A hinge that extends its proboscis to feed on the bottom of the tank
Allyson Evans
A fish found only in West African rivers and forest pools, it can protrude a tree-like snout to suck up food or inhale air like a snorkel.
The hinge (Phractolemus ansorgii) has what biologists call a deployable proboscis, a tubular structure folded over its head that can be extended up or down. The lips of the proboscis are lined with tooth-like structures made of keratin, which fish use to scrape off algae or other detritus.
Allyson Evans at George Washington University in Washington DC and her colleagues used dissection, videography and CT scans to reveal the complex construction of its jaws.
Unlike all other fish, whose jaw joint is at the back of their mouth, the hinge jaw joint is at the front of its head. The lower jaw is tipped back, pointing towards the throat. The upper jaw, connected to the lower jaw by a ligament, is part of the proboscis. “You can think of the upper jaws as more or less suspended in the skin of the proboscis,” says Evans. This is why the structure can be extended to the outside of the head.
The hingemouth offers “a novel mechanical solution to a ubiquitous challenge that all fish face, which is how to get food into a viscous, liquid medium,” Evans said.
He also observed fish swimming at the surface and drinking air, using the proboscis “like a snorkel”, he said. This can be a useful ability in forest pools where oxygen levels can be depleted.
Scientists aren’t sure how the hingemouth evolved, because fossil evidence is sparse. “That family Phractolaemus among the, Kneriidae, are believed to have originated in the mid-Cretaceous, but no intermediate forms have yet been seen in the fossil record,” Evans said.
“What’s most important to me is that scientists remain curious about the little freaks of the world,” he said.
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