Sharks have spiral-like intestines that resemble one of Nikola Tesla’s inventions

A CT scan image of the spiral intestine of a Pacific spiny dogfish shark (Squalus suckleyi). The structure is similar to the design of the Tesla one-way valve. Credit: Samantha Leigh/California State University Dominguez Hills.

Despite years of research, the exact structure and mechanics of shark intestines have been poorly understood until now. In a new study, researchers employed modern imaging tools to peer through the guts of various sharks, revealing that the spiral-shaped intestines remarkably function in a similar way to a Nikola Tesla patent for an unusual valve with no moving parts.

Marine biologists have had to rely on 2D sketches in order to study sharks’ digestive systems. Understanding how sharks’ intestines function has far-reaching consequences because these top-of-the-food-chain apex predators impact other species through what they eat and excrete.

Researchers at California State University Dominguez Hills, the University of Washington, and the University of California, Irvine embarked on a new study to fill in gaps in our knowledge that have eluded scientists for more than a century.

The reason why shark intestines are difficult to study has to do with their complex structure, with many overlapping layers. Dissecting a shark can destroy the context and connectivity of the tissue, which would be like “trying to understand what was reported in a newspaper by taking scissors to a rolled-up copy. The story just won’t hang together,” said Adam Summers, a professor at the University of Washington’s Friday Harbor Labs and co-author of the new study.

But this is where modern tools come in. The researchers used computerized tomography (CT) scanner to create 3D scans of intestines from nearly three dozen shark species. This machine takes X-ray images from different angles, then a computer algorithm stitches these different images together to create a 3-D image without harming or disturbing the tissues in any way or form. Before scanning them, the researchers made sure to fill the intestines and freeze-dried them in order to preserve their natural shape.

“It’s high time that some modern technology was used to look at these really amazing spiral intestines of sharks,” said lead author Samantha Leigh, assistant professor at California State University Dominguez Hills. “We developed a new method to digitally scan these tissues and now can look at the soft tissues in such great detail without having to slice into them.”

These scans serve to explain the odd shape of the shark’s intestines. Unlike most animals that have tubular intestines, sharks have spiral-shaped intestines that slow down the food as it moves downward through the gut due to gravity and peristalsis (the contraction of the gut’s smooth muscles). This shape allows food to move in only one way aided by gravity with virtually no energy expenditure.

That’s a very similar design to the “valvular conduit”, also known as a “Tesla valve”, patented by Nikola Tesla in 1920. Tesla’s invention is a one-way fluid valve with no moving parts consisting of a pipe with an intricate series of diverting teardrop-shaped loops.

Not all shark intestines are the same, though. There are four different kinds of spiral intestines, the researchers found. These organs can be columnar (like a spiral staircase), scroll (like a rolled-up sheet of paper), and either upward-facing or downward-facing funnels.

A CT scan image of a dogfish shark spiral intestine, shown from the top looking down. Credit: Samantha Leigh/California State University Dominguez Hills.

Sharks have wacky intestines for a reason. The researchers believe the spiral-shaped intestines allow sharks to hold food in their system for a longer time and absorb more nutrients than animals with tube-shaped digestive systems. Sharks are known to go days or even weeks without food if they have to, and their Tesla-valve-like intestines may play an important role.

The researchers also performed experiments to see with their own eyes how this spiral structure works. Intestines from five recently euthanized Pacific spiny dogfish sharks (Squalus suckleyi) were filled with colored liquids with different thicknesses. This allowed them to observe that the intestines “mix and churn” the liquids rather than pushing them along.

Beyond learning more about how sharks function, these findings have important implications for marine ecology. As top predators of the open ocean, sharks eat anything from fish and mammals to seagrass.

“The vast majority of shark species, and the majority of their physiology, are completely unknown. Every single natural history observation, internal visualization and anatomical investigation shows us things we could not have guessed at,” Summers said. “We need to look harder at sharks and, in particular, we need to look harder at parts other than the jaws, and the species that don’t interact with people.”

The researchers claim that this research could also inspire new technology that mimics shark intestine function, which may prove useful in certain industries where matter needs to flow in one direction with only minimal energy use. Potential applications include filtering microplastics from the water and industrial fluid-pump technologies.

The findings appeared in the journal Proceedings of the Royal Society B.

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