RESEARCH: The bulging eyes of a stingray make it swim fast

The more streamlined an animal, the faster it is. To be streamlined means that the shape of the animal has smooth, flowing lines that enable it to reduce resistance to movement (called drag), such as through water or air. 

Research scientists have found that the bulging eyes and mouth of a stingray makes it a faster swimmer. This seems impossible, because any part of a body that is protruding (sticking out) usually makes an animal slower.

Bluespotted Ribbontail Ray

A stingray has smooth flexible fins which makes it an efficient swimmer. However, its eyes and mouth stick out, which was thought to create drag and slow it down. Scientists have found that the bulging eyes actually makes it swim faster.  

Scientists at the Korea Advanced Institute of Science and Technology in South Korea conducted research on stingrays. They were studying the hydrodynamic actions of their swimming movements.

Hyung Jin Sung and his colleagues did computer simulations of the stingray body shape in three different ways: (1) bulging eyes and mouths (2) either bulging eyes and not a bulging mouth or not bulging eyes and a bulging mouth – i.e. only one or the other was bulging, and (3) completely smooth with no bulging eyes or mouth.

The scientists found that the bulges, or protrusions, created a set of vortices when the water moved over the stingrays. A vortex is a mass of water that spins very fast and pulls water into its centre. (Vortex is one and vortices is several of them.)

The scientists found that one vortex pushed water to the back of the stingray, leaving a zone of low pressure in front of it, and this zone allowed the simulated rays to swim faster. Another vortex pushed water to the sides of the stingray, increasing the pressure below its body and decreasing the pressure above its body. The made each stroke of the fins generate more thrust, which increased the efficiency and speed of the swimming motion. 

When Hyung Jin Sung analysed the results, he said that the magnitude (great extent or great difference) was strong. The analysis showed that the stingray’s eye protrusions increased its propulsion efficiency (thrust through the water) by 20.5% more than a completely smooth stingray. Also, the stingray’s bulging mouth increased its propulsion efficiency by about 10.6% more than a completely smooth stingray.

Hyung Jin Sung says he was “very surprised” at the results. 

The researchers say that the vortices – the masses of spinning water – occur because the stingray has a ‘bendy’ flexible body with rigid (stiff) eyes and mouth. 

The research team at the Korea Advanced Institute of Science and Technology will now try to incorporate their knowledge of water propulsion into designs for efficient water vehicles that can mimic the motions of the stingrays. 

Journal reference: Physics of FluidsDOI: 10.1063/5.0061287

Bluespotted Ribbontail Ray
Bluespotted Ribbontail Ray

Photographer: Martina Nicolls


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