Miniature sea snails “fly” gracefully underwater like winged insects

limacina helicina.
limacina helicina.
Image: Alexander Semenov/Flickr, All rights reserved
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As it speeds through the ocean, the delicate limacina helicina looks more like a butterfly than a snail. And in fact, it does use the same “flying” motion as terrestrial insects, but according to mechanical engineer David Murphy, the 3mm-long sea snail likely developed the trick entirely separately, in a stunning example of convergent evolution.

On Feb. 17, Murphy revealed how the zooplankton manages to “fly” through the water, in the Journal of Experimental Biology. It uses a motion that biologists call the “clap-and-fling”: In the sea snail, as in terrestrial butterfly species, “wings” slap together behind the back and are then pulled apart quickly to create a vortex that produces lift.

The sea snail, Murphy found, accomplishes its insect-like wing stroke by rotating its body to an extreme degree. This motion is called “hyper-pitching” and no other known animal pitches to such an extent. It’s a finding that could pose interesting implications for the design of micro-drones.

Prior to his research, Murphy expected to find that the critter paddles through the water, he told BBC News. But it turns out the sea snail’s appendages function almost exactly like the wings of flying insects. ”I was really surprised,” Dr. Murphy said. “It turns out to be more of an honorary insect.”

The sea snail’s ”wings” are derived from the “foot” that other sea snails use to travel the seafloor, Murphy explained.

Murphy and his team used four high-speed cameras to take pictures of the sea snail as it moved through a tiny volume of fluid littered with shiny particles to trace flow movements.

“I was putting all of this information into my code and finally it came out with this plot—and I saw this beautiful figure-of-eight pattern, which I immediately recognized as something more like what a fruit fly does,” said Murphy.