evans said.

When Kory Evans first learned about the jaws of the hingemouth, he imagined the fish would suck their food through the top of their head. So the feeding videos documented in the paper were "a real revelation" to him. "It is incredibly bizarre and still difficult to believe that a vertebrate feeds in this way," he added.

But a mouth is everything to a fish. Without limbs, fish often do many things with their mouths: breathing, brooding young, even climbing waterfalls. "Fishes have done insanely weird things with their faces," Kory Evans said. "And we are still just scratching the surface with how they evolved these adaptations and how they use them in their daily lives." A multipurpose mouth can make a fish's life easier, and the hingemouth is no exception. Their swampy home often runs out of oxygen, creating conditions that might doom other fish. But Allyson Evans observed that the hingemouth can breathe air from the surface with the help of its proboscis, a little like using a snorkel. And Corn pointed out that the hingemouth's fleshy, cartilaginous jaws are more flexible when feeding on uneven surfaces than thick, bony jaws. "What [the hingemouth] shows us is that animals can evolve solutions to problems that we never would have imagined," Corn said.

Evans started out her research examining hingemouth specimens from museums. She'd found a few helpful papers that described the skeleton and muscles of the head, but none that focused on the fish's nonsensical mouth. So Evans dissected museum specimens and took new CT scans of their heads. Her work revealed new structures and muscular arrangements that had never been reported in the hingemouth, such as a hair-thin tendon that operates like a pulley system to help the fish pull their jaw back into their skull. And unlike the mouths of other fishes, the hingemouth's mouth is not supported by any bone, just cartilage and an origami-like folding of skin.

But museum specimens never tell the whole story. In 2021, as the aquarium trade picked up after a pause during the COVID-19 pandemic, the team got their hands on live hingemouths collected from African rivers. But the fish had to be trained before filming could begin. "It was all honestly a bit of a nightmare to get these fish to eat on demand," Evans said. The hingemouths were skittish without the cloudy comfort of swamp water. The tanks' crystal clear water spooked them. Evans ended up building opaque dividers to block the fish's views and added some rocks for refuge. In 2022, Evans finally managed to film three of the more gregarious fish, which slurped up a slurry of algae pellets, spirulina powder, and frozen brine shrimp under the bright lights and on camera.

The researchers observed the hingemouth could extend their jaws past more than 30 percent of their head length. This is somewhat unremarkable for a fish, easily outshone by the sling-jaw wrasse, which can extend its jaws up to 65 percent the length of its head. As it turns out, the hingemouth's extremely innovative, unsettlingly showstopping, one-of-a-kind proboscis performs just about as well as other fish mouths. So why go through all that evolutionary trouble? "It seems extreme to change your face so much to feed on detritus," Kory Evans said.

Unfortunately for the hingemouth, the researchers suggest this extreme anatomical reorganization means the fish may be an evolutionary dead end. "Once you've kind of committed to flipping your jaws around and growing a tube, there's maybe not many places you can go from there," Allyson Evans said. She pointed out that people often describe evolution as survival of the fittest. "But it really is survival of the good enough," she said. "If you're good enough at eating, you're probably going to make it."

The hingemouth certainly meets the standards of being good enough at eating. It doesn't need to hunt because all of its prey is not only already dead but has essentially become subsumed into the flocculent slush rotting at the bottoms of swamps. So despite having a mouth that is completely flipped around, with an backwards jaw and lips on their forehead, the hingemouth sups on ooze with the best of them."I think it says something really beautiful about evolution—that lineages can evolve wild diversity beyond what we think can or should be possible," Corn said. Perhaps by disregarding more conventional notions of evolutionary success, the hingemouth succeeds on its own terms—proving you don't have to be the best to be the most extreme. And being an evolutionary dead-end is only a problem for your descendants.