The inside of a shark is full of curiosities, starting with rows of hardworking teeth that can be replaced by fresh ones throughout its life. But quite a bit farther down the digestive tract — in fact, right before the shark ends — lies another odd structure: the spiral intestine, an intricate staircase made of shark flesh.
Scientists have speculated that sharks have such intricately shaped intestines to slow down digestion, eking every last calorie out of their prey. It may even be one reason sharks can go a long time between meals.
But on Wednesday in the Proceedings of the Royal Society B, researchers published one of the most detailed looks at those spiral intestines so far by turning a CT scanner on them, revealing the complex inner geographies of more than 20 species of sharks. After filling the intestines with fluid, they also made a discovery: Some of them function like natural versions of a valve that Nikola Tesla patented in 1920, drawing fluid ever onward in one direction without moving parts.
Samantha Leigh, an assistant professor at California State University, Dominguez Hills who led the new study, said that researchers who study sharks’ spiral intestines often refer to a set of 1885 anatomical drawings. Or they may dissect the intestines themselves, marring the organ’s structural integrity in the service of getting a closer look. To see the structures whole, she and her colleagues carefully removed the intestines of numerous shark species and imaged them in a CT scanner.
Sharks’ spiral intestines come in four flavors — a basic spiral, a nested series of funnels pointing one way, a nested series of funnels pointing the other way, and what’s called a scroll intestine, where layered sheaths nestle within each other. In the CT scans, the whorls and folds of the structures come through clearly.
It did not seem to matter what a shark ate when it came to the shape of their intestine — bonnethead sharks, which eat both plants and other animals, had a scroll intestine, as did carnivorous hammerheads.
Then the researchers hooked up some spiral intestines to tubes and watched as a mixture of water and glycerol flowed through them. They found that indeed, the fluid moved more slowly through the spiral than through a straight section of the shark’s intestine, supporting the idea that spiral intestines help sharks stretch their digestion time out.
However, they also found that the funnel intestines had a preferred direction for flow. Fluid entering one end flowed much more slowly than fluid entering the other, implying that within the animal, the intestine functions like a one-way street. In mammals, muscular contractions produce this effect. But in sharks, the structure of the intestine itself may be helping.
In fact, the shape of the funnel intestine recalls the loops of the Tesla valve, a kind of pipe patented by the Serbian-American inventor.
“The purpose of the valve was to produce flow in one direction without the use of extra mechanical parts or extra added energy,” Dr. Leigh said. “That seems very similar to how these shark intestines are shaped.”
Structures honed by eons of evolution can provide inspiration to engineers — the spectacularly uncloggable filters of the manta ray, for example, may provide a way to sift out plastic pollution before it reaches waterways. In the case of shark intestines, said Dr. Leigh, who also studies the effects of microplastic pollution on fish, it may be that further information about how the intestines work can inform filters as well.
“My hope is to figure out what these particular morphologies are good at moving along, what they’re good at filtering out,” Dr. Leigh said. Perhaps somewhere along the line, shark intestines could inspire tools to help remove plastics from water passively, just by virtue of the way they are built.
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