Anatomical Oddity Helps Some Fish Power Their Pucker

GW researchers reveal mechanics, advantages of a curious bone found in 25 percent of freshwater fish.

The sight of a goldfish grazing—and grazing, and grazing—can start to feel like a reminder that some animals get to drift along, puckering away in simple, rent-free peace.

But Patricia Hernandez sees something else. She sees velocity and hydrodynamics, and nature’s age-old obsession with streamlining design.

In a trio of studies published this year Dr. Hernandez, a GW biologist, and her collaborators reported a new layer of complexity in that puckering, illuminating the role of a curious bone that’s found in one-quarter of all freshwater fish species.

This group of fish, called Cypriniformes—which includes minnows, carp, goldfish and more than 3,000 other species—use the bone to pucker unlike any other fish.

Known as a kinethmoid, the bone is centered behind the upper jaw and is nested in ligaments, rather than linked to other bones. When the lower jaw opens the kinethmoid lunges forward to extend the reach of the mouth, in some cases adding as much as 20 percent to a fish’s head size.

It’s what scientists call jaw protrusion, and while other fish can do this they do so with an attached bone. For them, a long reach means accommodating a long bone. But the kinethmoid is “a really crafty way of getting a much longer jaw” without adding to overall head size, said Dr. Hernandez.

And that has at least one obvious advantage: “When you’re this far away from a worm they probably figure, ‘I’m safe’—and then you have a mouth flinging out at you,” said Dr. Hernandez, whose work was funded by the National Science Foundation. She also is among the researchers expected to be in the new Science and Engineering Hall.

Although the kinethmoid bone had been identified in the past, and Dr. Hernandez had previously traced its development, she said there was little understanding of how the bone works; how this mechanism differed from the protrusion in other fish; and how the bone varies among the fish that have it.

That’s because the kinethmoid’s positioning, obscured by surrounding bones, prevented studies of it in action, she said. Dr. Hernandez and post-doctoral scientist Katie Lynn Staab finally were able to do this with the aid of new visualization technology developed recently at Brown University.

The technology merges data from X-ray videos with 3D modeling, which allowed the researchers to clearly visualize the kinethmoid in motion.

Together with the Brown researchers, in an article published in the Journal of Experimental Biology, Dr. Hernandez and Dr. Staab unraveled the mechanics of the kinethmoid, and found that the fish can even protrude their jaws while the mouth is closed. That’s key for bottom-feeders, like goldfish, that sift mouthfuls of “muck” for food, said Dr. Hernandez.

To see how this type of jaw protrusion differs from that of other fish, the researchers compared goldfish to bluegills, a member of another huge group of fish that feature a much simpler mechanism.

In an article, published in the Journal of Experimental Biology, the researchers reported that for both types of fish the jaw extension enabled them to increase the force they use to suck in prey. And the two types of jaws each seemed geared toward the fish’s food supplies.

The bluegill’s “explosive, really dynamic” jaw motion is better suited to its diet of insect larvae than the goldfish’s sifting of detritus, said Dr. Hernandez. Goldfish “have a more extended, Hoovering action,” to vacuum along the bottom.

In looking at the differences between kinethmoid bones, for an article in the journal Zoology, the researchers were surprised to find that the two fish with the longest kinethmoid bones also were the fastest on the draw.

In those fish, Dr. Hernandez and her colleagues found an extra jaw muscle. It’s launched their research into a new direction, and she’s planning to work with engineers to isolate the basic mechanical elements “that drive this machinery.”

“I think it’s important to understand the results of evolutionary experiments,” she said. Sometimes “you have one evolutionary novelty that essentially … opens up the door for evolution of other parts of the animal. And that’s likely what happened there.”

Dr. Hernandez also is chasing the origins of a tongue-like structure, called a palatal organ, found on the roof of the mouth that some of these fish use to identify and grasp food inside a mouthful of water. It was thought only to be in goldfish and carp, but Dr. Hernandez recently has found a palatal organ in each of the few dozen species of Cypriniformes she’s investigated so far.

Her lab is now trying to determine which stem cells give rise to this organ, in the hopes that it will reveal clues about the full nature of the organ, and the full complexity of a pucker.

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