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Sharks’ spiral-shaped intestines resemble a Nikola Tesla invention

A CT scan of the spiral intestine of a Pacific spiny dogfish shark (Squalus suckleyi)

Samantha Leigh/California State

Sharks have spiral-shaped intestines that work in a similar way to an unusual valve designed by Nikola Tesla. Studying their anatomy could help improve industrial fluid-pump technology.

Most animals have tubular intestines that use muscle contractions to push food along like squeezing a tube of toothpaste. But sharks slowly channel their meals through spirals without needing muscles to push the food. Their intestines are also shaped in a way that only allows food to flow one way – like a performance-enhanced Tesla valve, says Samantha Leigh at California State University Dominguez Hills.

“Sharks have all these different little tweaks to the Tesla valve design that could be making them more efficient,” she says.

Leigh and her colleagues surgically removed the entire digestive tubes from 32 dead sharks that had either been donated to their project or conserved in the Natural History Museum of Los Angeles County, representing 22 shark families. Each has one of four different kinds of spiral intestine: columnar, which looks like a spiral staircase; scroll, which looks like a rolled-up sheet of paper; and two types with either upward-facing or downward-facing funnels.

The researchers filled all the intestines with fluid and freeze-dried them to keep their natural shape before scanning them using computed tomography. From those scans, they created virtual 3D models, offering the first glimpse of how these “natural” Tesla valves are structured and function, says Leigh.

Then, using an unfrozen sample of each of the four types of intestine, the researchers found that liquids took about 35 minutes to run through the spirals in the normal direction of flow. However, when they turned the spiral intestines upside down – opposite the normal flow – liquids took up to twice as long to pass through the two funnel types as they did previously.

The team also tested spiral intestine function in five recently euthanised Pacific spiny dogfish sharks (Squalus suckleyi) by running coloured liquids of various thicknesses through the spirals. Because the sharks had just died, their spiral muscles still reacted to the presence of the liquid. The researchers determined that these contractions “mix and churn” fluids as opposed to pushing them along, says Leigh. “Just the structure of the intestine itself will do that work of moving things along unidirectionally.”

Shark-inspired technology could be especially useful for industries requiring flow moving in one direction with minimal energy use, like filtering microplastics from waste water, she says.

While scientists still don’t know why sharks have these special intestines, they suspect they might extract the maximum amount of nutrients from food, helping them survive for weeks without a meal, says Leigh.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2021.1359

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