Unlike the majority of zooplankton, the species Limacina helicina, a diminutive snail that lives in the cold oceans, utilizes a different type of locomotion to explore the waters. As their common name alludes to, these 3-mm marine mollusks flitter about their environment, flapping their wing-like appendages as an insect would in the air.
However, snails and flying insects split evolutionarily some 550 million years ago. The new research, performed by Georgia Institute of Technology (Georgia Tech) researchers and led by David W. Murphy, was recently published in the Journal of Experimental Biology, and is the first investigation into how these creatures “fly” beneath the ocean waves.
While Murphy was studying for his doctorate at Georgia Tech, one of his advisors—biological oceanographer Jeannette Yen—was interested in the sea butterfly’s locomotion. Previously, she recorded high-speed videos of the creatures moving about while tethered.
“It’s hard to draw conclusions from animals when they’re tethered, since they often behave quite differently than when they are freely swimming,” Murphy told R&D Magazine. “I built a system (a tomographic particle image velocimetry system) which could measure the flow around tiny, peppercorn-sized creatures like the sea butterfly as part of my PhD, and so we were eager to measure the flow around a freely swimming sea butterfly. In sum, we had suspected that the sea butterfly might be generating lift like an insect, but we weren’t sure until we measured its swimming kinematics and the flow it produced while swimming.”
As opposed to paddling, the sea butterfly generates lift in two ways similar to flying insects. First, it generates circulation around the wings, but it also uses transient vortices at its wingtips to generate lift during a power stroke, which is achieved through the clap-and-fling mechanism.
“In the clap-and-fling mechanism,” Murphy explained, “the wings approach each other closely (the clap) and the(n) rapidly rotate away from each other. This forms an opening V-shape which sucks flow into the gap. The flow of water into the gap causes flow separation over the wingtips, which creates lift-generating vortices.”
The technique, according to Murphy, is thought to be more efficient at generating lift than drag-based paddling.
Murphy, who is now a postdoctoral fellow at Johns Hopkins University, and colleagues were able to capture the sea butterfly’s swimming on high-speed cameras during their experiment.
Unlike the fruit fly, which generates lift by rotating its wings, the sea butterfly rotates its body up to 60 degrees with each stroke. Since nearly two-thirds of its body is covered in shell, the creature sinks to the ocean floor when not moving.
“This body rotation, or hyper-pitching, places the wing in the proper position to perform the next half-stroke,” Murphy concluded. “Hyper-pitching could also help the sea butterfly evade predators, such as the sea angel. Even though it is larger and can swim faster, the sea angel is not as agile as the sea butterfly. By changing directions 4-5 times per seconds, the sea butterfly can dart off in a different direction if it senses a sea angel approaching.”
Next, the researchers hope to study how shell composition and fluid viscosity affect the sea butterfly’s locomotion, according to Georgia Tech.
Additionally, understanding the sea butterfly’s locomotion may have implications for micro aerial vehicles (MAVs). “Pitching is usually avoided in MAVs, but the sea butterfly shows that it could actually be incorporated to provide some passive aerodynamic benefit,” said Murphy. “This is an example of biomimicry, where we can draw design inspiration from nature to solve human engineering problems.”
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