How stingrays became such sleek swimmers

With their smooth bodies and flexible fins, stingrays are among the slickest swimmers in the sea – but do their protruding eyes and mouths hinder their movement?

Scientists and stingray enthusiasts alike have long wondered this, and physics now has the answer: no.

“The influence of 3D protruding eyes and mouth on a self-propelled flexible stingray and its underlying hydrodynamic mechanism are not yet fully understood,” says Hyung Jin Sung, from the Korea Advanced Institute of Science and Technology.

“In the present study, the hydrodynamic benefit of protruding eyes and mouth was explored for the first time.”

Along with Chinese and South Korean colleagues, Sung created two computer models of stingrays swimming, where the “body” was clamped at the front and the rest was forced to oscillate up and down to mimic a stingray’s movements as it swims. One model gave the stingray eyes and a mouth, and the other didn’t.

Published in Physics of Fluids, the study then used an “immersed boundary method” to compare the interactions between the water and the stingray in each case. Such types of models are also used by scientists studying filamentary swimmers (like flagella) and other microorganisms.

Interestingly, the team found that when a stingray swims, its eyes and mouth generate beneficial changes in pressure and vorticity (a fluid’s tendency to rotate – if fluid doesn’t swirl around, vorticity is zero).

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“With no eyes and mouth, you get a large negative-pressure region on the top and a large positive-pressure region underneath the stingray body,” explains Sophie Calabretto, a fluid mechanics expert and Honorary Senior Lecturer at Macquarie University, who was not involved in the study.

A schematic showing a flexible rectangle oscillating like a stingray.
Schematic of the self-propelled flexible plate (the stingray) with eyes and mouth. Credit: Qi-an Mao

“Essentially, the negative-pressure zone generated at the leading edge enhances acceleration, while the positive-pressure zone limits acceleration.”

But the presence of eyes and mouth caused a pressure imbalance that also changed the vorticity; as the pressures tried to balance out, it created a spiralling flow at the body’s edges.

“This is the same thing that happens on plane wings and you get wingtip vortices,” Calabretto says.

“The net effect increases the negative-pressure zone and weakens the positive-pressure region, which increases the cruising speed.”

This means a stringray’s protruding features actually streamline it further. The researchers even put a number on it – the eyes and mouth increased overall propulsion efficiency by more than 20.5% and 10.6%, respectively.

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The study also found that there was an optimum shape of the eye and mouth for the biggest speed increase while swimming.

“One could probably speculate and suggest that evolution has had an effect in helping stingrays develop an optimal ‘shape’ for cruising,” Calabretto says.

“Nature is way ahead of us in terms of optimal design of things (thanks evolution!) so if we can use physics to understand the underlying mechanisms at play, not only does it shed light on how things work in our world at a fundamental level, it also means we can also pinch nature’s ideas when designing things for ourselves.”

The researchers of this study suggest that their findings could aid in the design of next-generation water vehicles.

“The high swimming efficiency of aquatic animals has received substantial attention in biomimetic designs, eg bioinspired underwater robots,” the authors write in their paper. “An in-depth understanding of the hydrodynamic role of the protruding eyes and mouths in self-propelled locomotion is highly desirable.”

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