JUL 19, 2017 6:48 AM PDT

Why Do Some Sharks 'Shrug' Their Shoulders When Swallowing?

WRITTEN BY: Anthony Bouchard

Not all sharks swallow their food in the same way, and researchers from Brown University are starting to take notice.

A new study published in the journal Proceedings of the Royal Society B reveals how bamboo sharks exhibit a distinct gesture when swallowing their food: a shrug of the shoulders.

A bamboo shark in its natural habitat.

Image Credit: Steve Childs/Wikimedia Commons

While bamboo sharks were feeding, the researchers used X-ray Reconstruction of Moving Morphology (XROMM), which is a fancy way of saying CT scans and high-resolution moving X-rays combined into a single package, to capture the behavior. XROMM was developed by Brown University to study biomechanics and see how bones and muscles move and interact with each other.

In particular, they noticed a slight 11º swinging motion of the shoulder girdle, a U-shaped system of bones, cartilage, and muscles that scientists once thought served no real purpose in sharks:

An illustration of how the bamboo shark's shoulder girdle "shrugs" as it swallows food.

Image Credit: Camp et. al.

Almost like saying “IDK," the shoulder girdle appears to ‘shrug’ while the food moves from the front of the mouth further into the digestive tract. Without a doubt, it was having an impact on the movement of the food.

“They have this long pharynx, and they have to keep food moving down it,” study lead author Ariel Camp explained. “We think this is part of a ‘hydrodynamic tongue.’ Sharks and fishes that don’t have a tongue control the motion of fluid within their mouths to manipulate food.”

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The study goes on to explain how this slight movement creates a powerful suction effect that helps the food get where it needs to go for digestion to take place. The process appears to be necessary since bamboo sharks lack a tongue, just like several other fish do.

Not only was this an important lesson learned about the shoulder girdle in bamboo sharks and potentially other suction-feeding fish, but it could pave the way for additional research into how the structure evolved in sharks.

“The girdle shows up [in the fossil record], around the time that jaws evolved,” Camp continued. “We aren’t sure exactly what structures it evolved from or how that happened. Part of understanding that history is understanding what were the functions this structure had to carry out.”

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It goes without saying that Brown’s XROMM technology played a significant role in discovering this fascinating clue, but there’s still so much to learn if we are to understand how the shoulder girdle works. A key detail is to identify whether it behaves in the same manner in other fish species with similar suction-feeding habits.

Source: Brown University

About the Author
Fascinated by scientific discoveries and media, Anthony found his way here at LabRoots, where he would be able to dabble in the two. Anthony is a technology junkie that has vast experience in computer systems and automobile mechanics, as opposite as those sound.
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