Watch a largemouth bass vacuum up a goldfish

"It's like they are doing a stomach crunch to open their mouth," says coauthor Thomas Roberts. (Credit: iStockphoto)

The same muscles that make bass powerful swimmers let them suck up their dinner.

A new study of the fish shows large muscles used for swimming help the fish double its mouth volume quickly, creating a vacuum. Mouth muscles have almost no role; mouth bones act like the ribs of an opening umbrella.

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animation (8)

In the 1950s, researchers first posited that the body might contribute to suction feeding, but that idea had never been tested and measured in fish as they feed.

“People have been tossing this around for as long as they’ve been playing with fish heads, which is a surprisingly long time,” says Ariel Camp, who earned her PhD at Brown University this spring based on the research.

Moreover, in the debate about how fish generate their suction, few if any scientists have given the swimming muscles this much of a role.

“I think everyone would be surprised by the extent to which the swimming muscles are really the source of power,” says Elizabeth Brainerd, professor of ecology and evolutionary biology and coauthor of the study published in the Proceedings of the National Academy of Sciences.

The researchers suspect that the same thing is going on in many of the more than 30,000 species of ray-finned fishes.

X-ray videos

The researchers recorded highly precise 3D X-ray videos of three bass as they gulped down goldfish.

This allowed them to track and visualize how all the bones in each fish’s head were moving around. That gave them the means to calculate the change in volume of the mouth many times a second as a fish captured its prey.

Those measurements, combined with more conventional data on the water pressure in the mouth, allowed them to calculate the power involved over the course of feeding.

Then the researchers calculated the power produced by each of the muscles in the mouth, as well as the swimming muscles in the body, during the suction action.

The fish head

Their analysis showed that up to 95 percent of the power required for the suction came from the swimming muscles, rather than the mouth. The mouth muscles were simply too weak to produce anything but small amounts of suction.

The findings have intriguing implications for fish evolution and neurobiology and illustrate the limits of muscle in many species.

For example, the results may explain why these fish have evolved the rather complex arrangement of bones in their mouths the way they have. The array apparently works quite well for rapidly opening when yanked upon by those big body muscles via the linkage.

“Our paper is the starting point for answering that question: With the fundamental design of the fish head, should it really be primarily explained on the basis of transmitting this power from the swimming muscles to the head?” Brainerd says.

Like a stomach crunch

Meanwhile in the paper, the researchers marvel at the feat of the neuromuscular control that the fish must achieve to be able to abruptly switch from swimming in pursuit of prey to using the very same muscles for producing suction. The muscles must execute very different movements for each activity.


“It’s like they are doing a stomach crunch to open their mouth,” says coauthor Thomas Roberts, professor of ecology and evolutionary biology.

In broader considerations of anatomy, the study highlights a point made not only by bass, but also by baseball. Smaller muscles can only do so much on their own, Roberts adds. Just as bass apparently evolved to draw upon their body’s swimming muscles to produce suction, so, too, do pitchers and hitters learn how to move much more than just their arms to throw and hit as powerfully as possible.

“There’s only so much power you can get out of muscle,” Roberts says. “If you need a really powerful activity, as these fish do, you need a really elaborate pattern of evolution that allows you to recruit more muscles from outside of the head.”

The National Science Foundation funded the study.

Source: Brown University