The flight of the sea butterfly

Sea butterflies — free-swimming mollusks that belong to the order Pteropoda — use their unique winged structures for movement, propelling themselves around in the water by “flapping.”

Even with detailed optical imaging, the hydrodynamics surrounding these pteropods are hard to understand, so Li et al. decided to simulate the sea butterfly’s swimming motion.

“One of our motivations was to understand how these organisms generate propulsion in water and how the surrounding vortex structures contribute to their swimming efficiency,” said author Wenbin Mao.

Using detailed imaging data from author David Murphy’s lab, the researchers reconstructed the sea butterfly’s wing motion and simulated how the wing motion affects the surrounding flow.

They observed that pteropods work like airplanes, primarily relying on lift-based propulsion — even at lower Reynolds numbers. Swimming pteropods typically sit at intermediate Reynolds numbers, where neither inertia nor viscosity dominates the behavior of the surrounding flow.

“We also found that the flexibility of the wings and the interaction between the two wings help delay aerodynamic stall — sudden loss of lift — and enhance lift production during the flapping cycle,” Mao said.

These findings suggest that, sometimes, flapping wing movements are more efficient than traditional propellors. This can help engineers designing underwater vehicles.

In addition to refining their model, the researchers want to study more swimming methods of pteropods.

“These animals may adopt different flapping strategies depending on environmental conditions such as turbulence, migration behavior, or escaping predators,” Mao said. “Understanding how flexibility and different swimming strategies influence propulsion will help us build a more complete picture of how these animals move through water.”

Source: “Flapping hydrodynamics and efficiency in a pteropod: A computational study of lift-based swimming,” by Zongze Li, David Murphy, and Wenbin Mao, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0313164 .