Engineers at the University of California, Riverside (UCR) have discovered key insights into how stingrays swim, revealing that their undulating movements aren’t just for show—they’re a refined adaptation to maintain stability in challenging underwater environments. A custom-built robotic fin, designed to mimic ray locomotion, was tested in controlled water tunnels, leading to the surprising finding that rays near the seafloor actively counteract downward forces by subtly tilting their fins upwards.
The Evolution of Ray Locomotion
Stingrays exhibit distinct swimming styles depending on their habitat. While pelagic (open-ocean) rays glide with flapping motions, benthic (bottom-dwelling) rays utilize a wave-like undulation that aligns with seafloor currents. This undulating style is notably efficient, recycling energy from the water to reduce drag. Researchers suspected this divergence was an evolutionary response to the physics of different environments, a theory they set out to prove.
The Robotic Fin Experiment
To test their hypothesis, the UCR team created a 9.5-millimeter-thick silicone robotic fin and submerged it in a specialized water tunnel simulating ocean flow. The intention was to observe how lift affected the fin at varying depths. Unexpectedly, the fin experienced a downward pull near the seafloor—the opposite of what was predicted.
By adjusting the fin’s angle, they found that tilting it upward by just a few degrees neutralized the negative lift. This suggests that natural stingrays instinctively swim with a slight upward fin angle, allowing them to overcome the pressure pushing them towards the seabed. The undulating motion also consistently provided greater clearance from the seafloor than flapping, reinforcing its effectiveness in benthic environments.
Implications for Robotics and Beyond
These findings have significant implications for underwater vehicle design. The principles behind stingray swimming could inspire more energy-efficient and stealthier robots. Researchers are already exploring these possibilities, with prior work including tissue-based and biohybrid robots powered by heart cells, electrodes, or even rat muscles. The ultimate goal is to create submersible vehicles that mimic a ray’s natural efficiency and quiet operation.
“Nature seems to have already solved the problem,” Yuanhang Zhu, a UCR mechanical engineer, stated. This emphasizes the enduring relevance of biological systems as blueprints for future technologies.
The study highlights how evolution optimizes solutions to physical challenges, and how understanding those solutions can drive innovation in robotics and underwater engineering.
