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There are two types of ray: pelagic rays live high above the ocean floor and flap their fins in a smooth, bird-like motion, while benthic rays – which hug the seabed – undulate with the motions of the waves.
Yuanhang Zhu, an assistant professor of mechanical engineering at the University of California, Riverside, built a robotic fin that mimics the movement of rays. They tested it in a large water tunnel to simulate ocean flow and measured how different swimming motions affected lift.
They were surprised by the findings. Near the seafloor, rays experienced negative lift – that is, being sucked downward. This effect is the opposite of what birds experience when flying steadily near the ground. That difference, the researchers believe, is due to the dynamic way rays oscillate their fins, a phenomenon they describe as an “unsteady ground effect”.
“There are many reasons that rays might swim differently near the seabed that are unrelated to lift and thrust, so we weren’t sure whether we’d measure any differences between the swimming styles,” said Daniel Quinn, paper co-author and associate professor of mechanical engineering at the University of Virginia. “But the results were striking.”
“This wasn’t what we expected,” Zhu said. “Instead of gaining extra lift near the ground, the rays were pulled downward. But nature seems to have already solved the problem.” Real rays swim with a slight upward tilt – and the negative lift disappeared when the researchers mimicked the same tilt with their robots.
The research team observed that rays employing undulatory, wave-like propulsion achieved significantly better ground clearance than those using oscillatory, flapping strokes – a finding that mirrors how bottom-dwelling rays navigate cramped seafloor environments. During simulations where robots were released near the bottom of a tunnel, the undulatory models maintained a stable, level flight path, whereas the oscillatory swimmers quickly lost altitude and collided with the base.
Zhu suggests a future for marine robotics defined by “style-switching”. By toggling between different modes of movement, a single vehicle could mimic a pelagic ray to glide efficiently through open water, then transition to a benthic ray’s undulatory gait when approaching the seafloor. This versatility would be invaluable for underwater surveillance, deep-sea exploration and environmental research.
The ability to mimic the stingray’s low-altitude stability is vital for preventing floor-impact accidents. Even a minor collision can be devastating, potentially crippling delicate sensors, breaking mechanical fins or kicking up clouds of silt that obscure navigation cameras. For missions requiring high levels of stealth, the capacity to hover just inches above the seabed without making contact is often the deciding factor in a successful deployment.
“The results from this project help us think about why benthic rays may swim differently, and may also guide the design of ray-inspired robots that could be used to, say, map the ocean floor,” Quinn said.
“Usually, human-made robots cannot both swim in the middle of the ocean and manoeuvre easily near the ground,” Zhu said. “But, by watching how rays adjust between undulation and oscillation, we understand now how it is possible.”
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